UNIVERSITY OF CALIFORNIA MEDICAL CENTER LIBRARY SAN FRANCISCO Digitized by the Internet Archive in 2007 with funding from IVIicrosoft Corporation http://www.archive.org/details/dancingmouseOOyerkrich THE DANCING MOUSE THE MACMILLAN COMPANY NEW YOKK • BOSTON • CHICAGO ATLANTA • SAN FRANCISCO MACMILLAN & CO., Limited LONDON • BOMBAY • CALCUTTA MELBOURNE THE MACMILLAN CO. OF CANADA, Ltd. TORONTO Dancing Mice — Sniffing and Eating. THE ANIMAL BEHAVIOR SERIES. VOLUME I THE DANCING MOUSE A Study in Animal Behavior BY ROBERT M. lYERKES, Ph.D. INSTRUCTOR IN COMPARATIvF^SYCHOLOGY IN HARVARD UNIVERSITY The Cartwright Prize of the Alumni Association of the College of Physicians and Surgeons, Columbia University, was awarded, in rgoj, for an Essay which comprised the first twelve chapters of this volume. THE MACMILLAN COMPANY 1907 All rights reserved Copyright, 1907, By the MACMILLAN COMPANY. Set up and electrotyped. Published October, 1907. • • • c • •• e c a • • • c • • • f « « • • • • • • •• • ■ • • »• • • • • Norinootj ^i^xtw J. 8. Gushing Co, — Berwick & Smith Co. Norwood, Mass., U.S.A. /9c7 IN LOVE AND GRATITUDE THIS BOOK IS DEDICATED TO MY MOTHER m\ PREFACE This book is the direct result of what, at the time of its occurrence, seemed to be an unimportant incident in the course of my scientific work — the presentation of a pair of dancing mice to the Harvard Psychological Laboratory. My interest in the peculiarities of behavior which the crea- tures exhibited, as I watched them casually from day to day, soon became experiment-impelling, and almost before I real- ized it, I was in the midst of an investigation of their senses and intelligence. The longer I observed and experimented with them, the more numerous became the problems which the dancers presented to me for solution. From a study of the senses of hearing and sight I was led to investigate, in turn, the various forms of activity of which the mice are capable ; the ways in which they learn to react adaptively to new or novel situations ; the facility with which they acquire habits ; the duration of habits; the roles of the various senses in the acquisition and performance of certain habitual acts; the efficiency of different methods of training ; and the inheritance of racial and individually acquired forms of behavior. In the course of my experimental work I discovered, much to my surprise, that, no accurate and detailed account of this curiously interesting animal existed in the English language, and that in no other language were all the facts concerning it available in a single book. This fact, in connection with my appreciation of the exceptional value of the dancer as a pet and as material for the scientific study of animal behavior, has led me to supplement the results of my own observation. viii Preface by presenting in this little book a brief and not too highly technical description of the general characteristics and his- tory of the dancer. The purposes which I have had in mind as I planned and wrote the book are three : first, to present directly, clearly, and briefly the results of my investigation ; second, to give as complete an account of the dancing mouse as a thorough study of the literature on the animal and long-continued observation on my own part should make possible ; third, to provide a supplementary text-book on mammalian behavior and on methods of studying animal behavior for use in con- nection with courses in Comparative Psychology, Compara- tive Physiology, and Animal Behavior. It is my conviction that the scientific study of animal be- havior and of animal mind can be furthered more just at present by intensive special investigations than by extensive general books. Methods of research in this field are few and surprisingly crude, for the majority of investigators have been more deeply interested in getting results than in per- fecting methods. In writing this account of the dancing mouse I have attempted to lay as much stress upon the de- velopment of my methods of work as upon the results which the methods yielded. In fact, I have used the dancer as a means of exhibiting a variety of methods by which the be- havior and intelligence of animals may be studied. As it happens the dancer is an ideal subject for the experimental study of many of the problems of animal behavior. It is small, easily cared for, readily tamed, harmless, incessantly active, and it lends itself satisfactorily to a large number of experimental situations. For laboratory courses in Compara- tive Psychology or Comparative Physiology it well might hold the place which the frog now holds in courses in Com- parative Anatomy. Preface ix Gratefully, and with this expression of my thanks, I ac- knowledge my indebtedness to Professor Hugo Miinsterberg for placing at my command the resources of the Harvard Psychological Laboratory and for advice and encouragement throughout my investigation; to Professor Edwin B. Holt for valuable assistance in more ways than I can mention ; to Professor Wallace C. Sabine for generous aid in connec- tion with the experiments on hearing ; to Professor Theobald Smith for the examination of pathological dancers ; to Miss Mary C. Dickerson for the photographs of dancing mice which are reproduced in the frontispiece; to Mr. Frank Ash- more for additional photographs which I have been unable to use in this volume ; to Mr. C. H. Toll for the drawings for Figures 14 and 20; to Doctors H. W. Rand and C. S. Berry for valuable suggestions on the basis of a critical read- ing of the proof sheets; and to my wife, Ada Watterson Yerkes, for constant aid throughout the experimental work and in the preparation of this volume. R. M. Y. Cambridge, Massachusetts, August, 1907. CONTENTS PAGE List of Illustrations xvii Literature on the Dancing Mouse xix CHAPTER I Characteristics, Origin, and History i Peculiarities of the dancing mouse — Markings and method of keeping record of individuals — The dancer in China and Japan (Kishi, Mitsukuri, Hatai) — Theories concerning the origin of the race : selectional breeding ; the inheritance of an acquired character ; mutation, inheritance, and selectional breeding ; pathological changes ; natural selection — In- stances of the occurrence of dancers among other kinds of mice — Results of crossing dancer with other kinds of mice. CHAPTER II Feeding, Breeding, and Development of the Young . i6 Methods of keeping and caring for dancers — Cages, nest- boxes, and materials for nest — Cleansing cages — Food supply and feeding — Importance of cleanliness, warmth, and pure food — Relations of males and females, fighting — The young, number in a litter — Care of young — Course of de- velopment — Comparison of young of dancer with young of common mouse — Diary account of the course of develop- ment of a typical litter of dancers. CHAPTER III Behavior : Dance Movements 29 Dancing — Restlessness and excitability — Significance of restlessness — Forms of dance : whirling, circling, and figure- eights — Direction of whirling and circling: right whirlers, left whirlers, and mixed whirlers — Sex differences in danc- ing — Time and periodicity of dancing — Influence of light on activity — Necessity for prolonged observation of behavior. xii Contettls CHAPTER IV PAGE Behavior: Equilibration and Dizziness .... 41 Muscular coordination — Statements of Cyon and Zoth concerning behavior — Control of movements, orientation, equilibration, movement on inclined surfaces, climbing — The tracks of the dancer — Absence of visual dizziness — Comparison of the behavior of the dancer with that of the common mouse when they are rotated in a cyclostat — Be- havior of blinded dancers (Cyon, Alexander and Kreidl, Kishi) — Cyon's two types of dancer — Phenomena of be- havior for which structural bases are sought: dance move- ments ; lack of response to sounds ; deficiency in equilibrational ability ; lack of visual and rotational dizziness. CHAPTER V Structural Peculiarities and Behavior .... 52 The functions of the ear — Structure of the ear of the dancer as described by Rawitz, by Panse, by Baginsky, by Alexander and Kreidl, and by Kishi — Cyon's theory of the relation of the semicircular canals to space perception — Con- dition of the auditory organs — Condition of the equilibra- tional organs — Condition of the sound-transmitting organs — The bearing of the results of anatomical investigations upon the facts of behavior. CHAPTER VI The Sense of Hearing 73 Experiments on hearing in the dancer made by Rawitz, by Panse, by Cyon, by Alexander and Kreidl, by Zoth, and by Kishi — Hearing and the voice — Methods of testing sen- sitiveness to sounds — Results of tests with adults — Im- portance of indirect method of experimentation — Results of tests with young — The period of auditory sensitiveness — Individual differences. CHAPTER VII The Sense of Sight: Brightness Vision .... 91 What is known concerning sight in the dancer — Bright- ness vision and color vision — Methods of testing brightness Contents xiii PAGE vision, the visual discrimination apparatus — Motives for discrimination and choice — Punishment versus reward as an incentive in animal experiments — Hunger as an incentive — An electric stimulus as an incentive — Conditions for bright- ness vision tests — White-black vision — Evidence of prefer- ence — Check experiments — Conclusion. CHAPTER Vni The Sense of Sight: Brightness Vision {Continued) . 113 The delicacy of brightness discrimination — Methods of testing the dancer's ability to detect slight differences in brightness — Results of tests with gray papers — Relation of intensity of visual stimuli to the threshold of discrimina- tion — Weber's law apparatus and method of experimentation — Results of Weber's law tests — Practice effects, the train- ing of vision — Description of the behavior of the dancer in the discrimination box experiments — Modes of choice : by afifirmation ; by negation ; by comparison — Evidence of indiscriminable visual conditions. CHAPTER IX The Sense of Sight : Color Vision 133 Does the dancer see colors? — The food-box method of testing color vision — Waugh's food-box method — Results of tests — Tests by the use of colored papers in the visual discrimination box — Yellow-red vision — Blue-orange vision — Brightness vision versus color vision — Brightness check tests — Green-blue vision — Violet-red vision — Conclusions. CHAPTER X The Sense of Sight: Color Vision {Continued) . .151 The use of color filters — Testing color vision by the use of transmitted light — Green-blue vision — Green-red vision — Blue-red vision — Stimulating value of different portions of the spectrum — Does red appear darker to the dancer than to us ? — Conclusions concerning color vision — Structure of the retina of the dancer and its significance. xiv C 071 tents CHAPTER XI The Role of Sight in the Daily Life of the Dancer . Sight and general behavior — Behavior of blinded dancers — Experimental tests of ability to perceive form — Visual guidance in mazes — Following labyrinth paths in the dark — The relative importance of visual, olfactory, and kinaesthetic stimuli — Conditions for the acquisition of a motor habit — Conditions for the execution of an habitual act. CHAPTER Xn Educability: Methods of Learning The modifiability of behavior — Educational value of ex- perimental studies of modifiability — Methods: the problem method; the labyrinth method; the discrimination method — Relation of method to characteristics of animal — Simple test of the docility of the dancer — Lack of imitative tendency — Persistence of useless acts — Manner of profiting by expe- rience — Individual differences in initiative. CHAPTER XIII Habit Formation: The Labyrinth Habit . . . . The labyrinth method — Problems — Preliminary tests — Comparison of the behavior of the dancer in a maze with that of the common mouse — Evolution of a labyrinth method — Records of time and records of errors — Simple and effec- tive method of recording the path — Curves of habit formation — Regular and irregular labyrinths — Points for a standard labyrinth — Values and defects of the labyrinth method. CHAPTER XIV Habit Formation: The Discrimination Method Quantitative versus qualitative results — Motives — Pre- cautions — Preference — Results of systematic habit-forming experiments — Curves of habit formation — Meaning of ir- regularity in curve — Individual differences — Comparison of curves for discrimination habits with those for labyrinth habits — Averages — The index of modifiability as a measure of docility — Reliability of the index. Contents xv CHAPTER XV PAGE The Efficiency of Training Methods .... 239 Importance of measuring the efficiency of educational methods — Rapidity of learning and permanency of modifica- tions wrought by training — Results of a study of the efficiency of discrimination methods — Comparison by means of indices of modifiability — Number of tests per series versus number of series — Efficiency as measured by memory tests. CHAPTER XVI The Duration of Habits: Memory and Re-learning . 251 Measures of the permanency of modifications in behavior — The duration of brightness and color discrimination habits — The relation of learning to re-learning — Can a habit which has been lost completely be re-acquired with greater facility than it was originally acquired? — Relation of special train- ing to general efficiency — Does the training in one form of labyrinth aid the dancer in acquiring other labyrinth habits? CHAPTER XVII Individual, Age, and Sex Differences in Behavior . . 264 Individual peculiarities in sensitiveness, docility, and initia- tive — The relation of docility to age — The individual result and the average — How averages conceal facts — Sex differ- ences in docility and initiative — Individual differences of motor capacity which seem to indicate varieties — Is the dancer pathological ? CHAPTER XVIII The Inheritance of Forms of Behavior .... 278 Characteristics of the race — Inheritance of the tendency to whirl in a particular way — Tests of the inheritance of individually acquired forms of behavior. Index 285 ILLUSTRATIONS Dancing Mice — sniffing and eating .... Frontispiece FIGURE PAGB 1. Color patterns of dancers. Record blanks .... 4 2. Double cage, with nest-boxes and water dishes . . • 17 3. Double cages in frame .18 4. Photographs of dancers climbing (After Zoth) ... 43 5. Tracks of common mouse (After Alexander and Kreidl) . 44 6. Tracks of dancer (After Alexander and Kreidl) ... 44 7. The inner ear of the rabbit (Retzius) 54 8. The membranous labyrinth of the ear of the dancer (After Rawitz) 56 9. Same 57 10. Same ........... 57 11. Model of the ear of the dancer (After Baginsky) ... 58 12. Ear of the dancer (After Kishi) ...... 66 13. Ear of the dancer (After Kishi) 67 14. Discrimination box 92 1 5 . Ground plan of discrimination box ..... 93 16. Nendel's gray papers 115 17. Weber's law apparatus 119 18. Food-box apparatus . . . . . . . .134 19. Waugh's food-box apparatus . . . , . '135 20. Color discrimination apparatus 153 21. Ground plan of color discrimination apparatus . . .154 22. Cards for form discrimination 182 23. Labyrinth B • . . . 184 24. Labyrinth B on electric wires . . . . . .188 25. Labyrinth A 211 26. Curves of habit formation for labyrinth B . . . .217 27. Plan of labyrinth C, and path records . . . . .219 28. Labyrinth D 222 29. Curve of learning for white-black discrimination, twenty individuals 231 xvii xviii Illustrations FIGURE PAGE 30. Curve of learning for white-black discrimination, thirty individuals 233 31. Curve of habit formation for labyrinth D .... 235 32. Curves of learning and re-learning 257 33. Plasticity curves . . . . . . . , . 273 LITERATURE ON THE DANCING MOUSE 1. Alexander, G. und Kreidl, A. "Zur Physiologie des Laby- rinths der Tanzmaus." Archiv fur die gesammte Physiologie, Bd. 82: 541-552. 1900. 2. Alexander, G. und Kreidl, A. " Anatomisch-physiologische Studien uber das Ohriabyrinth der Tanzmaus." II Mittheilung. Archiv fUr die gesafnmte Physiologie, 'Qd. %^ : 509-563. 1902. 3. Alexander, G. und Kreidl, A. "Anatomisch-physiologische Studien iiber das Ohriabyrinth der Tanzmaus." Ill Mitthei- lung. Archiv fur die gesammte Physiologie, Bd. 88: 564-574. 1902. 4. Baginsky, B. "Zur Frage iiber die Zahl der Bogengange bei japanischen Tanzmausen. Centralblatt fur Physiologie, Bd. 1 6 : 2-4. 1902. 5. Bateson, W. " The present state of knowledge of colour-heredity in mice and rats." Proceedings of the Zoological Society of Lon- don,Wo\.2: 71-99. 1903. 6. Brehm, A. E. "Tierleben." Dritte Auflage. Saugetiere^ Bd. 2 : 513-514. 1890. 7. Brehm, A. E. " Life of Animals." Translated from the third German edition of the "Tierleben" by G. R. Schmidtlein. Mammalia, p. 338. Marquis, Chicago. 1895. 8. Cyon, E. de. " Le sens de Tespace chez les souris dansantes japonaises." Cinquantettaire de la Sociiti de Biologie (Volume jubilaire). p. 544-546. Paris. 1899. 9. Cyon, E. von. "Ohriabyrinth, Raumsinn und Orientirung." Archiv fur die gesammte Physiologie, Bd. 79 : 211-302. 1900. 10. Cyon, E. de. "Presentation de souris dansantes japonaises." Comptes rendus du XJII Congres International de Paris, Section de physiologie, p. 1 60-1 61. 1900. XX Literature on the Dancing Mouse 11. Cyon, E. von. "Beitrage zur Physiologic des Raumsinns." I Theil. " Neue Beobachtungen an den japanischen Tanzmau- sen." Archiv /lir die gesamnite FhystologieyBd.2><^: 427-453. 1902. 12. Cyon, E. DE. « Le sens de Tespace." Richef s " Dictionnaire de physiologie," T. 5 : 570-571. 1901. 13. Darbishire, a. D. Note on the results of crossing Japanese waltzing mice with European albino races. Biometrica, Vol. 2 : 101-104. 1902. 14. Darbishire, A. D. Second report on the result of crossing Japanese waltzing mice with European albino races. Biotnet- rtca,Yo\. 2: 165-173. 1903. 15. Darbishire, A. D. Third report on hybrids between waltzing mice and albino races. Biometrica, Vol. 2 : 282-285. 1903. 16. Darbishire, A. D. On the result of crossing Japanese waltzing with albino mice. Biometrica^YoX. '^•. 1-5 1. 1904. 17. GuAiTA, G. V. "Versuche mit Kreuzungen von verschiedenen Rassen der Hausmaus." Berichte der naturforschenden Gesell- schaft zu Freiburg /. ^., Bd. 10 : 317-332. 1898. 18. GuAlTA, G. V. "Zweite Mitteilung iiber Versuche mit Kreuzun- gen von verschiedenen Hausmausrassen." Berichte der natur- forschenden Gesellschaft zu Freiburg i. B.j Bd. 11 : 131-138. 1900. 19. Haacke, W. "Ueber Wesen, Ursachen und Vererbung von Albinismus und Scheckung und iiber deren Bedeutung fiir vererbungstheoretische und entwicklungsmechanische Fragen." Biologisches Centralblatt, Bd. 15: 44-78. 1895. 19a. Hunter, M. S. "A Pair of Waltzing Mice." The Century Magazine^ Vol. 73: 889-893. April, 1907. 20. Kammerer, p. " Tanzende Waldmaus und radschlagende Haus- maus." Zoologische Garten, Bd. 41 : 389-390. 1900. 21. KiSHi, K. "Das Gehororgan der sogenanntenTanzmaus." Zeit- schrift fur wissenschaftliche Zoologie, Bd. 71 : 457-485. 1902. 22. Landois, H. "Chinesische Tanzmause." Jahresbericht des Westfdlischen Provinzial-Vereins, Miinster, 1 893-1 894: 62-64. 22a. Lose, J. "Waltzing Mice." Country Life in America, Sep- tember, 1904. p. 447. Literature on the Dancing Mouse xxi 23. Panse, R. Zu Herrn Bernhard Rawitz' Arbeit : " Das Gehororgan der japanischen Tanzmause." Archiv fur Anatomie und Physi- ologie, Physiologische Abtheilung, 1901 : 139-140. 24. Panse, R. *'Das Gleichgewichts- und Gehororgan der japan- ischen Tanzmause." Munchener inedicinische Wochenschrift^ Jahrgang 48, Bd. i: 498-499. 1901. 25. Rawitz, B. "Das Gehororgan der japanischen Tanzmause." Archiv fur Anatomie und Physiologie, Physiologische Abthei- lung, 1899: 236-243. 26. Rawitz, B. " Neue Beobachtungen iiber das Gehororgan japan- ischer Tanzmause." Archiv fur Anatomie und Physiologie^ Physiologische Abtheilung, 1901, Supplement: 171-176. 27. Rawitz, B. "Zur Frage uber die Zahl der Bogengange bei japan- ischen Tanzmausen." Centralblatt fur Physiologic^ Bd. 15: 649-651. 1902. 28. Saint-Loup, R. " Sur le movement de manege chez les souris." Bulletin de la Societt Zoologique de France, T. 18: 85-88. 1893. 29. SCHLUMBERGER, C. "A propos d\m netzuke japonais." Me- moir es de la Sociite Zoologique de France, T. 7 : 63-64. 1894. 30. Weldon, W. F. R. Mr. Bateson's revisions of Mendel's theory of heredity. Biometrica,Yo\. 2: 286-298. 1903. 31. ZOTH, O. " Ein Beitrag zu den Beobachtungen und Versuchen an japanischen Tanzmausen." Archiv fur die gesammte Physi- ologic, ^di, ^d: 147-176. 1901. 32. Anonymous. "Fancy Mice: Their Varieties, Management, and Breeding." Fourth edition. London : L. Upcott Gill. No date. THE DANCING MOUSE CHAPTER I Characteristics, Origin, and History The variety of mouse which is known as the Japanese dancing or waltzing mouse has been of special interest to biologists and to lovers of pets because of its curious move- ments. Haacke in Brehm's "Life of Animals" (7 p. 337) ^ writes as follows concerning certain mice which were brought to Europe from China and Japan: "From time to time a Hamburg dealer in animals sends me two breeds of common mice, which he calls Chinese climbing mice (Chinesische Klettermause) and Japanese dancing mice (Japanische Tanzmause). It is true that the first are distinguished only by their different colors, for their climbing accomplishments are not greater than those of other mice. The color, how- ever, is subject to many variations. Besides individuals of uniform gray, light yellow, and white color, I have had specimens mottled with gray and white, and blue and white. Tricolored mice seem to be very rare. It is a known fact that we also have white, black, and yellow mice and occasionally pied ories, and the Chinese have profited by these variations of the common mouse also, to satisfy their fancy in breeding animals. The Japanese, however, who are no less enthu- siastic on this point, know how to transform the common ^ The reference numbers, of which 7 is an example, refer to the numbers in the bibliographic list which precedes this chapter. B I 2 The Dancing Mouse mouse into a really admirable animal. The Japanese dancing mice, which perfectly justify their appellation, also occur in all the described colors. But what distinguishes them most is their innate habit of running around, describing greater or smaller circles or more frequently whirling around on the same spot with incredible rapidity. Sometimes two or, more rarely, three mice join in such a dance, which usually begins at dusk and is at intervals resumed during the night, but it is usually executed by a single individual." As a rule the dancing mouse is considerably smaller than the common mouse, and observers agree that there are also certain characteristic peculiarities in the shape of the head. One of the earliest accounts of the animal which I have found, that of Landois (22 p. 62), states, however, that the peculiari- ties of external form are not remarkable. Landois further remarks, with reason, that the name dancing mouse is ill chosen, since the human dance movement is rather a rhyth- mic hopping motion than regular movement in a circle. As he suggests, they might more appropriately be called "circus course mice" (22 p. 63). Since 1903 I have had under observation constantly from two to one hundred dancing mice. The ori^nal pair was presented to the Harvard Psychological Laboratory by Doctor A. G. Cleghorn of Cambridge. I have obtained speci- mens, all strikingly alike in markings, size, and general behavior, from animal dealers in Washington, Philadelphia, and Boston. Almost all of the dancers which I have had, and they now number about four hundred, were wKite with patches, streaks, or spots of black. The black markings occurred most frequently on the neck, ears, face, thighs, hind legs, about the root of the tail, and occasionally on the tail itself. In only one instance were the ears white, and that in the case of one of the offspring of a male w^hich was distin- Characteristics, Origin, and History 3 guished from most of his fellows by the possession of one white ear. I have had a few individuals whose markings were white and gray instead of white and black. The method by which I was able to keep an accurate record of each of my dancers for purposes of identification and reference is illustrated in Figure i. As this method has proved very convenient and satisfactory, I may briefly de- scribe it. With a rubber stamp ^ a rough outline of a mouse, like that of Figure i A, was made in my record book. On this outline I then indicated the black markings of the individual to be described. Beside this drawing of the ani- mal I recorded its number, sex,^ date of birth, parentage, and history. B, C, and D of Figure i represent typical color patterns. D indicates the markings of an individual whose ears were almost entirely white. The pattern varies so much from individual to individual that I have had no trouble whatever in identifying my mice by means of such records as these. All of my dancers had black eyes and were smaller as well as weaker than the albino mouse and the gray house mouse. The weakness indicated by their inability to hold up their own weight or to cling to an object curiously enough does not manifest itself in their dancing; in this they are indefatigable. Frequently they run in circles or whirl about with astonishing rapidity for several minutes at a time. Zoth (31 p. 173), who measured the strength of the dancer * For the use of the plate from which this stamp was made, I am in- debted to Professor W. E. Castle, who in turn makes acknowledgment to Doctor G. M. Allen for the original drawing. ^ I have found it convenient to use the even numbers for the males and the odd numbers for the females. Throughout this book this usage is followed. Wherever the sex of an individual is not specially given, the reader therefore may infer that it is a male if the number is even ; a female if the number is odd. The Dancing Mouse in comparison with that of the common mouse, found that it can hold up only about 2.8 times its own weight, whereas the common white mouse can hold up 4.4 times its weight. Figure i. — Typical markings of dancers. -4, blank outline of mouse for record. B, markings of No. 2 cT, born September 7, 1905, of unknown parents, died March 30, 1907. C, markings of No. 43 9, born November 10, 1906, of 212 and 211. D, markings of No. 151 ?, born February 28, 1906, of 1000 and 5, died Febniary 26, 1907. Characteristics^ Origin^ and History 5 No other accurate measurements of the strength, endurance, or hardiness of the dancer are available. They are usually supposed to be weak and delicate, but my own observations cause me to regard them as exceptionally strong in certain respects and weak in others. What the Japanese have to say about the dancing mouse is of special importance because Japan is rather commonly supposed to be its home. For this reason, as well as because of the peculiar interest of the facts mentioned, I quote at length from Doctor Kishi (21 p. 457). " The dancing mouse has received in Europe this name which it does not bear in its own home, because of the fact that the circular move- ments which it makes are similar to the European (human) dance. Sometimes it is also called the Japanese or Chinese mouse ; originally, however, China must have been its home, since in Japan it is mostly called ''Nankin nesumi,^ the mouse from Nankin. When this animal came from China to Japan I shall inquire at a later opportunity. There were origi- nally in Japan two different species of mouse, the gray and the white; therefore in order to distinguish our daj^ing mouse from these it was necessary to use the name of its native city. "In Japan, as in Europe, the animal lives as a house animal in small cages, but the interest which is taken in it there is shown in quite another way than in Europe, where the whirling movements, to which the name dancing mouse is due, are of chief interest. For this reason in Europe it is given as much room as possible in its cage that it may dance conveniently. In Japan also the circular movements have been known for a long time, but this has had no influence upon our interest in the animal, for the human fashion of dancing with us is quite different from that in Europe. What has lent interest to the creature for us are its prettiness, its 6 The Dancing Mouse cleverness in tricks, and its activity. It is liked, therefore, as an amusement for children. For this purpose it is kept in a small cage, usually fifteen centimeters square, sometimes in a somewhat broader wooden box one of whose walls is of wire netting. In this box are built usually a tower, a tunnel, a bridge, and a wheel. The wheel is rather broad, being made in the form of a drum and pierced with holes on one side through which the animal can slip in and out. Run- ning around on the inside, the mouse moves the wheel often for hours at a time, especially in the evening. Moreover, there are found in the box other arrangements of different kinds which may be set in motion by the turning of the wheel. No space remains in the box in which the animal may move about freely, and therefore one does not easily or often have an opportunity to observe that the animal makes circular movements, whether voluntarily or involuntarily. This is the reason that in its home this interesting httle animal has never been studied by any one in this respect." It is odd indeed that the remarkable capacity of the dancer for the execution of quick, graceful, dextrous, bizarre, and oft-repeated movements has not been utilized in America as it has in Japan. The mice are inexhaustible sources of amusement as well as invaluable material for studies in animal behavior and intelligence. Concerning the origin and history of this curious variety of mouse little is definitely known. I have found no men- tion of the animal in scientific literature previous to 1890. The fact that it is called the Chinese dancing mouse, the Japanese dancing mouse, and the Japanese waltzing mouse is indicative of the existing uncertainty concerning the origin of the race. Thinking that Japanese literature might furnish more in- formation bearing on the question of racial history than was Characteristics, Origin, and History 7 available from European sources, I wrote to Professor Mitsukuri of the University of Tokyo, asking him whether any reliable records of the dancer existed in Japan. He replied as follows: "I have tried to find what is known in Japan about the history of the Japanese waltzing mice, but I am sorry to say that the results are wholly negative. I cannot find any account of the origin of this freak, either authentic or fictitious, and, strange as it may seem to you, no study of the mice in a modern sense has been made, so you may consider the literature on the mouse in the Japanese language as absolutely nilP In explanation of this some- what surprising ignorance of the origin of the race in what is commonly supposed to be its native land. Professor Mit- sukuri adds: "The breeders of the mice have mostly been ignorant men to whom writing is anything but easy." In response to similar inquiries, I received the following letter, confirmatory of Professor Mitsukuri's statements, from Doctor S. Hatai of Wistar Institute, Philadelphia: "If I remember rightly the so-called Japanese dancing mouse is usually called by us N ankin-nedzumi {Nankin means any- thing which has been imported from China, and nedzumi means rat-Hke animal, or in this case mouse) or Chinese mouse. I referred to one of the standard Japanese diction- aries and found ttfe following statement: 'The N ankin- nedzumi is one of the varieties of Mus spiciosus {Hatszuka- nedzumi), and is variously colored. It was imported from China. These mice are kept in cages for the amusement of children, who watch their play.' Mus spiciosus, if I remem- ber correctly, is very much like Mus musculus in color, size, and several other characteristics, if not the same altogether." In Swinhoe's list of the mammals of China, which appeared in the Proceedings of the Zoological Society oj London for 1870, Mus musculus L. is mentioned as occurring in houses 8; The Dancing Mouse in South China and in Formosa. It is further stated that black and white varieties which are brought from the Straits are often kept by the Chinese (p. 637). The statements of Kishi, Mitsukuri, and Hatai which have been quoted, taken in connection with the opinions expressed by various European scientists who have studied the dancer, make it seem highly probable that the race appeared first in China, and was thence introduced into Japan, from which country it has been brought to Europe and America. Accept- ing for the present this conclusion with reference to the place of origin of the dancer, we may now inquire, how and when did this curious freak, as Professor Mitsukuri has called it, come into existence? Concerning these matters there is wide divergence of opinion. Haacke (6 p. 514), as quoted in Brehm's "Tierleben," says that an animal dealer with whom he discussed the question of the possible origin of the dancer maintained that it came from Peru, where it nests in the full cotton capsules, arrang- ing the cotton fibers in the form of a nest by running about among them in small circles. Hence the name cotton mouse is sometimes applied to it. Haacke himself believes, how- ever, that the race originated either in China or japan as the result of systematic selectional breeding. Of this he has no certainty, for he states that he failed to find any literature on the "beautiful mice of China and Japan." Whether Haacke' s description of the dancing mouse was pubHshed elsewhere previous to its appearance in Brehm's ''Tierleben" I am unable to state ; I have found nothing written on the subject by him before 1890. Zoth (31 p. 176) also thinks that the race was developed by systematic breeding, or in other words, that it is a product of the skill of the Asiatic animal breeders. Another account of the origin of the race is that accepted Characteristics^ Origin, and History 9 by Kishi (21 p. 481) and some other Japanese biologists. It is their behef that the forms of movement acquired by the individual as the result of confinement in narrow cages are inherited. Thus centuries of subjection to the conditions which Kishi has described (p. 6) finally resulted in a race of mice which breed true to the dance movement. It is only fair to add, although Kishi does not emphasize the fact, that in all probability those individuals in which the dancing tendency was most pronounced would naturally be selected by the breeders who kept these animals as pets, and thus it would come about that selectional breeding would supple- ment the inheritance of an acquired character. Few indeed will be willing to accept this explanation of the origin of the dancer so long as the inheritance of acquired characters remains, as at present, unproved. Still another mode of origin of the mice is suggested by the following facts. In 1893 Saint Loup (28 p. 85) advanced the opinion that dancing individuals appear from time to time among races of common mice. The peculiarity of movement may be due, he thinks, to an accidental nervous defect which possibly might be transmissible to the offspring of the exceptional individual. Saint Loup for several months had under observation a litter of common mice whose quick, jerky, nervous movements of the head, continuous activity, and rapid whirling closely resembled the characteristic move- ments of the true dancers of China. He states that these mice ran around in circles of from i to 20 cm. in diameter. They turned in either direction, but more frequently to the left, that is, anticlockwise. At intervals they ran in figure- eights (00 ) as do the true dancers. According to Saint Loup these exceptional individuals were healthy, active, tame, and not markedly different in general intelligence from the or- dinary mouse. One of these mice produced a litter of seven lo The Dancing Mouse young, in which, however, none of the peculiarities of behavior of the parents appeared. In view of this proof of the occurrence of dancing indi- viduals among common mice, Saint Loup believes that the race of dancers has resulted from the inheritance and ac- centuation of an "accidental" deviation from the usual mode of behavior. It is scarcely necessary to say that this opinion would be of far greater weight had he observed, instead of postulating, the inheritance of the peculiarities of movement which he has described. It might be objected, to the first of his so-called facts, that the litter resulted from the mating of mice which possessed dancer blood. Until the occurrence of dancers among varieties of mice which are known to be unmixed with true dancers is established, and further, until the inheritance of this peculiar deviation from the normal is proved, Saint Loup's account of the origin of the dancing mouse race must be regarded as an hypothesis. The occurrence of dancing individuals among common mice has been recorded by several other observers. Kammerer (20 p. 389) reports that he found a litter of young wood mice {Mus sylvaticus L.) which behaved much as do the spotted dancers of China. He also observed, among a lot of true dancers, a gray individual which, instead of spinning around after the manner of the race, turned somersaults at frequent intervals. It is Kammerer's opinion, as a result of these observations, that the black and white dancers of China and Japan have been produced by selectional breed- ing on the basis of this occasional tendency to move in circles. Among albino mice Rawitz (25 p. 238) has found individuals which whirled about rapidly in small circles. He states, however, that they lacked the restlessness of the Chinese dancers. Some shrews {Sorex vulgaris L.) which exhibited whirling movements and in certain other respects resembled Characteristics, Origin, and History 1 1 the dancing mouse were studied for a time by Professor Hacker of Freiburg in Baden, according to a report by von Guaita (17 p. 317, footnote). Doctor G. M. Allen of Cam- bridge has reported to me that he noticed among a large number of mice kept by him for the investigation of problems of heredity ^ individuals which ran in circles ; and Miss Abbie Lathrop of Granby, Massachusetts, who has raised thousands of mice for the market, has written me of the ap- pearance of an individual, in a race which she feels confident possessed no dancer blood, which whirled and ran about in small circles much as do the true dancers. Although it is possible that some of these cases of the unexpected appearance of individuals with certain of the dancer's peculiarities of behavior may have been due to the presence of dancer blood in the parents, it is not at all probable that this is true of all of them. We may, there- fore, accept the statement that dancing individuals now and then appear in various races of mice. They are usually spoken of as freaks, and, because of their inability to thrive under the conditions of life of the race in which they happen to appear, they soon perish. Another and a strikingly different notion of the origin of the race of dancers from those already mentioned is that of Cyon (11 p. 443) who argues that it is not a natural variety of mouse, as one might at first suppose it to be, but instead a pathological variation. The pathological nature of the animals is indicated, he points out, by the exceptionally high degree of variability of certain portions of the body. Ac- cording to this view the dancing is due to certain pathological structural conditions which are inherited. Cyon's belief raises the interesting question, are the mice normal or ab- » Allen, G. M. "The Heredity of Coat Color in Mice." Proc. Amer. Academy, Vol. 40, 59-163, 1904. 12 The Dancing Mouse normal, healthy or pathological ? That the question cannot be answered with certainty off-hand will be apparent after we have considered the facts of structure and function which this volume presents. Everything organic sooner or later is accounted for, in some one's mind, by the action of natural selection. The dancing mouse is no exception, for Landois (22 p. 62) thinks that it is the product of natural selection and heredity, favored, possibly, by selectional breeding in China. He further maintains that the Chinese dancer is a variety of Mus musculus L. in which certain pecuharities of behavior appear because of bilateral defects in the brain. This author is not alone in his belief that the brain of the dancer is de- fective, but so far as I have been able to discover he is the only scientist who has had the temerity to appeal to natural selection as an explanation of the origin of the race. Milne-Edwards, as quoted by Schlumberger (29 p. 63), is of the opinion that the Chinese dancer is not a natural wild mouse race, but instead the product of rigid artificial selec- tion. And in connection with this statement Schlumberger describes a discovery of his own which seems to have some bearing upon the problem of origin. In an old Japanese wood carving which came into his possession he found a group of dancing mice. The artist had represented in minute detail the characteristics of the members of the group, which consisted of the parents and eight young. The father and mother as well as four of the little mice are represented as white spotted with black. Of the four remaining young mice, two are entirely black and two entirely white. The two pure white individuals have pink eyes, as has also the mother. The eyes of all the others are black. From these facts Schlumberger infers that the dancer has resulted from the crossing of a race of black mice with a race of albinos ; the Characteristics, Origin, a^id History 13 two original types appear among the offspring in the carving. Experimental studies of the inheritance of the tendency to dance are of interest in their bearing upon the question of origin. Such studies have been made by Haacke (19), von Guaita (17, 18), and Darbishire (13, 14, 15, 16), and the important results of their investigations have been well sum- marized by Bateson (5). By crossing dancing mice with common white mice both Haacke and von Guaita obtained gray or black mice which are very similar to the wild house mouse in general appear- ance and behavior. The characteristic movements of the dancers do not appear. As the result of a long series of breeding experiments, Darbishire (16 pp. 26, 27) says : "When the race of waltzing mice is crossed with albino mice which do not waltz, the waltzing habit disappears in the resulting young, so that waltzing is completely recessive in Mendel's sense; the eye-color of the hybrids is always dark; the coat- color is variable, generally a mixture of wild-gray and white, the character of the coat being distinctly corre- lated with characters transmitted both by the albino and by the colored parent." When hybrids produced by the cross described by Darbishire are paired, they produce dancers in the proportion of about one to five. Bateson (5 p. 93, footnote), in discussing the results ob- tained by Haacke, von Guaita, and Darbishire, writes: ''As regards the waltzing character, von Guaita's experiments agree with Darbishire's in showing that it was always reces- sive to the normal. No individual in Fj [thus the first hybrid generation is designated] or in families produced by crossing Fi with the pure normal, waltzed. In Darbishire's experi- ments Fj X Fj [first hybrids mated] gave 8 waltzers in 37 off- spring, indicating i in 4 as the probable average. From von 14 The Dancing Mouse Guaita's matings in the form DR x DR the totals of families were 117 normal and 2 1 waltzers. . . . There is therefore a large excess of normals over the expected 3 to i. This is possibly due to the delicacy of the waltzers, which are cer- tainly much more difficult to rear than normals are. The small number in von Guaita's litters makes it very likely that many were lost before such a character as this could be determined." Bateson does not hazard a guess at the origin of the dancer, but merely remarks (5 p. 86) that the exact physiological basis of the dancing character is uncertain and the origin of this curious variation in behavior still more obscure. "Mouse fanciers have assured me," he continues, "that something like it may appear in strains inbred from the normal type, though I cannot find an indubitable case. Such an occurrence may be nothing but the appearance of a rare recessive form. Certainly it is not a necessary consequence of inbreeding, witness von Guaita's long series of inbred albinos." (von Guaita (17 p. 319) inbred for twenty-eight generations.) From the foregoing survey of the available sources of in- formation concerning the origin and history of the race of dancing mice the following important facts appear. Therp are four theories of the origin of the race: (i) origin by selec- tional breeding (Haacke, Zoth, Milne-Edwards) ; (2) origin through the inheritance of an acquired character (Kishi); (3) origin by mutation, inheritance, and selectional breeding (Saint Loup, Kammerer, Cyon) ; (4) origin by natural selec- tion, and inheritance, favored by selectional breeding (Landois). Everything indicates that the race originated in China. It is fairly certain that individuals with a tendency to move in circles appear at rare intervals in races of common mice. It seems highly probable, in view of these facts, that the Chinese took advantage of a deviation from the usual form Characteristics, Origin, and History 15 of behavior to develop by means of careful and patient selec- tional breeding a race of mice which is remarkable for its dancing. Even if it should be proved that the mutation as it appears among common mice is not inherited, the view that slight deviations were taken advantage of by the breeders would still be tenable. The dancing tendency is such in nature as to unfit an individual for the usual conditions of mouse existence, hence, in all probability human care alone could have produced and preserved the race of dancers. In answer to the question, how and when did the race of dancers originate, it may be said that historical research indicates that a structural variation or mutation which occa- sionally appears in Mus musculus, and causes those peculiari- ties of movement which are known as dancing, has been preserved and accentuated through selectional breeding by the Chinese and Japanese, until finally a distinct race of mice which breeds true to the dance character has been established. The age of the race is not definitely known, but it is supposed to have existed for several centuries. CHAPTER II Feeding, Breeding, and Development of the Young In this chapter I shall report, for the benefit of those who may wish to know how to take care of dancing mice, my experience in keeping and breeding the animals, and my observations concerning the development of the young. It is commonly stated that the dancer is extremely delicate, subject to diseases to an unusual degree and difficult to breed. I have not found this to be true. At first I failed to get them to breed, but this was due, as I discovered later, to the lack of proper food. For three years my mice have bred frequently and reared almost all of their young. During one year, after I had learned how to care for the animals, when the maximum number under observation at any time was fifty and the total number for the year about one hundred, I lost two by disease and one by an accident. I very much doubt whether I could have done better with any species of mouse. There can be no doubt, however, that the dancer is delicate and demands more careful attention than do most mice. In March, 1907, I lost almost all of my dancers from what appeared to be an intestinal trouble, but with this exception I have had remarkably good luck in breeding and rearing them. My dancers usually were kept in the type of cage of which Figure 2 is a photograph.^ Four of these double cages, * This cage was devised by Professors W. E. Castle and E. L. Mark, and has been used in the Zoological Laboratories of Harvard University for several years. • , 16 Feeding, Breeding, and Development 17 70 cm. long, 45 cm. wide, and 10 cm. deep in front, were supported by a frame as is shown in Figure 3. The fact that the covers of these cages cannot be left open is of practical importance. A similar type of cage, which I have used to some extent, consists of a wooden box 30 by 30 cm. by 15 cm. deep, without any bottom, and with a hinged cover made Figure 2. — Double cage, with nest boxes and water dishes. in part of i cm. mesh wire netting. Such a cage may be placed upon a piece of tin or board, or simply on a news- paper spread out on a table. The advantage of the loose bottom is that the box may be lifted off at any time, and the bottom thoroughly cleansed. I have had this type of cage constructed in blocks of four so that a single bottom and cover sufficed for the block. If the mice are being kept for show or for the observation of their movements, at least one side of the cages should be of wire netting, and, as Kishi suggests, such objects as a wheel, a tower, a tunnel, a bridge, and a turntable, if placed in the cage, will give the animals excellent opportunity to exhibit their capacity for varied forms of activity. i8 The Dancing Mouse The floors of the cages were covered with a thin layer of sawdust for the sake of cleanliness, and in one corner of each cage a nest box of some sort was placed. During the warm months I found it convenient and satisfactory to use berry boxes, such as appear in Figure 2, with a small en- trance hole cut in one side ; and during the cold months cigar boxes, with an entrance hole not more than 5 cm. in diameter at one end. In the nest box a quantity of tissue paper, torn into fragments, fur- nished material for a nest in which the adults could make themselves comfort- able or the female care for her young. Cotton should never be used in the nest box6s, for the mice are likely to get it Figure 3.-Double cages in frame. wOUnd about their legs with serious results. Apparently they are quite unable to free themselves from such an incumbrance, and their spin- ning motion soon winds the threads so tightly that the circulation of the blood is stopped. The cages and nest boxes were emptied and thoroughly cleaned once a week with an emulsion made by heating to- gether one part of kerosene and one part of water containing a Httle soap. This served to destroy whatever odor the cages had acquired and to prevent vermin from infesting the nests. In hot weather far greater cleanliness is necessary for the Feedings Breedings and Development 19 welfare of the mice than in cold weather. The animals attend faithfully to their own toilets, and usually keep them- selves scrupulously clean. For water and food dishes I have used heavy watch glasses* 5 cm. in diameter and \ cm. deep. They are convenient because they are durable, easily cleaned, and not large enough for the young mice to drown in when they happen to spin into one which contains water. It is said that mice do not need water, but as the dancers seem very fond of a little, I have made it a rule to wash the watch glasses thoroughly and fill them with pure fresh water daily. The food, when moist, may be placed in the cages in the same kind of watch glass. There is no need of feeding the animals oftener than once a day, and as they eat mostly in the evening and during the night, it is desirable that the food should be placed in the cage late in the afternoon. For almost a year I kept a pair of dancers on " force "^ and water. They seemed perfectly healthy and were active during the whole time, but they pro- duced no young. If the animals are kept as pets, and breed- ing is not desired, a diet of "force," ''egg-o-see,"^ and crackers, with some bird-seed every few days, is Hkely to prove satis- factory. As with other animals, a variety of food is beneficial, but it appears to be quite unnecessary. Too much rich food should not be given, and the mice should be permitted to dictate their own diet by reveahng their preferences. They eat surprisingly little for the amount of their activity. I have had excellent success in breeding the mice by feeding them a mixture of dry bread-crumbs, '' force," and sweet, clean oats slightly moistened with milk. The food should never be made soppy. A little milk added thus to the food every other day greatly increases fertility. About once a week a small ^ Minot watch glasses. ^ A cereal food. 20 The Dancing Mouse quantity of some green food, lettuce for example, should be given. It is well, I have found, to vary the diet by replacing the bread and ''force" at intervals with crackers and seeds. Usually I give the food dry every other day, except in the case of mice which are nursing litters. One person to whom I suggested that lettuce was good for the dancers lost four, apparently because of too much of what the mice seemed to consider a good thing. This suggests that it should be used sparingly. Success in keeping and breeding dancing mice depends upon three things: cleanliness, warmth, and food supply. The temperature should be fairly constant, between 60° and 70° Fahr. They cannot stand exposure to cold or lack of food. If one obtains good healthy, fertile individuals, keeps them in perfectly clean cages with soft nesting materials, maintains a temperature of not far above or below 65°, and regularly supplies them with pure water and food which they like, there is not Kkely to be trouble either in keeping or breed- ing these delicate little creatures. Several persons who have reported to me difficulty in rearing the young or in keeping the adults for long periods have been unable to maintain a sufficiently high or constant temperature, or have given them food which caused intestinal trouble. The males are likely to fight if kept together, and they may even kill one another. A male may be kept with one or more females, or several females may be kept together, for the females rarely, in my experience, fight, and the males seldom harm the females. Unless the male is removed from the cage in which the female is kept before the young are born, he is likely to kill the newborn animals. When a female is seen to be building a nest in preparation for a litter, it is best to place her in a cage by herself so that she may not be disturbed. Feedings Breedijig^ and Development 2i The sex of individuals may be determined easily in most cases, at the age of lo to 12 days, by the appearance of teats in the case of females. The period of gestation is from 18 to 21 days. The maxi- mum number born by my dancers in any single litter was 9, the minimum number 3. In 25 litters of which I have accurate records, 135 individuals were born, an average of 5.4. The average number of males per litter was precisely the same, 2.7, as the number of females. On the birth of a litter it is well to see that the female has made a nest from which the young are not likely to escape, for at times, if the nest is carelessly made, they get out of it or under some of the pieces of paper which are used in its construction, and perish. Several times I have observed nests so poorly built that almost all of the young perished because they got too far away to find their way back to the mother. It is surprising that the female should not take more pains to keep her young safe by picking them up in her mouth, as does the common mouse, and carrying them to a place where they can obtain warmth and nourishment. This I have never seen a dancing mouse do. For the first day or two after the birth of a litter the female usually remains in the nest box almost constantly and eats little. About the second day she begins to eat ravenously, and for the next three or four weeks she consumes at least twice as much food as ordinarily. Alexander and Kreidl (3 p. 567) state that the female does not dance during the first two weeks after the birth of a litter, but my experience contradicts their statement. There is a decreased amount of activity during this period, and usually the whirling movement ap- pears but rarely; but in some cases I have seen vigorous and long-continued dancing within a few hours after the birth of a litter. There is a wide range of variability in this matter, 22 The Dancing Mouse and the only safe statement, in the Hght of my observations, is that the mother dances less than usual for a few days after a litter is bom to her. The development of the young, as I have observed it in the cases of twenty litters, for ten of which (Table i) sys- tematic daily records were kept, may be sketched as follows. At birth the mice have a rosy pink skin which is devoid of hair and perfectly smooth; they are blind, deaf, and irresponsive to stimulation of the vibrissae on the nose. During the first week of post-natal life the members of a litter remain closely huddled together in the nest, and no dance movements are exhibited. The mother stays with them most of the time. On the fourth or fifth day colorless hairs are visible, and by the end of the week the body is covered with a coat which rapidly assumes the characteristic black and white markings of the race. For the first few days the hind legs are too weak to support the body weight, and whatever movements ap- pear are the result of the use of the fore legs. As soon as the young mice are able to stand, circling movements are exhib- ited, and by the end of the second week they are pronounced. Somewhere about the tenth day the appearance of the teats in the case of the females serves to distinguish the sexes plainly. Between the tenth and fifteenth days excitabihty, as indicated by restless jerky movements in the presence of a disturbing condition, increases markedly; the auditory meatus opens, and, in the case of some individuals, there are signs of hearing. On or after the fifteenth day the eyes open and the efforts to escape from the nest box rapidly become more vigorous. About this time the mother re- sumes her dancing with customary vigor, and the young, when they have opportunity, begin to eat of the food which is given to her. They now dance essentially as do the adults. From the end of the third week growth continues without Feeding, Breeding, and Development 23 notev^orthy external changes until sexual maturity is at- tained, between the fourth and the sixth v^eek. For several weeks after they are sexually mature the mice continue to increase in size. TABLE I Development of the Young Parents Number in Litter Hair Visible Teats Visible Jerky Move- ments Appear Ears Open React TO Sound Eyes Open cf ^ 152+151 152 + 151 410 + 415 410 + 415 420 + 425 210 + 215 210 + 215 212+ 211 220 + 225 220 + 225 5 I 2 4 3 I 2 3 3 I 4 2 I 3 3 4 3 4th day 4th day 5 th day 5th day 4th day 5th day 4th day 4th day 4th day 9th day nth day loth day loth day nth day loth day loth day loth day 13th day loth day 14th day 13th day 1 2th day 1 7th day nth day 15th day 1 6th day I '/th day 14th day 12th day 15th day 14th day 14th day 13th day 14th day X4th day 14th day 13th day 14th day 13th day 15th day 14th day 14th day 17th day No No No No 1 6th day 15 th day 1 7th day 1 6th day 1 6th day 15th day 1 6th day 15th day 1 5th day 1 5th day A course of development very similar to that just described was observed by Alexander and Kreidl (3 p. 565) in three litters of dancing mice which contained 3,5, and 7 individuals respectively. These authors, in comparing the development of the dancer with that of the common mouse, say that at birth the young in both cases are about 24 mm. in length. The young common mouse grows much more rapidly than the dancer, and by the ninth day its length is about 43 mm. as compared with 31 mm. in the case of the dancer. Accord- ing to Zoth (31 p. 148) the adult dancer has a body length of from 7 to 7.5 cm., a length from tip of nose to tip of tail of from 12 to 13 cm., and a weight of about 18 grams. The movement of the dancer from the first tends to take the form of circles toward the middle of the nest ; that of the common mouse has no definite tendency as to direction. When the 24 The Dancing Mouse common mouse does move in circles, it goes j5rst in one direc- tion, then in the other, and not for any considerable period in one direction as does the true dancer. Neither the young dancer nor the common mouse is able to equilibrate itself well for the first few days after birth, but the latter can follow a narrow path with far greater accuracy and steadiness than the former. The uncertain and irregular movements of the common mouse are due to muscular weakness and to blindness, but the bizarre movements of the young dancer seem to demand some additional facts as an explanation. A brief account of the development of the dancer given by Zoth (31 p. 149) adds nothing of importance to the descrip- tion given by Alexander and Kreidl. As my own observa- tions disagree with their accounts in certain respects, I shall now give, in the form of a diary, a description of the important changes observed from day to day in a normal litter. The litter which I have selected as typical of the course of develop- ment in the dancer grew rapidly under favorable conditions. I have observed many litters which passed through the various stages of development mentioned in this description anywhere from a day to a week later. This was usually due to some such obviously unfavorable condition as too little food or slight digestive or bowel troubles. According to the nature of the conditions of growth the eyes of the dancer open any- where from the fourteenth to the twentieth day. This state- ment may serve to indicate the degree of variability as to the time at which a given stage of development is reached by different litters. On July 14, 1906, No. 151 (female) and No. 152 (male) were mated, and on August 3 a litter of six was born to them. The course of the development of this litter during the first three weeks was as follows : — Feeding, Breeding, and Development 25 I First day. The skin is pink and hairless, several vibrissae are visible on the nose and lips, but there is no definite re- sponse when they are touched. The mice are both blind and deaf, but they are able to squeak vigorously. The mother was not seen to dance or eat during the day. Second day. There is a very noticeable increase in size. The vibrissae are longer, but touching them still fails to cause a reaction. No hairs are visible on the body. The mother danced rapidly for periods of a minute several times while the record was being made. She ate very little to-day. Third day. Scales began to appear on the skin to-day. The animals are rapidly increasing in strength ; they can now crawl about the nest easily, but they are too weak to stand, and constantly roll over upon their sides or backs when they are placed on a smooth surface. Because of their inability to progress it is impossible to determine with certainty whether they have a tendency to move in circles. The mother was seen out of the nest dancing once to-day. She now eats ravenously. Fourth day. One of the six young mice was found under a corner of the nest this morning dead, and the others were scattered about the nest box. I gathered them together into a nest which I made out of bits of tissue paper, and the mother immediately began to suckle them. They are very sensitive to currents of air, but they do not respond to light or sound and seldom to contact with the vibrissae. Fijth day. When placed on a smooth surface, they tend to move in circles, frequently rolling over. When placed on their sides or backs, they immediately try to right themselves. They do not walk, for their legs are still too weak to support the weight of the body; instead they drag themselves about by the use of the fore legs. Fine colorless hairs are visible over the entire body surface. When the vibrissae are touched, 26 The Dancing Mouse the head is moved noticeably. The mother dances a great deal and eats about twice as much as she did before the birth of the litter. Sixth day. Certain regions of the skin, which were slightly darker than the remainder on the fourth and fifth days, are now almost black. It is evident that they are the regions in which the black hair is to appear. The movement in circles is much more definite to-day, although most of the individuals are still too weak to stand on their feet steadily for more than a few seconds at a time. Most of their time, when they are first taken from the nest, is spent in trying to maintain or regain an upright position. The hair is now easily visible, and the skin begins to have a white appearance as a result. Seventh day. Although they are strong enough to move about the nest readily, none of the young has attempted to leave the nest. They huddle together in the middle of it for warmth. The epidermal scales, which have increased in number since the third day, are dropping oflf rapidly. Contact with the vibrissae or with the surface of the body frequently calls forth a motor reaction, but neither light nor sound produces any visible change in behavior. The black and white regioQs of the skin are sufficiently definite now to enable one to distinguish the various individuals by their markings. The mother was seen to dance repeatedly to-day, and she ate all the food that was given to her. Eighth day. A fold is plainly visible where later the eyelids will separate. The black pigment in the skin has increased markedly. Ninth day. The eyelids are taking form rapidly, but they have not separated. The body is covered with a thick coat of hair which is either pure white or black. Standing on the four legs is still a difficult task. Feedhig^ Breeding, and Development 27 Tenth day. To-day teats are plainly visible in the case of four of the five individuals of the litter. Up to this time I had thought, from structural indications, that there were three males and two females ; it is now evident that there are four females and one male. The external ear, the pinna, is well formed, and has begun to stand out from the head, but no opening to the inner portion of the ear is present. The eyelids appear to be almost fully formed. Eleventh day. There are no very noticeable changes in ap- pearance except in size, which continues to increase rapidly. They are able to regain their normal upright position almost immediately when they happen to roll over. The mother dances as usual. Tweljth day. It appears to-day as if the eyes were about to open. The ears are still closed, and there is no evidence of a sense of hearing. They squeaked considerably when in the nest, but not at all when I took them out to note their development. The mother stays outside of the nest box much of the time now, probably to prevent the young ones from sucking continuously. Thirteenth day. One of the little mice came out of the nest box while I was watching the litter this morning, and was able to find his way back directly despite the lack of sight. The mice are still dependent upon the mother for nourishment. I have not seen any of them attempt to eat the food which is given to the mother. They are extremely neat and clean. I watched one of them wash himself this morning. Each foot was carefully Hcked with the tongue. There seems to be special care taken to keep the toes perfectly clean. Fourteenth day. An opening into the ear is visible to-day. When tested with the Galton whistle, all five responded with quick, jerky movements of the head and legs. They evi- dently hear certain tones. During the past two days the ears 28 The Dancing Mouse have changed rapidly. In one of the females, which seems to be a little in advance of the others in development, certain peculiarities of behavior appeared to-day. She jumped and squeaked sharply when touched and sprang, out of my hand when I attempted to take her up. This is in marked contrast with her behavior previously. Fifteenth day. The eyes are partly opened. All of the members of the litter came out of the nest box this morning and ran around the cage, dancing frequently and trying to eat with the mother. Three out of the five gave auditory reactions on first being stimulated ; none of them responded to repetitions of the stimulus. All appeared to be less sensi- tive to sounds than yesterday. The quick, nerv^ous, jerky movements are very noticeable. Sixteenth day. The eyes of all five are fully opened. They dance vigorously and are outside the nest much of the time. Seventeenth day. No reactions to sound could be detected to-day. The sense of sight gives evidence of being well developed. The nervous jumping movements persist. Eighteenth day. The young mice continue to suck, al- though they eat of the food which is given to the mother. They are now able to take care of themselves. Nineteenth day. There are no noteworthy changes except increase in size and strength. Twentieth day. No auditory reactions were obtained to- day, but other forms of stimulation brought about unmis- takable responses. Twenty- first day. They are now about half grown and there is no other change of special interest to be recorded. Growth continues for several weeks. The statement made by Alexan- der and Kreidl to the efifect that the dancer is almost full grown by the thirty-first day of life is false. At that age they may be sexually mature, but usually they are far from full grown. CHAPTER III Behavior: Dance Movements The peculiarities of behavior of the dancing mouse are responsible alike for the widespread interest which it has aroused, and for its name. In a little book on fancy varieties of mice, in which there is much valuable information con- cerning the care of the animals, one who styles himself "An old fancier" writes thus of the behavior of the dancer: "I believe most people have an idea that the waltzing is a stately dance executed on the hind feet; this is not so. The per- former simply goes round and round on all fours, as fast as possible, the head pointing inwards. The giddy whirl, after continuing for about a dozen turns, is then reversed in direc- tion, and each performance usually occupies from one to two minutes. Whether it is voluntary or not, is difficult to determine, but I am inclined to think the mouse can refrain if it wishes to do so, because I never see them drop any food they may be eating, and begin to waltz in the midst of their meal. The dance, if such it can be called, generally seizes the mouse when it first emerges from its darkened sleeping place, and this would lead one to suppose that the light con- veys an impression of shock to the brain, through the eyes, which disturbs the diseased centers and starts the giddy gyrations. The mice can walk or run in a fairly straight line when they wish to do so." Some of the old fancier's state- ments are true, others are mere guesses. Those who have studied the mice carefully will doubtless agree that he has not adequately described the various forms of behavior of 29 30 The Dancing Mouse which they are capable. I have quoted his description as an illustration of the weakness which is characteristic of most popular accounts of animal behavior. It proves that it is not sufficient to watch and then describe. The fact is that he who adequately describes the behavior of any animal watches again and again under natural and experimental conditions, and by prolonged and patient observation makes himself so familiar with his subject that it comes to possess an individu- ality as distinctive as that of his human companions. To the casual observer the individuals of a strange race are almost in- distinguishable. Similarly, the behavior of all the animals of a particular species seems the same to all except the observer who has devoted himself whole-heartedly to the study of the subject and who has thus become as familiar with their life of action as most of us are with that of our fellow- men; for him each individual has its own unmistakable characteristics. I shall now describe the behavior of the dancing mouse in the light of the results of the observation of scores of indi- viduals for months at a time, and of a large number of ex- periments. From time to time I shall refer to points in the accounts of the subject previously given by Rawitz (25 p. 236), Cyon (9 p. 214), Alexander and Kreidl (i p. 542), Zoth (31 p. 147), and Kishi (21 p. 479). The most striking features of the ordinary behavior of the dancer are restlessness and movements in circles. The true dancer seldom runs in a straight line for more than a. few centimeters, although, contrary to the statements of Rawitz and Cyon, it is able to do so on occasion for longer distances. Even before it is old enough to escape from the nest it begins to move in circles and to exhibit the quick, jerky head movements which are characteristic of the race. At the age of three weeks it is able to dance vigorously, and is incessantly active when not washing itself, eating, or sleeping. I Behavior: Dance Movements 31 According to Zoth (31 p. 149) the sense of sight and es- pecially the sense of smell of the dancer "seem to be keenly developed; one can seldom remain for some time near the cage without one or another of the animals growing lively, looking out of the nest, and beginning to sniff around in the air (windet). They also seem to have strongly developed cutaneous sensitiveness, and a considerable amount of curiosity, if one may call it such, in common with their cousin, the white mouse." I shall reserve what I have to say concerning the sense of sight for later chapters. As for the sense of smell and the cutaneous sensitive- ness, Zoth is undoubtedly right in inferring from the behavior of the animal that it is sensitive to certain odors and to changes in temperature. One of the most notice- able and characteristic activities of the dancer is its sniffing. Frequently in the midst of its dancing it stops suddenly, raises its head so that the nose is pointed upward, as in the case of one of the mice of the frontis- piece, and remains in that position for a second or two, as if sniffing the air. The restlessness, the varied and almost incessant move- ments, and the peculiar excitability of the dancer have re- peatedly suggested to casual observers the question, why does it move about in that aimless, useless fashion? To this query Rawitz has replied that the lack of certain senses compels the animal to strive through varied movements to use to the greatest advantage those senses which it does pos- sess. In Rawitz's opinion the lack of hearing and orienta- tion is compensated for by the continuous use of sight and smell. The mouse runs about rapidly, moves its head from side to side, and sniffs the air, in order that it may see and smell as much as possible. In support of this interpretation of the restlessness of the dancer, Rawitz states that he once 32 The Dancing Mouse observed similar behavior in an albino dog which was deaf. This suggestion is not absurd, for it seems quite probable that the dancer has to depend for the guidance of its move- ments upon sense data which are relatively unimportant in the common mouse, and that by its varied and restless move- ments it does in part make up for its deficiency in sense equipment. The dancing, waltzing, or circus course movement, as it is variously known, varies in form from moment to moment. Now an individual moves its head rapidly from side to side, perhaps backing a little at the same time, now it spins around like a top with such speed that head and tail are almost in- distinguishable, now it runs in circles of from 5 cm. to 30 cm. in diameter. If there are any objects in the cage about or through which it may run, they are sure to direct the expression of activity. A tunnel or a hole in a box calls forth endless repetitions of the act of passing through. When two individuals are in the same cage, they frequently dance to- gether, sometimes moving in the same direction, sometimes in opposite directions. Often, as one spins rapidly about a vertical axis, the other runs around the first in small circles ; or again, both may run in a small circle in the same direction, so that their bodies form a living ring, which, because of the rapidity of their movements, appears perfectly continuous. The three most clearly distinguishable forms of dance are (i) movement in circles with all the feet close together under the body, (2) movement in circles, which vary in diameter from 5 cm. to 30 cm., with the feet spread widely,' and (3) movement now to the right, now to the left, in figure- eights (00 ). For convenience of reference these types of dance may be called whirling, circling, and the -figure eight dance. Zoth, in an excellent account of the behavior of the dancer (31 p. 156), describes "manege movements," "solo I Behavior: Dance Movements 33 dances," and "centre dances." Of these the first is whirling, the second one form of circling, and the third the dancing of two individuals together in the manner described above. Both the whirling and the circling occur to the right (clock- wise) and to the left (anticlockwise). As certain observers have stated that it is chiefly to the left and others that it is as frequently to the right, I have attempted to get definite information concerning the matter by observing a number of individuals systematically and at stated intervals. My study of this subject soon convinced me that a true conception of the facts cannot be got simply by noting the direction of turning from time to time. I therefore planned and carried out a series of experimental observations with twenty dancers, ten of each sex. One at a time these individuals were placed in a glass jar, 26 cm. in diameter, and the number of circle movements executed to the right and to the left during a period of five minutes was determined as accurately as possible. This was repeated at six hours of the day : 9 and 1 1 o'clock A.M., and 2, 4, 6, and 8 o'clock p.m. In order that habitua- tion to the conditions under which the counts of turning were made might not influence the results for the group, with ten individuals the morning counts were made first, and with the others the afternoon counts. No attempt was made in the counting to keep a separate record of the whirling and circling, although had it been practicable this would have been de- sirable, for, as soon became evident to the observer, some individuals which whirl in only one direction, circle in both. In Table 2 the results of the counts for the males are re- corded; in Table 3 those for the females. Each number in the column headed "right" and "left" indicates the total number of circles executed by a certain dancer in a period of five minutes at the hour of the day named at the head of the column. I may point out briefly the curiously interesting 34 The Dmtcing Mouse and entirely unexpected new facts which this method of observation revealed to me. First, there are three kinds of dancers : those which whirl almost uniformly toward the right, those which whirl just as uniformly toward the left, and those which whirl about as frequently in one direction as in the other. To illustrate, No. 2 of Table 2 may be characterized as a ''right whirler," for he turned to the right almost uniformly. In the case of the 6 P.M. count, for example, he turned 285 times to the right, not once to the left. No. 152, on the contrary, should be characterized as a "left whirler," since he almost always turned to the left. From both of these individuals No. 210 is distinguished by the fact that he turned now to the left, now to the right. For him the name "mixed whirler" seems appropriate. Second, the amount of activity, as indicated by the number of times an individual turns in a circle within five minutes, increases regularly and rapidly from 9 a.m. to 8 p.m. Ac- cording to the general averages which appear at the bottom of Table 2, the average number of circles executed by the males at 9 a.m. was 89.8 as compared with 207.1 at 8 p.m. In other words, the mice dance more in the evening than during the day. Third, as it appears in a comparison of the general aver- ages of Tables 2 and 3, the females dance more than the males, under the conditions of observation. At 9 a.m. the males circled 89.8 times, the females 151.0 times; at 8 p.m. the males circled 207.1 times, the females, 279.0 times. Fourth, according to the averages for the six counts made with each individual, as they appear in Table 4, the males turn somewhat more frequently to the left than to the right (the difference, however, is not sufficient to be considered significant) ; whereas, the females turn much more frequently Behavior : Dance Movements 35 TABLE 2 Number of Whirls to the Right and to the Left during Five-minute Intervals as Determined by Counts made at Six Different Hours, for each of Ten Male Dancers Number OF Animal 2 30 34 36 152 156 210 220 410 420 Averages Gen. Av. Right II 20 2 194 7 63 3 168 2 15 48.5 Left 2 I 16 21 48 8 9 105 61 142 41.3 89.8 Right 23 134 2 180 3 53 7 39 10 5 45-6 Left 4 I 48 II 171 9 41 43 27 214 56.9 102.5 Right 194 109 4 143 6 27 225 47 8 16 77-9 Left I 2 92 65 79 6 21 5 103 238 61.2 1391 Number 4 P.M. 6 P.M. 8 P.M. Animal Right Left Right Left Right Left 2 70 3 285 237 10 30 154 107 6 134 5 34 7 158 5 .118 6 147 36 173 14 170 II 325 19 152 91 16 210 9 223 156 85 2 72 26 139 . 26 210 159 18 31 82 47 201 220 45 38 78 17 69 2,2> 410 9 ■ 155 9 394 24 94 420 18 243 16 291 3 320 Averages 72.0 72.2 78.9 II5-5 99-3 107.8 Gen. Av. 144.2 194.4 207.1 36 The Dancing Mouse TABLE 3 Number of Whirls to the Right and to the Left during Five-minute Intervals as Determined by Counts made at Six Different Hours, for each of Ten Female Dancers Number Q A.M. II A.M. 2 P.M. OF Animal Right Left Right Left Right Left 29 9 18 17 30 7 22 33 287 329 I 352 3 35 48 15 198 46 208 14 151 13 88 7 75 3 167 157 57 6 50 45 53 12 211 218 21 31 55 66 .5 215 67 216 33 105 37 226 225 46 39 72 49 143 44 415 ^Z 156 34 3 425 43 296 69.9 12 201 12 210 Averages 81.1 90-5 60.7 91-5 70.6 Gen. Av. 151.0 151.2 162. 1 Number 4 P.M. 6 P.M. 8 p.m. OF Animai- Right Left Right Left Right Left 29 33 114 31 36 45 99 33 436 7 408 3 364 2 35 279 6 165 24 353 10 151 3 8 2 285 2 217 157 52 15 19 125 51 104 211 190 7 86 31 67 250 215 15 292 45 336 150 232 225 133 86 48 39 177 81 415 268 3 437 7 382 8 425 12 242 19 210 4 192 Averages 142. 1 78.0 126.0 109.6 1595 1 19- 5 Gen. Av. 220.1 235-6 279.0 Behavior: Da7tce Movements 37 to the right than to the left. I do not wish to emphasize the importance of this difference, for it is not improbable that counts made with a larger number of animals, or even with another group of twenty, would yield different results. The most important results of this statistical study of turn- ing are the demonstration of the existence of individual tendencies to turn in a particular direction, and of the fact that the whirling increases in amount from morning to evening. In order to discover whether the distribution of the dancers among the three groups which have been designated as right, left, and mixed whirlers agrees in general with that indicated by Table 4 (approximately the same number in each group) I have observed the direction of turning in the case of one hundred dancers, including those of the foregoing tables, and have classified them in accordance with their behavior as is indicated below. Right Whirlers Left Whirlers Mixed Whirlers Males 19 12 19 42 12 Females Totals 15 27 The left whirlers occur in excess of both the right and the mixed whirlers. This fact, together with the results which have already been considered in connection with the counts of turning, suggests that a tendency to whirl in a certain way may be inherited. I have examined my data and conducted breeding experiments for the purpose of ascertaining whether this is true. But as the results of this part of the investigation more properly belong in a special chapter on the inheritance of behavior (XVIII), the discussion of the subject may be closed for the present with the statement that the prepon- 38 The Dancing Mouse TABLE 4 Average Number of Whirls to the Right and to the Left for THE Six Intervals of Tables 2 and 3, with a Characteriza- tion OF THE Animals as Right Whirlers, Left Whirlers, or Mixed Whirlers. Average No. Average No. Males Age OF Whirls TO Right OF Whirls TO Left Characterization 2 12 mo. 136.7 3-3 Right whirler 30 2 mo. 109.7 2.5 Right whirler 34 2 mo. 4-3 96.5 Left whirler 36 2 mo. 197-5 23.5 Right whirler 152 6 mo. 6.8 I37-0 Left whirler 156 I mo. 73-2 12.8 Right whirler 210 3 mo. 78.7 62.0 Mixed whirler 220 4 mo. 74-3 40.2 Mixed whirler 410 3 wio. 10.3 139.0 Left whirler 420 3 mo. 12.2 241.3 Left whirler Ave rage 70.4 75.8 4 Right whirlers 4 Left whirlers 2 Mixed whirlers Females 29 2 mo. 23-7 53.2 Left whirler 33 2 mo. 362.7 2.7 Right whirler 35 2 mo. 208.5 19.2 Right whirler 151 6 mo. 50 140.0 Left whirler 157 I mo. 47.0 51-2 Mixed whirler 211 3 mo. 109.7 61.5 Mixed whirler 215 3 mo. 57.8 234-5 Left whirler 225 4 mo. 103.2 56.3 Mixed whirler 415 3 mo. 216.7 3.5 Right whirler 425 3 mo. 17.0 225.2 Left whirler Ave rage 115.1 84.7 3 Right whirlers 4 Left whirlers 3 Mixed whirlers Behavior: Dance Moveme7its 39 derance of left whirlers indicated above is due to a strong tendency to turn to the left which was exhibited by the in- dividuals of one line of descent. The tendency of the dancer's activity to increase in amount toward evening, which the results of Tables 2, 3, and 4 ex- hibit, demands further consideration. Haacke (7 p. 337) and Kishi (21 p. 458) agree that the dancing is most vigorous in the evening; but Alexander and Kreidl (i p. 544) assert, on the contrary, that the whirling of the individuals which they observed bore no definite relation to the time of day and apparently was not influenced in intensity thereby. Since the results of my own observations contradict many of the state- ments made by the latter authors, I suspect that they may not have watched their animals long enough to discover the truth. The systematic records which I have kept indicate that the mice remain quietly in their nests during the greater part of the day, unless they are disturbed or come out to obtain food. Toward dusk they emerge and dance with vary- ing intensity for several hours. I have seldom discovered one of them outside the nest between midnight and daylight. The period of greatest activity is between 5 and 10 o'clock p.m. Zoth states that he has observed the adult dancer whirl 79 times without an instant's interruption, and I have counted as many as no whirls. It seems rather absurd to say that an animal which can do this is weak. Evidently the dancer is exceptionally strong in certain respects, although it may be weak in others. Such general statements as are usually made fail to do justice to the facts. The supposition that light determines the periodicity of dancing is not borne out by my observations, for I have found that the animals continue to dance most vigorously toward evening, even when they are kept in a room which is constantly illuminated. In all probabihty the periodicity 40 The Dancing Mouse of activity is an expression of the habits of the mouse race rather than of the immediate influence of any environmental condition. At some time in the history of the dancer hght probably did have an influence upon the period of activity; but at present, as a result of the persistence of a well-estab- lished racial tendency, the periodicity of dancing depends to a greater extent upon internal than upon external conditions. During its hours of quiescence it is possible to arouse the dancer and cause it to whirl more or less vigorously by stimu- lating it strongly with intense light, a weak electric current, or by placing two individuals which are strangers to one another in the same cage; but the dancing thus induced is seldom as rapid, varied, or as long-continued as that which is characteristic of the evening hours. One of the most interesting results of this study of the direction of turning, from the observer's point of view, is the demonstration of the fact that the truth concerning even so simple a matter as this can be discovered only by long and careful observation. The casual observer of the dancer gets an impression that it turns to the left more often than to the right ; he verifies his observation a few times and then asserts with confidence that such is the truth about turning. That such a method of getting knowledge of the behavior of the ani- mal is worse than valueless is clear in the light of the results of the systematic observations which have just been reported. But, however important the progress which we may have made by means of systematic observation of the phenomenon of turning, it must not for one moment be supposed that the whole truth has been discovered. Continued observation • will undoubtedly reveal other important facts concerning circling, whirling, and the periodicity of dancing, not to men- tion the inheritance of peculiarities of dancing and the sig- nificance of the various forms of activity. CHAPTER IV Behavior: Equilibration and Dizziness Quite as interesting and important as the general facts of behavior which we have been considering are the results of experimental tests of the dancer's ability to maintain its position under unusual spatial conditions — to climb, cross narrow bridges, balance itself on high places. Because of its tendency to circle and whirl, to dart hither and thither rapidly and apparently without control of its movements, the study of the mouse's ability to perform movements which demand accurate and delicate muscular coordination, and to control its expressions of activity, are of peculiar scientific interest. That observers do not entirely agree as to the facts in this field is apparent from the following comparison of the state- ments made by Cyon and Zoth (31 p. 174). Cyon states that the dancer Cannot run in a straight line. Cannot turn in a narrow space, Cannot run backward, Cannot run up an incline, Cannot move about safely when above the ground, because of fear and visual dizziness, Can hear certain tones. Zoth, on the contrary, maintains that the animal Can run in a straight line for at least 20 cm., Can and repeatedly does turn in a narrow space, Can run backward, for he has observed it do so, 41 42 The Dancing Mouse Can run up an incline unless the surface is too smooth for it to gain a foothold, Can move about safely when above the ground, and gives no signs of fear or dizziness, Cannot hear, or at least gives no signs of sensitiveness to sounds. Such contradictory statements (and unfortunately they are exceedingly common) stimulated me to the repetition of many of the experiments which have been made by other investigators to test the dancer's behavior in unusual spatial relations. I shall state very briefly the general conclusions to which these experiments have led me, with only sufficient reference to methods and details of results to enable any one who wishes to repeat the tests for himself to do so. For the sake of convenience of presentation and clearness, the facts have been arranged under three rubrics: equilibrational ability, dizziness, and behavior when blinded. To our knowledge of each of these three groups of facts important contributions have come from the experiments of Cyon (9 p. 220), Alexander and Kreidl (i p. 545), Zoth (31 p. 157), and Kishi (21 p. 482), although, as has been stated, in many instances their results are so contradictory as to demand reexamination. All in all, Zoth has given the most satis- factory account of the behavior and motor capacity of the dancer. If the surface upon which it is moving be sufficiently soft or rough to furnish it a foothold, the dancer is able to run up or down inclines, even though they be very steep, to cross narrow bridges, to balance itself at heights of at least 30 cm. above the ground, and even to climb up and down on rods, as is shown by certain of Zoth's photographs which are re- produced in Figure 4. Zoth himself says, and in this I am able fully to agree with him on the basis of my own obser- vations, "that the power of equilibration in the dancing [JRE 4. — Zoth's photographs of dancers crossing bridges and climbing rods. Reproduced from P finger's Archiv, Bd. 86. 44 Tlie Dancing Mouse mouse, is, in general, very complete. The seeming reduction which appears under certain conditions should be attributed, not to visual dizziness, but in part to excitabihty and rest- lessness, and in part to a reduced muscular power" (31 p. i6i). Figure 5 . — Tracks of common mouse. Reproduced from Alexander and Kreidl's figure in Pjluger's Archiv, Bd. 82. The dancer certainly has far less grasping power than the common mouse, and is therefore at a disadvantage in moving about on sloping surfaces. One evidence of this fact is the character of the tracks made by the animal. Instead of raising its feet from the substratum and placing them neatly, as does the common mouse (Figure 5), it tends to shuffle along, Figure 6. — Tracks of dancing mouse. Reproduced from Alexander and Kreidl's figure in Pfliiger's Archiv, Bd. 82. dragging its toes and thus producing on smoked paper such tracks as are seen in Figure 6. From my own observations I am confident that these figures exaggerate the differences. My dancers, unless they were greatly excited or moving under conditions of stress, never dragged their toes as much as is indicated in Figure 6. However, there can be no doubt that they possess less power of grasping with their toes than do common mice. The animal is still further incapacitated Behavior: Equilibration and Dizziness 45 for movement on inclined surfaces or narrow places by its tendency to move in circles and zigzags. The results of my own experiments indicate that the timidity of the adult is greater than that of the immature animal when it is placed on a bridge i or 2 cm. wide at a distance of 20 cm. from the ground. Individuals three weeks old showed less hesitation about trying to creep along such a narrow pathway than did full-grown dancers three or four months old ; and these, in turn, were not so timid apparently as an individual one year old. But the younger animals fell off more frequently than did the older ones. Additional support for these statements concerning equili- brational ability is furnished by the observations of Kishi (21 p. 482). He built a wooden bridge 60 cm. long, i cm. wide at one end, and J cm. at the other, and supported it at a height of 30 cm. above the ground by posts at the ends. On this bridge ten dancers were tested. Some attempted to move sidewise, others began to whirl and fell to the ground ; only one of the ten succeeded in getting all the way across the bridge on the first trial. The second time he was tested this individual crossed the bridge and found the post; and the third time he crossed the bridge and cHmbed down the post directly. The others did not succeed in descending the post even after having crossed the bridge safely, but, instead, finally fell to the floor from awkwardness or exhaustion. On the basis of these and other similar observations, Kishi says that the dancer possesses a fair degree of ability to orient and balance itself. Inasmuch as equilibration occurs similarly in darkness and in daylight, Zoth thinks that there is neither visual dizzi- ness nor fear of heights. But it is doubtful whether he is right concerning fear. There is no doubt in my mind, in view of the way the mice behave when placed on an elevated 46 The Dancing Mouse surface, that they are timid ; but this is due probably to the uncomfortable and unusual position rather than to percep- tion of their distance from the ground. That they lack visual dizziness seems fairly well established. When rotated in a cyclostat ^ the dancer, unhke the com- mon mouse, does not exhibit symptoms of dizziness. The following vivid description of the behavior of both kinds of mice when rotated is given by Alexander and Kreidl (i p. 548). I have not verified their observations. The common mouse at first runs with increasing rapidity, as the speed of rotation of the cyclostat cylinder is increased, in the direction opposite to that of the cylinder itself. This continues until the speed of rotation has increased to about 60 revolutions per minute. As the rotation becomes still more rapid the mouse begins to crawl along the floor, its body stretched out and clinging to the floor. At a speed of 250 revolutions per minute it lies flat on the floor with its limbs extended obHquely to the movement of rotation, and at times with its back bent against the axis of the cylinder; in this position it makes but few and feeble efforts to crawl forward. When the rotation is suddenly stopped, the animal pulls itself together, remains for some seconds with extended limbs lying on the floor, and then suddenly falls into convul- sions and trembles violently. After several attacks of this kind, cramps appear and, despite its resistance, the animal is thrown about, even into the air at times, as if by an external force. This picture of the position assumed during rapid rotation, and of cramps after the cessation of rotation (the typical picture of rotation dizziness), is repeated with great uniformity in the case of the common mouse. Within fif- teen minutes after being returned to its cage the animal re- ^ An apparatus consisting of a glass cylinder with a mechanism for turn- ing it steadily and at different speeds about its vertical axis. Behavior: Equilibratio7i and Dizziness 47 covers from the effects of its experience. This description of the symptoms of rotation dizziness in the common mouse apphes equally well to the blinded and the seeing animal. In sharp contrast with the behavior of the common mouse in the cyclostat is that of the dancer. As the cylinder begins to rotate the dancer runs about as usual in circles, zigzags, and figure-eights. As the speed becomes greater it naturally becomes increasingly difficult for the mouse to do this, but it shows neither discomfort nor fear, as does the common mouse. Finally the centrifugal force becomes so great that the animal is thrown against the wall of the cylinder, where it remains quietly without taking the oblique position. When the cyclostat is stopped suddenly, it resumes its dance move- ments as if nothing unusual had occurred. It exhibits no signs of dizziness, and apparently lacks the exhaustion which is manifest in the case of other kinds of mice after several repetitions of the experiment. The behavior of the blinded dancer is very similar. If these statements are true, there is no reason to believe that the dancer is capable of turning or rotation dizziness. If it were, its daily life would be rendered very uncomfortable thereby, for its whirling would constantly bring about the condition of dizziness. Apparently, then, the dancer differs radically from most mammals in that it lacks visual and rotational dizziness. In the next chapter we shall have to seek for the structural causes for these facts. The behavior of the blinded animal is so important in its bearings upon the facts of orientation and equilibration that it must be considered in connection with them. Cyon in- sists that the sense of vision is of great importance to the dancer in orienting and equilibrating itself. When the eyes are covered with cotton wads fastened by collodion, this writer states (9 p. 223) that the mice behave as do pigeons and frogs 48 The Dancing Mouse whose semicircular canals have been destroyed. They per- form violent forced movements, turn somersaults forward and backward, run up inclines and fall over the edges, and roll over and over. In a word, they show precisely the kind of disturbances of behavior which are characteristic of ani- mals whose semicircular canals are not functioning nor- mally. Cyon, however, observed that in certain dancers these peculiarities of behavior did not appear when they were blinded, but that, instead, the animals gave no other indication of being inconvenienced by the lack of sight than do common white mice. This matter of individual differences we shall have to consider more fully later. No other observer agrees with Cyon in his conclusions concerning vision, or, for that matter, in his statements con- cerning the behavior of the blind dancer. Alexander and Kreidl (i p. 550) contrast in the following respects the behavior of the white mouse and that of the dancer when they are blinded. The white mouse runs less securely and avoids obstacles less certainly when deprived of vision. The dancer is much disturbed at first by the shock caused by the removal of its eyes, or in case they are covered, by the presence of the unusual obstruction. It soon recovers sufficiently to become active, but it staggers, swerves often from side to side, and frequently falls over. It moves clumsily and more slowly than usual. Later these early indications of blindness may wholly disappear, and only a slightly impaired ability to avoid obstacles remains. It was noted by Kishi (21 p. 484) that the dancer when first blinded trembles violently, jumps about wildly, and rolls over repeatedly, as Cyon has stated ; but Kishi believes that these disturbances of behavior are temporary effects of the strong stimulation of certain reflex centers in the nervous system. After having been blinded for only a few minutes Behavior: Equilibratio7i and Dizziness 49 the dancers observed by him became fairly normal in their behavior. They moved about somewhat more slowly than usually, especially when in a position which required accu- rately coordinated movements. He therefore fully agrees with Alexander and Kreidl in their conclusion that vision is not so important for the guidance of the movements of the dancer as Cyon believes. In summing up the results of his investigation of this subject Zoth well says (31 p. 168), "the orientation of the positions of the body with respect to the horizontal and verti- cal planes seems to take place without the assistance of the sense of sight." And, as I have already stated, this excellent observer insists that the ability of the dancer to place its body in a particular position (orientation), and its ability to maintain its normal relations to its surroundings (equiH- bration) are excellent in darkness and in daylight, provided only the substratum be not too smooth for it to gain a foot- hold. It must be admitted that the contradictions which exist in the several accounts of the behavior of the dancer are too numerous and too serious to be explained on the basis of careless observation. Only the assumption of striking in- dividual differences among dancers or of the existence of two or more varieties of the animal suffices to account for the discrepancies. That there are individual or variety differences is rendered practically certain by the fact that Cyon himself worked with two groups of dancers whose peculiarities he has described in detail, both as to structure and behavior. In the case of the first group, which consisted of three in- dividuals, the snout was more rounded than in the four individuals of the second group, and there were present on the head three large tufts of bristly black hair which gave the CO The Dancing Mouse mice a very comical appearance. The animals of the second group resembled more closely in appearance the common albino mouse. They possessed the same pointed snout and long body, and only the presence of black spots on the head and hips rendered them visibly different from the albino mouse. In behavior the individuals of these two groups differed strikingly. Those of the first group danced frequently, violently, and in a variety of ways ; they seldom climbed on a vertical surface and when forced to move on an incline they usually descended by sliding down backwards or sidewise instead of turning around and coming down head first; they gave no signs whatever of hearing sounds. Those of the second group, on the contrary, danced very moderately and in few ways; they climbed the vertical walls of their cage readily and willingly, and when descending from a height they usually turned around and came down head first ; two of the four evidently heard certain sounds very well. No wonder that Cyon suggests the possibihty of a different origin ! It seems not improbable that the individuals of the second group were of mixed blood, possibly the result of crosses with common mice. As I shall hope to make clear in a subsequent discussion of the dancer's peculiarities of behavior, in a chapter on individual differences, there is no sufficient reason for doubt- ing the general truth of Cyon's description, although there is abundant evidence of his inaccuracy in details. If, for the present, we accept without further evidence the statement that there is more than one variety of dancer, we shall be able to account for many of the apparent inaccuracies of descrip- tion which are to be found in the literature on the animal. As a result of the examination of the facts which this chapter presents we have discovered at least six important Behavior: Equilibration and Dizziness 51 peculiarities of behavior of the dancer which demand an explanation in terms of structure. These are: (i) the dance movements — whirling, circhng, figure- eights, zigzags; (2) restlessness and the quick, jerky movements of the head; (3) lack of responsiveness to sounds; (4) more or less pronounced deficiency in orientational and equilibrational power; (5) lack of visual dizziness; (6) lack of rotational dizziness. Naturally enough, biologists from the first appearance of the dancing mouse in Europe have been deeply interested in what we usually speak of as the causes of these peculiarities of behavior. As a result, the structure of those portions of the body which are supposed to have to do with the control of movement, with the phenomena of dizziness, and with the ability to respond to sounds, have been studied thor- oughly. In the next chapter we shall examine such facts of structure as have been discovered and attempt to correlate them with the facts of behavior. CHAPTER V Structural Peculiarities and Behavior The activities of an animal are expressions of changes which occur in its structure, and they can be explained satis- factorily only when the facts of structure are known. Such pecuHarities of activity as are exhibited by the dancing mouse, as contrasted with the common mouse, suggest at once that this creature has a body which differs in important respects from that of the ordinary mouse. In this chapter I shall present what is known concerning the structural bases for the whirhng, the lack of equilibrational ability and of dizziness, the quick jerky head movements, the restlessness, and the partial or total deafness of the dancing mouse. Comparative physiologists have discovered that the ability of animals to regulate the position of the body with respect to external objects and to respond to sounds is dependent in large measure upon the groups of sense organs which collec- tively are called the ear. Hence, with reason, investigators who sought structural facts with which to explain the forms of behavior characteristic of the dancer turned their atten- tion first of all to the study of the ear. But the ear of the ani- mal is not, as might be supposed on superficial examination, a perfectly satisfactory natural experiment on the functions of this groupof sensory structures, for it is extremely uncertain whether any one of the usual functions of the organ is totally lacking. Dizziness may be lacking, and in the adult hearing also, but 52 Structural Peculiarities and Behavior 53 in general the functional facts lead the investigator to expect modifications of the sense organs rather than their absence. I shall now give an account of the results of studies concern- ing the structure of the ear and brain of the dancer. Since the descriptions given by different anatomists contradict one another in many important points, the several investigations which have been made may best be considered chronologi- cally. Bernhard Rawitz (25 p. 239) was the first investigator to describe the structure of the ear of the Japanese or Chinese dancers, as he calls them. The definite problem which he proposed to himself at the beginning of his study was, what is the structural basis of the whirling movement and of the deafness of the mice? In his first paper Rawitz described the form of the ears of five dancers. His method of work was to make micro- scopic preparations of the ears, and from the sections, by the use of the Born method, to reconstruct the ear in wax. These wax models were then drawn for the illustration of the author's papers (Figures 8, 9, 10). The principal results of the early work of Rawitz are summed up in the following quotation from his paper: ''The Japanese dancing mice have only one normal canal and that is the anterior vertical. The horizontal and posterior ver- tical canals are crippled, and frequently they are grown together. The utriculus is a warped, irregular bag, whose sections have become unrecognizable. The utriculus and sacculus are in wide-open communication with each other and have almost become one. The utriculus opens broadly into the scala tympani, and the nervous elements of the cochlea are degenerate. "The last- mentioned degeneration explains the deafness of the dancing mice; but in my opinion it is a change of 54 The Dancing Mouse secondary nature. The primary change is the broad open« ing between the utriculus and the scala tympani from which results the streaming of the endolymph from the semicircular canals into the cochlea. When, as a consequence of the rapid whirling movements, a great part of the endolymph is hurled into the scala tympani, the organ of Corti in the scala vestibuli is fixed and its parts are rendered incapable of vibration. The condition of atrophy which is observable in the sense cells and in the nerve elements is probably due Ampulla externa Ampulla anterior Eanlus utriculi Membrana basilaris Lagena Canalis utriculo- saccularis __ __ _ Membrana Ampiilla Macula acustica basilaris posterior sacculi Figure 7. — The inner ear of the rabbit. Reproduced from Selenka after Retzius, to the impossibility of functional activity; it is an atrophy caused by disuse " (25 p. 242). To render the terms which occur in this and subsequent descriptions of the ear of the dancer somewhat more intelli- gible to those who are not familiar with the general anatomy of the vertebrate ear, a side view of the inner ear of the rabbit is reproduced from a drawing by Retzius (Figure 7). I have chosen the ear of the rabbit for this purpose, not in preference Structural Peculiarities and Behavior 55 to that of the common mouse, but simply because I failed to find any reliable description of the latter with drawings which could be reproduced. The rabbit's ear, however, is sufficiently like that of the mouse to make it perfectly satis- factory for our present purpose. This drawing of the rabbit's ear represents the three semi- circular canals, which occur in the ear of all mammals, and which are called, by reason of their positions, the anterior vertical, the posterior vertical, and the horizontal. Each of these membranous canals possesses at one end, in an enlargement called the ampulla, a group of sense cells. In Figure 7 the ampullae of the three canals are marked respec- tively, ampulla anterior, ampulla posterior, and ampulla externa. This figure shows also the cochlea, marked lagena, in which the organ of hearing of mammals (the organ of Corti) is located. The ear sac, of which the chief divisions are the utriculus and the sacculus, with which the canals communicate, is not shown well in this drawing. Within a few months after the publication of Rawitz's first paper on the structure of the dancer's ear, another European investigator, Panse (23 and 24) published a short paper in which he claimed that previous to the appearance of Rawitz's paper he had sectioned and mounted ears of the common white mouse and the dancing mouse side by side, and, as the result of careful comparison, found such slight differences in structure that he considered them unworthy of mention. Panse, therefore, directly contradicts the state- ments made by Rawitz. In fact, he goes so far as to say that he found even greater differences between the ears of different white mice than between them and the ears of the dancer (23 p. 140). In a somewhat later paper Panse (24 p. 498) expresses his belief that, since there are no peculiarities in the general form, 56 The Dancing Mouse ..-C.s. sensory structures, or nerve supply of the ear of the dancer, which serve to explain the behavior of the animal, it is prob- able that there are unusual structural conditions in the brain, perhaps in the cerebellum, to which are due the dance move- ments and the deafness. The work of Panse is not very con- vincing, however, for his figures are poor and his descriptions meager ; neverthe- less, it casts a cer- tain amount of doubt upon the reliability of the descriptions given by Rawitz. The unfavorable light in which his report was placed by Panse's state- ments led Rawitz to examine ad- ditional prepara- tions of the ear of the dancer. Again he used the recon- struction method. The mice whose ears he studied were sent to him by the physiologist Cyon. As has been noted in Chapter IV, Cyon discovered certain differences in the structure and in the behavior of these dancers (ii p. 431), which led him to classify them in two groups. The individuals of one group climbed readily on the vertical walls of their cages and responded vigorously to sounds ; those of the other group could not climb at all and gave no evidences of hearing. After he had completed his study of their behavior, Cyon killed the mice and sent their heads to Rawitz ; but unfortunately those of the two groups became mixed, and Rawitz was unable to distinguish them. When Figure 8. — The membranous labyrinth of the dancer's ear. Type I. This figure, as well as 9 and ID, are reproduced from Rawitz's figures in the Archiv fur Anatomic und Physiologic, Physiologischc Abthei- lung, 1899. C.S., anterior vertical canal; C.p., posterior vertical canal; C.c, horizontal canal; U., utriculus. Structural Peculiarities and Behavior 57 C.e-.-'- FiGURE 9. — The membranous labyrinth of the dancer's ear. Type II. he examined the structure of the ears of these mice, Rawitz did find, according to his accounts, two structural types between which very marked differ- ences existed. Were it not for the carelessness which is indicated by the confusion of the materials, and the influ- ence of Cyon's suggestion that there should be different structures to account for the differences in behavior, Ra- witz's statements might be accepted. As matters stand there can be no doubt of in- dividual differences in behavior, external appearance, and the structure of the ear ; but until these have been correlated on the basis of thorough- going, careful ob- servation, it is scarcely worth while to discuss their relations. To his previous de- scription of the con- ditions of the ear sacs, sense organs, and nerve elements of the dancer's ear, Rawitz adds nothing of importance in his second paper (26 p. 171). He merely reiterates his previous statements concerning the form of the canals, on the^Dasis of his findings in the case of six additional dancers. Figures 8, 9, and 10 are reproduced from Rawitz to show the anatomical conditions which he claims that he found. As these figures indicate, the canals were found to be extremely Figure io. — The membranous labyrinth of the dancer's ear. Type III. 58 The Da7icmg Mouse variable, as well as unusual in form, and the sacs distorted. In the ears of some specimens there were only two canals, and in all cases they were more or less reduced in size, distorted, or grown together. The work of Rawitz was unfavorably criticised by Alex- ander and Kreidl (2), Kishi (21), and Baginsky (4), as well Figure i i . — Photograph of a wax model of the membranous labyrinth of the ear of the dancer. Reproduced from Baginsky's figure in the Centralblatt fur Physiologic, Bd. i6. as by Panse (23 and 24). To their criticisms Rawitz replied by insisting that the other investigators could not with right attack his statements because they had not used the reconstruction method. In order to test the value of this contention, and if pos- sible settle the question of fact, Baginsky had a model of the ear of the dancer constructed by a skilled preparator (Herr Spitz) from sections which had been prepared by the best neurological Structural Peculiarities and Behavior 59 methods. This model was made eighty times the size of the ear. It was then reduced in the process of photographic reproduction to sixteen times the natural size of the ear in the mouse. Figure 1 1 is a photograph of Baginsky 's model. It shows beyond ques- tion the presence of three canals of the same general form and relations as those of the common mouse and of other mammals. Baginsky's paper is brief and to the point. His criticisms of the work of both Cyon and Rawitz are severe, but they are justified in all probability by the carelessness of these investi- gators in the fixation of their materials. Of the five skilled his- tologists who have examined the ear of the dancer, Rawitz alone found markedly abnormal canals. It is highly probable, there- fore, that the canals in his preparations in some way became distorted before the ears were sectioned. He doubtless described accurately the conditions which he found, but the chances are that those conditions never existed in the hving animals. The conflicting statements of Rawitz and Panse stimulated interest, and as a result two other investigators, without knowl- edge of one another's work, began careful researches on the dancer's ear. One, Alexander (2 and 3), worked in coop- eration with the physiologist Kreidl; the other, Kishi (21), worked independently. The anatomical papers of Alex- ander and Kishi appeared at about the same time, and since neither contains a reference to the other, it is evident that the investigations were carried on almost simultaneously. Alex- ander's descriptions are more detailed than those of Rawitz and Panse, and in certain respects Kishi's are even, more thoroughgoing. The first paper published by Alexander and Kreidl (i) contains the results of observations on the habits and behavior of the dancers. Having examined the chief facts of function, these investigators attempted to discover the structural conditions for the pecuHarities of behavior which they had observed. 6o The Dancing Mouse As material for their anatomical work they made use of four dancers, one albino mouse, and four common gray mice. The ears of these individuals were fixed, sectioned, and examined microscopically in connection with parts of the brain. In all, eight dancer ears and six common mouse ears were studied. Very extensive descriptions of these preparations, together with measurements of many important portions of the ear, are presented in their paper, the chief conclusions of which are the following : — 1. The semicircular canals, the ampullae, the utriculus, and the cristas acusticae of the canals are normal in their gen- eral form and relations to one another as well as in their histological conditions (2 p. 529). This is contradictory of the statements made by Rawitz. 2. There is destruction of the macula sacculi (2 p. 534). 3. There is destruction also of the papilla basilaris coch- leae, with encroachment of the surrounding tissues in vary- ing degrees. 4. There is diminution in the number of fibers of the branches and roots of the ramus superior and ramus medius of the eighth nerve, and the fiber bundles are very loosely bound together. 5. Similarly the number of fibers in the inferior branch (the cochlear nerve) of the eighth nerve is very much reduced. 6. There is moderate reduction in the size of the two vestibular ganglia as a result of the unusually small number of nerve cells. 7. The ganglion spirale is extremely degenerate. There is therefore atrophy of the branches, ganglia, and roots of the entire eighth nerve, together with atrophy and degeneration of the pars inferior labyrinthii. The nerve endings are especially degenerate (2 p. 534). Structural Peculiarities and Behavior 6i The above structural deviations of the ear of the dancer from that of the common mouse may be considered as pri- mary or secondary according as they are inherited or acquired. Since, according to Alexander and Kreidl, the dancers' peculiarities of behavior and deafness are directly and uni- formly inherited, it is obvious that certain primary structural deviations must serve as a basis for these functional facts. But it is equally clear, in the opinion of Alexander and Kreidl (2 p. 536), that other structural pecuKarities of the dancer are the result of the primary changes, and in no way the conditions for either the dancing or the deafness. These authors feel confident that the facts of behavior which are to be accounted for are almost certainly due to the pathological changes which they have discovered in the nerves, ganglia, and especially in the peripheral nerve endings of the ear of the mouse (2 p. 537). It is further claimed by Alexander and Kreidl that there are very marked individual differences among the dancers in the structure of the ear. In some cases the otoHths and the sensory hairs are lacking; in others, they are present in the state of development in which they are found in other varieties of mouse. Sometimes the cochlea is much reduced in size ; at other times it is found to be of normal size (2 p. 538). These variations in structure, if they really exist, go far toward justifying the tendency of Cyon and Alexander and Kreidl, as well as many other investigators, to regard the dancer as abnormal or even pathological. The functions of the ear as at present known to the com- parative physiologist are grouped as the acoustic and the non-acoustic. The cochlea is supposed on very good grounds to have to do with the acoustic functions, and the organs of the semicircular canals on equally good evidence are thought to have to do with such of the non-acoustic functions as 62 The Dancing Mouse equilibration and orientation. Just what the functions of the organs of the ear sacs are is not certainly known. These facts are of importance when we consider the attempts made by Alexander and Kreidl to correlate the various peculiar- ities of behavior shown by the dancer with the structural facts which their work has revealed. This correlation is indicated schematically below. The physiological facts to be accounted for in terms of structure are presented in the first column, and the anatomical facts which are thought to be explanatory, in the second (2 p. 539). Function I Lack of sensitiveness to auditory stimuli. 2 Defective equilibra- tional ability. 3 Lack of turning diz- ziness. Structure Destruction of the papilla basilaris coch- leae, etc. Diminution of the inferior branch of the eighth nerve. Marked degeneration of the ganglion spi- rale. Destruction of the macula sacculi. Diminution of the branches and roots of the superior and middle branches of the eighth nerve. Diminution of both gangUa vestibulii and of the nerve cells. 4 Normal reactions to galvanic stimulation. Alexander and Kreidl themselves believe that the partial deafness of the dancers (for they admit that the total lack of hearing has not been satisfactorily proved) is due to the defective condition of the cochlea. They account for the imperfect equilibrational ability of the animals by pointing out the structural peculiarities of the sacculus, the vestibular ganglia, and the peripheral nerves. Similarly, the lack of dizziness they suppose to be due to the diminution of the fibers of the nerves which supply the canal organs, the atrophied condition of the vestibular ganglia, and a disturb- Structural Peculiarities a7zd Behavior 63 ance of the peripheral sense organs. Furthermore, there are no anatomical facts which would indicate a lack of gal- vanic dizziness (2 p. 552). Despite the fact that they seem to explain all the functional peculiarities of the dancer, the statements made by Alexander and Kreidl are neither satisfying nor convincing. Their statements concerning the structure of the ear have not been verified by other investigators, and their correlation of struc- tural with functional facts lacks an experimental basis. In this connection it may be worth while to mention that a beautiful theory of space perception which Cyon (9) had constructed, largely on the basis of the demonstration by Rawitz that the dancers have only one normal canal, is totally destroyed by Panse, Baginsky, Alexander and Kreidl, and Kishi, for all of these observers found in the dancer three canals of normal shape. Cyon had noted that the most abnormal of the voluntary as \Mell as of the forced movements of the dancer occur in the plane of the canal which Rawitz found to be most strikingly defective. This fact he con- nected with his observation that the fish Petromyzon, which possesses only two canals, moves in only two spatial dimen- sions. The dancer with only one functional canal in each ear moves in only one plane, and neither it nor Petromyzon is able to move far in a straight line (11 p. 444). From these and similar surmises, which his eagerness to construct an ingenious theory led him to accept as facts quite uncritically, Cyon concluded that the perception of space depends upon the number and arrangement of the semicircular canals, and that the dancer behaves as it does because it possesses canals of unusual shape and relations to one another. The absurd- ity of Cyon's position becomes obvious when it is shown that the structural conditions of which he was making use do not exist in the dancer. 64 The Dancing Mouse The results obtained by Kishi in his study of the ear of the dancer differ in many important respects from those of all other investigators, but especially from those of Rawitz and Alexander and Kreidl. Kishi's work was evidently done with admirable carefulness. His methods in the preparation of his materials, so far as can be judged from his report, were safe and satisfactory, and his descriptions of results are minute and give evidence of accuracy and conscientious thoughtfulness. The material for his histological work he obtained from three different animal dealers. It consisted of fifteen adult and nineteen young dancers, and, as material for comparison, ten com- mon gray mice. The animals were studied first biologically, that their habits and behavior might be described accurately and so far as possible accounted for in the light of whatever histological results might be obtained subsequently; then they were studied physiologically, that the functional impor- tance of various organs which would naturally be supposed to have to do with the peculiarities of the mouse might be understood; and, finally, they were killed and their ears and portions of their brains were studied microscopically, that structural conditions for the biological and physiological facts might be discovered. The ear, which was studied by the use of several series of sections, as well as in gross dissections, is described by Kishi under three headings : — (i) The sound-receiving apparatus (auditory organs). (2) The static apparatus (equilibrational organs). (3) The sound-transmitting apparatus (ear drum, ear bones, etc.). The chief results of his structural investigation may be stated briefly under these three headings. In the sound-receiving or auditory apparatus, Kishi failed to find the important Structural Peculiarities and Behavior 65 deviations from the usual structure of the mammalian ear which had been described by Rawitz. The latter distinctly says that although the organ of Corti is present in all of the whirls of the cochlea, the auditory cells in it are noticeably degenerate. Kishi does not agree with Panse's statement (21 p. 476) that the auditory organ of the dancer differs in no important respects from that of the common mouse, for he found that in certain regions the hair cells of the organ of Corti were fewer and smaller in the dancer. He therefore concludes that the auditory organ is not entirely normal, but at the same time he emphasizes the serious discrepancy between his results and those of Rawitz. In not one of the ears of the twelve dancers which he studied did Kishi find the direct communication between the utriculus and the scala tympani which Rawitz described, and such differences as appeared in the organ of Corti were in the nature of slight deviations rather than marked degenerations. In the outer wall of the ductus cochlearis of the dancer the stria vasculosa is almost or totally lacking, while in the gray mouse it is prominent. This condition of the stria vasculosa Kishi was the first to notice in the dancer; Alex- ander and Kreidl had previously described a similar condi- tion in an albino cat. If, as has been supposed by some physiologists, the stria vasculosa is really the source of the endolymph, this state of affairs must have a marked influence on the functions of the auditory apparatus and the static ap- paratus, for pressure differences between the endolymph and the perilymph spaces must be present. And, as Kishi points out, should such pressure differences be proved to exist, the functional disturbance in the organ of hearing which the lack of responses to sounds seems to indicate might better be ascribed to them than to the streaming of the endo- lymph from the canals into the cochlea as assumed by Rawitz 66 TJie Dancing Mouse h.b. (21 p. 477). Kishi merely suggests that the condition of the stria may account for the deafness of the mouse ; he does ^ _ not feel at all o«c» 1 O- ^' confident of the truth of his ex- planation, and he therefore prom- ises in his first paper a contin- uation of his work in an inves- tigation of the functions of the stria. This, how- ever, he seems not to have ac- compHshed thus far. The static ap- paratus Kishi describes as closely similar in form to that of the gray mouse. In none of his twelve preparations of the ear of the dancer did he find such abnormalities of form and connections in the semicircular canals as Rawitz's figures and descriptions represent. Rawitz states that the anterior canal is normal except in its lack of connection with the posterior and that the posterior and horizontal are much reduced in size. Kishi, on the contrary, insists that all of the three canals are normal in shape and that the usual connection between the anterior and the posterior canals, the crus simplex, exists. He justifies these statements by presenting photographs of two dancer ears which he care- fully removed from the head. Comparison of these photo- graphs (Figures 12 and 13) with Rawitz's drawings of the Figure 12. — The inner ear of the dancer. Repro- duced from Kishi's figure in the Zeitschrift jur wissen- schaftliche Zoologie, Bd. 71. c.c. crus simplex ; o.b. anterior vertical canal; /j.6. posterior vertical canal; a.b. horizontal canal. Structural Peculiarities and Behavior 67 conditions of the canals and sacs as he found them (Figures 8, 9, and 10), and of both with the condition in the typical mamma- lian ear as shown by Figure 7, will at once make clear the meaning of Kishi's statements. That Rawitz's descriptions of the canals are not correct is rendered almost certain by the fact that Panse, Baginsky, Alexander and Kreidl, and Kishi all agree in describing them as normal in form. ^ -u ..^^mam^^' ' The only impor- tant respects in which Kishi found the membranous labyrinth, that is, the canals and the ear sacs, of the dancer to differ ^^B^» ^ .MxmiMmy a.b. from that of the gray mouse are the following. In the dancer the CUDola Figure 13. — The inner ear of the dancer, showing . the spiral form of the cochlea. After Kishi. 01 the crista acustica is not so plainly marked and not so highly developed, and the raphae of the ampuUas and canals, which frequently are clearly visible in the gray mouse, are lacking (21 p. 478). The sound-transmitting apparatus of the dancer, according to Kishi, differs only very slightly from that of the gray mouse, and there is no reason to consider the differences which appear as important (21 p. 478). Almost as amusing as the way in which Cyon's theory of space perception disappears in the light of critical research is Panse's explanation of the deafness of the dancer. Fail- ing to find any defects in the auditory apparatus of the inner ear which seemed adequate to account for the obvious lack 68 The Dancing Mouse of responsiveness to sounds, this investigator concluded that plugs of wax which he had noticed in the auditory meatus of the dancer excluded sounds or in some way interfered with the functioning of the tympanic membrane. Kishi reports that he found such plugs of wax in the ears of one gray mouse, but in none of the dancers which he examined did he discover them (21 p. 479). Panse's explanation of the defective hear- ing of the dancer neither needs nor deserves further comment. As one result of his investigation, Kishi is convinced that the dance movements are not due to peculiarities in the semi- circular canals and their sense organs, as Rawitz claimed, for the general form and finer structure of these organs in the dancer is practically the same as in the common mouse. Kishi is just as certain that the whirling is not due to defects in the canal organs, as Rawitz is that it is due to such struc- tural conditions ! It is rather surprising that any one should feel confident of the power of the microscope to reveal all those structural conditions which are important as conditions of function. Probably there are histological differences between the ear of the dancer and that of the gray mouse, which, although undetectable by scientific means at present, furnish the structural basis for the marked differences in behavior. As has been set forth already (p. 9), Kishi ac- counts for the dance movements by assuming the inheritance of an acquired character of behavior. This inherited ten- dency to dance, he thinks, has been accentuated by the con- finement of the mice in narrow cages and their long-continued movement in the wheels which are placed in the cages (21 p. 481). Rawitz, Cyon, and Alexander and Kreidl felt themselves under the necessity of finding peculiarities of behavior in the dancer which could be referred to the various abnormal- ities of structure which they had either seen or accepted on Structural Peculiarities and Behavior 69 faith ; Kishi found himself in a very different predicament, for he had on his hands the commonly accepted statement that the animals are deaf, without being able to find any structural basis for this defect. To avoid the difficulty he questions the existence of deafness ! If perchance they are deaf, he thinks that it is possibly because of the defect in the stria vasculosa. This suggestion Kishi makes despite the fact that our ignorance of the function of the stria renders it impossible for us to do otherwise than guess at its relation to hearing. We have now briefly reviewed the results of the various important investigations of the behavior and structure of the dancer. The observations of Cyon, Zoth, and the writer estabHsh beyond doubt the existence of important individual differ- ences in behavior if not of distinct divisions within the species of mouse, and the general results of the several anatomical investigations make it seem highly probable that the structure of the ear, as well as the externally visible structural features of the animals, vary widely. Unfortunately, the lack of agree- ment in the descriptions of the ear given by the different students of the subject renders impossible any certain corre- lation of structural and functional facts. That the whirling and the lack of dizziness and of hearing have their structural bases no one doubts, but whether it is in the brain itself, in the sense organs, or in the labyrinth, our knowledge does not permit us to say. With this statement Rawitz, Cyon, and Alexander and Kreidl would not agree, for they beheve that they have discovered structural peculiarities which fully explain the behavior of the dancer. Panse and Kishi, on the other hand, contend that the ear gives no structural signs of such peculiarities as the dancing and deafness suggest; they therefore look to the cerebellum for the seat of the dis- 70 The Dancing Mouse turbance. With the same possibility in mind the author of ** Fancy Varieties of Mice" writes: "These quaint little creatures make amusing pets for any one who is not scientific, or very fond of knowing 'the reason why.' In their case, the reason of the peculiarity which gives them their name is rather a sad one. It is now pretty conclusively established that they are no more Japanese than they are of any other country in particular, but that the originators of the breed were common fancy mice which were suffering from a disease of the brain analogous to the ' gid ' in sheep. In the latter, the complaint is caused by a parasite in the brain; in the case of the Waltzing Mouse, it is probably due to an hereditary malformation therein. Be this as it may, the breed is now a firmly established one, and the children of waltzing mice waltz hke their parents" (32 p. 45). Although it is quite possible that peculiarities in the central nervous system, rather than in the peripheral nervous system, may be respon- sible for the forms of behavior exhibited by the dancer, it must be remembered that no such peculiarities have been re- vealed by the examination of the central nervous system. The old fancier has neither better nor worse grounds for his belief than have Panse and Kishi. So far as the reliability of the anatomical work which has been discussed is in question, it would seem that Rawitz's results are rendered somewhat unsatisfactory by the careless- ness of Cyon in fixing the materials ; that Panse's descriptions and comparisons are neither careful nor detailed enough to be convincing ; that the work of Alexander and Kreidl, as well as that of Kishi, gives evidence of accuracy and trustworthiness. The fact that the statements of Alexander and Kreidl frequently do not agree with those of Kishi proves that there are serious errors in the work of one or another of these investigators. Cyon's discussion of the anatomy of the dancer is not to be Structural Peculiarities and Behavior 71 taken too seriously, for by his theory of space perception and of a sixth sense he was unduly biased in favor of the struc- tural peculiarities described by Rawitz. Nevertheless, his discussion is not without interest, for the way in which he succeeded in making every structural fact which Rawitz sug- gested fit into his theories and help to account for the func- tional peculiarities which he had himself observed, is extremely clever and indicates a splendid scientific imagination. To sum up : All the facts of behavior and physiology which have been established lead us to expect certain marked struc- tural differences between the dancer and the common mouse. The bizarre movements, lack of equilibrational ability, and the nervous shaking of the head suggest the presence of peculiar conditions in the semicircular canals or their sense organs; and the lack of sensitiveness to sounds indicates defects in the cochlea. Yet, strange as it may seem to those who are not familiar with the difficulties of the study of the minute structure of these organs, no structural conditions have been discovered which account satisfactorily for the dancer's peculiarities of behavior. That the ear is unusual in form is highly probable, since three of the four investiga- tors who have studied it carefully agree that it differs more or less markedly from that of the common mouse. But, on the other hand, the serious lack of agreement in their several descriptions of the conditions which they observed renders their results utterly inconclusive and extremely unsatisfactory. The status of our knowledge of the structure of the central nervous system is even less satisfactory, if possible, than that of our knowledge of those portions of the peripheral nervous system which would naturally be supposed to have to do with such functional peculiarities as the dancer exhibits. So far as I have been able to learn, no investigator has carefully examined the brain and spinal cord in comparison with those 72 The Dancing Mouse of the common mouse, and only those who have failed to find any structural basis for the facts of behavior in the organs of the ear have attempted to account for the dancer's whirl- ing and deafness by assuming that the cerebellum is unusual in structure. We are, therefore, forced to conclude that our knowledge of the nervous system of the dancing mouse does not at present enable us to explain the behavior of the animal. It seems highly probable to me, in the light of my observa- tion of the dancer and my study of the entire literature con- cerning the animal, that no adequate explanation of its ac- tivities can be given in terms of the structure of the peripheral or the central nervous system, or of both, but that the struc- ture of the entire organism will have to be taken into account. The dancer's physiological characteristics, in fact, suggest multitudinous structural peculiarities. I have confined my study to its behavior, not because the problems of structure seemed less interesting or less important, but simply because I found it necessary thus to limit the field of research in order to accomplish what I wished within a Hmited period. That there are structural bases for the forms of behavior which this book describes is as certain as it could be were they definitely known; that they, or at least some of them, are discoverable by means of our present-day histological methods is almost as certain. It is, therefore, obvious that this is an excellent field for further research. It is not an agreeable task to report inconclusive and contradictory results, and I have devoted this chapter to a brief account of the work that has been done by others on the structure of the ear of the dancer rather for the sake of presenting a com- plete account of the animal as it is known to-day than because of the value of the facts which could be stated. CHAPTER VI The Sense of Hearing Repeatedly in the foregoing chapters mention has been made of the dancer's irresponsiveness to sounds, but it has not been definitely stated whether this pecuHarity of behavior is due to deafness or to the inhibition of reaction. This chap- ter is concerned with the evidence which bears upon the problem of the existence of a sense of hearing. Again I may be permitted to call attention to the observations of other investigators before presenting the results of my own experiments and stating the conclusions which I have reached through the consideration of all available facts. By the results of various simple tests which he made, Rawitz (25 p. 238) was convinced that the adult dancer is totally deaf. He did not experiment with the young, but he says he thinks they may be able to hear, since the necessary structural conditions are present. This guess which Rawitz made on the basis of very indefinite and uncertain knowledge of the histology of the ear of the young dancer is of special interest in the light of facts revealed by my own experiments. Unfortunately the study of hearing made by Rawitz is casual rather than thorough, and although it may turn out that all of his statements are justified by his observations, the reader is not likely to get much satisfaction from his discus- sion of the subject. Inasmuch as he could discover no structural basis for deafness, Panse (23 p. 140) expressed himself as unwilling 73 74 The Dancing Mouse to believe that the mice are deaf, and this despite the fact that he observed no responses to the sounds made by a series of tuning forks ranging from C5 to Cg. He believes rather that they are strangely irresponsive to sounds and that their sensitiveness is dulled, possibly, by the presence of plugs of wax in the ears. Since another investigator, KIshi, has observed the presence of similar plugs of v^ax in the ears of common mice which could hear, there is but slight probability that Panse is right in considering the plugs of wax as the cause of the dancer's irresponsiveness to sounds. Far more thoroughgoing tests than those of Rawitz or Panse were made by Cyon (9 p. 218), who holds the unique position of being the only person on record who has observed the adult dancer give definite reactions to sounds. To a Konig Galton whistle so adjusted that it gave a tone of about 7000 complete vibrations per second, which is said to be about the pitch of the voice of the dancer, some of the ani- mals tested by Cyon responded unmistakably, others not at all. In one group of four mice, two not only reacted markedly to the sound of the whistle but apparently listened intently, for as soon as the whistle was blown they ran to the side of the cage and pressed their noses against the walls as if attempting to approach the source of the stimulus. The remaining two mice gave not the slightest indication that the sound acted as a stimulus. By the repetition of this sound from eight to twelve times Cyon states that he was able to arouse the mice from sleep. When thus disturbed, the female came out of the nest box before the male. Simi- larly when the mice were disturbed by the whistle in the midst of their dancing, the female was first to retreat into the nest box. There is thus, according to Cyon, some indi- cation of sex, as well as individual, differences in sensitiveness The Sense of Hearing 75 to the sound of the whistle, Cyon's statement that in order to evoke a response the whistle must be held above the head of the dancer suggests at once the possibility that currents of air or odors instead of sounds may have been responsible for the reactions which he observed. The work of this investi- gator justifies caution in the acceptance of his statements. Neither the conditions under which the auditory tests were made nor the condition of the animals is described with suffi- cient accuracy to make possible the comparison of Cyon's work with that of other investigators. As will appear later, it is of the utmost importance that the influence of other stimuli than sound be avoided during the tests and that the age of the mouse be known. The conclusion reached by Cyon is that some dancers are able to hear sounds of about the pitch of their own cries. The fact, emphasized by Cyon, that the mice respond to tones of about the pitch of their own voice is of pecuHar interest in its relation to the additional statements made by the same author to the effect that the female dancer is more sensitive to sounds than the male, and that the males either do not possess a voice or are much less sensitive to disagree- able stimuli than the females. In the case of the dancers which he first studied (9 p. 218), Cyon observed that certain strong stimuli evoked pain cries; but later in his investiga- tion he noticed that four individuals, all of which were males, never responded thus to disagreeable stimulation (11 p. 431). He asks, therefore, does this mean that the males lack a voice or that they are less sensitive than the females? The fact that he did not succeed in getting a definite answer to this simple question is indicative of the character of Cyon's work. My dancers have provided me with ample evidence concerning the matter. Both the males and the females, among the dancers which I have studied, possess a voice. 76 The Dancing Mouse The females, especially during periods of sexual excitement, are much more likely to squeak than the males. At such times they give their shrill cry whenever they are touched by another mouse or by the human hand. A slight pinching of the tail will frequently cause the female to squeak, but the male seldom responds to the same stimulus by crying out. The most satisfactory way to demonstrate the existence of a voice in the male is to subject him to the stimulating effect of an induced current, so weak that it is barely appreciable to the human hand. To this unexpected stimulus even the male usually responds by a sudden squeak. There can be no doubt, then, of the possession of a voice by both males and females. The males may be either less sensitive or less given to vocal expression, but they are quite able to squeak when favorable conditions are presented. The possession of a voice by an animal is presumptive evidence in favor of a sense of hearing, but it would scarcely be safe to say that the mice must be able to hear their own voices. Cyon, how- ever, thinks that some dancers can. What further evidence is to be had? Although they obtained no visible motor reactions to such noises as the clapping of the hands, the snapping of the fingers, or to the tones of tuning forks of different pitches and the shrill tones of the Galton whistle, Alexander and Kreidl (i p. 547) are not convinced of the total deafness of the dancer, for, as they remark, common mice which un- doubtedly hear do not invariably respond visibly to sounds. Furthermore, the anatomical conditions revealed by their investigation of the ear of the dancer are not such as to render sensitiveness to sounds impossible. They recognize also that the existence of the ability to produce sounds is an indication of hearing. They have no confidence in the re- sults reported by Cyon, for they feel that he did not take * The Sense of Hearing 77 adequate precautions to guard against the action of other than auditory stimuli. Zoth (31 p. 170) has pointed out with reason and force that testing the sensitiveness of the mice is especially diffi- cult because of their restlessness. They are almost con- stantly executing quick, jerky movements, starting, stopping, or changing the direction of movement, and it is therefore extremely difficult to tell with even a fair degree of certainty whether a given movement which occurs simultaneously with a sound is a response to the sound or merely coincident with it. With great care in the exclusion of the influence of extraneous stimuli, Zoth tried a large number of experiments to test the hearing of both young and adult dancers. Not once did he observe an indubitable auditory reaction. As he says, "I have performed numerous experiments with the Galton whistle, with a squeaking glass stopper, with caps and cartridges, without being able to come to any certain conclusion. With reference to the Galton whistle and par- ticularly to the tone which was said to have been heard extremely well by Cyon's mice, I believe I am rather safe in asserting that my mice, young (12-13 days) as well as old, do not react to the Konig Galton whistle (7210 Vs.). They could not be awakened out of sleep by repetitions of the sound, nor enticed out of their nests, and their dancing could not be interrupted" (31 p. 170). Zoth's experiments appear to be the most careful and critical of those thus far considered. Last to be mentioned, but in many respects of greatest interest and value, is the work of Kishi (21 p. 482) on the problem of hearing. To this acute observer belongs the credit of calhng attention emphatically to the ear movements which are exhibited by the dancer. Frequently, as he re- marks, the ears move as if the animal were listening or trying 78 The Dancing Mouse to determine the direction whence comes a sound, yet usually the mouse gives no other sign of hearing. That the absence of ordinary reactions to sounds is due to deafness, Kishi, like Panse, is led to doubt because his anatomical studies have not revealed any defects in the organs of hearing v^hich would seem to indicate the lack of this sense. This historical survey of the problem of hearing has brought out a few important facts. No one of the several investigators of the subject, with the exception of Cyon, is certain that the dancer can hear, and no one of them, with the exception of Rawitz, is certain that it cannot hear! Cyon almost certainly observed two kinds of dancing mice. Those of his dancers which exhibited exceptional abihty to climb in the vertical direction and which also gave good evi- dence of hearing certain sounds may have been hybrids result- ing from the crossing of the dancer with a common mouse, or they may have been exceptional specimens of the true dancer variety. A third possibility is suggested by Rawitz's belief in the ability of the young dancer to hear. Cyon's positive results may have been obtained with immature individuals. I am strongly inclined to believe that Cyon did observe two types of dancer, and to accept his statement that some of the mice could hear, whereas others could not. It is evident, in the light of our examination of the experimental results thus far obtained by other investigators, that neither the total lack of sensitiveness to sounds in the adult nor the presence of such sensitiveness in the young dancer has been satisfactorily proved. I shall now report in detail the results of my own study of the sense of hearing in the dancer As the behavior of the young differs greatly from that of the adult, by which is meant the sexually mature animal, I shall present first the results of my experiments with adults and later, in contrast, The Sense of Hearing 79 the results obtained with mice from one to twenty-eight days old. My preliminary tests were made with noises. While care- fully guarding against the interference of visual, tactual, temperature, and olfactory stimuli, I produced noises of vary- ing degrees of loudness by clapping the hands together suddenly, by shouting, whistling, exploding pistol caps, striking steel bars, ringing an electric bell, and causing an- other mouse to squeak. To these sounds a common mouse usually responds either by starting violently, or by trem- bling and remaining perfectly quiet for a few seconds, as if frightened. The adult dancers which I have tested, and I have repeated the experiment scores of times during the last three years with more than a hundred different indi- viduals, have never given unmistakable evidence of hearing. Either they are totally deaf or there is a most surprising lack of motor reactions. Precisely the same results were obtained in tests made with the Galton whistle throughout its range of pitches, and with Appuun whistles which, according to their markings, ranged from 2000 Vs. (C4) to 48,000 (G9), but which un- doubtedly did not correspond at all exactly to this range, and with a series of Konig tuning forks which gave tones vary- ing in pitch from 1024 to 16,382 complete vibrations. I am willing to trust these experimental results the more fully because during all the time I have had adult dancers under observation I have never once seen a reaction which could with any fair degree of certainty be referred to an auditory stimulus. Never once, although I have tried re- peatedly, have I succeeded in arousing a dancer from sleep by producing noises or tones, nor have I ever been able to observe any influence of sounds on the dance movements. All of Cyon^s signs have failed with my mice. Occasionally 8o The Dancing Mouse what looked like a response to some sound appeared, but critical observation invariably proved it to be due to some other cause than the auditory stimulus. A sound produced above the animal is very likely to bring about a motor re- action, as Cyon claims ; but I have always found it to be the result of the currents of air or odors, which usually influence the animal when the experimenter is holding any object above it. I do not wish to maintain that Cyon's conclusions are false; I merely emphasize the necessity for care in the exclusion of other stimuli. The mice are extremely sensitive to changes in temperature, such, for^ example, as are produced by the breath of the experimenter, and one must constantly guard against the misinterpretation of behavior. In a single experiment with mice over a month old, I observed what might possibly indicate sensitiveness to sound. While holding a mouse, thirty-five days old, in my hand I pursed my lips and made a very shrill sound by drawing in air; the mouse seemed to start perceptibly according to the indications given by my sense of touch. I repeated the stimulus several times and each time I could see and feel the animal start slightly. With two other individuals which I tested the reaction was less certain, and with several others I failed to get any indication of response. This would seem to prove that the three individuals which responded happened to be sensitive to that particular tone at the age of five weeks. The test is unsatisfactory because the vibrations from my own body may have brought about the reaction instead of the air vibrations produced by my lips, and I therefore merely mention it in the enumeration of the various experimental tests which I have made. If we should conclude from all the negative evidence that is available, or that could be obtained, that the dancer is totally deaf, it might fairly be objected that the conclusion The Sense of Hearing 8i is unsafe, since an animal does not necessarily respond to stimuli by a visible change in the position or relations of its body. Death feigning may fairly be considered a response to a stimulus or stimulus complex, yet there may be no sign of movement. The green frog when observed in the labora- tory usually gives no indication v^hatever, by movements that are readily observable, that it hears sounds which occur about it, but I have been able to show by means of indirect methods of study that it is stimulated by these same sounds.* Its rate of respiration is changed by the sounds, and although a sound does not bring about a bodily movement, it does very noticeably influence movements in response to other stimuli which occur simultaneously with the sound. I discovered that under certain rather simple experimental conditions the green frog would regularly respond to a touch on the back by drawing its hind leg up toward the body. Under the same conditions the sound of an electric bell caused no visible movement of the leg, but if at the instant the back was touched the bell was rung, the leg movement was much greater than that brought about by the touch alone. This suggests at once the desirability of studying the sense of hearing in the dancer by some indirect method. The animal may be stimulated, and yet it may not give any visible sign of the influence of the auditory stimulus. Were not the dancing so extremely variable in rapidity and duration, it might be used as an index of the influence of auditory stimuli. Cyon's statements would indicate that sounds interfere with the dancing, but as I obtained no evidence of this, I worked instead with the following indirect method, which may be called the method of auditory choice. The apparatus which was used is described in detail in ^ "The Sense of Hearing in Frogs." Journal of Comparative Neurology and Psychology, Vol. XV, p. 288, 1905. G 82 The Dancing Mouse Chapter VII, p. 92. Figures 14 and 15 will greatly aid the reader in understanding its essential features. Two small wooden boxes, identical in form and as closely similar as possible in general appearance, were placed in a larger box in such positions that a mouse was forced to enter and pass through one of them in order to get to the nest- box. On the bottom of each of these small boxes was a series of wires through which an electric current could be made to pass at the will of the experimenter. The boxes could readily be interchanged in position. At one side of the large wooden box and beyond the range of vision of the mouse was an electric bell which could be caused to ring whenever the mouse approached the entrance to one of the small boxes. The point of the experiment was to determine whether the dancer could learn to avoid the box-which-rang when it was approached. The method of conducting the tests was as follows. Each day at a certain hour the mouse was placed in that part of the large box whence it could escape to the nest-box only by passing through one of the small boxes. If it approached the wrong box (whether it happened to be the one on the right or the one on the left depended upon the experimenter's decision), the bell began to ring as a warning against entering ; if it approached the other box, all was silent. As motives for the choice of the box-which-did- not-ring both reward and punishment were employed. The reward consisted of freedom to return to the nest-box via the passage which led from the box-which-did-not-ring ; the punishment, which consisted of a disagreeable electric shock, was given whenever the mouse entered the wrong box, that is, the one which had the sound as a warning. Entering the wrong box resulted in a disagreeable stimulus and in the necessity of returning to the large box, for the exit to the nest-box by way of the passage from this box was closed. The Sense of Hearing 83 My assumption, on the basis of extended study of the ability of the dancer to profit by experience, was that if it could hear the sound of the bell it would soon learn to avoid the box-that-rang and enter instead the one which had no sound associated with it. Systematic tests were made with No. 4 from the 3d to the 12th of February, inclusive, 1906. Each day the mouse was permitted to find his way to the nest-box through one of the small boxes ten times in succession. Usually the experimenter rang the bell alternately for the box on the left and the box on the right. The time required for such a series of experiments varied, according to the rapidity with which the mouse made his choice, from ten to thirty minutes. If in these experiments the animal approached and entered the right, or soundless box, directly, the choice indicated nothing so far as ability to hear is concerned; if it entered the wrong, or sounding box, despite the ringing of the bell, it indicated either the lack of the influence of experience or inability to hear the sound; but if it regularly avoided the box-which-sounded it thus gave evidence of ability to hear the sound of the bell. The purpose of the test was to deter- mine, not whether the mouse could learn, but whether it could hear. For ten successive days this experiment was carried on with No. 4 without the least indication of increasing abihty to avoid the wrong box by the association of the sound of the bell with the disagreeable electric shock and failure to escape to the nest-box. In fact, the experiment was dis- continued because it became evident that an impossible task had been set for the mouse. Day by day as the tests were in progress I noticed that the animal became increasingly afraid of the entrances to the small boxes; it seemed absolutely helpless in the face of the situation. Partly because of the 84 The Dancing Mouse definiteness of the negative results obtained with No. 4 and partly because of the cruelty of subjecting an animal to dis- agreeable conditions which it is unable to avoid, the experi- ment was not repeated with other individuals. I have never conducted an experiment which gave me as much discomfort as this; it was like being set to whip a deaf child because it did not learn to respond to stimuli which it could not feel. By a very similar method No. 18 was tested for his sen- sitiveness to the noise and jar from the induction apparatus which was used in connection with many of my experiments on vision and the modifiability of behavior. In this experi- ment the wrong box was indicated by the buzzing sound of the apparatus and the slight vibrations which resulted from it. Although No. 18 was tested, as was No. 4, for ten suc- cessive days, ten trials each day, it gave no evidence of ability to avoid the box-which-buzzed. Since both direct and indirect methods of testing the hear- ing of the dancer have uniformly given negative results, in the case of mice more than five weeks old, I feel justified in concluding that they are totally deaf and not merely irre- sponsive to sounds. Rawitz's statements, and the fact that what may have been auditory reactions were obtained with a few individuals of five weeks of age, suggest that the mice may be able to hear at certain periods of life. To discover whether this is true I have tested the young of twenty different litters from the first day to the twenty-eighth, either daily or at intervals of two or three days. In these tests Konig forks, steel bars, and a Galton whistle were used. The results obtained are curiously interesting. During the first two weeks of life none of the mice which I tested gave any visible motor response to the various sounds used. During the third week certain of the individuals re- The Sense of Hearing 85 sponded vigorously to sudden high tones and loud noises. After the third week I have seen only doubtful signs of hear- ing. I shall now describe in detail the method of experi- mentation, the condition of the animals, and the nature of the auditory reactions. Between the twelfth and the eighteenth day the auditory canal becomes open to the exterior. The time is very variable in different litters, for their rate of growth depends upon the amount of nourishment which the mother is able to supply. Without exception, in my experience, the opening to the ear appears before the eyes are open. Consequently visual stimuli usually are not disturbing factors in the auditory tests with mice less than sixteen days old. There is also a sudden and marked change in the behavior of the mice during the third week. Whereas, for the first fourteen or eighteen days they are rather quiet and deliberate in their movements when re- moved from the nest, some time in the third week their be- havior suddenly changes and they act as if frightened when taken up by the experimenter. They jump out of his hand, squeak, and sometimes show fight. This is so pronounced that it has attracted my attention many times and I have studied it carefully to determine, if possible, whether it is due to some profound change in the nervous system which thus suddenly increases the sensitiveness of the animal or to the development of the sexual organs. I am inclined to think that it is a nervous phenomenon which is intimately connected with the sexual condition. Within a day or two after it appears the mice usually begin to show auditory re- actions and continue to do so for three to five days. I shall now describe the results obtained with a few typical litters. A Htter born of Nos. 151 and 152 gave uniformly negative results in all auditory tests up to the fourteenth day. On that day the ears were open, and the following observa- 86 The Dancing Mouse tions were recorded. The five individuals of the htter, four females and one male, were taken from the nest one at a time at 7 a.m. and placed on a piece of paper in the bright sunlight. The warmth of the sun soon quieted them so that auditory tests could be made to advantage. As soon as an individual had become perfectly still, the Galton whistle was held at a distance of about four inches from its head in such a position that it could not be seen nor the currents of air caused by it felt, and suddenly blown. Each of the five mice responded to the first few repetitions of this stimulus by movements of the ears, twitchings of the body, and jerky movements of the legs. The most violent reactions resulted when the individual was lying on its back with its legs ex- tended free in the air. Under such circumstances the four legs were often drawn together suddenly when the whistle was sounded. Similar responses were obtained with the lip sound already mentioned. Two other observers saw these experiments, and they agreed that there can be no doubt that the mice responded to the sound. The sounds which were effective lay between 5000 and 10,000 complete vibrations. On the fifteenth day the eyes were just beginning to open. Three of the mice responded definitely to the sounds, but the other two slightly, if at all. On the sixteenth day they were all too persistently active for satisfactory auditory tests, and on the seventeenth, although they were tested repeatedly under what appeared to be favorable conditions, no signs of sensitiveness were noted. Although I continued to test this litter, at intervals of three or four days, for two weeks longer, I did not once observe a response to sound. This was the first litter with which I obtained perfectly definite, clear-cut responses to sounds. That the reactive ability had not been present earlier than the fourteenth day I am confident, for I had conducted the tests in precisely The Sense of Hearing 87 the same manner daily up to the time of the appearance of the reactions. To argue that the mice heard before the fourteenth day, but were unable to react because the proper motor mechanism had not developed sufficiently would be short-sighted, for if the response depended upon the develop- ment of such a mechanism, it is not likely that it would disappear so quickly. I am therefore satisfied that these reactions indicate hearing. With another litter the following results were obtained. On the thirteenth day each of the eight members of the litter responded definitely and uniformly to the Galton whistle, set at 5 (probably about 8000 complete vibrations), and to a Konig steel bar of a vibration rate of 4096 Vs. The largest individuals, for almost always there are noticeable differences in size among the members of a fitter, appeared to be most sensitive to sounds. On the fifteenth day and again on the seventeenth unmis- takable responses to sound were observed ; on the eighteenth the responses were indefinite, and on the nineteenth none were obtained. I continued the tests up to the twenty-eighth day without further indications of hearing. Certain individuals in this litter reacted so vigorously to the loud sound produced by striking the steel bar a sharp blow and also to the Galton whistle, during a period of five days, that I have no hesitation in saying that they evidently heard during that period of their lives. Other members of the litter seemed to be less sensitive; their reactions were sometimes so indefinite as to leave the experimenter in doubt about the presence of hearing. A third litter, which developed very slowly because of lack of sufficient food, first showed unmistakable reactions to sound on the twenty-first day. On this day only two of the five individuals reacted. The reactions were much more 88 The Dancing Mouse obvious on the twenty-second day, but thereafter they be- came indefinite. Still another litter, which consisted of one female and four males, began to exhibit the quick, jerky movements, already mentioned, on the fourteenth day. On the morning of the fifteenth day three members of the litter definitely reacted to the tone of the steel bar, and also to the hammer blow when the bar was held tightly in the hand of the experimenter. My observations were verified by another experimenter. Two individuals which appeared to be very sensitive were selected for special tests. Their reactions were obvious on the sixteenth, seventeenth, and eighteenth days; on the nine- teenth day they were indefinite, and on the twentieth none could be detected. Some individuals of this litter certainly had the ability to hear for at least five days. A sixth litter of four females and two males first gave indi- cations of the change in behavior which by this time I had come to interpret as a sign of the approach of the period of auditory sensitiveness, on the seventeenth day. I had tested them almost every day previous to this time without obtaining evidence of hearing. The tests with the steel bar and the Galton whistle were continued each day until the end of the fourth week without positive results. To all appearances the individuals of this fitter were unable to hear at any time during the first month of life. Practically the same results were obtained with another litter of four females. The change in their behavior was obvious on the eighteenth day, but at no time during the first month did they give any satisfactory indications of hearing. In the accompanying table, I have presented in condensed form the results of my auditory tests in the case of twelve litters of young dancers. The Sense of Hearing 89 TABLE 5 Period of Auditory Reaction in Young Dancers No. IN Litter Change in Behavior Ears Open Auditory Reactions Appear Disappear 152 ^- 151 5 13th day 14th day 14th day 1 6th day 152 + 151 8 (?) 13th day 13th day 17th day 152+ 151 5 13th day 13th day 13th day 17th day 152 + 151 4 loth day 1 2th day 13th day 15th day 410 + 415 5 14th day 15th day 15th day 19th day 410 + 415 6 13 th day 14th day 14th day i8th day 420 + 425 2 1 2 th day 14th day 14th day 17th day 210+ 215 5 17th day 13th day 17th day 19th day 210 + 215 6 nth day 14th day No reactions 220 + 225 6 i6th day 14th day No reactions 220 + 225 6 17th day 13th day No reactions 212 + 211 4 15th day 14th day No reactions Certain of the litters tested responded definitely to sounds, others gave no sign of hearing at any time during the first four weeks of life. Of the twelve litters for which the re- sults of auditory tests are presented in Table 5, eight evi- dently passed through an auditory period. It is important to note that all except one of these were the offspring of Nos. 151 and 152, or of their descendants Nos. 410 and 415 and Nos. 420 and 425. In fact every one of the fitters in this line of descent which I have tested, and they now num- ber fifteen, has given indications of auditory sensitiveness. And, on the other hand, only in a single instance have the litters born of Nos. 210 and 215, or of their descendants, given evidence of ability to hear. These two distinct lines of descent may be referred to hereafter as the 400 and the 200 lines. I have observed several important differences between the individuals of these 90 The Dancing Mouse groups in addition to the one already mentioned. The 200 mice were sometimes gray and white instead of black and white; they climbed much more readily and danced less vigorously than those of the 400 group. These facts are particularly interesting in connection with Cyon's descriptions of the two types of dancer which he observed. In criticism of my conclusion that the young dancers are able to hear certain sounds for a few days early in life, and then become deaf, it has been suggested that they cease to react because they rapidly become accustomed to the sounds. That this is not the case, is evident from the fact that the re- actions often increase in definiteness during the first two or three days and then suddenly disappear entirely. But even if this were not true, it would seem extremely improbable that the mouse should become accustomed to a sudden and startlingly loud sound with so few repetitions as occurred in these tests. On any one day the sounds were not made more than five to ten times. Moreover, under the same external condition, the common mouse reacts unmistakably to these sounds day after day when they are first produced, although with repetition of the stimulus at short intervals, the reactions soon become indefinite or disappear. The chief results of my study of hearing in the dancer may be summed up in a very few words. The young dancer, in some instances, hears sounds for a few days during the third week of life. The adult is totally deaf. Shortly be- fore the period of auditory sensitiveness, the young dancer becomes extremely excitable and pugnacious. CHAPTER VII The Sense of Sight: Brightness Vision The sense of sight in the dancer has received little atten- tion hitherto. In the literature there are a few casual state- ments to the effect that it is of importance. Zoth, for example (31 p. 149), remarks that it seems to be keenly developed ; and other writers, on the basis of their observation of the animal's behavior, hazard similar statements. The descriptions of the behavior of blinded mice, as given by Cyon, Alexander and Kreidl, and Kishi (p. 47), apparently indicate that the sense is of some value ; they do not, however, furnish definite information concerning its nature and its role in the daily life of the animal. The experimental study of this subject which is now to be described was undertaken, after careful and long- continued observation of the general behavior of the dancer, in order that our knowledge of the nature and value of the sense of sight in this representative of the Mammalia might be in- creased in scope and definiteness. The results of this study naturally fall into three groups: (i) those which concern brightness vision, (2) those which concern color vision, and (3) those which indicate the role of sight in the life of the dancer. Too frequently investigators, in their work on vision in animals, have assumed that brightness vision and color vision are inseparable; or, if not making this assumption, they have failed to realize that the same wave-length prob- 91 92 The Dancing Mouse ably has markedly different effects upon the retinal elements of the eyes of unlike organisms. In a study of the sense of sight it is extremely impor- tant to discover whether difference in the quality, as well as in the in- tensity, of a visual stimulus influ- ences the organ- ism; in other words, whether color sensitive- ness, as well as brightness sensi- tiveness, is pres- ent. If the dancer perceives only brightness or lu- minosity, and not color, it is evident that its visual world is strikingly Figure 14. — Discrimination box. W, electric-box dllierent irom with white cardboards; B, electric-box with black card- that of the normal boards. Drawn by Mr. C. H. Toll, 1 1 • human being. The experiments now to be described were planned to show what the facts really are. As a means of testing the ability of the dancer to distin- guish differences in brightness, the experiment box repre- sented by Figures 14 and 15 was devised. Figure 14 is the box as seen from the position of the experimenter during the The Sense of Sight: Brightness Vision 93 tests. Figure 15 is its ground plan. This box, which was made of wood, was 98 cm. long, 38 cm. wide, and 17 cm. deep, as measured on the outside. The plan of construction and its significance in connection with these experiments on vision will be clear from the following ac- count of the experimental procedure. A mouse whose brightness vision was to be tested was placed in the nest-box, A (Figure 15). Thence by pushing open the swinging door at /, it could pass into the en- trance chamber, B. Having entered B it could return \.oA only by passing through one of the electric- boxes, marked TF, and following the alley to O, where by pushing open the swing door it could enter the nest-box. I n E 1 ■ 1 —- w W ""l R B 6 ^^ I A Figure 15, — Ground plan of discrimination box. ^, nest-box; B, entrance chamber; W,W, rpv 1 .7- electric-boxes; L, doorway of left electric-box; R, 1 ne door at 1 swung doorway of right electric-box; E, exit from electric- box to alley; 7, swinging door between A and 5; O, swinging door between alley and ^ ; /C, induc- tion apparatus; C, electric cell; K, key in circuit. inward, toward B, only; those at O, right and left, swung outward, toward A, only. It was therefore impossible for the mouse to follow any other course than A-I-B-L-W-E-0 or A-I-B-R-W-E-0. The doors at / and O were pieces of 94 The Dancing Mouse wire netting of \ cm. mesh, hinged at the top so that a mouse could readily open them, in one direction, by pushing with its nose at any point along the bottom. On the floor of each of the electric-boxes, W^ was an oak board i cm. in thickness, which carried electric wires by means of which the mouse could be shocked in W when the tests demanded it. The interrupted circuit constituted by the wires in the two electric- boxes, in connection with the induction appa- ratus, /C, the dry battery, C, and the hand key, K, was made by taking two pieces of No. 20 American standard gauge copper wire and winding them around the oak board which was to be placed on the floor of each electric- box. The wires, which ran parallel with one another, \ cm. apart, fitted into shallow grooves in the edges of the board, and thus, as well as by being drawn taut, they were held firmly in position. The coils of the two pieces of wire alternated, forming an interrupted circuit which, when the key K was closed, was completed if the feet of a mouse rested on points of both pieces of wire. Since copper wire stretches easily and becomes loose on the wooden base, it is better to use phosphor bronze wire of about the same size, if the surface covered by the interrupted circuit is more than three or four inches in width. The phosphor bronze wire is more difficult to wind satisfactorily, for it is harder to bend than the copper wire, and it has the further disadvantage of being more brittle. But when once placed properly, it forms a far more lasting and satisfactory interrupted circuit for such experiments as those to be described than does copper wire. In the case of the electric-boxes under consideration, the oak boards which carried the interrupted circuits were separate, and the two circuits were joined by the union of the wires between the boxes. The free ends of the two pieces of wire which con- stituted the interrupted circuit were connected with the The Sense of Sight: Brightness Vision 95 secondary coil of a Porter inductorium whose primary coil was in circuit with a No. 6 Columbia dry battery. In the light of preliminary experiments, made in preparation for the tests of vision, the strength of the induced current re- ceived by the mouse was so regulated, by changing the posi- tion of the secondary coil with reference to the primary, that it was disagreeable but not injurious to the animal. What part the disagreeable shock played in the test of brightness vision will now be explained. An opportunity for visual discrimination by brightness difference was provided by placing dead black cardboard at the entrance and on the inside of one of the electric-boxes, as shown in Figure 14, 5, and white cardboard similarly in the other box. These cardboards were movable and could be changed from one box to the other at the will of the experi- menter. The test consisted in requiring the mouse to choose a certain brightness, for example, the white cardboard side, in order to return to the nest- box without receiving an electric shock. The question which the experimenter asked in connection with this test really is. Can a dancer learn to go to the white box and thus avoid discomfort? If we assume its ability to profit by experience within the limits of the number of experiences which it was given, such a modi- fication of behavior would indicate discrimination of bright- ness. Can the dancer distinguish white from black; light gray from dark gray; two grays which are almost of the same brightness ? The results which make up the remainder of this and the following chapter furnish a definite answer to these questions. To return to the experimental procedure, the mouse which is being tested is placed by the experimenter in the nest-box, where frequently in the early tests food and a comfortable nest were attractions. If it does not of its own 96 The Dajicing Mouse accord, as a result of its abundant random activity, pass through / into B within a few seconds, it is directed to the doorway and urged through. A choice is now demanded of the animal ; to return to the nest- box it must enter either the white electric-box or the black one. Should it choose the white box, it is permitted to return directly to A by way of the doorway jE, the alley, and the swinging door at O, and it thus gets the satisfaction of unobstructed activity, freedom to whirl, to feed, and to retreat for a time to the nest. Should it choose to attempt to enter the black box, as it touches the wires of the interrupted circuit it receives a shock as a result of the closing of the key in the circuit by the experimenter, and further, if it continues its forward course instead of retreating from the ''stinging" black box, its passage through E is blocked by a barrier of glass temporarily placed there by the experimenter, and the only way of escape to the nest- box is an indirect route by way of B and the white box. Ordinarily the shock was given only when the mouse entered the wrong box, not when it retreated from it; it was never given when the right box was chosen. The box to be chosen, whether it was white, gray, or black, will be called the right box. The electric shock served as a means of forcing the animal to use its discriminating ability. But the question of motives in the tests is not so simple as might appear from this statement. The reader will wonder why the mouse should have any tendency to enter 5, and why after so doing, it should trouble to go further, knowing, as it does from previous experiences, that entering one of the electric- boxes may result in dis- comfort. The fact is, a dancer has no very constant tendency to go from A \.Q> B dX the beginning of the tests, but after it has become accustomed to the box and has learned what the situation demands, it shows eagerness to make the trip The Sense of Sight: Brightness Vision 97 from A to B, and thence by way of either the right or the left route to A. That the mouse should be willing to enter either of the electric-boxes, after it has experienced the shock, is even more surprising than its eagerness to run from A to B, When first tested for brightness discrimination in this apparatus, a dancer usually hesitated at the entrance to the electric-boxes, and this hesitation increased rapidly un- less it were able to discriminate the boxes by their difference in brightness and thus to choose the right one. During the period of increasing hesitancy in making the choice, the experimenter, by carefully moving from / toward the en- trances to the electric- boxes a piece of cardboard which extended all the way across -S, greatly increased the mouse's desire to enter one of the boxes by depriving it of dancing space in B. If an individual which did not know which entrance to choose were permitted to run about in B^ it would often do so for minutes at a time without approaching the entrance to the boxes ; but the same individual, when con- fined to a dancing space 4 or 5 cm. wide in front of the entrances, would enter one of the electric-boxes almost im- mediately. This facilitation of choice by decrease in the amount of space for whirling was not to any considerable extent the result of fear, for all the dancers experimented with were tame, and instead of forcing them to rush into one of the boxes blindly and without attempt at discrimination, the narrowing of the space simply increased their efforts to discriminate. The common mouse when subjected to simi- lar experimental conditions is likely to be frightened by being forced to approach the entrances to the boxes, and fails to choose ; it rushes into one box directly, and in consequence it is as often wrong as right. The dancer always chooses, but its eagerness to choose is markedly increased by the restriction of its movements to a narrow space in front of H 98 The Dancing Mouse the entrances between which it is required to discriminate. It is evident that the animal is uncomfortable in a space which is too narrow for it to whirl in freely. It must have room to dance. This furnished a sufficiently strong motive for the entering of the electric-boxes. It must avoid dis- agreeable and unfavorable stimuli. This is a basis for attempts to choose, by visual discrimination, the electric-box in which the shock is not given. It may safely be said that the success of the majority of the experiments of this book depended upon three facts: (i) the dancer's tendency to avoid disagreeable external conditions, (2) its escape-from- confinement-impelling need of space in which to dance freely, and (3) its abundant and incessant activity. Of these three conditions of success in the experiments, the second and third made possible the advantageous use of the first. For the avoidance of a disagreeable stimulus could be made use of effectively in the tests just because the mice are so restless and so active. In fact their eagerness to do things is so great that the experimenter, instead of having to wait for them to perform the desired act, often is forced to make them wait while he completes his observation and record. In this respect they are unlike most other animals. My experiments with the dancer differ from those which have been made by most students of mammalian behavior in one important respect. I have used punishment instead of reward as the chief motive for the proper performance of the required act. Usually in experiments with mammals hunger has been the motive depended upon. The animals have been required to follow a certain devious path, to escape from a box by working a button, a bolt, a lever, or to gain entrance to a box by the use of teeth, claws, hands, or body weight and thus obtain food as a reward. There are two very serious objections to the use of the desire for food The Sense of Sight: Brighhiess Vision 99 r ff as a motive in animal behavior experiments — objections W which in my opinion render it almost worthless in the case of many mammals. These are the discomfort of the animal and the impossibility of keeping the motive even fairly con- stant. However prevalent the experience of starvation may be in the Hfe of an animal, it is not pleasant to think of sub- jecting it to extreme hunger in the laboratory for the sake of \ finding out what it can do to obtain food. Satisfactory re- sults can be obtained in an experiment whose success depends ■ chiefly upon hunger only when the animal is so hungry that \ it constantly does its best to obtain food, and when the desire for food is equally strong and equally effective as a spur to action in the repetitions of the experiment day after day. It is easy enough to get almost any mammal into a condition of utter hunger, but it is practically impossible to have the desire for food of the same strength day after day. In short, the desire for food is unsatisfactory as a motive in animal behavior work, first, because a condition of utter hunger, as has been demonstrated with certain mammals, is unfavorable for the performance of complex acts, second, because it is impossible to control the strength of the motive, and finally, because it is an inhumane method of experi- mentation. In general, the method of punishment is more satisfactory than the method of reward, because it can be controlled to a greater extent. The experimenter cannot force his subject to desire food; he can, however, force it to discriminate between conditions to the best of its knowledge and ability by giving it a disagreeable stimulus every time it makes a mistake. In other words, the conditions, .upon which the avoidance of a disagreeable factor in the/ enyitonment de- pends are far simpler and much More" -constant 'than ^. those upon which the seeking of an agreeable ciaetoi: depends. lOO The Dancmg Mouse Situations which are potentially beneficial to the animal attract it in varying degrees according to its internal condi- tion; situations which are potentially disagreeable or in- jurious repel it with a constancy which is remarkable. The favorable stimulus solicits a positive response; the unfavor- able stimulus demands a negative response. Finally, in connection with the discussion of motives, it is an important fact that forms of reward are far harder to find than forms of punishment. Many animals feed only at long intervals, are inactive, do not try to escape from con- finement, cannot be induced to seek a particular spot, in a word, do not react positively to any of the situations or con- ditions which are employed usually in behavior experiments. It is, however, almost always possible to find some disagree- able stimulus which such an animal will attempt to avoid. As it happens, the dancer is an animal which does not stand the lack of food well enough to make hunger a possi- ble motive. I was driven to make use of the avoiding re- action, and it has proved so satisfactory that I am now using it widely in connection with experiments on other animals. The use of the induction shock, upon which I depended almost wholly in the discrimination experiments with the dancer, requires care ; but I am confident that no reasonable objection to the conduct of the experiments could be made on the ground of cruelty, for the strength of the current was carefully regulated and the shocks were given only for an instant at intervals. The best proof of the hu- maneness of the method is the fact that the animals continued in perfect health during months of experimentation. The brightness^ discrimination tests demanded, in addition to motives f6r'