COMPLIMENTS Or

THE WRITER

Responses of Young Toads to Light and
Contact

By C. F. CURTIS RILEY

Reprinted from the Journal of Animal Behavior
May-June, 1913, vol. 3, no. 3, pp. 179-214.

23Je 13'

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RESPONSES OF YOUNG TOADS TO LIGHT
AND CONTACT1
C. F. CURTIS RILEY

One figure

CONTENTS Page

I. Introduction 179

II. Response to Intense Artificial Light 180

III. Response to Less Intense Artificial Light 184

IV. Response to Strong Diffuse Daylight 186

V. Response to Weak Diffuse Daylight 189

VI. Response to Sunlight 191

VII. Response to Colored Light 193

VIII. Response to Contact 198

IX. Discussion 205

X. Summary 210

XI. Bibliography 213

I. INTRODUCTION

Several years' ago the opportunity arose to investigate the
responses of a number of young toads, Bufo americanus Le
Conte.2 The toads averaged about 14 mm. in length. The
animals were kept in a glass aquarium jar in the laboratory,
the vessel being placed 3 m. from an east window in a shaded
place. A piece of filter paper, dampened with water, was placed
in the bottom of the jar in order to keep the animals moist.
They were fed each day on small flies and mosquitoes.

As the toads displayed marked responses to light and contact,
it was decided to commence experimentation with photic and con-
tact stimuli. The entire series of experiments were carried on in a
dark room at an approximate temperature of 220 C. For experi-
mental purposes, a glass dish with parallel sides was used. The
dimensions of the vessel were 60 x 25 x 6cm., inside measurements.
In order to eliminate the effects of reflection,- the experimentation
trough was painted a dull black on the inside with the excep-
tion of the two ends; each of which was covered with a strip

u Contributions from the Zoological Laboratory, University of Illinois, under the
direction of Henry B. Ward, No. 21.

2 The experimental Avork was done at Ann Arbor, Michigan, in the summer of
1904, and the first draft of the paper was made the following year. The literature
was reviewed, the paper recast and entirely rewritten at Urbana, Illinois, during
the winter of 1912-1913.

179

I

180

C. F. CURTIS RILEY

of paper having a dead black finish. This paper was so arranged
that it could be readily removed at either end, thus permitting
the rays of light to enter.

Unfortunately, the writer was unable either to complete the
work on the responses to photic and contact stimuli, or to com-
mence any experiments on other forms of reaction work. Through
an oversight, the water was allowed to dry up in the jar containing
the toads, and the next time they were needed for experimenta-
tion purposes they were found to be dead. As it was not pos-
sible, at the time, to obtain any more material, and as the
opportunity to complete the work has riot presented itself since
then, it was thought best to publish in a brief form the results
of the experiments incomplete as they are. The writer hopes,
at some future time, to continue the work outlined in this paper
in far more detail and to supplement it with experiments on
responses to other forms of stimuli.

Thanks are due to Miss F. J. Dunbar of the Zoological De-
partment of the University of Michigan, who kindly collected
the amphibians used in these experiments. Mr: S. A. Rowland,
of the Physics Department of the University of Illinois, calcu-
lated the intensity of the illumination within the field of ex-
perimentation employed in this work. His assistance is grate-
fully acknowledged. The writer also desires to express his
appreciation of the criticisms and- encouragement given by
Doctor C. C. Adams of the University of Illinois.

II. RESPONSE TO INTENSE ARTIFICIAL LIGHT

The first experiments undertaken were with reference to
intense artificial light as a source of stimuli. The light em-
ployed for this reaction work was obtained from the electric
arc of a Thomson “ 90° carbon ” projection lantern. The current
passed from the electric lighting circuit through a rheostat, and
from there to the lantern. The current was direct with an
approximate voltage of 210. Within the. field of experimenta-
tion the illumination was approximately 10,000 ca. m. In
order to eliminate the effect of the heat rays, the light was
allowed to filter through a cell containing distilled water. Hav-
ing placed the toads in the glass dish, it was then moved into
the beam of light that emerged from the lantern. In order to
prevent the organisms from becoming unnaturally dry during

RESPONSES OF YOUNG TOADS

181

experimentation, the trough was immersed in cool water from
time to time, thus keeping the vessel cool and moist.

When the dish is placed in the beam of light, it is noticed
that the toads are scattered promiscuously throughout the
entire length of the vessel. The first demonstrative movement
is a decided orientation. All the animals that are facing the
light immediately turn around until their heads are pointing
directly away from the source of illumination. The orientation
is of such a nature that the longitudinal axes of their bodies
become parallel with the rays of light. Those individuals which
are facing away from the source of illumination, in the first
instance, are already commencing to jump away from the light.
After all the toads have completed the orienting reaction, there
is a general movement towards that end of the dish farthest
from the source of light. The animals jump rapidly toward
the extremity of the vessel. The pauses between the jumps
are very brief, in many instances barely extending over a second
of time. Orientation with the longitudinal axis of the body
parallel with the incoming rays is retained while the organisms
travel the entire length of the dish. After reaching the end of
the receptacle, the toads usually remain oriented with their
heads touching the glass. If they are left in this position for
some time, many of the individuals climb up the perpendicular
end of the vessel as if to move away as far as possible from the
source of illumination. Those that climb to the top are not
in the most intense glare of the prejection lantern, their elevated
position placing them a little above the strong central beam of
light. Possibly this movement may be due, in part, to response
to contact stimuli. The dish is now turned around until the
end where the animals are congregated is again brought nearest
to the light. Those animals clinging to the upright end of the
vessel quickly drop to the bottom. All the toads promptly per-
form the reaction of orientation and jump rapidly to the far
end of the trough as in the former instance. This reversing of
the dish is continued for about eight trials and the animals
respond promptly each time to the photic stimuli, the response
being as previously described.

Some experiments on the influence of intense light upon the
swimming responses of the young toads were also undertaken.

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C. F. CURTIS RILEY

Water with a temperature approximately of 220 C. is poured
into the glass receptacle to a depth of 4 cm. The animals are
then placed in the water and the vessel moved into the beam of
light emerging from the projection lantern. The organisms
immediately orient themselves with the longitudinal axis of the
body parallel to the rays of light. Complete orientation is
quickly followed by every animal swimming vigorously away
from the source of illumination to the extremity of the dish.
The young toads retain their precision of orientation as they
move through the water for the entire length of the vessel.
After all the animals have assembled at the end of the dish, it
is turned around, thus placing the organisms again nearest the
source of light. The animals as before respond promptly, swim-
ming to the far end of the vessel. In this connection it may be
mentioned that Torelle (1903, p. 473) while engaged with some
very interesting experiments, demonstrated that in water Rana
virescens and Rana clamata move to the illuminated end of a
glass trough. Diffuse daylight was used in these experiments.

Many consecutive experiments of the nature of the above
were performed, the toads orienting themselves each time in
such a manner that the long axis of the body becomes parallel
to the rays of light; and they swim away from the source of
illumination as already explained. Frequently after reaching
the extremity of the glass trough, many of the toads climb up
its perpendicular wall. Attention has been drawn to this move-
ment in connection with the responses out of water. The ani-
mals respond as if they are attempting to recede as far as pos-
sible from the source of stimulation. When the end of the
dish, where the toads are congregated, is placed next to the
light, they very promptly drop off the perpendicular glass wall
into the water. Sometimes there is an attempt at orientation
even before leaving the wall of the trough for the water. The
results of the experiments in connection with both the jumping
and swimming responses of young toads differ, decidedly, from
the results of Parker’s (1903, p. 29) suggestive work on Rana
pipiens Schreber. He found that these animals were positively
phototropic in light, from a Nernst lamp, even at 20,480 ca.m.
Without regard to the side of the frog that was exposed to the
light, they turned and jumped toward its source. The frogs
oriented themselves until they faced the source of illumination

RESPONSES OF YOUNG TOADS

183

and they remained in this position although the light was en-
tirety unbearable to the human eye.

In connection with the responses of young toads both in and
out of water, it would have been extremely interesting to have
used a much longer experimentation trough in order to demon-
strate whether or not the animals were seeking a certain opti-
mum intensity of illumination. The writer planned to perform
experiments of this nature if more material had been available
at the time, and he hopes to do so in the future.

In connection with these responses of young toads to intense
light, it is of interest to recall the work of Pearse (1910) relative
to frogs and toads. This writer (l.c., p. 175) found that Rana
clamata moved toward a light of 225 ca.m. from a six-glower
Nernst lamp. Five specimens were subjected to the light.
He also states (l.c., pp. 175-176) that Rana sylvatica was ex-
posed to photic stimuli from a light of the same intensity. His
results are summed up in the following quotation (l.c.):

“This frog was more active than the last species, [R. clamata ] and
some individuals gave more decided phototropic reactions than did any
member of the preceding species. There were, however, such differences
in the reactions of the four animals used that they are tabulated sepa-
rately. Individual No. 1 never failed to move straight toward the light.

No. 2 was not as persistently positive after the eyes had been excised as
before this operation, though it continued to give a majority of positive
reactions. As individuals 3 and 4 were apparently indifferent to the light
in their normal conditions, their eyes were not removed. The reactions
of animals 1 and 2 were, however, strongly positive, and this condition
remained even after the eyes had been excised; hence their skins served
as photoreceptors as well as their eyes.”

These results with frogs are quite different from the results of
the writer with young toads. Such differences, however, are
not at all surprising when it is recalled that not only are ani-
mals of different genera being considered, but also of different
families ; and further it must be kept in mind that on the one
hand mature organisms are under observation and on the other
very immature ones. Pearse (l.c,. p. 176) also experimented with
Bufo americanus and Bufo jowleri. The records of the two spe-
cies were not kept separate. The toads respond positively to
a light of 225 ca.m., moving toward the source of illumination.
Most of the animals used were adults, but a few were immature.
None of them, however, were less than 2 cm. long. It has been
demonstrated that the present writer finds young toads to
respond negatively to the light from a projection lantern. The

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C. F. CURTIS RILEY

difference in these results from those of Pearse is probably due
to the fact that the writer employed more powerful stimuli in
his experiments. Further, the writer used animals of much
greater immaturity than was the case in Pearse ’s experiments.

III. RESPONSE TO LESS INTENSE ARTIFICIAL LIGHT
The young toads are next subjected to light of much lower
intensity. The vessel containing the animals is placed directly
in front of the bulb of an ordinary 16 c.p. incandescent electric
light. Within the field of experimentation the illumination is
approximately 44 ca.m. The first noticeable movement is the
orienting response, all the toads so placing themselves, with
respect to the light, that the longitudinal axes of their bodies
become parallel with the longitudinal axis of the dish, which
points directly toward the light. This response brings the
toads into such a position that their heads are turned directly
toward the source of illumination, and the median longitudinal
axes of their bodies lie parallel with many of the rays passing
through the bulb. However, with a light of this nature, it is
incorrect to state, without qualification, that the median longi-
tudinal axis of the body is parallel with the rays of light. Im-
mediately after completing orientation, the toads jump in the
direction of the light until they all reach the extremity of the
vessel nearest to the source of illumination. The organisms
remain oriented while traveling from one end of the glass trough
to the other. Parker ( l.c .) found that Rana pipiens Schreber
oriented itself with its head toward the light and also moved
toward the source of illumination. These were the responses
to the intermediate light intensities between 1 and 20,480 ca.m.
After the young toads had all reached the end of the vessel,
it is turned around until the animals are moved to a position
farthest from the source of light. They quickly perform the
orienting response and again jump away toward the light. In
this manner the animals are driven repeatedly from one end of
the dish to the other. Usually, orientation is not performed so
promptly nor do the toads jump so rapidly as in those experi-
ments with the projection lantern. When the animals reach
the end of the trough, they frequently climb up the end wall
of the vessel as if to move still nearer to the source of illumina-
tion. There is no definite evidence that the median longitu-

RESPONSES OF YOUNG TOADS

185

dinal axes of the toads lie parallel with all the rays of light,
for the rays emerge from the bulb at various angles and many
of them must cross within the area of experimentation.

These results seem to indicate that young toads respond
positively to incandescent light of 16 c.p. They orient them-
selves in such a manner that the long axis of the body lies
parallel to some of the incoming rays and the head is turned
directly toward the source of light. They retain this orienta-
tion with considerable precision while traversing the entire
length of the experimentation dish. The fact that the animals
move as near as possible to the light, and in some cases climb to
the top of the glass wall at the end of the vessel, leads one to infer
that the intensity of the light is a factor in causing the move-
ments, rather than the direction of the rays per se in the field.
In connection with the experiments on young toads just de-
scribed, it is interesting to compare the work of Parker ( l.c .)
on Rana pipiens Schreber. He noticed the interesting fact that,

“With the lower intensities the animals often did not react for from
five to ten minutes or even longer, and the jumping response was fre-
quently omitted; but their orientation was finally always with their
heads toward the source of light, that is, positive. In some instances
after a frog had remained ten minutes or more without changing its original
position, it was induced to jump by being touched from behind, and,
when this was done, the animal almost invariably turned first and then
jumped toward the source of light.”

It has been demonstrated that stimuli from a 16 c.p. incan-
descent electric light affects young toads in much the same
manner as Parker has observed with reference to frogs. The
toads orient more slowly to the weaker light than to the stimuli
from the projection lantern. Movement toward the incandes-
cent light is also more deliberate than toward the light from
the projection lantern.

Dickerson (1906, p. 66) also has noticed that toads respond
positively to artificial light of relatively low intensity. Her
statement is as follows:

“If we go to a pond at night, we shall have every opportunity both
to see and hear toads, especially if we carry a lantern. Instead of being
frightened by the light, they are attracted by it and may gather about
it. If the lantern is set on the ground, they sometimes try to climb to
its top.”

The results of my experiments with less intense artificial light
agree very largely with those of Cole (1907, p. 392) on Acris
gryllus Le Conte. This observer states that the source of the

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C. F. CURTIS RILEY

photic stimuli was an electric light situated 50 cm. above a
flat surface on which the frogs were placed. A. gryllus faces
the source of illumination and leaps toward it. When a frog
jumps past the light, the animal remains with its back turned
toward it for a short time. Then it turns in such a manner
as to face the light and again leaps toward it. Cole ( l.c ., pp.
393-401) also found that both A. gryllus and Rana clamata
Daudin respond positively to light with an intensity of from
1.25-5 ca.m. by turning toward the source of illumination, but
that individuals of the latter species were much the slower in
their responses. He noticed that the positive response of A.
gryllus occurs more quickly and uniformly when light of 5-20
ca.m. is used. This is in accord with t*he present writer’s ex-
periments on Bufo americanus as it is also in accord with the
results of Parker (l.c.) with Rana pipiens Schreber already
mentioned. It should be stated that during these experiments
of Cole with A. gryllus and R. clamata , the animals were con-
fined in glass boxes which were of such dimensions that the
amphibians could turn readily in any direction, but were unable
to jump away. My experiments are also essentially in agreement
with those of Mast (1911, p. 219-220). This worker subjected
seven toads to light from a single source. Two intensities were
used, one of 12.5 ca.m. and the other of 25 ca.m. Five of these
toads were small ones, but the exact size is not given. The
following quotation presents the results which have more direct
bearing upon the experiments described in the present paper:

“They all oriented directly and fairly accurately. If placed on the
table in the beam of light so that one side faced the glower they turned
slowly but directly until they faced the light and then hopped or walked
toward its source, stopping frequently for a few moments at intervals
on the way. * * * The toads always went directly toward one or

the other of the two sources.”

IV. RESPONSE TO STRONG DIFFUSE DAYLIGHT
It was found that young individuals of Bufo americanus
respond to diffuse daylight of relatively strong intensity, and
a number of experiments were performed in which light of this
nature was employed. The glass trough containing the toads
was placed on a table, having a black surface, at a distance
of 50 cm. from an east window. A glass plate, painted a dead
black on the under side, was placed over the top of the experi-
mentation dish. Then the strip of black paper, covering the ,

RESPONSES OF YOUNG TOADS

187

end of the vessel facing the light, was removed. Such an ar-
rangement modified, considerably, the effects of the cross rays.

It is noticed, after the dish containing the toads has been
near the window for a few seconds, that the animals orient
themselves with their heads toward the source of light and
with the long axes of their bodies parallel with the longitudinal
axis of the vessel used for the reaction work, but not parallel
with the great majority of the rays, for these enter the dish at
various angles notwithstanding the precautions already de-
scribed. The toads jump away in a comparatively straight line
toward the light. All the organisms are soon congregated at
the end of the trough nearest to the window. Not infrequently
some of them climb up the glass wall at the end of the dish and
cling in that position. The vessel is turned around until the
end where the toads are gathered is pointing directly away
from the window. Those that are clinging to the glass side
drop down to the floor of the trough. All the animals again
turn their heads toward the window and move off, with the
long axes of their bodies parallel with the longitudinal axis of
the experimentation dish, in the direction of the source of illu-
mination. In a short time they are all found at the extremity
of the vessel. Experiments of this nature are continued for
eight successive trials. Each time the animals move promptly
from one end of the dish to the other. As in the experiments
with incandescent electric light, there is evidence that orienta-
tion does not occur quite so promptly nor is locomotion so
rapid as in the experiments with the projection lantern.

These experiments appear to indicate that young toads respond
positively to diffuse daylight of somewhat strong intensity.
They also exhibit definite orientation with the anterior end of
the body turned directly toward the light. But it cannot be
said that the long axis of the body lies parallel to the incoming
rays, for it is evident after careful consideration that these must
enter the experimentation trough at many and varied angles,
numbers of them passing, more or less, along its entire length.

The responses of toads and frogs to strong diffuse daylight
have been observed by other workers, and a brief comparison
will be made with the results of some of their experiments. It
is of interest to notice that, Graber (1884, p. 124) found that
toads, Bufo vulgaris Laur, placed in a box with two compart-

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C. F. CURTIS RILEY

ments, one of which was darkened and the other exposed to
diffuse daylight, moved toward the darkened compartment and
tended to collect there. According to Graber’s ( l.c ., pp. 39,
40) description of his own methods used in experimentation,
the natural inference is that he used strong diffused daylight
as a source of stimulation, except in those instances where it
is stated specifically otherwise. The following quotation pre-
sents his results (l.c.) :

“Da die meisten Versuche, die ich mit diesen ekelhaften Tieren anfieng,
ein negatives Resultat ergaben, glaubte ich mich in eine genauere Licht-
gefuhl-Priifung nicht einlassen zu sollen und geh’ ich auch bei der Mitteil-
ung der erhaltenen Reactionswerte ganz summarisch zu Werke.

“Aus der Vergleichung von Weiss und Schwarz geht zunachst hervor,
dass, wie zu erwarten war, die dunkle Abteilung der hellen bei weitem
vorgezogen wird.

Weiss 1

“1) =— •

Schwarz 8 . 4

“Darnach ist also die Lichtscheu der Krote (obiger Quotient beruht
freilich nur auf 5 aber unter sich ubereinstimmenden Beobachtungen)
entschieden viel grosser wie jene des Frosches (aber kleiner wie die des
Triton).”

While it is evident that the results of Graber’s experiments
differ from those of the writer, yet attention should be drawn
to the fact, as indicated in the first paragraph quoted, that
there was evidently some doubt in the author’s mind as to the
correctness of his own results. Plateau (1889, p. 82), however,
working with Rana temporaria and Bufo calamata reached con-
clusions of an opposite nature. He demonstrated, when speci-
mens were liberated in an experimentation box lighted by
windows at one end only, that both specimens responded posi-
tively to the light and jumped toward the source of illumina-
tion. One infers from Plateau’s (l.c., p. 81) statement that
strong diffuse daylight was used in his experimental work with
amphibians. According to Loeb (1890, p. 90) frogs respond
negatively to strong diffuse daylight. Torelle (1903, p. 469)
experimented with Rana virescens virescens and Rana clamata
and demonstrated that both species oriented in such a manner
that their heads pointed toward the source of illumination,
diffuse daylight, and also that they moved toward the light.
Dickerson (l.c., p. 32) states that the frog moves toward diffused
light, probably meaning strong diffuse daylight.

RESPONSES OF YOUNG TOADS

189

V. RESPONSE TO WEAK DIFFUSE DAYLIGHT
A number of experiments were performed with weak diffuse
daylight. The light was obtained from a south window. The
incoming rays passed through a small slit-like opening in the
wall of the dark room before reaching the experimentation
•dish. The length of the opening corresponded with the width
of the vessel and the width of the former was equal to the depth
of the latter. The window through which the light entered
was 4 m. distant from the dark room (see Fig. i, A).

A

B

Figure 1. A. Sectional diagram showing plan of apparatus employed in testing
responses of young toads to stimulation from weak diffuse daylight. B. Ex-
perimentation dish enlarged more than is shown in A. a, Interior of dark
room; b, walls of dark room; c, experimentation dish containing young
toads; d. table; e, opening through which light enters dark room; f, window
through which light enters, situated in wall of outer room; + , positive;
— , negative; 0, indifferent. (Not drawn to scale.)

The glass trough containing the toads was placed in close
proximity to the slit-like opening. It was noticed that there
was no immediate response to the light, neither by orientation
nor by movement toward or away from the source of illumina-
tion. This being the case observations were taken every fifteen
minutes, and the positions of the animals in the dish noted. The

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C. F. CURTIS RILEY

vessel was divided by transverse lines into three equal divisions
(see Fig. i, B). Toads found in the section marked + were
considered to respond positively to the light. Those gathered
in the division marked — were enumerated as responding nega-
tively. The organisms congregated in the space marked o were
counted as being indifferent to the illumination. Table I indi-
cates the results of six experiments with twelve individuals used
in each experiment.

TABLE I

Responses of Young Toads to Weak Diffuse Daylight

Number of
Experiments

Positive

Indifferent

Negative

1

7

2

3

2

6

1

5

3

8

2

2

4

6

2

4

5

7

1

4

6

8

2

2

Totals

42

10

20

The above table shows that out of a total of 72 responses,
42 of them may be considered as being positive, 20 as being
negative, and 10 as being indifferent. The writer performed
many other experiments beside those indicated in Table I, some
of which showed a rather higher percentage of positive responses
than evidenced in the table; but it is believed that the results
indicated there present a fairly representative series. From
these results the inference is drawn that young toads respond
positively to weak diffuse daylight. However, the orientation
of the animals lacks the definiteness exhibited in the experi-
ments with light of stronger intensities. Neither the motor
response nor the orientation is so precise as is the case in the
experiments with strong diffuse light. Many of the toads that
collected in the section marked + faced the slit-like opening in
the wall of the dark room through which the light entered.
Those toads that faced the light have the median longitudinal
axes of their bodies parallel with the longitudinal axis of the
experimentation dish and also parallel with a portion of the
incoming light rays. But it must be remembered that the
rays cross at many and varied angles, and while it is true that

RESPONSES OF YOUNG TOADS

191

many of these cross lights are cut out as the rays enter the
dark room, yet it would be incorrect to state that all the rays
of light are parallel with each other and with the median longi-
tudinal axis of the bodies of the toads facing the light.

Graber ( l.c ., pp. 120, 12 1) while making observations on
Rana esculenta L., stimulated by means of weak diffuse day-
light, “meist triiber Himmel,” states results which lead to the
inference that these animals react negatively to photic stimuli
of such a nature. However, their responses were not so pre-
ponderantly negative as was the case of Bufo vulgaris Laur
already mentioned. The following quotation will give an idea
of his results {l.c., p. 121):

“Auch zeigt der Versuch, dass das Reactions- Verhaltnis ein sehr con-
stantes ist, indem die Weiss-Frequenz sich zwischen 11 und 18 und die
des Schwarz zwischen 22 und 29 bewegt, und grossere Extreme, wie
solche sonst sehr haufig sind, mit Ausnahme eines einzigen Falles (7:33)
gar nicht vorkamen.

“Das Mittel-Verhaltnis ist:

Weiss 1

1) — ==— ”

Schwarz 1 . 5

Loeb {l.c., p. 89) experimented with frogs subjected to weak
diffuse daylight, and found that they responded by moving
away from the light.

VI. RESPONSE TO SUNLIGHT

No systematic experiments were undertaken in order to deter-
mine the effect of sunlight on the behavior of young toads.
However, a few incidental observations were made and these
were recorded.

The young toads were kept in a large glass aquarium jar
when not under experimental observation. This chanced to be
placed near a window with an eastern exposure. A narrow
beam of sunlight entered the dish, and to one side of these
rays of bright light was diffuse daylight. The animals jumped
toward the side of the jar at which the sunlight entered. Usually
they remained in the sunlight for some time. This response
seemed to vary, for certain individuals remained in the direct
light longer than others. However this may be, there was cer-
tainly a response to sunlight, evidenced by the toads jumping
toward the window where the sunlight entered. Frequently the
animals were found in the less illuminated portions of the jar,

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C. F. CURTIS RILEY

but they usually remained at the side of the vessel nearest to
the window. On sunny days the writer often has observed that
young toads respond both to moving shadows and moving
objects. Sometimes they respond by crouching against the
ground, and at other times by the jumping response. The
response to moving objects has been noticed also on cloudy
days. The writer did not employ a heat screen during any of
these observations. Therefore there was no definite attempt to
isolate possible temperature responses from photic responses.
However, he infers from the experiments of Pearse ( l.c ., pp.
1 92-1 95) that these responses are probably not due to tempera-
ture. My observations were recorded in the middle of the
day, during the month of July. Similar responses occurring in
the early spring or late fall may be due largely to temperature.

Torelle (1903, p. 469) working with Rana virescens virescens
and Rana claniata noticed, in one series of experiments, that
specimens of both species responded positively to sunlight, but
that they did not stay in the circle of most intense illumination.
Some individuals moved away without turning. Others turned
and retreated some distance, and then oriented themselves with
their heads pointing toward the incoming beam of sunlight. In
another series of experiments in which specimens of the same
species of frogs were placed in a box, admitting sunlight at
one end and diffuse light at the other, Torelle (l.c., p. 471) found
that the animals “turned toward and moved to the end” where
the sunlight entered. They did not stay within the area of
strongest light, for they either moved to the opposite end of
the box, or else backed, without turning, into the region
where the light was less intense. These results correspond
more or less to my observations on young toads; although
unfortunately the present writer did not observe the kind of
response when the animals moved into the less illuminated
area. Dickerson (l.c., p. 71) has noticed the response of young
toads to sunlight. She states that, “They congregate in large
numbers on sunny brown earth patches.” Such places are near
or among the grass. The observations of Miller (1909, pp. 659—
660) are in accord with those of Dickerson. The former has
noticed that toads are rarely seen during the day unless it is
cloudy. Late in the fall, however, they are found in the sun-
light, among the grass. It should be stated that none of these

RESPONSES OF YOUNG TOADS

193

writers, Torelle, Dickerson and Miller, indicate clearly whether
the response is due to light or temperature.

VII. RESPONSE TO COLORED LIGHT

A few experiments of a very general nature were performed
with colored light as the source of stimulation. The young
toads were subjected to the stimuli from a 16 c.p. incandes-
cent light. At first the light was passed through glass of various
colors. Later the rays were transmitted through colored solu-
tions prepared according to the directions recommended by
Nagel (1898, pp. 649-655). Such light proved to be more
nearly monochromatic, as was seen on examination with the
spectroscope. The solutions were placed in a glass cell with
parallel sides. Red and blue are the only colors that will be
referred to here.

The young toads are seen to be scattered promiscuously
throughout the experimentation dish when it is placed in the
beam of light transmitted through the red solution. Frequently
there is observed to be considerable hesitancy before the animals
orient themselves with reference to the light. Some of them
turn toward the light, while others exhibit no definite orienta-
tion. In some experiments it is observed that the number of
toads responding positively to the light is slightly in excess of
those responding negatively ; but in other experiments the
reverse proves to be the case, for the number of animals exhib-
iting the negative response to the light is somewhat greater than
those exhibiting the positive response. Usually, however, it
may be stated that a small majority of the toads turn in such
a manner that their heads point toward the source of illumina-
tion. Some animals orient a few seconds after they are placed
in the beam of light. Others wait much longer than this before
they turn either away from or toward the red light. Certain
individuals jump in the direction in which their heads are point-
ing, immediately after orientation is completed, while others
wait from a few seconds to several minutes before they jump
away. The jumping movement toward the red light is far less
common than is the case with reference to the blue light. In
general it may be said that the movements of young toads away
from and toward the source of illumination are slower and with
more and longer pauses between the jumps than is the case when

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C. F. CURTIS RILEY

white light is used. It is evident that red light is not a very
effective form of stimulus, certainly the toads are not nearly
so responsive to it as they are to white light.

In the experiments with blue light the toads are observed to
be distributed at various different points in the experimentation
dish. This is placed in the beam of light emerging from the
blue solution. The animals turn with promptness toward the
source of illumination. In most cases they jump immediately
toward the light, or with a delay of but a few seconds. There
are comparatively few pauses between the jumps and these are
of short duration. Not only is the orienting response more
prompt, but the movement toward the light is also more vigor-
ous than is the case when the red solution is used. The orien-
tation of the toads and their movement toward the source of
illumination are very similar to the results observed with the
1 6 c.p. incandescent light when neither the red nor the blue
solution is used.

Torelle {Lc., p. 478) found when single colored lights were
used that specimens of Rana virescens virescens and Rana clamata
always jumped in the direction of the blue light and remained
with their heads pointed toward it, touching the glass. On a
frog being placed close to the red light, it usually turned away
from the source of illumination. In some cases the animal not
only turned, but jumped away from the red light. When a
frog was placed about 30 cm. away from the red light, the
animal generally remained there and did not jump toward the
light. According to the experiments of Pearse ( lx., p. 189)
with Rana palustris ,

“The results show that the blue is apparently the most effective in
the production of positively phototropic reactions, and that there is a
regular graduation from blue to red, both in the percentage of positive
reactions and in the rapidity with which the movements took place. *

* * It is probable that these differences in the reactions are due to

differences of the wave lengths, but they may be due to intensity
differences.”

These results of Pearse with R. palustris are largely in accord
with those of the writer with young toads. Results similar to
those of the writer on young toads when blue and red lights
were used were obtained by Laurens (1911, p. 267). He sub-
jected Bufo americanus and Bufo fowler i to the stimuli from
single monochromatic lights of equal intensity, using Nernst

RESPONSES OF YOUNG TOADS

195

glowers for the source of illumination, and showed that members
of both species reacted by giving motor responses, turning and
jumping toward or away from the light. In one series of experi-
ments with blue light, there were 251 positive responses and 37
negative ones. In another series of experiments with red light,
there were 167 positive responses and 12 1 negative ones. The
former series shows the percentage of responses to be 87 positive
to 13 negative, while the latter series shows the percentage of
responses to be 58 positive to 42 negative.

“By way of summarizing the results of the experiments with Isingle
monochromatic lights in which both the eye and the skin acted as recep-
tors* it may be stated that all four colored lights used produced positive
responses. Blue light was the most effective, and the other lights formed
a decreasing series, corresponding roughly to their relative position in
the spectrum, the red light being but slightly more effective than dark-
ness.” (Laurens, l.c.)

This summary presents results which are in extremely close
agreement with those of Pearse (l.c.) on Rana palustris.

Some observations were recorded when stimuli from light of
different colors, red and blue, were impinging upon the toads
at the same time. The glass vessel containing the animals was
so placed that the red light was situated at one extremity and
the blue light at the other. The position was such that one end
of the dish was in the beam of red light and the opposite end in
the beam of blue light.

The young toads are placed in the center of the dish, approx-
imately half way between the lights passing through the red
and blue solutions. There is little more exhibited in the result-
ing responses than has already been recorded with reference to
the single colored light. Orientation and movement toward the
blue light are more definite and more vigorous, than they are
with respect to the red light. By far the majority of the animals
jump toward the blue light. A few individuals jump toward
the red light. These make longer pauses, between the jumps
than is the case of those toads moving toward the blue light.
Frequently periods three minutes or more in duration elapse
between some of the jumps. At times one or two specimens
appear to be indifferent to either light. In several instances
observations were recorded after a period of fifteen minutes
from the time the toads were first placed in the experimen-
tation dish. In these experiments twelve toads are used each

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C. F. CURTIS RILEY

time. Table II indicates the position of the toads with respect
to the two colored lights for four sets of observations.

TABLE II

Responses of Young Toads to Colored Lights

Experiment

Blue

Red

Indifferent

1

10

2

0

2

7

3

2

3

10

1

1

4

9

2

1

Total No. of Responses

36

8

4

It is seen from the table that out of a total of 48 responses, 36
are toward the blue light, 8 are toward the red light, and 4 are
indifferent, apparently being no response to either of the col-
ored lights. The blue is evidently by far the more effective color
with respect to the responses of young toads when stimuli from
both red and blue light are impinging upon them.

The results recorded above are largely in accord with those
of Graber ( l.c ., p. 125) on Bufo vulgaris Laur as indicated in
the following quotation (l.c.) :

“ Dass auch bei diesen Reactionen wieder in erster Linie die Helligkeits-
verhaltnisse ausschlaggebend sind, geht schon aus dem Umstande hervor,
dass einerseits Dunkel-Rot, andererseits Dunkel-Blau vorgezogen wird.
Speciell aus der ersten Vergleichung, wo Rot sehr .bedeutend dunkler
als Blau und doch nur wenig starker als letzteres besucht ist, konnte
aber vielleicht geschlossen werden, dass diese Farbe als soloche der Krote
weniger angenehm als Blau sei. * * *

Rot 1

2) ?”

Blau m. uv. 1 . 2

While it is true that in one series of experiments when “ Dunkel-
Rot” and “Hell-Blau” lights were employed, the number of
responses were 305 toward the former and 288 toward the.
latter, or in proportion of 1 .‘0.9, yet in another series when
“Hell-Rot” and “Dunkel-Blau” were used, the responses were
108 to 142, or in the reaction-proportion of 1 : 1.3. Graber
(l.c., p. 122) experimenting with Rana esculenta L. obtained
results somewhat at variance with those of the writer on young
toads. The lights employed were “Hell-Rot” and “Dunkel-
Blau.” Three sets of experiments were performed, each con-

RESPONSES OF YOUNG TOADS

197

sisting of ten trials. There were 736 responses toward the red
and 464 toward the blue. The following quotation gives a brief
statement regarding the experiments (l.c.) :

“Da Rot, trotzdem es heller als Blau war, in 30 Fallen 26mal starker
als letzteres besucht wurde, unterliegt es wol keinem Zweifel, dass das-
selbe dem Frosch, ahnlich wie dem Triton, viel angenehmer als das
Blau ist. * * *

“Als (Minimal)-Verhaltnis ergibt sich

Rot 1

2) — .

„ Blau m. ult. 0.6

d. h. es kommen auf^lO Rot- durchschnittlich nur 6 Blau-Besuche.”

The results of Graber’s experiments with Rana esculenta L.,
according to Torelle {l.c., p. 487),

“Can be explained only on the ground of a confusion arising as a result
of using so many frogs (forty) at the same time in one receptacle."

As will be shown, Torelle {l.c., pp. 478, 479) obtained results
with frogs, Rana virescens virescens and Rana clamata, exposed
to the stimuli of red and blue lights both at the same time,
much at variance with those of Graber {l.c.). For experimental
purposes Torelle used a low, narrow box about 45 cm. in length
with a glass plate at each end. Red light was admitted at one
end of the box and blue at the other. The frog was then sub-
mitted to the influence of the two lights. The response toward
the blue light was immediate, the animal moving toward the
source of illumination. Frequently the frog remained with its
head against the glass and turned toward the light. Appar-
ently there was no movement on the part of the frogs toward
the red light. These results of Torelle ’s with frogs are largely
in agreement with those of the writer with specimens of young
Bufo americanus, as they are also with the statements of Holmes
in regard to frogs. According to this author (1907, p. 349)
the blue and the violet rays are the most effective in producing
phototactic responses. When two lights are used, red and blue,
frogs collect near the blue light.

The results of the writer with specimens of young Bufo amer-
icanus and those of Laurens {l.c., pp. 277-282) with Bufo
americanus and Bufo fowleri are in accord. He worked with
pairs of balanced colored lights and found that the toads reacted
by giving motor responses, either jumping toward or away from
the source of illumination. The animals also responded by

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C. F. CURTIS RILEY

orienting"£themselves so that their heads pointed toward or
away from the light. In one series of experiments with red
and blue lights, out of a total of 960 responses, 788 were toward
the blue light, 167 were toward the red light, and 5 were indif-
ferent. Viewing the responses from the percentage basis, 82
per cent were in favor of the blue light, while only 1 7 per cent
were in favor of the red light. It will be noticed that the results
in the main are similar to those obtained with single mono-
chromatic lights. However, it should be stated that the re-
sponses were not quite so quick as was the case with single
lights. There were movements toward both lights, but those
toward the blue far outnumbered those toward the red.

“It cannot be said that there is much evidence in favor of positive

phototropism for the red light.” (Laurens, l.c ., p. 280.)

In comparing these experiments of Laurens, with balanced
blue and red lights, with those of Pearse (l.c.) on Rana palus-
tris with single blue and red lights, it is of interest to observe
that the results in the two cases very largely agree, in that
there is a greater number of positive responses toward the blue
light than there is toward the red light. However, it should
be noticed that there is a much larger percentage of positive
responses with reference to the blue light in the former’s experi-
ments with balanced blue and red lights, than is found to be
the case in Pearse ’s experiments with blue and red lights used
singly. It should also be noticed that in Pearse ’s experiments
the results indicate many responses that were indifferent with
respect to the red light.

VIII. RESPONSE TO CONTACT

Some observations were made on young specimens of Bufo
americanus with reference to their responses to contact stimuli.
While no systematic experiments were undertaken in order to
study the effect of contact on the behavior of the animals, yet
certain incidental observations were recorded. Some data were
obtained regarding the influence of contact stimuli on the
response to light. Record was made of observations on a form
of a contact response resembling the death-feint among Arthro-
pods. Some description also was given of the work of other
writers with regard to the contact responses of toads and frogs.

During the experiments with light there were usually a num-

RESPONSES OF YOUNG TOADS

199

ber of young toads in the experimentation dish at the same
time. Occasionally in jumping toward or away from the source
of illumination two individuals come in contact with each other.
Frequently this contact appears to have no effect upon the
animals so far as their response to light is concerned. At other
times one or both of the toads pause for a few seconds before
again reacting with the motor response with reference to the
light. Generally, however, when contact occurs it seems to act
as a stimulus in inducing the motor response. This is noticeable
when a toad, in jumping, comes in contact with a stationary
individual, the latter’s motor response being invoked. If a
toad does not respond to the light for a few seconds, but remains
quietly resting in one position, and if then it is stimulated by
means of another animal jumping against it, the motor response
may result and this may be followed by the animal responding
to the photic stimuli. These statements apply more largely
to the experiments with the weaker intensities. Parker ( l.c.y
pp. 28, 29) found when subjecting frogs, Rana pipiens Schreber,
to the lower light intensities from a Nernst glower that,

“In some instances after a frog had remained ten minutes or more
without changing its original position, it was induced to jump by being
touched from behind, and, when this was done, the animal almost invari-
ably turned first and then jumped toward the source of light.”

Pearse (/.c., pp. 177, 178) in discussing “the influence of
mechanical stimulation on the photic reactions of the toad,”
Bufo americanus and Bufo jowleri, when subjected to a light
intensity of 220 ca.m. states that,

“In jumping about they stimulated each other in a mechanical way.

* * * It is evident * * * that mechanical stimulation exerts an

influence on the phototropism of the toad by enforcing the effect of light,
or, it could perhaps better be said, that the mechanical stimulation fur-
nishes the impulse to locomotion, while the light is effective in determining
the direction of the movement after locomotion has been established.”

The writer has noticed in his experiments with the less intense
illuminations that when several young toads move into the angles
formed by the bottom and sides of the experimentation dish
that the contact of their bodies and of the solid surfaces of the
vessel seem to inhibit the motor response for some time. The
animals remain quietly resting with their bodies in rather close
contact. During the experiments with weak diffuse daylight, it
was found that when stones were placed about 25 mm. apart

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C. F. CURTIS RILEY

in the experimentation dish that there was a tendency for some
of the toads to move into the spaces between the stones. The
writer has frequently observed a similar “habit” on the part
of mature and partially mature frogs. Large numbers of frogs,
chiefly Rana pipiens Schreber, were kept in two large tanks in
a basement room, the tanks being in diffuse daylight. It was
a very common sight to find as many as thirty individuals
crowded closely together in the angles formed by the sides
and bottom of each tank. Frequently they were congregated in
the more shaded corners, and the taking of such positions might
be due in part to a response to light. However, they were some-
times observed to be gathered in corners that were not so shaded.
At other times frogs were found to be grouped in the corners of
the tanks, both in shaded and unshaded situations. Not infre-
quently frogs were observed to be distributed sparsely about the
more central and open portions of the tanks. Such groupings as
have been described are in all probability due to responses to
contact stimuli. The movement into the shade may be due in
part to vision as Torelle (l.c.t p. 470) has suggested.

Other writers have recorded observations on the contact re-
sponses of toads and frogs. Torelle ( l.c ., p. 477) experimented
with specimens of Rana virescens virescens and Rana clamata in
a jar of water and found that the propensity of the frogs to
place themselves in contact with solid bodies “is apparently
stronger when the temperature is lowered.” The following inter-
esting facts are quoted from the author’s work cited above:

“When a rock was lowered into the jar in such a way that a small space
was formed between it and the wall of the jar, the frog crawled into this
space and remained there. When a space was formed between the bottom
of the jar and the rock, it crawled into that. This was tested several
times, and was also observed when the temperature of the water in the
aquarium in which the frogs were kept was lowered 10° C. and below.
When this was done, all the frogs responded, either by flattening their
bodies against the stone floor, or by creeping under the rocks usually
kept there. It therefore seems that the frog is stereotropic in tempera-
tures between 10° C. and 4° C.”

These experiments of Torelle ’s, considered with those ( l.c ., p.
476) on frogs out of water, seem to bear an interesting relation
to hibernation. It is probably true that this instinct is not
due to a single but to several causes. The increase of stereo -
tropism with a lowering of the temperature is an important
physiological change which may be related to the burrowing

RESPONSES OF YOUNG TOADS

201

response of hibernating frogs. Dickerson ( l.c ., p. 71) has
observed the tendency of young toads, Bufo americanus Le
Conte, to crawl “under stones and chips, in the cracks of board-
walks or under the protecting cover of leaves and grasses.” The
young toads to which she referred had left the water just re-
cently and were therefore very delicate creatures, and they
remained during the daytime in such protected situations as
have been mentioned. So that while these responses are prob-
ably due in part to contact stimuli, yet other stimuli also such
as light and temperature undoubtedly have considerable influ-
ence in bringing them about. The burrowing response of young
spadefoot toads, Scaphiopus holbrookii Harlan, exhibits itself
early in the life of the individual (Dickerson, l.c., p. 56), and
is in part at least a response to contact. According to Holmes
(l.c., p. 351) there is a tendency for frogs and toads to crawl
under stones and to place themselves between objects. In such
positions they remain quiet. This propensity to move into
such situations is more pronounced in the case of toads. He
considers such responses to be of a thigmotactic nature, although
from his discussion it may be inferred that at times light also
plays some r61e.

Considerable care was used in handling the young toads dur-
ing the experiments with light. They were removed from the
aquarium jar and placed in the experimentation trough by
hand. In order that such contact should modify the response
to light as little as possible, the toads were left undisturbed for
approximately fifteen minutes before being subjected to the
photic stimuli of the experiment. Should the toads while being
transferred from one vessel to another be handled with undue
pressure and roughness, they sometimes assume an immobile
state. As this response was a somewhat unfamiliar one to the
writer a number of observations were recorded concerning it.

Frequently when young specimens of Bufo americanus are
handled, the contact stimulus causes them to become motionless ;
they react with the death-feigning response. Certainly they
assume an attitude which is comparable to that assumed by
many Arthropods when they are said to feign death. The legs
are drawn up closely against the body and they assume a more
or less rigid condition, the animal remaining motionless. Some-
times a toad lies so absolutely quiet that even the respiratory

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C. F. CURTIS RILEY

movements are unobserved, and the eyes may be closed. The
attitude assumed by the death-feigning animal is not always as
just described. Instead of the legs being drawn up in close
contact with the body they may be somewhat extended. A
young toad may be made to feign death by being placed on
its back in the hand or on the laboratory table, and held in
that position for a few seconds. There is considerable variation
among different individuals regarding the length of time of the
death-feigning response. Some feign death for a few seconds
only, while others retain the death-feigning posture for a minute,
and occasionally even for a longer time. When young toads are
exposed to the beam of light from the projection lantern during
the death-feint, the length of the response seems to be some-
what shorter, than when the animals are induced to feign death
in weak diffuse daylight. Here again there is much variation,
for in some instances it appears to make no difference to the
toad as to the intensity of the light to which it is subjected.
If the animal is put into the death feint in diffuse daylight and
is then exposed to the bright beam of light from the projection
lantern, the length of the response is curtailed, in fact the toad
at times arouses immediately from the death-feint. A young
toad generally arouses from the death-feint rather suddenly.
If the animal is on its back, first one leg and then another is
extended until the legs are no longer pressed closely against the
body. If the eyes are closed while in the death-feint, they are
opened sometime during the process of arousing from the re-
sponse, while the legs are being extended. Immediately after
the eyes have been opened and the legs extended, the toad turns
over with the ventral side down. While it is true that the
young toad usually arouses from the death-feint rather abruptly,
there are individual variations, some animals being more delib-
erate in the process than is the case with others. Young toads
may be promptly aroused from the death-feint by sudden tact-
ual stimuli, as for example, a touch on the body, though this
may at times cause a continuance of the response, or by drop-
ping them into a jar of water. Dickerson ( lx ., pp. 71-72)
has observed the death-feigning response in young toads, as
indicated by the following quotation:

“When they are handled they play dead for seconds at a time and
finally ‘come to life’ sticking up their little orange paws in most ridic-
ulous fashion before they tumble over and hop away.”

RESPONSES OF YOUNG TOADS

203

Mature specimens of Bufo americanus will also feign death.
The writer has frequently caused toads to exhibit this response
by placing them with the back down and in close contact with
some solid surface, meanwhile holding them firmly in that posi-
tion for approximately thirty seconds more or less. They some-
times respond to stimuli of this nature by feigning death for
one or more minutes. Near Ann Arbor, Michigan, the leopard
frog, Rana pipiens Schreber, is very common. Mature specimens
taken in that locality frequently have been made to exhibit the
death-feigning response by rough handling and by placing them
on their backs on the laboratory table and holding them securely
for some time in such a position. Mature toads and frogs exhibit
much individual variation in reacting with the death-feigning
response. The response is elicited in some animals much more
readily than in others. In some instances it seems to be prac-
tically impossible to induce the response. The length of the
death-feint also varies considerably in different individuals.
The death-feigning response is undoubtedly of the same nature
as that which Verworn (1898) 3 calls hypnosis. According to
this writer specimens of Rana esculenta when turned on their
backs, become motionless. Sometimes the hind legs are drawn
close to the body, and the eyes are closed. While the animals
lie in this position, their muscles are in a condition of “tonic
contraction.” Hypnotized frogs may assume peculiar attitudes,
as if in attempting to right themselves the movements were
suddenly inhibited. Verworn (1899, pp. 358-359) in discussing
the hypnotic state of frogs makes the following statement:

“The phenomena of prolonged reflex tone after brief stimulation may-
be seen still more clearly in frogs that have been deprived of their cere-
brum. If such a frog sitting quietly in the customary squatting attitude
be gently stroked by two fingers along the sides of the spinal column, he
raises himself upon his extremities by contracting their muscles, and
stands, sometimes more than an hour, in this grotesque position.”

According to Verworn (l.c., p. 496) if a frog is seized suddenly
and held with a firm grip, and is then placed with its back down
the animal remains immobile. A very peculiar contact response
on the part of certain toads, Bombinator igneus and Bombinator

8 Beitrage zur Physiologie des Centralnervensystems I. Die sogenannte Hyp-
nose der Thieren. Jena, 1898, pp. iv + 92. This paper was not accessible, but
some discussion of it is given in the section on “Hypnotism,” Holmes (1907, pp.
59, 60, 61), and in a review by Gotch (1898).

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C. F. CURTIS RILEY

pachypus, appears to be somewhat akin to the death-feint, or
to the hypnotic state already described. The following quota-
tion taken from Gadow (1901, p. 156) gives a good description of
the posture assumed by these toads during the response:

“When these toads are surprised on land, or roughly touched, they
assume a most peculiar attitude. * * * The head is partly thrown

back, the limbs are turned upwards with their under surfaces outwards,
and the whole body is curved up. * * * The creature remains in this

strained position until all danger seems passed.”

The following interesting description of the death-feigning in-
stinct among toads and frogs is given by Dickerson ( l.c ., p. 34) :

“Many of the Salientia play dead in response to an unexpected tactual
stimulus. The common toad will often hold the legs tight against the
body and inhibit all movement — even the breathing vibrations of the
throat- — when seized by a dog or other enemy. The leopard frog may
stretch the legs backwards stiff and straight, fold the arms on the breast,
and inhibit the breathing movements. It certainly looks like a dead frog
as it lies motionless in one’s hand for fully a minute; suddenly, with a
lightning movement, it is gone before the hand can be closed over it. The
cricket frog plays dead in water. Taking a position with arms and legs
rigid and throat collapsed, it floats about helplessly like any stick or leaf.”

Dickerson (l.c., 87-88) considers the death-feigning response of
the toad to be a protection to the animal. She states that,

“The toad is fitted for his place in life by what he does, as well as by
what he is. Let an enemy seize him roughly, and he is a dead toad.
‘Playing dead’ saves him many a time. He will lie on his back with
scarcely any perceptible motion for minutes at a time. Even the breath-
ing movements seem to be suspended. Suddenly one leg is thrust out,
then another, the eyes open wide, and in an instant more, the toad has
turned over and is ready for new emergencies. Whether this habit is
a protective instinct, or whether the toad really is insensible from fright
during the time that it ‘plays dead,’ the resulting protection is the same,
for, as a rule, animals that feed upon living food associate motion with
life so firmly that they pay no attention to a motionless creature.”

Facts similar to those described by Dickerson have been ob-
served by Holmes (l.c., pp. 59, 60, 61), who considers this
immobile condition of the frog to be an hypnotic state. Accord-
ing to this author the position assumed varies at different times.
Some individuals are more easily hypnotized than others, and
the duration of the hypnotic state also varies in different frogs.
Sometimes a frog will remain immobile for hours. A frog may
be aroused from its condition of hypnosis by some sudden
stimulus, and the awakening often occurs immediately.

Sometimes when removing the young toads from the aquarium
jar to the experimentation dish, the writer observed that the

RESPONSES OF YOUNG TOADS

205

animals respond to contact in another manner than that of the
death-feint. The body of the toad is spread out and closely pressed
against the bottom of the aquarium, and the lungs are filled
with air until the animal becomes as wide as it is long. The
head is also bent downward and placed on the bottom of the
aquarium. As long as the animal retains this position it remains
motionless. Reactions similar to these may occur as responses
to a moving shadow or object, as for example the writer’s hand
when it is reached into the aquarium to remove the young
toads. So that these are responses to both tactual and visual
stimuli. Dickerson (Z.c., pp. 33-34, 86) also has drawn atten-
tion to this form of response in the toad. According to Holmes
(l.c., p. 32),

“Frogs sometimes swell the body before being seized as if in antici-
pation of their capture, and they are especially apt to do this after being
lightly touched. Touch a frog that is resting quietly, and if the creature
does not hop away, one may see the body puff up; and if the body is
touched two or three times, the swelling will continue until the lungs
contain their maximum amount of air. * * * Frogs often avoid

capture better by remaining perfectly quiet than by attempting to get
away by jumping. * * * Safety is also sought occasionally by crouch-

ing close to the ground, and more often by crawling under some object that
promises to afford shelter.”

Another interesting response to contact stimuli is the “sing-
ing” or croaking of frogs and toads. The croaking of frogs and
toads is readily induced by stroking the body, especially on
the back or sides. They also will croak when kept in an aquar-
ium. The contact of one animal against another is often suffi-
cient stimulus to produce this sound. It is not improbable that
light as well as contact may play some role in connection with
the croaking reflex. The writer frequently has observed hun-
dreds of toads in bright patches of moonlight along the shores
of ponds and marshes. On such occasions their heads are raised,
and the throat-sac is puffed out to a large size, owing to their
vigorous “singing.” At such times they give little “attention”
to the observer, and one may pick up a toad, placing it upon
the palm of the hand where it will continue to “sing” with
astonishing vigor.

IX. DISCUSSION

It has been pointed out, in the experiments with the projec-
tion lantern, that young specimens of Bufo americanus orient
in such a manner with reference to the light that the head points

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C. F. CURTIS RILEY

away from the source of illumination. After orientation is com-
pleted the animals retain the position assumed with reference
to the light, the medium longitudinal axis of the body being
kept practically parallel with the incoming rays. So far as the
writer’s observations are concerned, there is no definite evidence
that young toads orient to light according to the method of
“selection of random movements,” as advocated by -Holmes
(1905) (though the present writer believes that it may function
in some modified form), or by that of “trial” so ably propounded
by Jennings (1904) and (1906), also discussed by Mast ( l.c .,), and
many other writers. Usually the young toads orient promptly
and definitely. If the head is pointing toward the light, they
make a turn of 180° so that the anterior end of the body is
pointed directly away from the source of illumination. Should
the toad be in such a position that the median longitudinal
axis of the body lies at right angles to the rays and facing the
light, the animal makes a turn of 90 °, thus bringing the head
into such a position that it points directly away from the source
of illumination. There are no preliminary movements, either
“trial” or “random” ones, during or immediately preceding the
orienting response, so far as the writer observed. But as Mast
(l.c., p. 214) has suggested, these facts do not preclude the possi-
bility of preliminary movements when other forms of stimuli
impinge upon the toads. After orientation is completed the
young toads jump away from the source of illumination along
a comparatively straight path, the medium longitudinal axis of
the body being parallel with the rays of light. During such
motor responses the direction of the rays in the field may be a
guiding factor. It is not impossible that they may be both a
guiding and a correcting factor, if we should apply the thhory
advocated by Holmes (l.c., pp. 108-109). While there is little
evidence of such responses on the part of young toads, neverthe-
less the applicability of Holmes’s modified “trial and error
method” with reference to the responses of animals which
orient themselves “according to the usual scheme” is worthy
of careful consideration. The present writer believes that it is
absolutely futile to attempt to explain the responses of all
animals by any one theory, or from any one point of view.

Such a response on the part of young toads as has been
described by the writer is a tropic response in so far as it fitsv

RESPONSES OF YOUNG TOADS

207

the definition of Jennings (1909, p. 307), however, the writer
does not wish to be understood as believing that internal fac-
tors, changes in bodily states, play no role in the orientation
of young toads to light. This matter was not discussed in con-
nection with the responses to intense artificial light, largely
because the wr ter was attempting to record the responses as
they occurred in the majority of cases. There are occasional
examples when it is observed that one toad orients to the light
much more slowly and hesitatingly than another. Some toads
jump away from the light more rapidly than others and along
a straighter path. There also is found to be variations in such
responses on the part of the same animal on different occasions.
These facts seem to indicate modifications in the bodily condi-
tions of the animals concerned, especially when it is remem-
bered that both the environment and the kind of stimuli remain
unchanged. Loeb ( l.c ., p. 24) and (1912, p. 47) early recog-
nized the importance of differences in the physiological condi-
tions of animals as modifying factors in animal responses. This
subject has been discussed by the present writer (1912, pp.
281-283). It is not improbable, in the writer’s experiments
with young toads, that before orientation the animals are, as
it were, in a condition of unstable equilibrium with refer-
ence to the light, and that the orienting response is one
of adjustment; and further, after orientation is completed the
toads are then in a condition of relative stability toward the
light, so far as orientation is concerned, and they exhibit further
response by jumping away from the source of illumination.
With reference to orientation in general, may it not be a fact
that previous to the orienting response the bodily state of the
animal differs from its condition after orientation is complete?

The responses of young toads to the light from the projec-
tion lantern seem to indicate that the unequal light intensity
on the two sides of the body is a factor in inducing the animals
to Orient so promptly and definitely. According to Holmes
(1907, p. 346), frogs orient with respect to light in general in
much the same manner. Pearse (l.c., pp. 172-205) apparently
takes a similar point of view with reference to the orientation
of specimens (some of them immature) of Bufo americanus and
Bufo jowleri to light of 220 ca.m. intensity (see particularly
l.c., pp. 204-205). The orientation of specimens of B. ameri-

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C. F. CURTIS RILEY

Canus (some of them immature), as described by Mast ( l.c .,
pp. 2 14-2 1 5, 219-220) appears to occur much in the same way
as observed by the writer in young toads. The intensities of
the lights employed by Mast in his experiments were 12.5 and
25 ca.m. It is probable, during the orienting response of young
toads, that the light acts in some such manner as described by
Loeb (1905, p. 32), and to which Mast (l.c., p. 223) practically
assents. Even so, though the light should act “at a constant
intensity/’ such a fact does not necessarily preclude the influ-
ence of the differences in intensity on the two sides of the ani-
mal’s body, during orientation.

In regard to the effect of light intensity upon the orientation
of young toads, mention should be made of Mast’s (l.c., pp.
219-220) extremely interesting experiments with specimens of
Bufo americanus, some being immature. Two lights of different
intensities were employed, one being 12.5 ca.m. and the other
25 ca.m., and the source of illumination was two Nernst glow-
ers, the two beams of light crossing at right angles. When a
toad was placed, with one side turned toward the glower, in the
beam of light of lesser intensity, it oriented directly and accu-
rately, and then jumped toward the source of illumination. How-
ever, when the animal reached the intersecting beam from the
light of greater intensity, instead of orienting toward this light,
it continued to jump toward the weaker light. Altogether 42
trials were made, 36 of these being as described. In 6 cases
only did the toad turn toward the stronger light when it reached
the point of intersection of the two beams, and these six trials
were all with the same individual. Seven toads only were used
in the experiments. It would seem in a series of experiments
exhibiting results of this nature that the effect of light intensity
was modified as an orienting factor, or why did not the animals
orient toward the light of stronger intensity. The present
writer offers two suggestions which may prove to be partially,
if not fully explanatory of this. First, the eyes are strongly
stimulated by the light from in front, and the response to such
stimulation in itself may result in producing an inhibitory effect
upon the toad in so far as its response to the stimuli from the
intersecting light is concerned. While it is true that in toads
both skin and eyes are photoreceptors as Pearse (l.c.) has
proved, it is evident that the cross rays from the stronger light \

RESPONSES OF YOUNG TOADS

209

would reach one eye only with full effectiveness. Second, if
animals with image -forming eyes go toward a source of light,
because they perceive the light itself and follow it “much as
an animal pursues any other object of interest” as indicated
by Graber ( l.c ., p. 248), Torelle ( l.c ., p. 471), Holmes (1905a,

PP- 34i. 344-345) and 09°8> P- 496). and Mast PP- 2I9> 223).
then such behavior may present a partial explanation as to the
reason why the toads used in Mast’s experiments did not go
toward the stronger light, for the “ attention ” of the animals may
have been so fully occupied with the light in front of them that
they did not turn into the intersecting beam, and jump toward
the source of the stronger light. Further, it must be recalled, as
Mast {l.c., p. 223) himself has suggested, that the direction of
the rays in the field may be a guidance to the toads, especially
if they go toward an object because they see it. Then, the
after effects of the directive weaker light may have been suffi-
cient to keep the toads moving along the path already taken,
even when they reached the strong intersecting beam of light.
In so far as this work of Mast’s applies to the writer’s experi-
ments with young toads, certain facts should be kept clearly
in mind regarding the latter’s experiments, that the animals
employed Were extremely immature, that light approximating
10,000 ca.m. intensity was used, and that the toads reacted nega-
tively to the photic stimuli. From what is known of the habits
of toads, it was to be expected that the animals would respond
negatively to the strong stimuli from the projection lantern.
Toads are very largely nocturnal animals, and are more com-
monly seen about twilight when they leave their places of
“concealment,” which they have occupied during the daytime.

The young toads respond positively to all the lesser light
intensities of white light. In these experiments it seems as if
the difference in the intensity of the stimuli on the two sides
of the body was an important factor in orientation. This sub-
ject has been discussed in connection with the responses to the
light from the projection lantern, and therefore will not be
dwelt upon here. In jumping toward the light it is hardly prob-
able that the rays per se in the field are a very important ele-
ment in guiding the toads, for it has been stated that there
must be many cross lights in the field of experimentation. Of
course there is a certain part of Holmes’s theory (1905, pp. 108-

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C. F. CURTIS RILEY

109) that might apply here as a partial explanation, for a por-
tion of the rays are parallel with each other and with the median
longitudinal axes of the bodies of the toads. In the experi-
ments in the dark room the principal object in the toad’s field
of vision is the light, and it is probable that the animals jump
toward it because they see it, an idea that has been mentioned
before. The light also may act continuously as Mast (l.c.) has
suggested, and somewhat as Loeb (1890, p. 90) stated more than
twenty years ago with reference to frogs when diffuse daylight
was used as a stimulus. The following quotation gives Loeb’s
point of view:

“Dass auch beim Frosch das Licht als konstante Reizursache wirkt,
geht daraus hervor, dass die Thiere dauernd an dem der Lichtquelle ent-
gegengesetzten Ende des Kastens sitzen bleiben.”

In experiments with weak diffuse daylight, orientation is not a
prominent feature in behavior, but there is some evidence of a
tendency for the toads to gather toward the source of illumina-
tion.

Many of the responses of young toads to contact stimuli are
probably adaptive ones, such as creeping under and between
objects. Even so peculiar a response as that of the death-feint
may be of such a nature, as the observations of some writers
seem to indicate. It is true that in the case of some of the'
Arthropods, it is rather more difficult to see how the death-
feigning response serves any adaptive purpose. The act of crouch-
ing against the ground and of inflating the lungs with air may
be another example of a protective device. It would be of con-
siderable scientific interest, for some investigator to make a long
and varied series of observations with reference to the contact
responses of young toads, and see in how many instances such
responses were adaptive in function.

X. SUMMARY

Specimens of Bufo americanus Le Conte, approximately 14 mm.
in length were collected near Ann Arbor, Michigan. They
respond negatively to the light from a projection lantern, with
an approximate illumination of 10,000 ca.m. within the field
of experimentation. The animals jump away from the light
toward the opposite end of the dish. If they are left in that

RESPONSES OF YOUNG TOADS

211

situation for some time, some of them climb up the perpendic-
ular glass wall at the end of the vessel. This places them out
of the most intense glare of the light. The young toads orient
promptly and definitely, by turning away from the source of
illumination and so place themselves that the longitudinal
axes of their bodies lie parallel with the incoming rays. This
position in relation to the rays of light is maintained while
traveling from one end of the dish to the other, and the pauses
between the jumps are brief, so that the animals move with a
fair degree of speed. The responses in water are similar to those
already described, the toads swimming away from the light.

The young toads respond positively to the light from a 16
c.p. incandescent light, with an illumination approximately of
44 ca.m. They also respond in a similar manner to strong
diffuse daylight, to weak diffuse daylight, and to sunlight. Except
in the case of diffuse daylight, the animals jump toward the
source of illumination in a comparatively straight path. It
cannot be said that the median longitudinal axes of the bodies of
the toads are parallel with all the incoming rays, because many
of the rays enter the experimentation dish at various angles
and there must be cross lights within the field of experimenta-
tion. While the animals jump toward the light with consider-
able promptness, their motor responses are perhaps not so
quick as in the experiments with the projection lantern. At
times it seems as if the toads do not travel in quite so straight
a path as is the case when the intense artificial light is used as
a source of stimulation. The toads orient fairly definitely and
accurately, but not so promptly as when the projection lantern
is employed. At times, when responding to sunlight, the animals
come to rest in diffuse daylight, if it is nearby. Neither the
movement toward the light nor the orientation is so definite as
in the case of the stronger intensities.

It is not improbable that both light intensity and ray direc-
tion in the field are factors in these photic responses. During
orientation light intensity may play the more important role,
while the rays in the field may act as a guiding factor after
orientation is complete, though this does not necessarily do
away with the effect of intensity. In the positive responses
vision is an element not to be ignored, and it is probable that

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C. F. CURTIS RILEY

the light acts continuously. Further, it is not impossible that
a modified form of “ trial and error ” may function as the animals
travel toward or away from the source of illumination. In the
experiments with weak diffuse daylight, ray direction in the
field probably exerts comparatively little influence upon young
toads.

Young toads react to light passed through red and blue solu-
tions by giving motor responses. When red light is employed
singly, it is noticed that the stimuli are not very effective. There
is a tendency for the animals to act negatively, both by turning
and jumping away from the light. The responses are less defi-
nite than when either white or blue light is used. When blue
light is employed the toads both turn and jump toward the
light, and they do so with more promptness, than is the case
when red light is used. When red and blue lights are used at
the same time, more animals jump toward the blue light than
they do toward the red light. Some toads jump toward the
red light, and a few individuals appear to be indifferent.

Young toads subjected to light react to contact stimuli by
giving the motor response. If a stationary individual is stimu-
lated by another animal jumping against it, the former responds
either by turning or by jumping away. Frequently after such
a motor response, the animal follows it up by reacting to the
light. In the experiments with the lesser light intensities, the
young toads may respond to contact by several of them gather-
ing in the angles formed by the bottom and sides of the experi-
mentation dish. They remain in such situations with their
bodies in close contact.

Young toads frequently react with the death-feigning re-
sponse when handled with undue pressure and roughness.
During such response they remain immobile, with their legs
drawn up against the body. A toad may be caused to respond
in this manner by placing the animal on its back and holding
it in that posture for a few seconds. The length of the death-
feint varies in different individuals. If a toad has been made
to feign death in diffuse daylight, the response may be cur-
tailed by suddenly flashing a beam of light from the projection
lantern upon the animal. Young toads usually arouse from the
death-feint abruptly. A sudden tactual stimulus will effect this.
The death-feigning response may probably result in producing

%

RESPONSES OF YOUNG TOADS 213

a bodily condition in young toads not far removed from that of
hypnosis described by Verworn in other animals.

Young toads will also respond to contact in another manner.

? Sometimes when touched the body is pressed closely against the

ground, and the lungs are inflated with air, the head also being
bent downward. While in this position an animal remains
> motionless. It is likely that many of these responses to con-

tact on the part of young toads are adaptive in function.

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