jljjji li jjj iljijljlj) Jii (jjjjijjjj IlliJfJIfnliilt ii lit it i?i UNIVERSITY OF ILLINOIS UiBRARY AT URBANA-CHAMPA1GN BIOLOGY APR 91992 B I X Zoology ). 43 APR * 3 t< Convergence and Morphological Constraint in Frogs: Variation in Postcranial Morphology Sharon B. Emerson A Contribution in Celebration of the Distinguished Scholarship of Robert F. Inger on the Occasion of His Sixty-Fifth Birthday March 31, 1988 Publication 1386 PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY Information for Contributors to Fieldiana General rimarily a journal for Field Museum staff members and research associates, although manuscripts from nonaffiliated authors may be considered as space permits. The Journal carries a page charge of >cr printed page or fraction thereof. Contributions from staff, research associates, and invited authors will be rdless of ability to pay page charges, but the full charge is mandatory for nonaffiliated authors of unsolicited manuscripts. 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Page Proofs: Fieldiana employs a two-step correction system. Each author will normally receive a copy of the edited manuscript on which deletions, additions, and changes can be made and queries answered. Only one set of page proofs will be sent. All desired corrections of type must be made on the of page proofs. Changes in page proofs (as opposed to corrections) are very' expensive. Author- generated changes in page proofs can only be made if the author agrees in advance to pay for them. FIELDIANA Zoology NEW SERIES, NO. 43 Convergence and Morphological Constraint in Frogs: Variation in Postcranial Morphology Sharon B. Emerson Division of Amphibians and Reptiles Field Museum of Natural History Chicago, Illinois 60605-2496 Present address: Department of Biology IniversitvofUtah Salt Lake City, Utah 84112 A Contribution in Celebration of the Distinguished Scholarship of Robert F. Inger on the Occasion of His Sixty-Fifth Birthday Accepted for publication March 3, 1986 March 31, 1988 Publication 1386 PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY © 1988 Field Museum of Natural History ISSN 0015-0754 PRINTED IN THE UNITED STATES OF AMERICA Table of Contents Abstract 1 Introduction 1 Historical Background 1 Materials and Methods 2 Results 3 Multivariate Interfamilial Study 3 Intrafamilial Comparisons 7 Locomotor Groups of Frogs 10 Coefficients of Variation 11 Discussion 11 Convergence 11 Variation 13 Acknowledgments 14 Literature Cited 14 Appendices Appendix 1 : Interfamilial Sample 16 Appendix 2: Intrafamilial Sample 17 Appendix 3: Master Equations for Inter- familial Sample 18 Appendix 4: Master Equations for Hylidae Intrafamilial Sample 18 Appendix 5: Master Equations for Lepto- dactylidae Intrafamilial Sample 18 Appendix 6: Master Equations for Ran- idae Intrafamilial Sample 19 List of Illustrations 4. Factor scores of 4 1 species for the third and fourth rotated axes of the principal component analysis of species of a given family 6 5. Factor scores of 41 species for the third and fourth rotated axes of the principal component analysis of species of a given locomotor mode 7 6. Factor scores of 41 species for the third and fourth rotated axes of the principal component analysis of species of a given articulation type 8 7. Factor scores of 41 species for the third and fourth rotated axes of the principal component analysis of species of a given locomotor mode 9 8. Separation of locomotor modes along the first two canonical variates of the canonical discrimination analysis of species of a given locomotor mode 9 9. Diagrammatic representation of frog limb morphospace 10 10. Factor scores of hylid species for the fifth and sixth rotated axes of the princi- pal component analysis 12 1 1 . Morphological distance plotted as a function of taxonomic level for three groups of vertebrates 12 12. Factor scores of 41 species for the first two rotated axes of the principal com- ponent analysis 13 Factor scores of 4 1 species for the first two rotated axes of the principal com- ponent analysis of species of a given family 4 Factor scores of 4 1 species for the first two rotated axes of the principal com- ponent analysis of species of a given lo- comotor mode 5 Factor scores of 4 1 species for the first two rotated axes of the principal com- ponent analysis of species of a given ar- ticulation type 6 List of Tables 1 . Loadings of transformed variables on first four rotated axes, interfamilial data set 3 2. Variables loading on first four factors and the variance explained by each factor .... 8 3. Standardized canonical coefficients for the first two canonical variates 1 1 4. Coefficients of variation (inter- and intra- familial samples) 1 1 ill Convergence and Morphological Constraint in Frogs: Variation in Postcranial Morphology Abstract Morphological variation within the order Anura is often thought to be less than that in other ver- tebrate orders. Two explanations have been sug- gested for this repetitive occurrence of similar morphotypes among distantly related species: con- vergence and morphological constraint. Principal component analysis of postcranial features con- firms that morphological variation correlates with locomotor mode. This form-function correlation is taken as evidence of convergence. Furthermore, comparisons of patterns of morphological varia- tion among and within families show that the same features account for similar levels of variation within families and across the order. Similarity of coefficients of variation at different taxonomic levels is taken as evidence that morphological variation is constrained within frogs. Introduction Similar postcranial morphological features have appeared repeatedly and independently within and across the major families of frogs (Noble, 1931; Parker, 1932; Emerson, 1976, 1979). Some writers have suggested that the high incidence of conver- gence is related to the reduced number of skeletal elements of frogs (Parker, 1932;Inger, 1967). This idea follows from the assumption that a smaller number of bones allows a lesser number of pos- sible skeletal variants. Although morphological variation within the order is thought to be slight (Inger, 1967), there have been few studies specif- ically examining patterns of morphological vari- ation in frogs, other than studies focusing on sys- tematics. Another explanation for the convergence is that similar selective pressures have produced common morphologies (Lewontin, 1978) among unrelated taxa. But, while the functional signifi- cance of the postcranial morphology of frogs is well known (Emerson, 1976, 1978, 1979, 1982; Emerson & Diehl, 1980; Emerson & DeJongh, 1980), few studies (Emerson, 1976; Emerson & Diehl, 1980) have explicitly tested the hypothesis of morphological convergence in the context of adaptation. The two foregoing ideas are not mutually ex- clusive. Both similar selective pressures and lim- ited skeletal variation could be involved in the pattern of repetitive morphotypes seen in frogs. The object of this paper is to consider both ex- planations. Form-function correlations will be used to test the explanation of convergence through ad- aptation. Functional locomotor groupings will be mapped on patterns of variation in postcranial morphology. The presence of a form-function cor- relation will be taken as evidence for adaptation. Second, patterns of variation in postcranial mor- phology will be compared across different system- atic levels. The same features accounting for sim- ilar levels of variation within families and across the order will be taken as evidence for constraint in skeletal variation. Historical Background The postcranial morphology of frogs has pro- vided a fertile field of research for both functional morphologists and systematists. In general, we know that differences in both pelvic morphology and hind limb proportions are correlated with dif- ferences in locomotor mode, and that these mor- phological features are useful as characters in the analysis of systematic relationships among frogs at diverse taxonomic levels. For systematics stud- EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS ies, differences in relative hind limb length are used to characterize closely related species. Func- tionally, the length of the hind limb is the distance through which the force acts during jump takeoff and is strongly correlated with jump performance (Dobrowolska, 1973; Zug, 1972; Emerson, 1978). The tibiofibula is the out-lever of the lower leg extensor muscle, and frogs which are specialized for burrowing have a relatively short tibiofibula (Emerson, 1976). There are three patterns of ar- ticulation between the vertebral column and pelvis in frogs. Types of articulation tend to be family specific (Emerson, 1979), but there are some in- teresting exceptions. The three types of iliosacral articulation are correlated with differences in the lengths of the pelvic girdle and presacral transverse processes (Emerson, 1982). Functionally, the type of articulation mediates degree of movement at the joint (Emerson, 1979). The direction and de- gree of movement are correlated with different lo- comotor patterns (Emerson, 1979). Materials and Methods Forty-one species representing 36 genera from 1 5 families were selected for the interfamilial study (see Appendix 1 for a list of species). Represen- tatives from all the frog families and locomotor types were included. These particular species were chosen because their articulation type could be confirmed by dissection of museum specimens and because they represent the broadest possible range of frog postcranial morphology and locomotor be- havior. For intrafamilial comparisons, 28 species of Hylidae, 26 species of Leptodactylidae, and 32 species of Ranidae were examined (see Appendix 2). The object of these samples was to include as many genera within each family as possible, even if articulation type and locomotor mode could not be determined. One specimen of each species was measured (see Radinsky, 1981, and Emerson, 1982, for a justi- fication of using single specimens in studies which focus on familial level differences). Measurements on dried skeletons were taken to the nearest one- tenth of a millimeter with dial calipers. The fol- lowing measurements were taken: length of snout- vent, femur, tibiofibula, tarsus, foot, humerus, radioulna, hand, girdle, urostyle, and ilium; width of ilium; entire length of the last presacral trans- verse processes; and maximum height of the dorsal crest of the ilium. (See Zug [ 1 972] and Trueb [ 1 977] for definitions and illustrations of these measure- ments.) The original measurements were transformed to correct for the potentially confounding effects of allometry and size-related variation. First, the measurements were converted to logarithms (base 1 0), and for each group being studied, each mea- surement was plotted against ilium length, girdle length, or snout-vent length. Reduced major axis equations were calculated of the form: log Y = log a + b log X. The reduced major axis equations then became the master equations for the trans- formation (Appendices 3-6). Subsequently, each measurement for each species was changed to a dimensionless variable by inserting the ilium, gir- dle, or snout- vent length of that species into the master equation for that measurement to derive an "expected" Y value. The actual Y value was then divided by the expected Y value obtained from measurement. The result is the antilog of the residual from the log-transformed equation (see Emerson & Radinsky, 1980; Radinsky, 1981; Emerson, 1982, for details on this technique). The families Ranidae, Leptodactylidae, and Hy- lidae as defined for the principal component anal- ysis in this study include genera which are often placed in separate families of their own. The gen- era were lumped into these three families rather than split into a larger number of smaller families in order to have a large enough sample size for comparison. I consider this procedure to be jus- tified because the same range of locomotor types and morphological variation exists in the smaller family units as found in the pooled families used in this study. For calculation of dimensionless variables for the study of variation in the family Leptodactylidae, the master equation was derived after first excluding the myobatrachids. This was done because the small number of myobatrachids in the sample did not make it feasible to calculate separate equations. In the case of the Ranidae, master equations were initially generated sepa- rately for all three groups, ranids, hyperolids, and rhacophrids. The slopes were so similar among the three groups, however, that the master equa- tions from the ranid subsample were chosen to represent master equations for all three groups (Appendix 6). The choice of different standard variables against which to regress the other measurements was based on wanting both an independent estimator of size (snout- vent length) and a functionally significant FIELDIANA: ZOOLOGY Table 1 . Loadings of transformed variables on first four rotated axes, interfamilial data set. Character* Factor 1 Factor 2 Factor 3 Factor 4 FEMSV 0.90957 0.20876 -0.10023 -0.07565 TIBFIBSV 0.93605 0.03970 -0.12554 -0.01711 TARSV 0.89434 0.15630 -0.12270 0.06849 FOOTSV 0.51843 0.08004 0.11787 -0.04532 HUMSV 0.21834 0.84078 -0.02710 0.11254 RADULSV 0.12791 0.93404 -0.03407 0.08708 HANDSV 0.54186 0.28134 -0.15793 -0.02196 GIRDLSV -0.16848 -0.05212 0.95158 -0.19761 ILSV -0.11894 -0.03626 0.92089 -0.00546 UROSTSV 0.22529 -0.20653 0.28543 -0.32563 TVWGIRDL 0.20981 0.54554 -0.30109 0.08874 TVWSV 0.18781 0.60788 -0.04010 0.02387 ILWIDIL -0.18430 0.32488 -0.14984 0.3636 HTCRSTIL 0.12169 0.03928 -0.28639 -0.05492 VCLGIRDL 0.06819 0.08004 -0.62331 0.75945 VCLSV -0.04723 0.11780 -0.03614 0.98043 % Variance 23.0% 17.6% 17.2% 11.8% * Acronyms are defined in Appendix 3. standard (girdle and ilium lengths). Previous work (Emerson, 1979, 1982) has shown that locomotor mode was better correlated with girdle and ilium lengths than with snout- vent length. The four sets of transformed variables, those of the families Ranidae, Hylidae, and Leptodactyli- dae and the interfamilial data set, were first ex- amined by principal component analysis of the correlation matrix followed by varimax rotation. Varimax rotation was chosen so that each factor would have only a few important variables, thus facilitating comparisons among groups. With var- imax rotation the axes are orthogonal and there- fore uncorrelated (SAS Institute, Inc., 1982). Vari- ation on one axis is not correlated with variation on the others. Subsequently, morphological sep- aration of the major locomotor types of frogs was studied with discriminant function analysis. This was done by classifying the interfamilial data set into nine locomotor types (Appendix 1 ) and run- ning a canonical discriminant analysis. Acronyms are defined in Appendix 3. Computer analysis was done with an SAS program (SAS Institute, Inc., 1982). Morphological variation within and across fam- ilies was compared by use of multivariate coeffi- cients of variation (Van Valen, 1978). For this analysis, the Ranidae were subdivided into three separate family units, the Ranidae, Rhacophori- dae, and Hyperolidae (Appendix 2). The myoba- trachids were removed from the Leptodactylidae and not included in the analysis (Appendix 2). This regrouping was done to increase the likelihood that measurements of variation were done on mono- phyletic families. The raw data were log transformed and the multivariate coefficient of variation CV = 100 V2 variance/2 (mean)2 was calculated for the 14 measured variables (Ap- pendix 1). The resulting coefficients of variation were compared between the interfamilial data set and each of the following family data sets: the Leptodactylidae, Ranidae, Rhacophoridae, Hy- perolidae, and Hylidae. Locomotor sequences were filmed at 25, 32, and 64 frames/second with a Beaulieux 16-mm cam- era. Films were analyzed on a Lafayette stop-frame projector. For purposes of comparison, hopping is defined as locomotion in which the animal's jump is less than ten times its body length in a single bound. All quadrupedal locomotion is considered walking. Results Multivariate Interfamilial Study The results of the varimax rotation of the prin- cipal components are summarized in Table 1 and Figures 1-6. The loadings of the 16 variables on the first four rotated axes are given in Table 1. The first four axes account for about 69.6% of the total variation. The first axis is determined pri- EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS 2.0 # CD +^ CM O if 0.0 -2.0 HUMERUS RADIOULNA FEMUR TIBIOFIBULA TARSUS -1.4 -0.2 1.0 2.2 FAC 1 (23%) Fig. 1 . Factor scores of 4 1 species for the first two rotated axes of the principal component analysis. Polygons enclose all species of a given family: O = bufonids; • = ranids; » = hylids; © = leptodactylids; • = scores of species in other families. High loading variables for each factor are indicated on the graph. marily by femur, tibiofibula, and tarsus length, the second by humerus and radioulna length, the third by ilium and girdle length, and the fourth by ver- tebral column length. Figure 1 shows the factor scores for the 4 1 species for the first two rotated axes. In this figure, min- imum convex polygons have been drawn around each of the major families represented in the study, the Bufonidae, Leptodactylidae, Ranidae, and Hy- lidae. Notice that the polygon is distorted for each family by the presence of at least one outlier species. The outlier genera are Atelopus (Bufonidae), Eleutherodactylus (Leptodactylidae), Kassina (Ranidae), Pachymedusa (Hylidae), and Agalych- nis (Hylidae). Atelopus, Pachymedusa, and Aga- lychnis are characterized by having longer fore- limbs than other members of their families. These genera use walking locomotion. The locomotor mode of Eleutherodactylus was not filmed. Aga- lychnis, Atelopus, and Eleutherodactylus appear to have relatively long legs in relation to other mem- bers of their respective families, while Kassina has short legs relative to other family members. Kas- sina also uses a walking locomotion. Figure 1 also shows that, while members of the Bufonidae have a unique combination of hind limb and forelimb morphology, the other families are characterized by factor loadings which are quite similar. Variation in hind limb and forelimb length does not sort out by family. Figure 2 shows the distribution of locomotor types across the factor scores. The first two axes sort out the 4 1 species into two locomotor types. Minimum convex polygons have been drawn around swimmer-jumpers and walker-hopper- burrowers. Jumpers travel about 10 times their body length in a single bound; hoppers do not. Hoppers and jumpers also have different landing patterns. During hop landing the hind limbs are folded against the body, and the body swings through the forelimbs into the next hop. In jump- ing, the hind limbs are splayed posterolaterally and the entire body is used to absorb the force of land- ing. The frog with the shortest hind limbs is Pseu- dophryne occidentalis. Pseudophryne is character- ized by walking locomotion. The jumper-swim- mers with the shortest hind limbs are Pipa and Xenopus, two totally aquatic genera. The walker- hopper-burrowers with the longest hind limbs are FIELDIANA: ZOOLOGY ~ 2.0 CD CM O < IL 0.0 -2.0 - HUMERUS RADIOULNA FEMUR TIBIOFIBULA TARSUS -1.4 -0.2 1.0 2.2 FAC 1 (23%) Fig. 2. Factor scores of 41 species for the first two rotated axes of the principal component analysis. Polygons enclose all species of a given locomotor mode: O = jumper-swimmers; • = walker-hopper-burrowers. Atelopus, Gastrophryne, and Bombina. Gastro- phryne travels about eight and one-half times its body length in a single bound (Zug, 1978). It is not surprising, therefore, that it has one of the highest relative hind limb lengths of any of the hoppers. Interestingly, Bombina is a modest hop- per (Wermel, 1 934) despite its relatively long hind limbs; it travels about four times its body length in a single bound (Dobrowolska, 1 973). The tarsus of Bombina is long for a walker-hopper-burrower (Dobrowolska, 1973), and that limb segment is, in theory, an important contributor to jump per- formance. Why, then, is Bombina such a poor jumper? Bombina has a very low takeoff angle during hopping (Emerson, pers. obs.) and often places its feet lateral to the body during takeoff, rather than close to the side of the body. While a longer arm increases potential stride length during walking, Dendrobates and Atelopus are unusual among walker-hopper-burrowers in the extreme length of their forelimbs. Atelopus has an unusual walking mode. Unlike most walkers, Atelopus does not flex the vertebral column when the forelimb is protracted (see Barclay, 1 946, and Emerson, 1979, for descriptions of the common quadrupedal locomotion pattern of frogs); instead, the back is held straight. Second, the long legs and arms are not folded as tightly at the joints during protraction and retraction, and the body is held higher off the ground than with the more typical walking locomotion. The hopping locomotion of Dendrobates is not unusual, but the animals do often hold the anterior part of the body off the substrate, and the males have intraspecific combat involving use of the forelimbs. It is possible that the length of the forelimbs is related to the ag- gressive intraspecific behavior of the males. Aga- lychnis and Pachymedusa, two hylid genera with long forelimbs, appear to use slow walking loco- motion similar to that of Atelopus; they do not appear to flex the vertebral column, the arms and legs are less protracted than those of walkers with lateral undulation, and the walk speed is much slower (Emerson, 1979). Among walkers, Melanophryniscus, Oreophry- nella, and Atelopus show an interesting morpho- logical and behavioral cline. Melanophryniscus has the shortest hind limbs of the three genera and typical walking locomotion with vertebral flexure. Oreophrynella has a shortened presacral vertebral column and longer hind limbs. Behaviorally, it lacks vertebral flexure, shows a more lateral po- sitioning of the limbs, and also has a more elevated body. Oreophrynella has a relatively shorter stride EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS I eg O < U- 2.0 0.0 -2.0 HUMERUS RADIOULNA FEMUR TIBIOFIBULA TARSUS -1.4 -0.2 1.0 2.2 FAC 1 (23%) Fig. 3. Factor scores of 41 species for the first two rotated axes of the principal component analysis. Polygons enclose all species of a given articulation type: © = type I; • = type IIA; O = type IIB. g VERTEBRAL COLUMN 1.5 - 0.5 -0.5 -1.5 -2.5 PELVIC GIRDLE -2.2 -1.0 0.2 1.4 FAC 3 (17.2%) Fig. 4. Factor scores of 41 species for the third and fourth rotated axes of the principal component analysis. Polygons enclose all species of a given family: O = bufonids; • = ranids; » = hylids; © = leptodactylids; • = scores of species in other families. High loading variables for each factor are indicated on the graph. FIELDIANA: ZOOLOGY 1.5 -VERTEBRAL COLUMN • • ~~^ 0.5 f • • • \ o o^^ \ •• ^v • •> cq -0.5 O 0 ° \ ° • ^ O < -1.5 -2.5 I I o • y^ / o / PELVIC QIRDl I I -2.2 -1.0 0.2 1.4 FAC 3 (17.2%) Fig. 5. Factor scores of 41 species for the third and fourth rotated axes of the principal component analysis. Polygons enclose all species of a given locomotor mode: O = jumper-swimmers; • = walker-hopper-burrowers. length than Melanophryniscus because it does not protract the limbs as far during locomotion, and consequently is a slower walker. Atelopus has the longest hind limbs and forelimbs of the three gen- era. It also lacks vertebral flexion. Atelopus is char- acterized by having a slow walking locomotion with relatively laterally placed limbs and by hav- ing the body elevated off the ground. Figure 3 shows the distribution of the three ma- jor articulation types (Emerson, 1979) along the first two axes. The articulation types are not dis- tinguished by differences in forelimb and hind limb. In Figure 4, minimum convex polygons have been drawn around factor scores of individuals from the four major families. Clearly, differences in vertebral column length and pelvic girdle length do not distinguish the different family groups. On the average, ranids have shorter pelvic girdles than bufonids. The leptodactylid and hylid species in this study show overlapping pelvic girdle lengths. Figure 5 shows the distribution of the two major locomotor types along the third and fourth axes. The locomotor groups designated in this study are not distinguished by differences in vertebral col- umn and pelvic girdle length. Figure 6 shows the distribution of the three ma- jor articulation types among the factor scores. While there is a great deal of overlap among ar- ticulation types, individuals of types I and II A tend to have longer girdles than type IIB. I nt rafamil ial Comparisons Table 2 summarizes the results of the principal component analysis on three anuran families, the Leptodactylidae, Hylidae, and Ranidae. The amount of variance explained on each of the first four axes and the factors with the highest loadings on each axis are included. The same variables load most heavily on each axis for the Hylidae and Leptodactylidae. A similar amount of variance is explained by each axis. In the Ranidae the highest loadings on the fourth axis are humerus and ra- dioulna rather than vertebral column. All three families show the same variables con- tributing most heavily to the first three axes: length of the femur, tibiofibula, tarsus, pelvic girdle, and transverse process. A similar amount of variance is accounted for by each of the four major axes within each of the families and among all frogs in the interfamilial sample. The interfamilial sample EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS CO o 1.5 0.5 -0.5 -1.5 -2.5 - VERTEBRAL COLUMN PELVIC GIRDLE -2.2 -1.0 0.2 1.4 FAC3(17.2%) Fig. 6. Factor scores of 41 species for the third and fourth rotated axes of the principal component analysis. Polygons enclose all species of a given articulation type: © = type I; • = type IIA; O = type IIB. differs in that forelimb length is more variable than within families. Many of the morphological features listed above appear as variable at the intrafamilial level as at the interfamilial level. In the interfamilial sample, however, only a few representative genera were chosen for each family. It is important, therefore, to examine whether these genera encompass the entire range of variation found within the family when a larger sample is analyzed. For the Bufon- idae and Leptodactylidae, the entire range of vari- ation in hind limb length is included in the inter- familial subsample. For the Hylidae and Ranidae it is more complicated. The range of variation represented by the 4 1 species interfamilial subsample is a good reflection of the total range of variation in hind limb length across frogs, but the mean factor scores for the Table 2. Variables loading on first four factors and the variance explained by each factor. Interfamilial sample Intrafamilial samples % Vari- Variables ance Hylidae Leptodactylidae Ranidae Fac- tors % Vari- Variables ance % Vari- Variables ance Variables % Vari- ance I II III IV Femur 23.0% Tibiofibula Tarsus Humerus 17.6% Radioulna Pelvie girdle 17.2% Ilium Vertebral 11.8% column Tibiofibula 21.0% Femur Tarsus Pelvic girdle 17.0% Ilium Transverse 15.7% process Vertebral 15.0% column Femur 26.9% Tibiofibula Tarsus Pelvic girdle 17.8% Ilium Transverse 16.2% process Vertebral 13.0% column Femur Tibiofibula Tarsus Pelvic girdle Ilium Transverse process Humerus radioulna 26.9% 17.2% 16.4% 13.4% FIELDIANA: ZOOLOGY CO CM O < Li- FORELIMB 3.0 y y y / s 2.0 y y i : * y / / / / / 1.0 1 \ "• y-'' / 1 WT' / **.^y / Jf. / / / 0.0 - / JT^*' / •* t * / a y*^ \ / .• \ '* ' / / / 1.0 v» 2.0 I 1 1 HIND LIMB -1.4 -0.2 1.0 2.2 FAC 1 (23%) Fig. 7. Factor scores of 41 species for the first two rotated axes of the principal component analysis. Polygons enclose all species of a given locomotor mode: • = walker-hoppers; • = hopper-burrowers; • = walker-hopper-burrowers; • = hoppers; O = jumpers; O = jumper- walkers; • = all species classified as walker-hopper-burrowers in Figure 2; O = all species called jumper-swimmers in Figure 2. z < o -2 -4 - \ / t / \ I \ I -2.1 0.3 2.7 CAN 1 Fig. 8. Separation of locomotor modes along the first two canonical variates of the canonical discrimination analysis. Polygons enclose all species of a given locomotor mode. Symbols are the same as in Figure 7. EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS CO 2 W DC O u. WALKER-HOPPER WALKER-JUMPER JUMPER HOPPER-BURROWER HOPPER WALKER-HOPPER-BURROWER HIND LIMB Fig. 9. Diagrammatic representation of frog limb morphospace. Locomotor modes are mapped according to results from principal component analysis. Ranidae and the Hylidae in the interfamilial sam- ple are lower than those for the families when a larger number of species is analyzed. The hylid genera sampled in the interfamilial study have shorter hind limbs than many members of the family. The low endpoint is accurate, but not the high. Ranids are similar; there are ranids with longer hind limbs than those in the interfamilial sample, but the shortest hind limb genera have been included. The range of forelimb, pelvic gir- dle, vertebral column, and transverse process vari- ation within the large sample of each family is similar to that in the interfamilial sample, with the exception of a couple of extreme outliers. Hemiphractus has unusually long transverse processes among hylids, while Ooedoziga has un- usually long transverse processes and a short pel- vic girdle among ranids. Locomotor Groups of Frogs While Figure 2 shows that differences in hind limb and forelimb are correlated with locomotor mode, only two general categories were recog- nized: swimmer-jumpers and walker-hopper-bur- rowers. In fact, there are nine locomotor modes among the 4 1 species in the interfamilial sample (Appendix 1). When locomotor type is divided more finely, hind limb and forelimb differences— the characters explaining the most variation in the principal component analysis— do not map di- rectly on locomotor mode (fig. 7). For example, jumpers and hoppers have overlapping hind limb lengths and walker-hoppers and walker-hopper- burrowers show some overlap in forelimb and hind limb lengths. Nonetheless, there are some striking examples of convergence (fig. 7). The three hop- pers belong to three separate families but have converged toward a common hind limb length. The walker-hopper-burrowers from four different families have similar limb proportions. The jump- ers include species from four families. In the canonical discriminant analysis, separa- tion of the locomotor groups is along somewhat different morphological axes than in the principal component analysis (fig. 8). Femur, tarsus, hu- merus, girdle, and transverse process lengths have 10 FIELDIANA: ZOOLOGY Table 3. Standardized canonical coefficients for the first two canonical variates. Character Canonical I Canonical II FEMSV 0.8523 2.0953 TIBFIBSV 0.1657 2.3443 TARSV 1.4295 0.0175 FOOTSV -0.3364 -0.5710 HUMSV -1.0080 -0.4069 RADULSV -0.0023 0.8558 HANDSV -0.2300 0.1450 GIRDLSV -0.7834 0.1603 UROSTSV 0.3538 -0.5506 TVWGIRDL -0.6759 1.7144 ILWIDIL 0.2815 0.0632 HTCRSTIL 0.3093 -0.4832 VCLSV -0.7438 -0.3354 the highest standardized canonical coefficients for the first canonical variable (table 3). Femur, tibio- fibula, and transverse process lengths have the highest coefficients for the second canonical vari- able. When the variance within groups is mini- mized, the first canonical variable separates jump- ers and hoppers, and the second canonical variable separates the walker-hoppers from the walker-bur- rowers from the walker-hopper-burrowers. Dia- gram matically, the relationship between limb length and locomotor mode is summarized in Fig- ure 9. Outlines of taxa representing the major lo- comotor modes have been drawn to aid in the visualization. Coefficients of Variation Table 4 summarizes the multivariate coeffi- cients of variation for five frog families and the interfamilial data set. Two of the families have coefficients of variation that are slightly higher than that of the interfamilial sample. The other families have coefficients of variation that are slightly lower than that of the interfamilial sample. Discussion Convergence Hind limb and forelimb lengths are clearly the most important aspects of postcranial morphology in distinguishing among the locomotor modes of frogs, and also account for the largest amount of variation in postcranial morphology across fam- ilies. Figure 7 shows that there has been conver- gence in limb proportions among unrelated frogs with the same locomotor mode, but within any single locomotor mode there is still a fair amount of variation. This variation does not necessarily weaken the case for convergence; rather, it affirms, I think, the complexity of the biological world. All jumpers, by our definition, travel more than 10 times their body length in a single bound. That does not mean, however, that all jumpers must have the same hind limb length and jump capa- bility. Within a locomotor mode there are differ- ences in performance. Secondly, a locomotor mode may require a threshold morphological configu- ration but not be limited by a morphology that extends past that minimum. For example, to bur- row it may be necessary to have a certain mini- mum hind limb length (see Emerson, 1976, for the functional explanation), but when the hind limb length shortens beyond that threshold it does not restrict the animal from digging. There may be differences in digging performance, but they may not be significant in the natural history of the an- imal. I would suggest that much of the variation in pertinent morphology within a locomotor mode is related to the peculiar phylogenetic history of the unrelated animals. And, consequently, that convergence needs to be thought of as a relative phenomenon. The diagrammatic representation of locomotor types on limb morphospace suggests that walkers have longer forelimbs, yet the jump- ing-walking locomotor mode cannot be separated morphologically from the jumpers in the interfa- Table 4. Coefficients of variation. Interfamilial sample Intrafamilial sample Hylidae Leptodac- tylidae Ranidae Rhaco- phoridae Hyperolidae No. species No. variables Multivariate CV 41 14 20.83% 28 14 14.62% 19 14 21.9% 15 14 21.2% 10 14 15.2% 7 14 12.0% EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS 11 2.0 1.0 09 a> 0.0 CO o < u. -1.0 -2.0 - -TRANSVERSE PROCESS • •• Phyllomedusa f) Agalychnis Pachymedusa q HUMERUS RADIOULNA -1.0 0.0 1.0 2.0 FAC 5 (15%) Fig. 10. Factor scores of hylid species for the fifth and sixth rotated axes of the principal component analysis. Jumper-walker genera are identified by name; • = other species of the family. milial study (fig. 8). If, however, forelimb mor- phology of Pachymedusa and Agalychnis, two jumper- walkers, is compared with that of other members of the same family, a different picture emerges. Figure 10 plots the results of part of the principal component analysis run on the postcra- nial morphology of the family Hylidae (see Ap- pendix 2 for species). Forelimb variables load most heavily on the fifth axis. Notice that while Aga- lychnis and Pachymedusa do not have unusually Z < H CO 5 < o o o -I o X Q. rr o 20 ^ Mammals • Lizards Frogs SS S G SF F SPF SO O TAXONOMIC UNIT Fig. 11. Morphological distance plotted as a function of taxonomic level for three groups of vertebrates (data from Cherry et al., 1982). SS = subspecies; S = species; G = genus; SF = subfamily; F = family; SPF = superfamily; SO = suborder; O = order. 12 FIELDIANA: ZOOLOGY 3.5 CNJ o < -1.5 FORELIMB TIBIOFIBULA -1.4 2.2 FAC 1 Fig. 1 2. Factor scores of 4 1 species for the first two rotated axes of the principal component analysis. Lines enclose species sampled by Cherry et al., 1982; each dot represents a separate species. long forelimbs compared to other jumpers across different families, they do have long forelimbs rel- ative to other hylids. Variation An interesting outcome of this study is that sim- ilar aspects of postcranial morphology are the most variable within and between families, and these features account for about the same amount of variation on each axis. The most striking differ- ence is that in the interfamilial sample forelimb variation is much more important. Recently, an attempt has been made to compare morphological variation among vertebrate classes using a standardized set of measurements to cal- culate a "morphological distance" (Cherry et al., 1978; Cherry et al., 1982; Wilson et al., 1984). When such morphological distances are plotted for three vertebrate groups as a function of taxonomic level, two patterns emerge (fig. 1 1 ; data from Cher- ry et al., 1982). First, there is increasing morpho- logical distance at higher taxonomic categories within each vertebrate group. Secondly, frogs show less morphological distance at higher taxonomic levels than either lizards or mammals. This finding lends support to the notion that frogs are less di- verse morphologically than other vertebrate groups, but there have been a number of criticisms of the morphological distance measurement. Critics have pointed out both conceptual and statistical prob- lems (Findley, 1979; Atchley, 1980; Hafner et al., 1984). The data from this study highlight yet another problem. While over 1,200 frogs were measured for the 1982 study (Cherry et al., 1982), the results of the present work indicate that the authors seriously undersampled morphological di- versity. Two components of the morphological distance measurement are forelimb length and tib- iofibula length. These variables have high loadings on the first two axes of the principal component analysis from this study. Figure 12 shows the di- versity sampled in the 1982 study compared to the range of morphology represented by the 41 species in this study. That underestimation, if unique to the frog sample, could account for the differences in morphological distance between frogs and lizards and mammals. A much more serious problem with the mor- EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS 13 phological distance studies is the fact that taxo- nomic categories above the species level have no objective biological definition. There is no reason, a priori, for a family of mammals to be based on the same criteria as a family of frogs. Higher cat- egories are defined by the systematists who do the work on the group. The categories do not have any intrinsic biological properties. For this reason, it seems unlikely that we can gain insight into the relative variability of frogs compared to other ver- tebrates by making comparisons across classes. We can, however, look at the question another way. Empirical data from bats show that the vari- ances of shape characters increase about an order to magnitude between the generic and familial levels (Lemen & Freeman, 1984). Increasing mor- phological variability with taxonomic level is con- sistent with how higher taxonomic levels are gen- erally defined. We can ask whether frogs conform to this pattern. Do frogs show the expected rela- tionship of increased variability with higher taxo- nomic levels? When the relative variability of the interfamilial data set is compared with that of the several frog family data sets, there is little differ- ence in the levels of variation. Within- and be- tween-family data sets show similar multivariate coefficients of variation (table 4). If increasing variation with taxonomic level is the common pat- tern among vertebrates, then frogs do appear to be an exception: Ordinal level variation is less than expected. The seemingly low level of morphological vari- ation within frogs has been ascribed to their high degree of morphological specialization (Inger, 1967). The striking similarity in patterns of post- cranial variation within and between families sug- gests that there is some set of constraints shaping the variation. The nature of those constraints, however, remains unknown. Acknowledgments I thank the curators at the following institutions for allowing me access to material: Field Museum of Natural History; Museum of Comparative Zo- ology, Harvard University; and National Museum of Natural History. A. Jaslow and R. McDiarmid provided live animals for the study. I thank David Sherman for computer computations. L. Radin- sky, J. Sepkoski, C. Renzulli, S. Swartz, R. Inger, J. Humphries, and P. Wainwright provided assis- tance and information at various times during the study. P. Wainwright supplied helpful comments on a preliminary draft of this manuscript. The comments of the two reviewers substantially im- proved the manuscript. This work was supported by the National Science Foundation under Grants DEB 77-21901 and BSR 83-05998. Literature Cited Atchley, W. 1980. M-statistics and morphometric di- vergence. Science, 208: 1059-1060. Barclay, O. 1946. The mechanics of amphibian lo- comotion. Journal of Experimental Biology, 23: 177— 205. Cherry, L., S. Case, and A. Wilson. 1978. Frog per- spective on the morphological difference between hu- mans and chimpanzees. Science, 200: 209-21 1. Cherry, L., J. Kunkel, J. Wyles, and A. Wilson. 1 982. Body shape metrics and organismal evolution. Evo- lution, 36: 914-933. Dobrowolska, H. 1973. Body part proportions in re- lation to mode of locomotion in anurans. Zoologica Poloniae, 23: 59-108. Emerson, S. 1976. Burrowing in frogs. Journal of Mor- phology, 149: 437-458. . 1978. Allometry and jumping in frogs: Helping the twain to meet. Evolution, 32: 551-564. 1979. The ilio-sacral articulation in frogs: Form and function. Biological Journal of the Linnean So- ciety, 11: 153-168. 1982. Frog postcranial morphology: Identifi- cation of a functional complex. Copeia, 1982: 603- 613. Emerson, S., and H. DeJongh. 1980. Muscle activity at the ilio-sacral joint in frogs. Journal of Morphology, 166: 129-144. Emerson, S., and D. Diehl. 1980. Toe pad morphol- ogy and mechanisms of sticking in frogs. Biological Journal of the Linnean Society, 13: 199-216. Emerson, S., and L. Radinsky. 1980. Functional anal- ysis of sabertooth cranial morphology. Paleobiology, 6:295-312. Findley, J. 1979. Comparisons of frogs, humans and chimpanzees. Science, 204: 434-435. Hafner, M., J. Remsen, and S. Lanyon. 1984. Bird versus mammal morphological diversity. Evolution, 38: 1154-1156. Inger, R. 1967. The development of a phylogeny of frogs. Evolution, 21: 369-384. Lemen, C, and P. Freeman. 1984. The genus: A macroevolutionary problem. Evolution, 38: 1219— 1237. Lewontin, R. 1978. Adaptation. Scientific American, 239:212-230. Noble, G. 1931. The Biology of the Amphibia. McGraw-Hill Book Co., New York, 577 pp. Parker, H. 1932. Parallel modifications in the skele- 14 FIELDIANA: ZOOLOGY ton of the Amphibia Salienta. Archivio Zoologico Ita- liano, 16: 1239-1248. Radinsky, L. 1981. Evolution of skull shape in car- nivores. 1. Representative modern carnivores. Bio- logical Journal of the Linnean Society, 15: 369-388. SAS Institute, Inc. 1982. SAS User's Guide: Statis- tics. SAS Institute, Inc., Cary, N.C. Trueb, L. 1977. Osteology and anuran systematics: Intrapopulational variation in Hyla lanciformis. Sys- tematic Zoology, 26: 165-184. Van Valen, L. 1978. The statistics of variation. Evo- lutionary Theory, 4: 33-43. Wermel, J. 1934. Uber die Korperproportioner der Wirbeltiere und ihre functionelle Bedeutung. Zeit- schrift fiir Anatomie und Entwicklungsgeschichte, 103: 645-659. Wilson, A., J. Kunkel, and J. Wyles. 1984. Mor- phological distance: An encounter between two per- spectives in evolutionary biology. Evolution, 38: 1 1 56- 1159. Zug, G. 1972. Anuran locomotion: Structure and func- tion. I. Preliminary observations of the relation be- tween jumping and osteometries of appendicular and postaxial skeleton. Copeia, 1972: 613-624. . 1978. Anuran locomotion: Structure and func- tion. II. Jumping performance of semi-aquatic, ter- restrial, and arboreal frogs. Smithsonian Contribu- tions to Zoology, 276: 1-31. EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS 15 Appendices Appendix 1: Interfamilial Sample Species Articulation type Locomotor mode Pipidae Xenopus laevis Pipa pipa Ascaphidae Ascaphus truei Discoglossidae Bombina orientalis Discoglossus pictus Rhinophrynidae Rhinophrynus dorsalis Pelobatidae Pelobates fuscus Scaphiopus couchii Megophrys monticola Bufonidae Bufo boreas Bufo blombergii Bufo americanus Bufo calamita Atelopus varius Melanophryniscus stelzneri Oreophrynella quelchii Rhinodermatidae Rhinoderma darwini Leptodactylidae Notaden bennetti Pseudophryne occidentalis Physalaemus pustulosus Leptodactylus melanonotus Eleuthewdactylus rhodopis Telmatobius marmoratus Hylidae Pseudacris triseriata Hyla cinerea Phrynohyas venulosa Agalychnis callidryas Pachymedusa dacnicolor Dendrobatidae Dendrobates tinctorius Ranidae Rana clamitans Polypedates leucomystax Rhacophorus pardalis Chiromantis rufescens Hyperolius marmoratus Leptopelis aubryi Leptopelis bocagii Kassina senegalensis Microhylidae Kaloula pulchra Hypopachus muelleri Gastrophryne carolinensis Phrynomerus bifasciata I I IIB I IIA IIA I I I IIA IIA IIA IIA IIA IIA I Swimmer Swimmer Jumper Hopper Jumper Walker-hopper-burrower Hopper-burrower Hopper-burrower Walker-hopper Walker-hopper-burrower Walker-hopper Walker-hopper Walker-hopper-burrower Walker-hopper Walker-hopper Walker Jumper IIA Walker-hopper-j umper IIA Walker-burrower IIA Hopper IIB Jumper IIB Jumper IIB Jumper IIA Jumper I Jumper IIA Jumper I Jumper-walker I Jumper-walker IIB Walker-hopper IIB Jumper IIB Jumper IIB Jumper-walker IIB Jumper IIB Jumper IIB Jumper IIB Hopper-burrower IIA Walker I Walker-hopper-burrower IIA Walker-hopper-burrower I Hopper I Walker-burrower 16 FIELDIANA: ZOOLOGY Appendix 2: Intrafamilial Samples Hylidae Acris crepitans Acris gryllus Pachymedusa dacnicolor Agalychnis callidryas Smilisca baudinii Smilisca phaeota Hyla arenicolor Hyla cinerea Hyla gratiosa Hyla lichenata Osteopilus septentrionalis Osteopilus brunneus Litoria caerulea Litoria infrafrenata Litoria lesuerii Litoria alboguttata Phrynohyas venulosa Pseudacris triseriata Pseudacris ornata Pternohyla fodiens Triprion spatulatus Hemiphractus scutatus Osteocephalus buckleyi Phyllomedusa iheringii Gastrotheca marsupiata Nyctimystes papua Nyctimystes narinosa Cyclorana platycephala Leptodactylidae Eleutherodactylus bufoniformis Eleutherodactylus rhodopis Physalaemus pustulosus Neobatrachus pictus* Leptodactylus labialis Leptodactylus melanonotus Leptodactylus ocellatus Pleurodema bibroni Pleurodema bufonia Notaden bennetti* Telmatobius culeus Telmatobius marmoratus Crossodactylus dispar Hylodes lateristrigatus Limnomedusa macroglossa Leptodactylidae {Continued) Lithodytes lineatus Eupsophus monticola Heleioporus albopunctatus* Limnodynastes dorsal is* Limnodynastes ornatus* Pseudophryne occidentalism Odontophrynus americanus Batrachophrynus macrostomus Ceratophryx ornata Hylorina sylvatica Caudiverbera caudiverbera Ranidae Pyxicephalus adspersa Philautus bimaculatus\ Amolops jerboa Afrixalus wittei% Chiromantis rufescensf Cryptothylax greshoffft Hyperolius marmoratus% Kassina senegalensis% Leptopelis aubryi% Leptopelis bocagii% Ooeidozyga laevis Nyctixalus pictusf Ceratobatrachus guentheri Rana clamitans Rana sylvatica Conraua goliath Arthroleptides martiensseni Arthroleptis stenodactylus Discodeles bufoniformis Kassina maculata% Platymatnis guppyi Staurois natator Phrynobatrachus krefftii Mantidactylus guttulatusf Rhacophorus appendiculatusf Rhacophorus dugriter\ Rhacophorus pardalisf Rhacophorus colletttf Polypedates leucomystaxf Polypedates dennysif Ptychadena mascareniensis Hemisus marmoratus Myobatrachids. t Rhacophorids. $ Hyperolids. EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS 17 Appendix 3: Master Equations for Interfamilial Sample Variable Slope (b) log a Y X FEMSV 1.000 +0.435 Femur Snout-vent TIBFIBSV 0.958 +0.352 Tibiofibula Snout-vent TARSV 0.887 +0.472 Tarsus Snout-vent FOOTSV 1.040 +0.626 Foot Snout-vent HUMSV 0.975 + 0.539 Humerus Snout-vent RADULSV 0.986 +0.695 Radioulna Snout-vent HANDSV 1.110 + 0.834 Hand Snout-vent GIRDLSV 1.060 +0.475 Girdle Snout-vent ILSV 1.050 +0.515 Ilium Snout-vent UROSTSV 1.050 +0.624 Urostyle Snout-vent TVWGIRDL 1.020 +0.526 Transverse process Pelvic girdle TVWSV 1.120 + 1.090 Transverse process Snout-vent ILWIDIL 0.941 +0.212 Ilium width Ilium HTCRSTIL 0.834 +0.867 Height of dorsal crest on ilium Ilium VCLGIRDL 0.939 +0.008 Vertebral column Pelvic girdle VCLSV 1.033 + 0.521 Vertebral column Snout-vent Appendix 4: Master Equations for Hylidae In- trafamilial Sample Appendix 5: Master Equations for Leptodactyli- dae Intrafamilial Sample Variable Slope (b) log a Variable Slope (b) log a FEMSV 0.942 +0.279 FEMSV 0.963 +0.342 TIBFIBSV 0.931 +0.230 TIBFIBSV 0.968 +0.324 TARSV 0.996 +0.588 TARSV 0.960 +0.613 FOOTSV 0.882 +0.271 FOOTSV 0.941 +0.354 HUMSV 1.044 +0.693 HUMSV 1.057 +0.667 RADULSV 0.986 +0.727 RADULSV 0.989 +0.689 HANDSV 1.100 + 0.806 HANDSV 1.376 + 1.368 GIRDLSV 1.069 +0.508 GIRDLSV 0.993 + 0.384 ILSV 1.093 +0.593 ILSV 0.938 +0.342 UROSTSV 1.094 +0.691 UROSTSV 0.966 +0.468 TVWGIRDL 0.752 +0.175 TVWGIRDL 1.081 +0.530 TVWSV 0.811 +0.569 TVWSV 1.102 +0.994 ILWIDIL 0.017 -0.646 ILWIDIL 0.198 -0.551 VCLGIRDL 0.963 +0.033 VCLGIRDL 0.960 +0.044 VCLSV 1.046 +0.549 VCLSV 0.970 +0.440 18 FIELDIANA: ZOOLOGY Appendix 6: Master Equations for Ranidae In- trafamilial Sample Variable Slope (b) log a FEMSV 1.031 +0.401 TIBFIBSV 1.037 +0.380 TARSV 1.002 +0.632 FOOTSV 0.985 +0.332 HUMSV 1.030 +0.640 RADULSV 0.978 +0.690 HANDSV 1.032 +0.668 GIRDLSV 1.114 +0.615 ILSV 1.009 +0.454 UROSTSV 0.982 +0.496 TVWGIRDL 0.966 +0.300 TVWSV 1.149 + 1.026 ILWIDIL 0.015 -0.667 VCLGIRDL 0.879 -0.090 VCLSV 1.026 +0.535 EMERSON: POSTCRANIAL MORPHOLOGY IN FROGS 19 Field Museum of Natural History Roosevelt Road at Lake Shore Drive Illinois 60605-2496 Telephone: (312) 922-9410 ; ; : 5 UNIVERSITY OF ILLINOIS-URBANA 590.5FIN.S. C001 FIELDIANA : ZOOLOGY $ NEW SERIES $CHGO 40-54 1988-89 3 01 009378735