MMMimmm uwmUi UNIVERSITY OF ^ BIOLOGY BIX FlELDlANA Zoology NEW SERIES, NO. 31 Diet and Arboreality in the Emerald Monitor, Varanus prasinus, with Comments on the Study of Adaptation Harry W. Greene A Contribution in Celebration of the Distinguished Scholarship of Robert F. Inger on the Occasion of His Sixty-Fifth Birthday BIOLOGY LIBRARY 101 BURR1LL HALL DEC 1 1985 October 31, 1986 Publication 1370 PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY Information for Contributors to Fieldiana General: Fieldiana is primarily 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 $65 per printed page or fraction thereof. Contributions from staff, research associates, and invited authors will be con- sidered for publication regardless of ability to pay page charges, but the full charge is mandatory for nonaffiliated authors of unsolicited manuscripts. 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Inger on the Occasion of His Sixty-Fifth Birthday Accepted for publication March 4, 1986 October 31, 1986 Publication 1370 PUBLISHED BY FIELD MUSEUM OF NATURAL HISTORY © 1986 Field Museum of Natural History Library of Congress Catalog Card Number: 86-81772 ISSN 0015-0754 PRINTED IN THE UNITED STATES OF AMERICA Table of Contents Abstract 1 Introduction 1 Conceptual and Systematic Perspectives Adaptation and Phylogeny 2 Evolutionary Relationships in the Angui- morpha 4 Methods 4 Results Diet of Adults 4 Nest and Hatchlings 5 Feeding Behavior 5 Habitat and External Morphology 5 Background Observations on Other Taxa . . 7 Discussion General Remarks 7 Relative Prey Size 8 Hunting Adaptations 8 Acknowledgments 9 Addendum 9 Literature Cited 10 List of Illustrations 1 . Origins of features and performance ad- vantages as inferred from the distribu- tion of derived and primitive attributes among four taxa 3 2. Ventral view of left hind foot of Varanus prasinus from 8 miles north of Bulolo, Papua New Guinea 6 3. Ventral view of left hind foot of Varanus acanthurus from Charter's Towers, Queensland, Australia 7 in Diet and Arboreality in the Emerald Monitor, Varanus prasinus, with Comments on the Study of Adaptation Abstract Introduction Forty-seven prey items from 29 museum spec- imens demonstrate that Varanus prasinus eats mainly katydids and other small arthropods. An "importance index," used to consider prey fre- quency and relative prey mass simultaneously, demonstrates that infrequent predation on large walkingsticks and small rodents also might make significant energetic contributions to the diet. Prey typically were swallowed headfirst, and a large, spinose walkingstick evidently was dismembered prior to ingestion. Hatchlings emerged from ter- mite nests and fed there. Varanus prasinus is unique among living var- anids in having bright green coloration, a prehen- sile tail, and feet apparently specialized for grasp- ing. It thereby resembles other arboreal, tropical forest lizards of the family Chamaeleontidae and of the iguanid genus Polychrus, probably as a result of adaptive convergence. Number of species per lineage is correlated positively with degree of mor- phological specialization among these taxa. Varanus prasinus, chamaeleontids, and Poly- chrus are used to illustrate a modification of Gould and Vrba's conceptual treatment of adaptation. If natural selection and adaptation are separated conceptually, the latter can be identified by the coincidence of features and performance advan- tages as derived attributes within a clade. As such, the study of adaptation is independent of and com- plementary to investigations of optimal design and convergence. Lizards of the family Varanidae vary in adult total length from ca. 0.3 to 3 m, occur in habitats as different as tropical wet forests and temperate deserts, and exhibit substantial diversity in diet. Several species are known to eat vertebrates (e.g., Cowles, 1930; King & Green, 1979; Auffenberg, 1981; Pianka, 1982); others eat insects (e.g., Pian- ka, 1970); and one might be largely a frugivore (Auffenberg, 1979). Popular literature and mor- phological studies abound with statements that particular features of varanids are adaptations for feeding on large prey, despite the fact that very little evidence supports this point and some con- tradicts it (Greene, 1982; Auffenberg &Ipe, 1983). A thorough consideration of the role of feeding in varanid evolution is currently hampered by the lack of detailed natural history information on many species in this interesting family. The present paper describes aspects of the feed- ing biology of Varanus prasinus, one of the most distinctive and poorly known living varanids. In- formation is provided on diet, prey-predator size relationships, and feeding behavior in adults and hatchlings. Comparisons with other monitors and with ecologically similar lizards in two other fam- ilies are then used to support a hypothesis of adap- tive convergence for certain morphological attri- butes of these species. The concept of adaptation has come under heavy criticism in recent years, so I first present a mod- ification of Gould and Vrba's (1982) approach to GREENE: DIET AND ARBOREALITY IN VARANUS PRASINUS the topic. Because this method requires phyloge- netic information, I also discuss the relationships of Varanus prasinus with certain other taxa. My findings suggest that the exquisite green coloration, unusual foot structure, and long, prehensile tail of V. prasinus are adaptations for arboreal hunting. Conceptual and Systematic Perspectives Adaptation and Phylogeny Gould and Vrba (1982) proposed that a feature be regarded as an adaptation only if its origin was associated with a specified task and an increase in fitness. Operationally, this is tantamount to saying that a feature and a performance advantage are derived simultaneously within a population or a clade, except that Gould and Vrba also specified the action of natural selection in their concept of adaptation. In their terminology, an exaptation is a feature that promotes increased performance of a task that was not associated with the origin of that feature. Gould and Vrba suggested that ad- aptations often would be difficult to identify, im- plied that exaptations might be widespread, and coined the term aptation to encompass the two. Reactions to the approach taken by Gould and Vrba (1982) have been mixed, perhaps in part because they emphasized the potential role of ex- aptations in evolution and only implicitly iden- tified a method for distinguishing them from ad- aptations. Kiltie (1985) found their conceptual distinction useful, but used the default term ap- tation in discussing his data. Brown (1982) and Reif (1984) criticized them for confusing pread- aptation and exaptation (but see Gould & Vrba, 1982, p. 11), and Brown asserted that most evo- lutionary biologists are interested in ". . . natural populations instead of fossils, and variation among individuals rather than among species and higher taxa . . . ." Dobson (1985) did not cite Gould and Vrba, but pitted "historical origin" (as indicated by the presence of an attribute throughout all or most of a monophyletic group) against "superior adaptiveness in recent environments" (as indicat- ed by convergent features among species). If the goal of evolutionary biology is to under- stand the history of life, the views expressed by Brown (1982) and Dobson (1985) are unnecessar- ily narrow. After all, cetaceans are characterized by finlike limbs, but the fact that this diverse monophyletic group originated millions of years ago (Gingerich et al., 1983) does not make an aquatic lifestyle in extant whales irrelevant to un- derstanding the evolution of their appendages. Surely most evolutionary biologists are interested in diversification, adaptive or otherwise, beyond the level of populations in the present! Even if the distinction between preadaptation and exaptation does not prove useful, Gould and Vrba's (1982) paper is important because it emphasized the his- torical context for adaptation and the need to sep- arate factors involved in the origin of a feature from its current utility. Adaptation and natural selection should be sep- arated conceptually (cf. Burian, 1983; Fisher, 1985) so that it is not necessary to confirm or assume the past action of the latter in order to study the former. Adaptation and exaptation then can be distinguished by accepting a particular phylogeny; examining the distribution of morphological, functional, and ecological character states among related taxa; and determining if features and per- formance advantages are coincident as shared, de- rived traits (fig. la; see also Wanntorp, 1983; Cod- dington, 1985; Gauthier & Padian, 1985; Luke, 1986). Enhanced performance should be mea- sured in the living members of a monophyletic group and compared to that exhibited by organ- isms possessing the antecedent condition, as seen in sister taxa or experimentally altered individu- als. In this framework, an adaptive hypothesis can be rejected if a performance advantage is shown to be derived at a level either more restricted or more inclusive than that of the feature with which the advantage is associated. If the performance advantage is more restricted within a group, the feature is an exaptation for that task (fig. lb). If it is more widely distributed (fig. lc), the feature is irrelevant to the task (i.e., neither an adaptation nor an exaptation for that task). The approach taken here entails a common problem in phylogenetic analysis, namely paral- lelism. As with any other characteristic of organ- isms, we only can assume parsimoniously that a derived attribute common to all or most members of a higher taxon is homologous (e.g., the four- chambered heart in mammals, flapping flight in birds, etc.), and abandon this conclusion if there is evidence to the contrary. For example, an adap- tive hypothesis can be rejected if it is shown that the performance advantage was irrelevant when the feature originated, as with paleontological evi- dence that a habitat or prey type relevant to the advantage was not present (fig. Id). FIELDIANA: ZOOLOGY + + +- advantage — — + + + + -+- feature — + + + -+- advantage — + feature — Fig. 1. (a-c), Origins of features (solid bars) and performance advantages (open bars), as inferred from the distribution of derived (+) and primitive (— ) attributes among four taxa. Same symbols in (d), except that the dotted line indicates independent evidence of the earliest time at which a performance advantage could possibly have originated, regardless of its distribution among taxa. See Adaptation and Phylogeny for additional details. There are at least two advantages to this mod- ification of Gould and Vrba's (1982) formulation. First, by decoupling patterns and processes, the concepts of adaptation and exaptation become empirically tractable for historical analyses in evo- lutionary biology (Lauder, 1982). Evolutionary processes and adaptive patterns merge at the level of differences among individuals in a population (Fisher, 1985), but there remain real difficulties in studying past selection. Given a phylogeny for the organisms in question, however, this problem need not preclude the study of adaptive patterns among taxa. The possible roles of natural selection, het- erochronic processes, chance, and other phenom- ena in producing and sorting variation— in cre- ating adaptive patterns— should be relegated to other analytic approaches (e.g., Arnold, 1986) and to inference. A second advantage of a phylogenetic approach to studying adaptations is that it removes the ne- cessity for using optimality as a criterion in iden- tifying adaptations (cf. Levins & Lewontin, 1985). This does not negate the value of optimality ap- proaches in evolutionary biology (e.g., Oster & Wilson, 1978; Kingsolver & Koehl, 1985). For example, with independent criteria for perfection (in the sense of engineering design) and adaptation, it might be possible to determine under what con- ditions the former is most closely approached (cf. Gans, 1983). In the same vein, it is traditional to view convergence as evidence for adaptation, be- cause independently derived similarity is pre- sumed to indicate parallel selection for a particular solution to a problem (Mayr, 1983; Dobson, 1985). That assumes the burdens of an optimality ap- proach and can be circular (Levins & Lewontin, 1985), and convergence also might implicate fac- tors other than adaptation (e.g., constraint). Those problems are removed if convergence is viewed only as independently derived similarity (Wake, GREENE: DIET AND ARBOREALITY IN VARANUS PRASINUS 1982), selection is not associated necessarily with adaptation, and the latter is identified separately in each taxon (Fisher, 1985). Evolutionary Relationships in the Anguimorpha Although intrageneric groupings have been pro- posed (Holmes et al., 1976; Branch, 1982), a de- tailed, rigorously supported phylogeny is not available for varanids. I therefore cannot exclude on the basis of intrafamilial comparisons alone the possibility that any particular feature of Varanus prasinus is primitive for a higher taxon (either Varanidae or more inclusive), the rest of whose members lack ecological traits unique to the em- erald monitor. The closest living relative of var- anids is the monotypic Lanthanotidae, and the sister taxon of these two families is the Heloder- matidae; together, the three constitute the Vara- noidea. The relationship of varanoids to other families in the Anguimorpha is uncertain (Gau- thier, 1982; Estes, 1983). Accordingly, I noted col- oration, use of the tail, and foot structure for rep- resentatives of the families Anguidae (Abronia, Diploglossus, Elgaria, Gerrhonotus), Helodermat- idae (Heloderma), and Xenosauridae (Xenosau- rus) on the basis of specimens in the Museum of Vertebrate Zoology, University of California, Berkeley, literature accounts, and personal obser- vations. For the Lanthanotidae, I based my no- tations on the literature and examination of a live Lanthanotus borneensis in the Senckenberg Mu- seum, Frankfurt am Main, West Germany. I con- sider an attribute to be derived in Varanus pra- sinus if it is absent in other varanoids and rare or absent in other anguimorphs. Methods The stomach contents of all Varanus prasinus in the American Museum of Natural History, New York (amnh); Field Museum of Natural History, Chicago (fmnh); California Academy of Sciences, San Francisco (cas); Museum of Vertebrate Zo- ology, University of California, Berkeley (mvz); Museum of Comparative Zoology, Harvard Uni- versity, Cambridge, Massachusetts (mcz); Nation- al Museum of Natural History, Washington, D.C.; and Bernice P. Bishop Museum, Honolulu, were examined. To the extent possible, I recorded col- lecting data, snout-vent length (SV), total length (TL), and head width (HW, across the retroarticu- lar processes of the mandibles) for the lizards; and orientation in the gut, identity, minimum number of items, and linear dimensions (LD, maximum length and diameter) for each prey (exclusive of legs, antennae, and ovipositor). If not otherwise stated, orientation in the gut and prey dimensions could not be determined. The mass (M) of items that were at least approximately intact and the lizards that had contained them were weighed after each was blotted briefly on a paper towel. I used a Wild dissecting microscope to study the feet of Varanus in the mvz, including V. acanthu- rus, V. bengalensis, V. exanthematicus, V. gouldii, V. griseus, V. indicus, V. komodoensis, V. niloti- cus, V. prasinus, V. rudicollis, V salvator, V. tris- tis, and V. varius (the V. komodoensis is a tanned skin; all others are intact specimens stored in al- cohol). The sample thus included more than one- third of the extant species and most of the eco- logical, morphological, and size diversity within the family. Specimens of Polychrus in the mvz were also examined because it appeared that this iguanid genus might resemble V. prasinus in cer- tain features. Finally, I handled two living adult emerald monitors at the Dallas Zoo and obtained observations on their behavior from J. B. Murphy (pers. comm.). Results Diet of Adults Twenty-nine Varanus prasinus contained 47 prey items. The monitors were apparently sub- adults or adults (SV = 150-335 mm, TL = 520- 830 mm; Loveridge [1948] recorded maximum TL = 845 mm for a specimen with SV = 295 mm). Four specimens came from three islands in the D'Entrecasteaux Group, off the southeastern tip of New Guinea; one from Aru Island; and 24 from Irian Jaya and Papua New Guinea. The localities thus spanned ca. 1 4 1 °- 1 5 1 °E longitude and ca. 3°- 10°S latitude, much of the range of this lizard in New Guinea. They included the subspecies V. p. prasinus, V. p. beccari, and V. p. bogerti (Mertens, 1950), and were collected between 1928 and 1967 from sea level to at least 830 m elevation. Thirty-two prey items were katydids (Orthop- tera, Tettigoniidae), 1 4 of which had been swal- FIELDIANA: ZOOLOGY lowed headfirst (LD = 8 x 28 mm to 1 1 x 63 mm, N = 9; M = 1 .0-4.6 g, N = 7). Thirteen other invertebrate prey included two grasshoppers (Or- thoptera, Acrididae); one iridescent green beetle (Coleoptera, ca. 24 mm in diameter, M = 1.9 g); three coleopteran larvae, one of which was swal- lowed headfirst (LD = 6.3 x 37.9 mm, M = 0.5 g); two roaches (Blattodea), both swallowed head- first (LD = 10 x 29 mm, 19 x 20 mm, M = 0.6 g); three unidentified insects; one centipede (Chi- lopoda, ca. 8 mm wide) swallowed doubled up; and one spider (Arachnida). The relatively largest invertebrate prey was a katydid (LD =11 x 63 mm, M = 4.6 g) in a subadult Varanus prasinus (mcz 101298; SV = 150 mm, HW = 14.2 mm, M = 41 g), swallowed headfirst. The largest invertebrate prey in an adult V. prasinus was a spectacular walkingstick (Phas- matodea), probably Eurycantha sp., in the stom- ach of a large male from Marienberg, Papua New Guinea (fmnh 14103; SV = 280 mm, HW = 27.5 mm, M = 313 g). The stick insect (LD = 19 x 124 mm, M = 12.2 g) had been swallowed head- first. Specimens of Eurycantha that I examined (at cas) have the body and legs armed grotesquely with sharp spines, and it is noteworthy that some legs of the prey were not in the stomach of the monitor. The only vertebrate prey was a murid rodent of the genus Melomys, probably M. moncktoni, in a monitor from Kubuna, southeastern Papua New Guinea (amnh 59051; SV = 255 mm, M = 135 g). The prey was largely digested, but based on comparisons of the teeth with intact Melomys, it weighed ca. 40 g. Rodents of this genus are ter- restrial and/or semiarboreal, and M. moncktoni is widespread in the lowlands of Papua New Guinea (W. Z. Lidicker, Jr., pers. comm.). Each monitor's stomach contained one to six items (mode = 1 , mean = 1 .62, N = 29). Stomachs with multiple prey items contained, respectively: four katydids; two katydids; five katydids and one rodent; one katydid and one centipede; three ka- tydids and one coleopteran larva; one spider and one coleopteran larva; and one katydid and one roach (N = 2). Prey/predator mass ratios (MR) were 0.0032-0.296 (x = 0.045, N = 12); excluding the large walkingstick and rodent, mean MR for 1 arthropods was 0.02 1 . None of the unweighable orthopterans appeared to have been larger than 4 g, so modal MR for all 47 prey was probably ca. 0.01. The relatively largest items were the walk- ingstick (MR = 0.04), the katydid in the subadult (MR = 0.1 1), and the rodent (MR = 0.296). Nest and Hatchlings Two specimens (mvz 74904 and 74905) have obvious umbilical scars, SVs of 83 and 84 mm, and TLs of 2 1 8 and 224 mm, respectively. They were collected 23 October 1962, on the upper Baiune River, Papua New Guinea. R. G. Allen saw the lizards "hatch in termite nests and feed there later to emerge as one of the large long- tailed species" (sic, field notes of A. H. Miller, 24 Oc- tober 1 962, on file in mvz). Their stomachs were empty, but the colons contained unidentifiable de- bris and several insect eggs, perhaps those of ter- mites. Two Varanus prasinus hatched at the Dal- las Zoo on 4 October 1978; they had TLs of 205 and 210 mm and weighed 10.0 and 8.4 g. One died the day after hatching, and the other fed readi- ly on crickets. Feeding Behavior J. B. Murphy observed at least 1 00 feeding events by Varanus prasinus in the Dallas Zoo. Two-week- old mice were seized by the nape of the neck, slammed against the substrate, raked and eviscer- ated with the claws, and swallowed headfirst. Mice were raked with the claws as swallowing com- menced, which appeared to align the prey with the long axis of the monitor's head. Approximately five minutes were required to subdue mice and five minutes to swallow them. Crickets were sim- ply seized and swallowed headfirst. Other captive varanids (V. acanthurus, V. gilleni, V. gouldii, V. indicus, V. mitchelli, V. m's^z's) of similar or small- er size did not eviscerate mice prior to ingestion. Once, an emerald monitor descended from tree limbs to a bowl of young mice, and briefly sus- pended itself from a branch using only its tail. Habitat and External Morphology The emerald monitor is found in extreme north- ern Australia and throughout much of New Guinea, in rain forests, palms, mangroves, and cocoa plan- tations (Schlegel, 1839; Room, 1974; Czechura, 1980; Cogger et al., 1983). On mainland New Guinea, Varanus prasinus is a brilliant green liz- ard, often with a dorsal pattern of irregular black crossbands (for color illustrations see Miiller & Schlegel, 1845; Kundert, 1974; Grzimek, 1984), while on certain offshore islands and in northern Australia, populations are uniformly black (Mer- GREENE: DIET AND ARBOREALITY IN VARANUS PRASINUS rr/kfii Fig. 2. Ventral view of left hind foot of Varanus prasinus (mvz 74906), from 8 mi N Bulolo, Papua New Guinea. Snout-vent length of the lizard is 275 mm. tens, 1 950; Czechura, 1 980). Although several var- anids are known to climb frequently (e.g., Smith, 1930; Pianka, 1982), no other living monitor is bright green (Mertens, 1942a). Lanthanotus bor- neensis is dark reddish brown in life. Among other anguimorphs, only a few species of anguids are green (e.g., Abronia taeniata, males of Barisia monticola; pers. obs.). I regard green color as de- rived in V. prasinus. The tail of Varanus prasinus is prehensile (i.e., able to support the animal's weight, terminology of Emmons & Gentry, 1 983), unusually long com- pared to those of most other varanids, and has a blunt tip (Mertens 1942b, 1950; Czechura, 1980). Live animals, while struggling to escape, readily coiled the distal part of their tails around my fin- gers. Prehensile tails are unknown in other var- anids, helodermatids, and xenosaurids, and only sporadically present in anguids (e.g., Abronia dep- pii, Gerrhonotus liocephalus; pers. obs.). Semipre- hensile tails are known for some anguids (e.g., Blair, 1950), helodermatids (Alvarez del Toro, 1982), and Lanthanotus (Proud, 1978). I interpret a fully prehensile tail as derived in V. prasinus. Czechura (1980, p. 105) noted that climbing in Varanus prasinus is also aided by ". . . the struc- ture of the surface tissue on the soles of fore and hind feet. These surfaces are covered by soft black tissue, which feels sticky on contact and appears to give additional support to the climbing animal." The mvz specimens of V. prasinus possess black subdigital scales, juxtaposed in transverse rows of three to four scales each, such that their distal edges are elevated from the surfaces of the fingers and toes (fig. 2). Subdigital scales on the distal phalanges have dark pigment reduced to the cen- ters, as do all scales on the hands and feet. The digits, palms, and soles on the live animals I ex- amined were not soft or sticky to the touch. There are enlarged subdigital, palmar, and plan- tar scales on Varanus acanthurus and V. tristis, but in those species only the centers of some scales are darkly pigmented (fig. 3). The feet were not sticky to the touch on a live V. tristis I examined. The hands and feet of V. indicus resemble those of V. prasinus, but the former species has an ir- regular light and dark color pattern and its dark subdigital scales are not arranged in transverse rows. The anguids, helodermatids, xenosaurids, Lanthanotus, and nine other varanids I examined FIELDIANA: ZOOLOGY Fig. 3. Ventral view of left hind foot of Varanus acanthurus (mvz 81648), from Charter's Towers, Queensland Australia. Snout-vent length of the lizard is 123 mm. exhibit several foot morphologies, but none of them has rows of darkly pigmented, juxtaposed scales as in V. prasinus. Background Observations on Other Taxa Neotropical iguanids of the genus Polychrus are green and have long, semiprehensile tails (Gorman etal., 1969; Hoogmoed, 1973; Vanzolini, 1983). Hoogmoed (1973) noted keels on the free margins of subdigital scales of P. marmoratus and sug- gested that they aid in climbing. Peterson (1983) illustrated these structures for P. marmoratus and confirmed their presence in P. acutirostris, P. gut- turosus, and P. peruvianus. I observed similar sub- digital keels in P. femoralis and P. liogaster, the remaining species in the genus. Vitt and Lacher (1981) found that stomachs of 105 Polychrus acutirostris from Brazil contained a variety of arthropods and fruit, but orthopterans predominated. Stomach contents of 16 P. mar- moratus from northern and western South Amer- ica indicate a similar diet for that species (Beebe, 1944; Hoogmoed, 1973; Duellman, 1978). One P. liogaster (mvz 36455, Bolivia, 49.3 g) contained a cicada (Homoptera, Cicadidae, MR = 0.028), a grasshopper (Orthoptera, Acrididae, MR = 0.01), a beetle (Coleoptera, Cerambycidae, MR = 0.006), and seven fruits with hard seeds (5x10 mm). Old World chamaeleontids (especially species of Chamaeleo) are often green and are character- ized as a family by strongly prehensile tails, highly projectile tongues, zygodactylous feet, and several visual specializations (Bellairs, 1969; Harkness, 1977). There is surprisingly little information available on the natural diets of chameleons, but a few species are known to feed frequently on or- thopterans (Burrage, 1973; Schifter, 1984). Discussion General Remarks Varanus prasinus feeds mainly on arthropods, usually relatively small katydids, but large walk- ingsticks and rodents are also taken occasionally. These data probably reliably represent the diet, GREENE: DIET AND ARBOREALITY IN VARANUS PRASINUS despite the modest sample size, because a single prey type predominates in specimens that were collected from diverse localities over a period of four decades. Consideration of stomachs with multiple items further suggests that the sample primarily mirrors variation within, rather than among, individuals (cf. Arnold, 1977). Although a captive V. prasinus ate bananas (Mertens, 1 97 1), there is no evidence of plants in the natural diet of this species. With the exceptions of Varanus bengalensis (Loop, 1974; Auffenberg, 1984) and V. komo- doensis (Auffenberg, 1981), the role of prey char- acteristics in the feeding behavior of monitors re- mains largely unknown. Stomach contents indicate that V. prasinus typically swallows prey headfirst (of 1 9 items, this orientation in the gut could be determined for 18; one was swallowed doubled up). A large, spinose walkingstick evidently was partially dismembered before ingestion, perhaps with the raking movements observed in captive monitors. These findings suggest an array of prey handling behavior that merits further study. Some larger monitors in more terrestrial habi- tats also lay eggs in termite mounds, nest in bur- rows, or exhibit intraspecific variation in this re- gard (Auffenberg, 1981, 1983; review of Riley et al. [1985] omitted Longley [1945], who reported the eggs of Varanus varius in a termite nest, high up in a tree). Cowles (1930) pointed out that ter- mitaria provide a favorable microclimate for in- cubation of the eggs of V. niloticus in Africa, and subsequent authors have emphasized that effect and protection from predators (Magnusson et al., 1985; Riley et al., 1985). The observations of V prasinus suggest that immediate accessibility of a rich food source for the young might be another advantage to reptiles nesting in termitaria. More- over, if the use of termite nests proves typical for several species, it might represent a behavioral homology with potentially interesting implica- tions for the early evolution of varanids (cf. Greene &Burghardt, 1978). Relative Prey Size Varanus prasinus is comparatively large among extant lizards (cf. Pough, 1980) and, because ap- proximately 98% of its diet by frequency consists of arthropods, this species might seem to contra- dict Pough's (1973) suggestion that a large lizard could not catch enough insects to make a living. Large prey, however, might be rare in the diet and yet significant. If average MRs of 0.021, 0.122, and 0.296 are multiplied by 44 small arthropods, one walkingstick, and one rodent (frequencies of those items in adult V. prasinus), respectively, the resulting "importance indices" (each of these products divided by the sum of the products) are 0.69, 0.09, and 0.22. This simulation is at best a crude approximation of the overall diet of an in- dividual (e.g., it ignores differences in digestibility among prey), but it demonstrates that even the occasional ingestion of relatively large walking- sticks and rodents might be energetically signifi- cant (in this case, roughly 31% by relative prey mass). Conversely, it is clear that fairly small arthro- pods are an important part of the diet of this mod- erately large lizard (at least 69% by relative prey mass). If the frequency of large items is exagger- ated by small sample size, arthropods might be the only significant prey type for Varanus prasi- nus. These comments emphasize the need for ad- ditional data to address conclusively the problem of functionally important variables in lizard feed- ing biology (Greene, 1 982; Estes& Williams, 1984). Hunting Adaptations I propose that the green coloration, prehensile tail, and unusual feet of Varanus prasinus facili- tate carefully controlled, unobtrusive movements on small limbs, vines, or leafy vegetation and thereby enable the lizards to capture katydids and other arboreal creatures. Prey that can propel themselves from a perch probably present special capture problems for a predator, and such escape tactics are widespread among animals in tropical forests (e.g., Robinson, 1969; Emmons & Gentry, 1983; pers. obs.). Furthermore, tropical katydids and walkingsticks often are colored cryptically and active nocturnally (Robinson, 1973; pers. obs.), and monitors are known to search methodically in particular microhabitats for hidden prey (e.g., V. bengalensis, Auffenberg, 1984; V. tristis, Pian- ka, 1982). Ridges on the feet of some birds are thought to enhance maneuverability on perches (Bock & Miller, 1959), and perhaps the raised sub- digital scales of V. prasinus also do that. A plau- sible mechanism is that they increase the number of frictional edges in contact with the substrate (Hecht, 1952; Peterson & Williams, 1981; Cart- mill, 1985). The function of dark pigment in this case is unknown, but melanin reduces wear on bird feathers (Burtt, 1979) and might do so for lizard FIELDIANA: ZOOLOGY skin. Prehensile tails also provide additional fac- tional support for a climbing lizard (Tornier, 1 899; Cartmill, 1985). The situation for Varanus prasinus is equivalent to Figure 1 a, except that here the derived features and performance advantage characterize a single taxon. Green coloration, a prehensile tail, a regular arrangement of black foot scales, and a diet of katydids are unique to this species among var- anids, and these attributes are lacking even in oth- er members of the subgenus Odatria (Mertens, 1942b) that climb regularly and occasionally eat arthropods (e.g., V. tristis, Pianka, 1982). For this reason, alternatives such as constraint and exap- tation (Gould & Vrba, 1982) seem less likely than adaptive divergence to explain their concordant presence in V. prasinus. It is important to note, however, that if V. acanthurus, V. prasinus, and V. tristis are related such that the dark aspects of their subdigital scales are homologous, then only the extreme development of that character can be interpreted as an adaptation for hunting in tropical forest foliage by V. prasinus. A brief comparison of Varanus prasinus with certain other arboreal tropical lizards is instruc- tive. The closest living relatives of chamaeleontids and Polychrus are agamids and other iguanids, respectively, which lack primitively green color- ation, prehensile tails, and specialized grasping feet (Etheridge, in Paull et al., 1976; Estes, 1983; Pe- terson, 1983). The independently derived mor- phological similarities among chamaeleontids, Polychrus, and V. prasinus are concordant with dietary similarities. As such they present a good prima facie case for adaptive convergence, and I propose that the relevant task for all three is to stalk stealthily and seize wary, arboreal, saltatory, and flying prey. It is interesting to note that species richness among these lineages parallels the extent of morphological specialization exhibited by each of them. Varanus prasinus is modified in minor ways relative to other varanids; the approximately 90 species of chamaeleontids share a number of features that are associated with arboreal hunting, and are highly derived relative to agamids; and, lastly, the six species of Polychrus are intermediate in their deviation from ancestral iguanid charac- teristics. My findings emphasize the likelihood that adap- tive convergence in external morphology is a wide- spread phenomenon in lizards (e.g., Williams & Peterson, 1982; Luke, 1986). Conclusive treat- ment of this generalization will require compre- hensive information on the functional morphol- ogy, natural history, and systematic relationships of these animals. Acknowledgments I thank P. Alberch, R. L. Drewes, C. H. Kish- inami, H. Marx, R. W. McDiarmid, E. S. Ross, and R. G. Zweifel for permission to study speci- mens in their care; W. Z. Lidicker, Jr., for infor- mation about the rodent prey; and A. M. Bauer, J. H. Carothers, K. de Queiroz, J. B. Losos, C. Luke, J. B. Murphy, K. Nishikawa, N. Staub, and D. B. Wake for comments on the manuscript. G. M. Christman prepared Figure 1 . Field experience with insects and Polychrus in Costa Rica and Pan- ama was made possible by the Smithsonian Trop- ical Research Institute, Organization for Tropical Studies, and World Wildlife Fund-U.S. W. E. Rai- ney and E. D. Pierson provided two live Varanus tristis collected under permits from the Queens- land and Australian National Parks and Wildlife Services. I am particularly grateful to J. B. Murphy for allowing me to examine live V. prasinus in the Dallas Zoo and to report his observations on these animals, and to K. Klemmer for the treat of han- dling a live Lanthanotus in the Senckenberg Mu- seum. Data for this paper were obtained incidental to other work supported by the Fourth Bremen Symposium on Biological Systems Theory; the Committee on Research, University of California, Berkeley; and the National Science Foundation (NSF BSR 83-00346). It is a pleasure to thank R. F. Inger for a decade of personal encouragement and more than four decades of professional example. Addendum I learned of Mitchell's (1964) illustration of the foot scalation of Varanus glebopalma after this manuscript was accepted for publication. That species is approximately the same size as V. pra- sinus, has a black and gray dorsal pattern, inhabits sandstone outcrops in xeric parts of northwestern Australia, and feeds primarily on scincid lizards (J. B. Losos, pers. comm.). The soles of its feet are characterized by polished dark areas in the center of each scale, and thus closely resemble the con- dition in V. prasinus. An evolutionary assessment of the similarities GREENE: DIET AND ARBOREALITY IN VARANUS PRASINUS between Varanus glebopalma and V. prasinus will require more information on varanid relationships than is available currently. As noted above, the darkened soles of V. prasinus cannot be adapta- tions for predation on orthopterans that live on tropical foliage if those features are retained from a non-rain forest ancestor (as might be the case if V. glebopalma and V. prasinus are sister taxa). 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