The cantilever abilities of snakes

-We quantified the abilities of snakes to extend the anterior body horizontally without support, and we evaluated data for 31 species representing five families. Generally, terrestrial snakes exhibit rather uniform cantilever ability and can extend the body 30-50% of total body length. Arboreal species exhibit statistically superior performance both within and among families, with some species extending the body to more than 50% of body length. Extreme divergence of cantilever abilities occurs between arboreal and aquatic species. Intraspecific comparisons also show that juvenile snakes cantilever better than adults, presumably due to a lower mass relative to body length. Musculoskeletal features of the vertebral column are generally conserved in evolutionary terms, thus possibly explaining the generally uniform cantilever abilities in large numbers of snake species. However, several modifications of vertebrae and associated epaxial muscles and their tendons appear to be related to cantilever performance. l f erpetology, Vol. 34, No. 4, p . 523-528, 200 i t 2000 Society for the Study of Amphib ans and Reptiles It is well known that some snakes can extend a considerable distance of their body length unsupported, although this behavior has not been studied quantitatively. The means by which this is accomplished-cantilevering-is presumably adaptive with respect to terrestrial and especially arboreal habitats where the ability to span gaps in the substratum is advantageous. Thus, scansorial species are expected to excel in their abilities to cantilever. Aquatic or burrowing species would presumably cantilever less well. Theoretically, there are two principal ways in which morphology might evolve to improve the abilities of snakes to cantilever: (1) reduction of body mass in relation to length, and (2) lateral compression of the body muscle mass during unsupported extension (Johnson, 1955). Thin colubrid snakes might characterize the former category. Using branches or other contact points these snakes traverse uneven surfaces or gaps in substratum using lateral undulatory locomotion (Cundall, 1987). The latter category is typified by boid snakes that constrict branches with their trunk and prehensile tail and thereby extend themselves from a constricted anchor (Hoffstetter and Gasc, 1969). Cantilevering ability might also vary with skeletal morphology and associated musculature. Johnson (1955) studied vertebral form in a variety of snakes and reported that arboreal species tended to possess elongated and comparatively narrow vertebrae with short zygapophyses. Arboreal species, and particularly colubrids, are also characterized by greater relative length of epaxial muscles and associated tendons (Jayne, 1982). The present study was undertaken to provide quantitative comparisons of the abilities of snake species to cantilever. We also consider functional and morphological variation that might relate to species differences in both cantilever ability and behavioral ecology. MATERIALS AND METHODS Experimental Animals.-Live snakes used in the cantilevering tests were either wild-caught or obtained from private collections and commercial sources. In this report we combine two data sets that were generated independently, one in Taiwan and one in Florida, USA. Tests were conducted only with specimens that were postabsorptive and appeared to be healthy. Cantilever Tests.-Snakes were tested for their ability to cantilever at body temperatures of, or close to, 25 C. For snakes that were tested in Florida, each individual was placed in an artificial tree constructed of wooden dowels and was observed until it appeared to be calm with ral form in a variety of sn kes and reported at arboreal sp cies t nded to possess elongatand comp ratively narrow vertebrae with ort zygapophyses. A boreal species, and pari ularly colubrids, are also cha acterized by eater relativ length of epaxial muscles and sociated tendons (Jayne, 1982). e present study was undertaken to provide antitative compari ons of the abilities of ake species to cantilever. We also consider ctional and morphological variation that ight relate to sp cies differences in both canilever ab lity and behaviora ecology. 523 This content downloaded from 157.55.39.243 on Thu, 06 Oct 2016 05:07:24 UTC All use subject to http://about.jstor.org/terms H. B. LILLYWHITE ET AL. slow and deliberate movements. Another artificial tree was placed nearby. Eventually (sometimes after prodding) the snake was allowed to extend its body in attempt to bridge the space separating the two trees. As the snake's head approached a branch of the unoccupied tree, the tree was slowly pulled away from the snake. The snake continued to extend itself unsupported and either dropped down or withdrew and aborted the attempt. This procedure was repeated to obtain maximal horizontal distances in three separate trials. The mass and total body length (snout-tail) of each snake were measured, and ratios of cantilevered distance to body length were calculated. In the aquatic species Acrochordus granulatus, it was not plausible to place the snake in the branches, so the snake was cantilevered from the hand. The snake was held close to an artificial tree. As the snake's head approached a branch, the snake was slowly withdrawn from the branch. This was also the case in testing smaller individuals of some species. For snakes tested in Taiwan, the methods were similar in conceptual design but differed in physical detail. Each snake was placed on a stage 175 cm above the ground and at even height with a second stage having a vertical white plastic plate with a hole. Snakes were attracted to the hole and moved toward it. The stage with snakes was then moved away from the hole and adjusted to determine the maximum cantilever distance, as in the Florida tests. This procedure was repeated three times. Data from either method were used to quantify the cantilever abilities of snakes. For each snake we calculated a "cantilever ratio" (CR), which was the straight-line, unsupported, horizontal body extension divided by the total body length. We calculated a mean and maximum CR for each snake. Statistics.-Statistical analyses were performed using StatView? software. In parametric tests, data for CR were log-transformed prior to analysis.