THE EVOLUTION OF VERTEBRAL NUMBER AND BODY-SIZE IN SNAKES

1. In snakes body size and vertebral number vary considerably among species. To investigate whether the two traits are correlated, and similar to a pattern found in fish and termed pleomerism, data from the literature were examined for 771 species and subspecies of advanced snakes (superfamily Colubroidea; families Colubridae, Elapidae and Viperidae). A strong tendency was found for species with many vertebrae to reach a large maximum body size; in 12 groups of species examined all showed a positive correlation coefficient and in nine cases it was significant. 2. To eliminate the possibility that this result is an effect of similarity owing to common ancestry, both a directional method and an independent comparison method were applied on groups for which a cladogram was available. The results were found to be robust, and it is concluded that the evolution of vertebral number has paralleled the evolution of body size in these groups of snakes. It appears that vertebral number, at least in part, has evolved as a correlated response to selection on body size. 3. Deviations from the general pattern of pleomerism were examined to investigate whether they could be explained by family affiliation, body shape or by various ecological factors. The three families did not differ in either slope or elevation of the relationship between vertebral number and body size, but species with a mere elongated body had more vertebrae per size unit than stouter ones whereas burrowing species had fewer vertebrae per size unit than species occupying other habitats. Furthermore, the habit of constriction also appeared to affect vertebral number; constricting species had more vertebrae per size unit than non-constricting ones. Thus, vertebral number per se is also a target of selection and has been adjusted according to specific ecological circumstances apart from being affected by body size. Finally I examined whether body size and vertebral number might be affected by altitude, and thus presumably temperature as has been suggested. However, no effect of altitude on vertebral number or body size was found.