THE EVOLUTION OF VERTEBRAL NUMBER AND BODY-SIZE IN SNAKES

1. In snakes body size and vertebral number vary considerably among sp ecies. To investigate whether the two traits are correlated, and simil ar to a pattern found in fish and termed pleomerism, data from the lit erature were examined for 771 species and subspecies of advanced snake s (superfamily Colubroidea; families Colubridae, Elapidae and Viperida e). A strong tendency was found for species with many vertebrae to rea ch a large maximum body size; in 12 groups of species examined all sho wed a positive correlation coefficient and in nine cases it was signif icant. 2. To eliminate the possibility that this result is an effect o f similarity owing to common ancestry, both a directional method and a n independent comparison method were applied on groups for which a cla dogram was available. The results were found to be robust, and it is c oncluded that the evolution of vertebral number has paralleled the evo lution of body size in these groups of snakes. It appears that vertebr al number, at least in part, has evolved as a correlated response to s election on body size. 3. Deviations from the general pattern of pleom erism were examined to investigate whether they could be explained by family affiliation, body shape or by various ecological factors. The t hree families did not differ in either slope or elevation of the relat ionship between vertebral number and body size, but species with a mer e elongated body had more vertebrae per size unit than stouter ones wh ereas 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 ve rtebrae per size unit than non-constricting ones. Thus, vertebral numb er per se is also a target of selection and has been adjusted accordin g to specific ecological circumstances apart from being affected by bo dy size. Finally I examined whether body size and vertebral number mig ht be affected by altitude, and thus presumably temperature as has bee n suggested. However, no effect of altitude on vertebral number or bod y size was found.