Sexual Size Dimorphism in Snakes Revisited

Published and original data on the degree of sexual size dimorphism (SSD) were collated for 374 species of snakes from eight families, to test for predicted associations between SSD and other aspects of reproductive biology. Data on the occurrence and form of male-male combat were also reviewed. Because phylogenetic conservatism was evident in most of the characteristics studied, the data were analyzed in two ways: (1) I looked for general patterns by treating each species as an independent unit; and (2) I superimposed the data onto a phylogenetic framework so that I could calculate independent contrasts within each lineage. Male-male combat has been reported in 124 species of snakes, and this combat takes many forms. For example, biting is common in nonvenomous species. Males grow larger, relative to conspecific females, in species with malemale combat than in species not recorded to show this behavior, and this association remains significant when analysis is restricted to independent phylogenetic contrasts. On a proximate level, the larger size of males in species with combat is due primarily to a prolongation of male growth after maturation. Fecundity advantages of larger body sizes in females may also influence SSD, because the degree of SSD correlates with the rate at which litter sizes increase with body size. The evolution of viviparity is associated with a consistent shift toward larger female size relative to male size. However, my analyses suggest that a previously documented correlation between SSD and geographic distribution is due to phylogenetic conservatism rather than to any functional relationship between the two variables.