Measuring Sexual Size Dimorphism and Sexual Body Component Dimorphism in Snakes: Sexual, Population,
Johnson, Scott
The Bahamas National Trust
Village Road
P. O. Box N 4105
Nassau, The Bahamas
Hayes, William K.
Department of Earth and Biological Sciences
Loma Linda University
Loma Linda, CA USA
Sexual dimorphism in animals exists in many forms, including overall size difference between the sexes (sexual size dimorphism, SSD) and size and structural differences in body components (sexual body component dimorphism, SBCD). Studies of sexual dimorphism seek to determine whether dimorphic traits result from sexual selection, natural selection, or non-adaptive processes. Characterizing sexual dimorphism depends on identifying an unbiased reference character for overall body size (SSD), which can then be used to assess SBCD. Most studies of snakes use snout-vent length (SVL) as the reference character, but SVL may itself be dimorphic. In this study, we used methods developed for scorpions and lizards to assess SSD and SBCD in three island populations of the Bahamian Racer (Cubophis vudii). Discriminant function analysis (DFA) showed that head width (females wider) and tail length (males longer) best discriminated the sexes, and total length provided the least discrimination. We therefore used total length as an unbiased character for SSD. Linear models revealed absence of SSD, but SBCD existed for head size (width 8.9% greater in females, length similar), trunk length (4.3% longer in females), and tail length (9.8% longer in males). The linear models also revealed differences among island populations for total length (New Providence < Eleuthera = Allen Cay) and head length (Allen Cay < Eleuthera < New Providence), but not head width or tail length. Body condition was statistically similar for the two sexes and three seasons (spring, summer-fall, winter), but the large and moderate effect sizes, respectively, suggest that differences may exist (females heavier, both sexes heaviest in spring). The results illustrate the utility of using DFA to identify an unbiased reference character for overall body size, and suggest that sexual and natural selection may interact to shape the morphology of these snakes.