Sexual dimorphism in snakes: different reproductive roles favour different body plans

Because snakes have a highly simplified morphology, and many species have a wide (and broadly overlapping) range of adult body sizes within each sex, they offer an excellent opportunity to compare body composition of males and females. Evolutionary theory predicts that particular body components should be differentially enlarged in the two sexes. For example, we might expect the reproductive success of females to be enhanced by enlargement of organ systems involved in the processing and storage of energy (e.g. alimentary tract, liver, fat stores) whereas males would benefit from the enlargement of systems important for mate–searching, male–male combat and sperm competition (e.g. larger mass of skeletal muscles, tail, and kidneys). Dissection of 243 specimens of three snake species (117 Vipera aspis, 43 Elaphe longissima, 83 Coluber viridflavus) broadly supported these predictions. Strong sex differences were apparent in relative sizes (masses) of all the non–gonadal body components that we weighed. For example, males consistently had more musculature (relative to body length) than did conspecific females. Dimorphism in relative muscle mass is likely to be one of the most fundamental and widespread morphological differences between males and females in the Animal Kingdom.