VARIATION IN OXYGEN-CONSUMPTION OF THE WESTERN DIAMONDBACK RATTLESNAKE (CROTALUS-ATROX) - IMPLICATIONS FOR SEXUAL SIZE DIMORPHISM

Variation in metabolism affects energy budgets of individuals and may serve as a mechanism that influences variation at whole organism or po pulation levels. For example, sex differences in metabolic expenditure may contribute to bioenergetic sources of sexual size dimorphism. We measured oxygen consumption rates of 48 western diamondback rattlesnak es (Crotalus atrox) from a sexually dimorphic population and tested th e effects of body mass, body temperature and time of day, in three gro ups of snakes: males, non-reproductive females, and vitellogenic femal es. Metabolic rates of male and non-reproductive female C. atrox were similar to rates reported for other rattlesnakes (mass exponents rangi ng from 0.645-0.670), Oxygen consumption was affected by body mass, bo dy temperature and time of day, and was approximately 1.4 times greate r in vitellogenic females than in non-reproductive females. No differe nces were found between males and non-reproductive females. Accordingl y, differences in metabolic rate apparently do not contribute directly to sexual dimorphism in this population. Nevertheless, estimates of s ize-dependent maintenance expenditure lead us to hypothesize that adul t female body size may represent a compromise between selection for in creased litter size (accomplished by increasing body size), and select ion for increased reproductive frequency (accomplished by decreasing b ody size, and, therefore inactive maintenance expenditure); this is a mechanistic scenario suggested previously for some endotherms.