ONTOGENIC VARIATION IN GROWTH AND SEXUAL SIZE DIMORPHISM IN A CENTRALARIZONA POPULATION OF THE WESTERN DIAMONDBACK RATTLESNAKE (CROTALUS-ATROX)

The western diamondback rattlesnake (Crotalus atrox) is sexually dimor phic (adult males larger than females) throughout its range, A compreh ensive analysis of the ecological and evolutionary sources of sex diff erences in growth and size requires a detailed understanding of growth trajectories during ontogeny, We studied a pop ulation of C, atrox in the upper Sonoran Desert of central Arizona. Specifically, we asked a t what point during ontogeny males and females diverge in size: at bir th, during juvenile growth, or as mature adults? We used mark-recaptur e data and a novel analysis of rattle characteristics to test for sex differences during each of these three ontogenetic stages, Our analyse s suggest that male and female neonates are similar in size. Juvenile growth rates also appear to be uniform between sexes. However, males a nd females begin to diverge in size beyond sexual maturity, when femal e but not male growth rates become undetectable given our recapture in tervals. Our results indicate that, when partitioned into its Lifecycl e components, sexual size dimorphism can be expressed at different sta ges and may be affected by a variety of behavioral, bioenergetic, and selective forces, Accordingly studies of proximate and ultimate causat ion in sexual size dimorphism may benefit from consideration of the on togeny of size separately in males and females.