We examined the influence of sex divergences in reproductive role and physiology on catchability and demographic patterns in a closed population of aspic viper (Vipera aspis Linné). During 8 years, there were 4800 captures of 988 adults. In both sexes, captures were more frequent in spring when climatic conditions and reproductive activities impose extended basking periods that make animals more detectable. On average, males were captured more than females, reflecting intense sexual activity (i.e., mate searching) in spring. Reproductive females were more catchable than nonreproductive females, illustrating a major increase in basking behavior associated with reproduction. Estimates of population size revealed a sexually dimorphic demographic system with marked year-to-year fluctuations in females contrasting with a more stable male population. This sex difference in population dynamic reflects sex divergences in the acquisition and allocation of energy for reproduction. In both sexes, reproduction is fueled by body reserves. Females, however, need to accumulate substantial body reserves to reach a high body condition threshold before reproduction, whereas the male pattern of energy allocation is more gradual (i.e., no fixed threshold). In addition, reproduction entails major survival cost in females (i.e., most females reproduce just once), whereas males are generally annual breeders. As a consequence of this sex divergence, food abundance, through its direct effect on body store dynamics, influenced major demographic parameters of females (e.g., proportion of reproducing individuals, annual changes in population size) but not males.