Reproductive biology, mating aggregations, and sexual dimorphism of the argentine boa constrictor (Boa constrictor occidentalis)

We provide data on sexual dimorphism, reproductive biology, and mating aggregations of the Argentine Boa Constrictor (Boa constrictor occidentalis), a poorly known, threatened species. Snakes were examined by ultrasound scanning between 1998 and 2001. Adult females were 14% longer and 51% more massive than adult males. Litter size averaged 25.05 and increased significantly with maternal body size. Reproductive activity was seasonal and associated, with vitellogenesis and spermatogenesis occurring during the dry season (April to September). The ratio of reproductive to nortreproductive females was 1:1, suggesting females do not reproduce annually in this population. Reproductive females were in better body condition (mass relative to body length) than nonreproductive females, indicating that a female's initial decision to reproduce in any given year may be driven by her body condition (storing enough energy for a long period before expending it on reproduction: capital breeder). Aggregated boas were found only during the dry season. The high proportion of solitary reproductive males and the operational sex ratio (male:female, 1.53:1) suggests a system of prolonged mate-searching polygyny.