Determinants of Offspring Number and Size in the Brown Snake, Storeria dekayi

Abstract

Determinants of reproductive variables in the brown snake, Storeria dekayi, are examined using path analysis, a method which reveals both direct effects (e.g., of female size on offspring number) and indirect effects (e.g., of female size on offspring size through offspring number). In S. dekayi, there is a clear trade-off between offspring number and offspring size. Part of this trade-off is mediated through the timing of parturition suggesting that placental nourishment contributes to offspring development. However, placental nourishment is not sufficient to result in an uncoupling of offspring number and size. Another clear result is that female size and female condition at ovulation (a size-corrected measure of mass) both influence offspring number. As a result, female size and female condition have indirect effects on offspring size such that overall, larger females and females in better condition produce smaller offspring. Larger females also produce offspring in poorer condition. Additional reproductive mass (mass lost by females at parturition beyond that accounted for by offspring) is determined directly by offspring number suggesting it represents a per-offspring allocation of resources. Captivity has marked effects on repro- duction; females held captive longer produced offspring which were smaller and in poorer condition. Comparisons among species reveals that female size consistently has positive effects on offspring number and offspring size in viviparous snakes but that the effect of offspring number on offspring size varies among species.