The influence of female body size and shape on the trade-off between offspring number and offspring size in two viviparous snakes

摘要

Although the trade-off between offspring size and the number of offspring is a critical component of life-history theory, many empirical tests fail to show that such trade-off exists. Although this may be due to statistical issues (i.e. failure to control for maternal body size), other complications such as female body shape may play a role as well. Here, we examined reproductive traits in two species of viviparous garter snakes with very different body morphologies (Thamnophis marcianus and T.proximus) to see how female body shape affects this trade-off. In the more slender species (T.proximus), we found a strong, negative relationship between brood size and offspring size, with the effect most notable in smaller females. However, in the more robust snake (T.marcianus), there was no significant trade-off seen in either the sample as a whole or for either larger or smaller females. Our data support earlier work on ectotherms, which indicates that body shape can act to constrain how offspring size and clutch or litter size are related.