PHENOTYPIC PLASTICITY FOR GROWTH IN THE COMMON SNAPPING TURTLE - EFFECTS OF INCUBATION-TEMPERATURE, CLUTCH, AND THEIR INTERACTION

We examined a critical component of the Charnov-Bull hypothesis of tem perature-dependent sex determination (TSD) by determining the reaction norms of hatchling growth to embryonic incubation temperature in the common snapping turtle, Chelydra serpentina. Hormone manipulations of eggs produced females at male temperatures and vice versa, which there by permitted same-sex comparisons of hatchling growth across a range o f incubation temperatures. In this way, the normally confounded effect s of incubation temperature and sex were dissociated experimentally. T he resultant hatchlings, including controls and experimentals, exhibit ed normal gonadal structure and sex steroid profiles. The subsequent g rowth of hatchlings monitored for 6 mo was strongly affected by embryo nic incubation temperature but not by sex. As predicted, growth was en hanced at incubation temperatures that produced males. Clutch effects and interaction effects (clutch by incubation temperature) on growth w ere significant. In addition, there was a positive genetic covariance among incubation temperatures, but incubation temperature effects vari ed among clutches. The variation in growth plasticity among clutches w as consistent with Charnov-Bull predictions. In this TSD species, incu bation temperature is likely to have differential fitness effects on t he sexes mediated via differences in growth.