The Charnov-Bull hypothesis is the leading explanation for the evolution of environmental sex determination (ESD), which includes temperature-dependent sex determination (TSD), the most common form of ESD. Charnov-Bull predicts a sex-by-incubation temperature interaction for fitness, matching offspring sex with thermal conditions that increase parental fitness. However, there is no general explanation for how the sex-by-temperature interaction arises. Two competing explanations for the interaction lie in the Survival to Maturity hypothesis (SM) and the Trivers-Willard extension (TW). Under SM, the sex that matures later is produced under optimal incubation regimes as the late-maturing sex accrues more mortality by maturation, while TW suggests that males are always produced under optimal incubation regimes as male mating success is more sensitive to condition (general health, vigor) than female fecundity. In a system where females mature later than males, as in the painted turtle (Chrysemys picta), SM and TW generate opposite predictions for the effect of incubation temperature on juvenile survival. We incubated C. picta eggs under either female-promoting temperatures (28±3°C) or male-promoting temperatures (25±3°C), then released the hatchlings into their natal pond. We used a Bayesian capture-mark-recapture approach to follow their survival over two growing seasons. We found a 2% depression of biweekly survival in individuals incubated under the cooler temperature, providing subtle support to SM. Incubation treatments did not influence growth. Large-scale field experiments such as this one will be necessary for understanding TSD evolution, and we underline general principles to execute such experiments successfully.

Painted turtle eggs were collected from Algonquin Provincial Park and incubated under two temperature regimes. The following year, individuals were released into their natal pond with unique identifiers painted on their back. Researchers returned every two weeks to the pond and sampled for individuals following a capture-mark-recapture methodology.