Egg Size, Incubation Temperature, and Posthatching Growth in Painted Turtles (Chrysemys picta)

Incubation temperature has profound and pervasive effects on phenotypes of developing embryos and offspring of oviparous reptiles (reviewed in Deeming and Ferguson, 1991). Perhaps the most compelling impact involves sex determination in many of these species. Indeed, this temperature-dependent sex determination (TSD) in reptiles has received considerable attention (reviewed in Bull, 1983; Ewert and Nelson, 1991; Janzen and Paukstis, 1991; Shine, 1999). Most adaptive explanations for the existence and persistence of TSD hypothesize that incubation temperatures benefit the sexes differentially (sensu Charnov and Bull, 1977). Incubation temperature does, in fact, influence many traits that may affect fitness and examining such traits may disclose sex-specific effects in accord with the Charnov-Bull model. A number of studies have adopted this exploratory approach, yet no general explanation for the adaptive significance of TSD in reptiles has been validated empirically (reviewed by Shine, 1999). One promising hypothesis suggests that the evolutionary persistence of TSD in reptiles could be explained by a covariance between egg size, nest thermal environment, and sexual size dimorphism (Roosenburg, 1996; Roosenburg and Niewiarowski, 1998). That is, a key trait (i.e., egg size) might have sex-specific (i.e., incubation temperature-specific) effects on fitness by influencing posthatching growth rates and ages of maturity. Specifically, (1) egg size must be correlated with eventual size of an individual; (2) incubation temperature must influence posthatching growth rates; (3) any covariance between egg size and temperature affecting growth must be sex-specific; (4) nesting females should be able to discriminate among among environmental cues related to nest temperature; and (5) these females should use such environmental cues to manipulate offspring sex ratio according to their egg sizes. The only attempt to evaluate this hypothesis produced considerable support. In diamondback terrapins (Malaclemys terrapin), a turtle with TSD and largefemale sexual size dimorphism in adulthood, egg size influenced posthatching growth rates and ages of maturity for females but not for males (Roosenburg and Kelley, 1996). Fieldwork showed subsequently that nesting terrapins generally laid larger eggs in warmer, female-producing microhabitats and smaller eggs in cooler, male-producing microhabitats (Roosenburg, 1996). Our study is a first attempt at testing the generality