The effects of constant and fluctuating incubation temperatures on sex determination, growth, and performance in the tortoiseGopherus polyphemus

Temperature-dependent sex determination is one of the best documented yet evolutionarily enigmatic sex-determining systems. The classical theoretical framework suggests that temperature-dependent sex determination will be adaptive when males and females benefit differentially from development at certain temperatures. Empirical evidence has not provided convincing support for this "differential-fitness" hypothesis. Furthermore, since most experiments utilize constant temperature incubation treatments to explore phenotypic response to temperature, few studies have addressed the consequences of incubation under natural conditions. In this study I utilized constant-temperature laboratory incubations and natural-nest incubations to determine the effects of temperature on sex, size, growth, and locomotor performance in the tortoise Gopherus polyphemus. Constant-temperature incubations do induce substantial growth and performance variation in these tortoises. However, the data do not clearly support the differential-fitness hypothesis because (i) growth variation does not result in adult size dimorphism, (ii) performance differences are confined to a very short period after hatching, and (iii) natural incubation temperatures do not vary sufficiently to produce significant phenotypic variation in traits other than sex.