Maternal nest-site choice and offspring fitness in a tropical snake (Tropidonophis mairii, Colubridae)

Do reproducing female reptiles adaptively manipulate phenotypic traits of their offspring by selecting appropriate nest sites? Evidence to support this hypothesis is indirect, mostly involving the distinctive characteristics of used (vs. available) nest sites, and the fact that physical conditions during egg incubation can modify hatchling phenotypic traits that plausibly might influence fitness. Such data fall well short of demonstrating that nesting females actively select from among potential sites based on cues that predict fitness-determining phenotypic modifications of their offspring. We provide such data from experimental studies on a small oviparous snake (the keelback, Tropidonophis mairii) from the wet-dry tropics of Australia. When presented with a choice of alternative nesting sites, egg-laying females selected more moist substrates for egg deposition. Incubation on wetter substrates significantly increased body size at hatching, a trait under strong positive selection in this population (based on mark-recapture studies of free-ranging hatchlings). Remarkably, the hydric conditions experienced by an egg in the first few hours after it was laid substantially affected phenotypic traits (notably, muscular strength) of the hatchling that emerged from that egg 10 weeks later. Thus, our data provide empirical support for the hypothesis that nesting female reptiles manipulate the phenotypic traits of their offspring through nest-site selection, in ways that enhance offspring fitness.