Nesting ecology in the freshwater turtle Podocnemis unifilis: spatiotemporal patterns and inferred explanations

1. Habitat selection has profound ecological and evolutionary consequences. For example, there may be strong selection for nest-site choice such that oviparous females, lacking parental care, may adaptively manipulate their offspring survival or phenotype. Alternatively, nesting decisions may be a passive by-product of other processes leading to similar nonrandom nesting patterns. 2. Here we examined the nesting ecology of the turtle Podocnemis unifilis at multiple spatiotemporal scales to determine whether randomness, adaptive nest-site selection or social facilitation best explained the observed population-level patterns. We addressed these alternative strategies by exploring how environmental and geographic variation in nest sites influences embryonic survival within and across four nesting beaches in three years. 3. We found nonrandom spatial and environmental patterns of nesting within beaches and years, consistent with both the adaptive nest-site selection and social facilitation hypotheses. However, nesting patterns were unpredictable among beaches and among years. Furthermore, environmental conditions at nest sites and offspring survival were not associated, and nests from the most gregarious nesting night clustered more tightly geographically and suffered lower predation than nests laid on other nights. 4. Together, our findings provide more extensive support for social facilitation as compared to the adaptive nest-site selection hypothesis. Our results suggest that selection for female nest-site choice in reptiles may be acting more strongly via offspring survival through nest clustering derived from conspecific cueing and less strongly via environmental cueing than previously anticipated. 5. Our findings underscore the importance of examining multiple sites during multiple seasons. This approach permits testing critical predictions about the consistency of population-level patterns across space and time that enable the distinction between models. 6. Our results support a shift with regards to the trait that is usually considered the target of selection for female nest-site choice in a way that exemplifies the classic dichotomy between selection for (survival of nests and perhaps of females) and selection of (offspring phenotype, such as sex).