The significance of predation in nest site selection of turtles: an experimental consideration of macro- and microhabitat preferences

Nest selection has important consequences for fitness in freshwater turtles, as nest predation is extremely high in most populations. However, nest predation has rarely been considered a major factor affecting nest site selection. Thermal qualities of a nest are often suggested as a factor influencing maternal choice of nest site because incubation temperature affects offspring performance and growth, as well as the sex ratio of the clutch in species with temperature dependent sex determination (TSD). Yet experimental manipulations of the nesting environment (habitat and threat) to test factors influencing maternal nest site selection in turtles are uncommon. In this paper, we show that nest site selection in an Australian turtle (Emydura macquarii) with genetically determined sex determination (GSD) exists at two spatial scales, with predation as the major factor affecting the location of a nest. Females prefer to nest in areas where nest predation is minimal but when the risk of direct predation is increased, they trade-off minimising nest predation with survival by locating their nests closer to shore. However, experimentally reducing cover demonstrated that females prefer to nest in open areas, which are more common closer to shore. Hence females are forgoing preferred microhabitat to locate their nests away from shore to minimise nest predation. Despite a preference for open nesting areas, females exhibit similar repeatability of microenvironment selection to a North American species with TSD. Repeatability of nesting overstory vegetation in the North American turtle, Chrysemys picta, suggests that females may exert some control over the thermal qualities of the nests; a key assumption in theoretical models of the microevolution and adaptive significance of TSD in reptiles. Incubation temperature is one factor that may affect microhabitat preferences in both turtles but other factors (e.g. predation and soil moisture) affected by microbabitat may be critical in the evolution of repeatability of overstory cover.