Experimentally Testing Nest Site Selection: Fitness Trade-Offs and Predation Risk in Turtles

The nesting habitat of most freshwater turtle species has been described, but factors influencing maternal nest site selection have rarely been tested experimentally. Offspring fitness is assumed to be the major factor influencing nest site selection because habitat characteristics and nest microenvironments affect offspring survival. However, two opposing factors drive maternal nest site selection: minimizing female mortality and maximizing offspring fitness. In Australia, introduced red foxes are the major predator of turtle nests, and they also destroy nesting females. Thus, females may trade off maximizing nest survival or offspring fitness to avoid predators. In this paper, I show that the risk of predation affects maternal nest site selection and has negative effects on reproductive success in a freshwater turtle. I also show that the mechanisms behind predator detection vary between native and introduced species. From 1996 to 2000, I observed female freshwater turtles, Emydura macquarii, nesting around four lagoons in southeastern Australia to determine nesting habitat characteristics. During 1997 and 1998, foxes were removed from two sites, and nest predation rates declined by >50%, but remained >85% in nonremoval sites. Foxes destroyed ~3% of the female population only in high-risk areas. Female turtles nest away from shore to maximize offspring fitness when foxes are removed from an area. The dilemma in high-risk areas is that predation risk limits females from nesting in preferred areas away from shore, where nest predation is reduced. However, females may sacrifice some offspring by nesting in inappropriate substrate, where incubation conditions are not optimal, but nest predation is significantly reduced. Nesting turtles do not detect foxes by chemical recognition, but they have an innate avoidance response to the odor of a native predator. Nesting habitat affects offspring fitness, but factors affecting female survival may ultimately drive maternal nest site selection in turtles.