Olfactory recognition of predators by nocturnal lizards: safety outweighs thermal benefits

Many prey species are faced with multiple predators that differ in the degree of danger posed. The threat-sensitive predator avoidance hypothesis predicts that prey should assess the degree of threat posed by different predators and match their behavior according to current levels of risk. To test this prediction, we compared the behavioral responses of nocturnal velvet geckos, Oedura lesueurii, to chemicals from 2 snakes that pose different threats: the dangerous broad-headed snake Hoplocephalus bungaroides that eats geckos and the less dangerous small-eyed snake Cryptophis nigrescens that eats skinks (i.e., does not consume geckos). We also tested whether predator avoidance by prey was modulated by thermal costs associated with retreat-site selection. In both the presence and absence of thermal costs, velvet geckos avoided crevices scented by both dangerous and less dangerous snake species. When given the choice between a crevice scented by a broad-headed snake and a crevice scented by a small-eyed snake, most geckos avoided either retreat site. These results suggest that velvet geckos treat both snake predators as equally dangerous. To further explore these results, we quantified patterns of retreat-site selection by free-living velvet geckos on 2 sandstone plateaux. As in the laboratory, velvet geckos avoided thermally suitable rocks previously used by both snake species. Hence, a gecko's choice of retreat site is influenced by the presence of snake chemicals but is independent of thermal costs or the level of danger posed by the predator. To minimize their risk of predation, geckos may use a simple rule of thumb: "all snakes are dangerous.