Is it better in the moonlight? Nocturnal activity of insular cottonmouth snakes increases with lunar light levels

Abstract

Both theoretical and empirical investigations suggest that predation risk and availability of resources interact as trade-offs to produce patterns of predation-sensitive foraging. Such interactions have been explored intensely in terrestrial predator–prey systems where both nocturnal prey and predators adjust their activity and foraging behaviour to levels of moonlight. In the case of prey, higher levels of moonlight increase predation risks, and thus prey display lower levels of activity and/or shifts in their use of microhabitat during full moon nights. Predators also display parallel modifications of their activity rhythms during full moon nights – but why? Are predators also sensitive to increased predation, or do predators adjust their behaviour to that of their prey in order to optimize foraging success? Herein, we examined nocturnal predatory behaviour in a natural system where insular cottonmouth snakes, Agkistrodon piscivorus conanti, forage for fish carrion that is indifferent to levels of moonlight, thereby removing complexities linked to the behavioural patterns of prey. Our results demonstrated that insular cottonmouths increase their activity during full moon nights. Predation pressure on snakes foraging in the open does not seem to drive their nocturnal behaviour insofar as small-sized individuals – presumably more susceptible to predation – are equally abundant as adult snakes irrespective of levels of moonlight. These results suggest that variation in predator's activity in natural predator–prey systems during risky (full moon) nights might be attributable principally to the availability and detectability of prey rather than a foraging-safety trade-off specific to the predator.