Intraguild predation, thermoregulation, and microhabitat selection by snakes

Intraguild (IG) predation, the killing and eating of potential competitors, can be a powerful force within faunal assemblages. If both the IG predator and its prey prefer similar microhabitats in spatially structured environments, avoidance of the predator may relegate IG prey to suboptimal habitats. In southeastern Australia, the broad-headed snake (Hoplocephalus bungaroides) is an endangered species sympatric with the small-eyed snake (Cryptophis nigrescens), an abundant and geographically widespread species known to eat other snakes. Both of these nocturnal ectotherms shelter diurnally beneath thermally distinctive "hot rocks," which are in limited supply. When selecting shelter sites, broad-headed snakes thus face a trade-off between predation risk and habitat quality. In laboratory experiments, we allowed broad-headed snakes to choose between retreat sites differing in thermal regimes, in scent cues from predators, and in the actual presence of the predator. Broad-headed snakes displayed an aversion to sites with live predators and predator scent, yet nonetheless frequently selected those sites to obtain thermal benefits. In trials with live predators, adult broad-headed snakes shared hot rocks with small-eyed snakes, but most juveniles did not; data from a 16-year field study likewise suggest that broad-headed snakes only cohabit with small-eyed snakes if the two snakes are similar in body size. Our results suggest that thermoregulatory considerations are sufficient to prompt juvenile (but not adult) broad-headed snakes to risk IG predation, emphasizing the importance of microhabitat quality and body size in mediating IG predator--prey interactions. Copyright 2009, Oxford University Press.