Fleeing and hiding under simultaneous risks and costs

Prey evaluate risk implied by a predator during its approach, intrinsic prey features, and environmental factors to weigh risk against costs. Most studies of escape and hiding time in refuge focus on a single factor, but prey that ignore other factors may soon be dead. Two hypotheses make different predictions about additivity versus interaction of risk--cost factors. The risk-sensitivity hypothesis predicts that multiple factors have additive effects, but optimality theory suggests that interaction is more common. In the only previous study, flight initiation distance (FID = predator--prey distance when escape begins) was affected interactively by 2 risk factors (predator approach speed and directness) but additively by a risk and an escape cost (approach speed and food presence). I predicted that effects of approach speed and other risk factors interact because differences in risk are likely to increase more rapidly with increase in approach speed than other factors. FID by lizards that I approached increased interactively with approach speed and directness in the Sceloporus virgatus and speed and predator persistence in Sceloporus jarrovii. During indirect approaches, fewer S. virgatus fled, and they ran shorter distances. A review of effects of multiple risks and costs on FID and hiding time revealed far more frequent interactive than additive effects between 2 risk factors, as predicted by optimality theory. Additive and interactive effects were equally common between a risk and a cost factor. Findings for some factors differ between FID and hiding time and in other ways. I propose hypotheses to explain some differences. Copyright 2009, Oxford University Press.