State-dependent decision making: educated predators strategically trade off the costs and benefits of consuming aposematic prey

Aposematic prey advertise their defences, such as toxins or stings, to visually hunting predators using conspicuous warning coloration. Both the conspicuousness and the chemical content of prey determine the speed of avoidance learning by naive predators, and it has long been assumed that predator education is the main selective pressure in the evolution of aposematism. However, recent theoretical models have considered how educated predators could also exert significant selection pressures on aposematic prey by increasing their attack rates on defended prey in times of food shortage. Currently, there are no clear experimental data to support these models. In this study, we show that European starlings (Sturnus vulgaris) increase their attack rates on chemically defended insect larvae when their body masses and fat stores are experimentally reduced. In addition, the increase in attack rate is not simply due to indiscriminate attacks made when energy reserves are low but is based on knowledge about the prey's defences. Taken together, these results suggest that educated adult predators will strategically trade off the energetic benefits of prey against their toxic costs according to their energetic needs. This result challenges classic theoretical models of the evolution of aposematism based purely on predator learning and forgetting rates and demonstrates the need to consider energy-toxin trade-offs in foraging decisions on defended prey. We discuss the implication of these results for the evolution of chemical defences and warning signals. Copyright 2007, Oxford University Press.