Alarm calls as costly signals of antipredator vigilance: the watchful babbler game

Alarm-calling behaviour is common in many species that suffer from predatio n. While kin selection or reciprocal altruism are typically invoked to expl ain such behaviours, several authors have conjectured that some alarm calls may instead be costly signals sent by prey to inform approaching predators that they have been detected. We develop a general game-theoretical model, the watchful babbler game, in which prey signal awareness to predators. We derive necessary and sufficient conditions for alarm calls to function as honest signals. We show that signals can honestly reveal prey awareness if (1) the prey's sense of predation risk accurately reflects the probability that the predator is present, and (2) greater awareness of the predator all ows the prey a greater chance of escape. When honest signalling is possible , the model predicts that prey will be more willing to signal when predator s are common than when predators are rare, and that greater pursuit costs t o the predator will allow cheaper signals by the prey. (C) 2001 The Associn tion for the Study of Animal Behaviour.