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.