Honest advertisement of multiple qualities using multiple signals

Animal displays are often complex, involving a variety of different visual, auditory and/or olfactory components. This observation poses a problem for models of signalling based on the handicap principle, which predict that displays generally serve to advertise quality, because it is not obvious why honest advertisement should require multiple signals. One possible explanation is that complex, multi-component displays provide information about many different aspects of the quality or condition of the signaller. Here, a game-theoretical model of signalling is described in which multiple signals serve to advertise multiple qualities in this way. When many different kinds of signal are available, there can be no guarantee that a particular signal will be less costly for a signaller of higher overall value. Nevertheless, the model demonstrates that honest signalling using multiple displays can be stable; multiple signal equilibria exist at which receivers acquire accurate information about the overall value of signallers. It is also shown that, at such equilibria, there need be no one-to-one relationship between signals and qualities. Even if the cost of a particular signal trait depends only on one particular quality, its expression is likely to be influenced by other qualities as well.