Efficacy and honesty in communication between relatives.

The evolution of honest communication has recently become the focus of intense theoretical attention. However, strategic models dealing with honesty have largely ignored the implications of noise and perceptual error for signal evolution (just as models dealing with signal detection in the presence of noise ignore strategic issues). Here, I analyze an extended version of Maynard Smith's strategic model of signaling of need between relatives, the Philip Sidney game, that incorporates the possibility of perceptual error. I show that even in the presence of noise, there exists over a wide range of parameter values a unique, continuously stable signaling equilibrium, at which the signaler employs a costly display when needy but refrains from doing so when healthy. For a subset of this range, there also exists a second, lower cost signaling equilibrium that is not continuously stable. At the former equilibrium, predicted signal cost is inversely related to the coefficient of relatedness (r) between signaler and receiver. Cost is not, however, predicted to drop to zero even when r = 1 and there is no conflict of interest between the two (as is the case in errofree models), because it serves to enhance the efficacy of communication as well as to discourage deceit. Equilibrium signal cost is inversely related to the probability that the signaler is needy, and tends to increase with the level of noise. If noise becomes too great (i.e., if a detectable signal is too costly to produce), signaling is no longer stable; surprisingly, it is also unstable if the level of noise is too low (i.e., if a detectable signal is too cheap to produce).