Signalling Among Relatives. I. When is signalling too costly?

Zahavi's handicap principle, originally proposed as an explanation for sexual selection of elaborate male traits (Zahavi, 1975) suggests that signalling can be honest if less desirable signallers must pay a higher cost in order to signal. Maynard Smith (1991) and Johnstone and Grafen (1992) introduce the Sir Philip Sidney Game in order to apply the handicap principle to the case of signalling among relatives and demonstrate the potential for stable costly signalling systems among relatives. In this paper, we determine the factors which dictate signal cost and signal value in a generalized version of the Sir Philip Sidney game. This allows us to demonstrate that in both the discrete and continuous versions of the Sir Philip Sidney game, there exist conditions under which costly signalling among relatives is "too costly" to be worthwhile for either participant. Although with costly signalling the benefits associated with honest information transfer become available, the cost incurred may leave all participants worse off, even when costly signalling is itself stable. This possibility raises concerns about the evolutionary pathways which could have lead to the existence of such equilibria in nature. The paper stresses the importance of comparing signalling equilibria with other possible strategies before drawing conclusions regarding the optimality of signalling