Handicap signalling: when fecundity and viability do not add up.

It is widely accepted that a requirement for honest handicap signalling is that higher-quality signallers pay lower marginal costs for advertising. This is a simple, powerful principle, but it is correct only if the fitness effects of fecundity and viability are strictly additive. Additivity would not be expected from most life history models. The general criterion for honest handicap signalling is that higher-quality signallers must have higher marginal fitness effects of advertising. This might result from higher benefits rather than lower costs. The general criterion implies the existence of a ridge on the fitness surface for two correlated characters, quality (or viability) and advertising. This has important implications for the design of experiments. Critical tests of the handicap hypothesis should establish that signallers of different quality are on a rising fitness ridge because of different cost-benefit trade-offs. The further question of whether receivers are maximizing their fitness requires additional experiments because handicap signalling does not require that the receivers maximize their fitness, only that they return benefits to signallers as an increasing function of the size of the signal. If receiver preferences are exaggerated by sensory bias or indirect selection, the resulting exaggerated signals may be consistent with the handicap principle. Copyright 1998 The Association for the Study of Animal Behaviour.