Increased signalling effort when survival prospects decrease: male-male competition ensures honesty.

For signalling to be honest the handicap principle claims that signals must impose fitness costs so that only the best individuals can afford the most exaggerated signals. The cost of signalling in terms of reduced survival decreases, however, towards the end of an individual's lifetime, which can induce an increase in signalling effort as a terminal effort. I show for the three-spined stickleback, Gasterosteus aculeatus, that a decrease in survival prospects through impaired condition leads to an increase in red nuptial coloration that makes the signal less reliable as an indicator of male parental ability. Males in poor condition with a large signal sometimes cannibalized all the eggs they received, probably to start a new breeding cycle with a higher energy reserve. However, the inclusion of socially imposed costs of signalling through male-male competition during courtship increased the honesty of the signal, as some males in poor condition and of poor parental ability decreased their signal expression. Some cheaters still occurred, but the signalling system was honest on average. This implies that socially imposed costs are important in the maintenance of honest sexual signalling. Dishonesty may occur under favourable conditions when the cost of signalling is reduced. This emphasizes the importance of considering the environmental conditions experienced by individuals when investigating the evolution and maintenance of honest sexual signals. Copyright 2000 The Association for the Study of Animal Behaviour.