Decoupling vigour and quality in the autumn colours game: weak individuals can signal, cheating can pay.

According to the coevolution theory, autumn colours are a warning signal to insects, signalling the level of chemical defences or availability of nutrients. Because in the original model tree vigour and defences were positively correlated, it is not clear whether signalling would still be stable when they are decoupled, and the fact that weak trees often display bright autumn colours is usually presented as evidence against the coevolution theory. I show that in a theoretical model of insect-tree coevolution, signalling is still stable when vigour and defences are decoupled. Weak trees can signal. Moreover, partial cheating is possible. The different equilibria depend on the importance of vigour and defences against insect attack, of vigour in the production of the signal, and of pleiotropic effects between colour and defences. These results provide precise predictions that can be used for planning future empirical test.