The effect of predator appetite, prey warning coloration and luminance on predator foraging decisions

Aposematic prey advertise their defence to visually hunting predators using conspicuous warning colouration. Established theory predicts that aposematic signals should evolve towards increased conspicuousness and similarity to enhance predator education. Contrary to theoretical expectations, there is often considerable within- and between-species variation in aposematic signals of animals sharing the same ecological niche, phylogeny and predators. This may be explained by varying responses of predators that weaken the selection pressure for a consistent signal. By presenting painted mealworm larvae as prey to great tits as predators we tested if different aposematic colour patterns have different values as a means of initial protection and learnt avoidance from predators, and how widely birds generalise their learnt avoidance to other colour patterns. We also investigated how the colour and luminance of the pattern elements affect predator attack decisions. Finally, we studied if hunger affects the predators' reaction to differently coloured prey. We found that similarity in colour was not crucial to the survival of aposematic prey, since learnt avoidance was not influenced by colour, and predators remembered and generalised widely in their learnt avoidance to other colours. We found that initial avoidance was, however, apparently influenced by luminance contrast. Interestingly, the predators' level of hunger was more important than the colour of the aposematic signal in determining birds' decisions to attack chemically-defended insect larvae. We discuss the implications of visual properties of prey colour pattern and predator appetite for the evolution of insect defences and warning signals. In addition we propose a methodological approach to effectively control for predator appetite in laboratory experiments.