Hairiness and warning colours as components of antipredator defence: additive or interactive benefits?

To deter predator attack, aposematic prey species advertise their unprofitability with one or more conspicuous warning signals that, in turn, enhance the avoidance learning of predators. We studied the costs and benefits of multicomponent signalling in Parasemia plantaginis moths. The hairy moth larvae have an orange patch on their otherwise black bodies. The patch varies phenotypically and genetically in size. We studied whether the detection risk associated with patch size varied against two backgrounds (green or brown) with two different predators: naïve chicks, Gallus gallus domesticus, and experienced great tits, Parus major. We also evaluated the signal value of different defence traits within a multicomponent signal by testing which combination of two traits, hairiness and the presence or size of the orange patch, most affected the avoidance learning rate of predators. Larvae with a large orange patch were at greater risk of detection by birds against both backgrounds. This higher detection risk was traded-off with enhanced avoidance learning rate. The orange patch had a higher signal value for the predators than did hairiness, which only slightly increased the survival of totally black or small-patched larvae but did not affect the defence of larvae with a large orange patch. Multicomponent defences are therefore not necessarily additive and variation in the warning coloration of aposematic animals may be partly explained by variation in the relative benefits of different components of a warning signal to different predators.