Different UV reflectance on genotypically different morphs affects predator choice

Maintenance of colour polymorphism is a flagship topic in evolutionary biology as it allows tracking evolution of visible traits. In aposematic colouration (in which the organism signals about its unprofitability through warning colouration), we expect to see a selection towards monomorphism due to a shared cost of predator education. However, aposematic polymorphism is common and it is not clear how nuanced the variations in the prey warning signals can be, in order to facilitate avoidance learning by the predators. A possible explanation for the aposematic polymorphism maintenance is that predators with different vision system than ours may impose differential predation upon the colour morphs. Here, I used blue tits (Cyanistes caeruleus) as predators and models of wood tiger moths (Arctia plantaginis) as prey. The models resembled homozygote and heterozygote genotypes that both appear as white but differ in their reflectance including UV-part of the spectrum, and I manipulated their chemical defences. I, then, tested the bird avoidance learning on these two morphs in full factorial design experiment. I found that birds showed strong individual differences in their predatory behaviour and learning. When chemical defence was associated with the higher chromatic contrast (i.e., heterozygotes), avoidance was generalized by blue tits to both morphs, and the overall moths’ survival probability decreased faster. However, the opposite was true when defence was associated with achromatic contrast (i.e., homozygotes). This suggests that predators may learn subtle colour differences in prey, but when they make a choice, birds plausibly rely on easy-to-remember categorization rules. Thus, if heterozygotes have higher chemical defences, and get less predated, also homozygotes can benefit from it within a local population (and vice versa).