Gamberale, G., and B. S. Tullberg. 1996. Evidence for a peak-shift in predator generalization among aposematic prey. Proceedings of the Royal Society of London B Biological Sciences 263: 1329-1334.
A previous theoretical model involving learning psychology and game theory has suggested how warning coloration in unprofitable prey could evolve and become stable. The model shows that in a dimension of increasing prey conspicuousness a displacement of the minimum in a predators' generalization gradient (peak-shift), can produce a strong enough selection pressure towards a more conspicuous coloration that would balance the increased risk of discovery and thus stabilize an aposematic strategy. Using naive domestic chicks (Gallus gallus domesticus) as predators on three different instars of larvae of the aposematic bug Tropidothorax leucopterus (Heteroptera: Lygaeidae), we first show that their unconditioned aversion increases towards larger prey. This aversion is caused by increased aposematism-conspicuousness with increasing size, as no aversion at all was found in a control experiment increasing the size of edible prey. We then show that after one experience of aposematic prey, the chicks exhibit a greater aversion towards prey that are larger and thus more aposematic than those they had experienced. Our results thus provide evidence of a peak-shift in a predators' generalization gradient in a stimulus dimension of increasing prey conspicuousness. The mechanisms behind this peak-shift are investigated and discussed.