Do the multiple defense chemicals of visually distinct species enhance predator learning?

Müllerian mimicry, where 2 unpalatable species share a warning pattern, is classically believed to be a form of mutualism, where the species involved share the cost of predator education. Birds learn to avoid a color signal faster when individual prey possesses 1 of 2 bitter-tasting chemicals rather than all having the same chemical, suggesting that Müllerian mimics that possess different defense chemicals are better protected than those that possess the same defense chemical. Using domestic chicks as predators and flavored, colored crumbs for prey, we investigated whether birds learn to avoid 2 visually distinct crumb types faster when each crumb type possesses a different defense chemical than when both crumb types share the same defense chemical. We found that birds learned to avoid 2 visually distinct color signals at a similar rate, irrespective of whether each color signal represented a different defense chemical or whether both color signals represented the same defense chemical. This experiment, therefore, indicates that in terms of predator avoidance learning, possessing 2 defense chemicals is more advantageous when prey look the same than when they look different. This suggests that Müllerian mimics with different defense chemicals not only are better protected than Müllerian mimics that share a single chemical but also benefit more from their mimetic resemblance. Copyright 2006.