Direction and strength of selection by predators for the color of the aposematic wood tiger moth

Conventionally, predation is assumed to select for conspicuousness and uniformity of warning signals in aposematic (i.e., chemically defended and warning signaling) prey because this enhances predators' initial and learned avoidance. On the other hand, it has been suggested that both variation in the background where the signal is displayed as well as variation in predators' probability to attack defended prey may favor intermediate signals or relax selection for signal monomorphism. We studied the direction and strength of selection for the hind wing color (orange vs. red) of female Parasemia plantaginis moths. Birds found the moths aversive and avoided them by sight both in laboratory and field experiments. A laboratory experiment with great tits showed that birds can discriminate between orange and red wings and attack red females less than orange females. This directional selection should decrease variation in hind wing color. However, the hind wing color did not significantly affect the "survival" of dead specimens under natural field conditions in a multipredator community. In addition, even though both orange and red were highly conspicuous against green leaves and silver birch trunks (backgrounds used in the studies), the magnitude of avian-perceived chromatic contrast (conspicuousness) differed against these backgrounds. Our results suggest that depending on the signal environment (background, predator community) directional selection for the warning signal monomorphism can be relaxed. Copyright 2011, Oxford University Press.