Importance of internal pattern contrast and contrast against the background in aposematic signals

Aposematic color patterns that signal prey unprofitability are suggested to work best when there is high contrast within the animal color pattern or between the animal and its background. Studies show that prey contrast against the background increases the signal efficiency. This has occasionally been extended to also explain the presence of internal patterns. We used domestic chicks, Gallus gallus domesticus, to investigate the relative importance for avoidance learning of within-prey pattern contrast and prey contrast against the background. In a series of trials, birds were first trained to avoid artificially made aposematic mealworms that were plain red or red with black stripes, and to discriminate them from palatable brown mealworms, on either a red or a brown background. Second, we investigated how the birds generalized between striped and nonstriped prey. The chicks showed faster avoidance learning when the basic color of the aposematic prey (red) contrasted with the background color (brown). However, there was no similar effect of internal pattern contrast. The generalization test showed a complete generalization between the nonstriped and the striped prey. We conclude that contrasting internal patterns do not necessarily affect predator avoidance learning the same way as shown for prey-to-background contrast in aposematic prey. Copyright 2009, Oxford University Press.