Prey species have evolved two different strategies to avoid predation by either having cryptic coloration, or aposematic coloration. The first technique utilizes camouflaging coloring, while the second uses bright warning coloration of unpalatability to predators. Generally, conspicuous coloration is considered to be an efficient signal, in that it is easy for the recipient to detect, and memorize. In addition, bright warning coloration is often paired with patterns of contrasting color. The functional benefit of patterns are not fully understood, however, many hypotheses have been suggested. One hypothesis suggests that patterns help trigger the memory in avoidance learning. Another hypothesis suggests that patterns increase signal detection by creating many contrasting edges via lateral inhibition.

Aronsson and Gamberale-Stille conducted a study in order to determine the significance of patterns in aposematic prey selection by domestic chicks, Gallus gallus domesticus. The authors initially trained the chicks to forage for mealworm prey. Then the chicks were subjected to trials where bright coloration and two different patterns were used to indicate unpalatable prey which was soaked in quinine. Palatable prey items were placed atop grey rectangles of paper, whereas unpalatable prey were always placed atop cyan colored paper, that had either a striped or dotted pattern. Chicks were divided into two groups. In the seven discrimination learning trials, group 1 was subject to grey positive stimuli, and cyan negative stimuli with stripes. Group 2 was subject to the same positive stimuli, and cyan negative stimuli with dots. These two groups were then subject to a generalization test in which each group was divided into three subgroups (e.g. 1a, 1b, 1c, and 2a, 2b, 2c). The subgroups were subject either to the same stimuli as in training, a “color only” stimulus, or a “pattern only” stimulus.

The chicks were placed in cages with 32 prey options in sequence, half of which were palatable and half of which were not. The number of each prey type attacked by each individual was recorded. Based on the results, it was determined that the chicks showed no difference in discrimination learning between the two different patterns, and that in fact they responded significantly to “color only” negative stimuli, but not to “pattern only” negative stimuli. The pattern appeared to have more of a silent effect in terms of prey choice, whereas bright color was the primary learned signal.

In class we discussed how signals evolve. Initially a signal must evolve, such as patterning, and must then be associated with a condition, such as unpalatability. Receivers of the signal must then be able to learn and associate the signal with the condition, and then modify their decision making based on the info received from the signal. This study demonstrated that while color is clearly a signal used by chicks to determine which prey to consume, a contrasting pattern cannot necessarily be considered a signal because it is not clear if or how it is used by the receiver. Although this study failed to determine the role patterns play in aposematic coloration, the possibility exists that contrasting patterns evolved for different reasons, such as functioning as a mating signal. However, patterning is fairly ubiquitous among aposematicly colored creatures. Therefore, patterning may be used as a signal by older predators more than younger ones. Further research will have to be conducted to determine if predators of different age groups further distinguish aposematicly colored prey.

Aronsson, M., & Gamberale-Stille, G. (2008). Domestic chicks primarily attend to colour, not pattern, when learning an aposematic coloration. Animal Behaviour, 75, 417-423.