mimicry The evolution of mimicrybiology

There is considerable experimental evidence to illustrate how effectively predators learn to avoid certain adverse stimuli. Chickens conditioned by electric shock to avoid drinking dark green water drank progressively more from paler solutions in proportion to the intensity of the colour. This experiment suggests that even an incomplete warning system provides a modicum of protection. The degree of protection provided is also affected by the strength of the punishment; after strong shocks the chickens drank only from very light coloured solutions. In the presence of severe punishment, an improved warning system made little additional effect once a threshold level was reached.

In other experiments, starlings (Sturnus vulgaris) were fed normal mealworms, two segments of which had been painted orange. To provide aposematic “models,” the experimenter made other mealworms distasteful and painted the same segments green. “Mimics” were marked with green but not rendered unpalatable. There is no known instance in nature in which animals employ green for warning; there was therefore no possibility that the birds had already learned to avoid the experimental colour pattern. Before long the green-marked worms were completely avoided, regardless of palatability, even when the ratio of edible to distasteful was 60:40. This indicates that the number of mimics can exceed that of the model, when the resemblance is close, without loss of protection. When the ratio was increased to 90:10, 17 percent of the mimics were avoided, probably sufficient to a selective advantage in nature. Although a test bird would occasionally peck at a model, then reject it, the same action was sometimes shown to a mimic that it had picked up, suggesting that a premature response had been subsequently corrected.