Mimicry
Animals that are toxic, armed with spines, or are otherwise unpalatable, or disagreeable, to predators often exhibit conspicuous colors or patterns. These superficial characteristics, called aposematic signals, are used to warn potential predators of the animal's physical or chemical defenses. The distinct colorations are highly conspicuous against certain backgrounds. This imposes a cost on the aposematic prey because the predators can more readily spot them. However, the predators will also be quick to learn and remember which prey to avoid because of the distinctiveness of the signals. Of course, if the prey is not sufficiently unpalatable, then the costs are greater than the benefits for the attacked animal.
To evaluate whether the benefits outweigh the costs of any predator/prey strategy, the Evolutionary Stable Strategy (ESS) should be taken intoaccount. The ESS is a strategy that, when common among members of a particular role—such as aposematic coloration of prey—is not invaded or displaced over evolutionary time by any rare alternative. In addition, at evolutionary stability, each role has its own ESS.
A monarch butterfly (left) and a viceroy butterfly. The viceroy evolved to look like the monarch as a defensive mechanism.An important aspect of aposematic signaling is the evolution of another defense, mimicry. A mimic is a predator or prey that bears a superficial resemblance to another species. The mimic resembles the model, which exhibits aposematic coloration. There are two forms of mimicry: Batesian mimicry and Mullerian mimicry.
In Batesian mimicry, a palatable species mimics an unpalatable model, thereby gaining protection through the traits of another species. For example, juveniles of the harmless lizard species Heliobolus lugubris, inhabitants of the Kalahari Desert of southern Africa, mimic the color and posture of the ooglister beetle, a species that sprays noxious fluids at predators. As the lizards mature and grow larger than the average beetle, they develop cryptic coloration as a line of defense.
Another example is the monarch butterfly. Monarch butterflies store cardiac poisons acquired from milkweed plants they eat as larvae and are therefore distasteful and potentially harmful to other species. Viceroy butterflies, by contrast, are harmless and palatable and so need a good defense to ward off predators. The viceroy is protected by having wings of the same coloration pattern as the monarch butterfly. Many innocuous snakes mimic the conspicuous red, white, and black markings of the poisonous coral snake in an attempt to protect themselves.
It is important to note that for Batesian mimicry to be effective, there must be a larger population of models than mimics so that predators are not clued in to the fact that they are being tricked.
Mullerian mimicry involves two or more unpalatable, aposematically colored species that resemble each other in appearance. This strategy evolved so that predators will learn more quickly to avoid animals with particular warning signs.
Some predators practice a reverse mimicry in order to trap their prey. Some species of snapping turtles, for example, have tongues that resemble wriggling worms. By sticking their tongues in the water, these turtles are able to lure small fish that are looking for a meal of worms.
Although mimicry seems a rather straightforward tactic, several conditions must be met in order for it to function as an ESS. The first condition for the strategy to be successful is that very conspicuous signals of aposematic coloration should be avoided so that it is somewhat difficult for predators to learn to avoid the aposematically patterned prey. Second, increasing prey unpalatability should increase the chances that any attacked prey will survive because the predator will quickly learn species avoidance after attacking the prey. It is important to note that the degrees of unpalatability and signal conspicuousness at ESS depend on the predators' patterns of learning.
Despite all the warning patterns and colors, predators do sample aposematic prey on occasion. And although Mullerian mimicry is found among quite a few species, Batesian mimicry is considered a rare defensive strategy. The balance between the employment of mimicry and its success rate supports the notion that these aposematic signals are a relevant ESS.
Bibliography
Begon, Michael, John L. Harper, and Colin R. Townsend. Ecology, 2nd ed. New York: Blackwell Scientific, 1990.
Bradbury, Jack W., and Sandra L. Vehrencamp. Principles of Animal Communication. Sunderland, MA: Sinauer, 1998.
Campbell, Neil A. Biology, 3rd ed. Redwood City, CA: Benjamin/Cummings Publishing Company, Inc., 1993.
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