Sunday, February 8, 2004

Masters of mimicry
Nutan Shukla

Some birds lay eggs in other birdsí nests
Some birds lay eggs in other birdsí nests

Animals who have no special means of protection against predators defend themselves by closely resembling and mimicking others that are distasteful or unpleasant to the attackers. Predators learn that it is best to avoid eating a distasteful animal with a particular colour pattern. But in avoiding the genuinely unpleasant prey, the predators avoid the mimics as well, without realising that some of them may be quite harmless and even make for a good meal. Familiar examples in temperate countries are flies that have striped bodies and look just like bees or wasps. The advantage to an edible animal in outwardly resembling a much-less edible animal, to which it may not at all be related, was first recognised in 1862 by H.W. Bates. Hence it came to be known as Batesian mimicry.

There will, of course, only be an advantage to the mimic if there are many more real models than the mimics, so that young predators not acquainted with the warning colours are more likely to try to attack the inedible models than the mimics. If there are more mimics, the predators might soon learn to associate a particular colouring with edibility and attack the mimics instead.

There are many examples of Batesian mimicry in the tropics. Among the distasteful and aggressive birds, the drongo, lives in savanna areas and is often found in the company of the similar looking black fly-catcher. The latter seems to be the mimic of the former. In South-East Africa, orioles are usually found in the company of unrelated but similar-looking friar birds, which are not only noisy but also highly aggressive. This is again a case of Batesian mimicry wherein orioles take advantage by mimicking their companions. Likewise, there are several examples in the world of insects, reptiles and fish.

Later, F. Muller pointed out a different type of mimicry in 1879. His argument was that while young predators learn to avoid distasteful species, a certain number of creatures with warning colours are bound to be killed by them. But if two species, equally distasteful and offensively coloured, resembled each other closely, fewer of each species would be killed than if the predators had to learn to avoid two separate warning colour patterns. This idea has been proved experimentally, for the black and yellow striped caterpillars of the cinnabar moth are not attacked even by those predators that are not familiar with them, provided they have already tried eating wasps. This is Mullerian mimicry, with all the species concerned not only warningly coloured but also having some special means of protection from attackers. Another difference from Batesian mimicry is that all the species involved, being equally distasteful, can be equally numerous.

In tropical Africa, there are two related species of butterflies, which are found in very many different forms with their own distinct colour patterns. In any one place, however, the patterns of the two species are very similar. Both are known to be extremely distasteful to birds, and provide a good example of Mullerian mimicry.

A further type of mimicry involves the eggs of birds such as cuckoos that are laid in the nests of other birds. These eggs closely resemble those laid by the owners of the nest, who are deceived into breeding the eggs as their own and subsequently bringing up the young birds. This brood parasitism, practised by several groups of birds, is most highly developed among cuckoos. The parasitic species of the European cuckoo, lays eggs that are rather smaller than those of their closest non-parasitic relatives - their eggs are about the same size as those of the usual foster parents, such as meadow pipits, reed warblers and redstarts. In some areas, the cuckoo will lay several distinctly different types of eggs, each resembling the eggs of one species of the unwitting foster parent.

This rather extreme example is typical of animal camouflage in that one animal is deceived by another to the advantage of the deceiver. This does not mean that a camouflaged species will automatically survive while the species it is deceiving dies out. One might ask why the mimics and their distasteful models have not starved their predators out. The reason is camouflage and mimicry are forms of adaptation to the environment and all successful species, including the predators of the mimics, are adapted to their environment or they would not be able to survive. Predators may have another source of food or perhaps, if very hungry, they will even eat something relatively distasteful, with warning colours.

Whatever the exact situation, camouflage and mimicry are very fine example of adaptation and show just what elaborate systems have developed to fit animals to their various ways of life.

This feature was published on February 1, 2004