The Folly of Fools by Robert Trivers Read Free Book Online Page B

COEVOLUTIONARY STRUGGLE
     
    A very rich illustration of coevolutionary principles is found in the relationships between brood parasites and their disadvantaged hosts, especially in birds but also in ants. A surprising percentage of all bird species, about 1 percent (usually cuckoos and cowbirds but also including a species of duck), is entirely dependent on other species to raise their young. Naturally this arrangement is rarely to the advantage of the “host” birds, who may end up raising unrelated young in addition to their own—or worse still, as is often the case, unrelated young instead of their own. This particular host/parasite relationship has been studied in unusual detail. Indeed, it is mentioned almost as early as human writing permits, some four thousand years ago in India, later described by Aristotle, and recently studied intensively by very clever field experiments designed to tease apart how the relationship works.
    The first move is for the deceiver to lay one of its eggs in the victim’s nest. This selects in the victim for the ability to recognize a strange-looking egg and eject it. This, in turn, selects for egg mimicry in the brood parasite—the tendency to produce eggs that have the same spotting and coloration as the eggs of the species whose parental care is being borrowed. Some parasitic species lay in the nests of multiple species, with individual species specialized to lay eggs that match in coloration the eggs of the species in whose nest they are laying. It is now advantageous for the host to be able to count total number of eggs, and reject nests with one too many. This is especially valuable if the parasite’s young hatches before those of the host, ejecting all of its eggs so as to monopolize parental investment, leaving the host no offspring of its own to rear. Better for the host to start over. This selects for parasites that remove one egg for each one laid, leaving total number the same, and the egg is eaten or moved some distance from the nest, perhaps to hide the crime.
    Once the egg has safely hatched, selection may favor brood-parasite mouth colors that resemble those of the host species, since parents feed more strongly mouth colors that resemble those of their own species. Within their own brood, evidence from other birds suggests that mouth color may be brighter for healthier chicks, so it is interesting that brood parasites make their mouth colors especially bright. By pushing out its foster siblings, the host young can monopolize parental investment, but since parents adjust their feeding to the total begging calls they hear, a single cuckoo chick may evolve to mimic the calls of an entire brood of the host. In an even more bizarre twist, a species of hawk cuckoo that parasitizes a hole-nesting species in Japan has evolved inner-wing patches that resemble the throat coloration of its host, so that when begging for food, a chick can flap its wings and simulate the begging of three offspring instead of one. The wing patches are even occasionally fed, a case of deception being too convincing for its own good.
    A very important selective factor is errors in recognizing a host’s own offspring—so-called false positives—that are an inevitable feature of any system of discrimination (see spam versus anti-spam in Chapter 8). For weak systems of discrimination, a host rarely rejects itself, but it is fooled too often into accepting cowbird chicks. Stronger systems of discrimination cut down on the host’s loss due to the cowbirds but also impose a cost on the host, as it inevitably accidentally rejects its own offspring more of the time. In reed warblers, parents learn the appearance of their own eggs and then reject those differing by a certain amount. If their nests are parasitized about 30 percent of the time, it makes evolutionary sense for them to reject strange eggs, but if they are parasitized less often, the cost in destruction of their own eggs is too great. Sure