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 enough, reed warblers are parasitized only 6 percent of the time in the UK and do not reject new eggs—unless a cuckoo is seen near the nest at about the right time (perhaps pushing probability above 30 percent). In one population, a drop in parasitism rate from 20 percent to 4 percent was matched by a one-third reduction in rejection rate, an effect too rapid to be genetic, so reed warblers probably often adjust their degree of discrimination to evidence of ongoing brood parasitism.
Note the important frequency-dependent effect. When almost all eggs are their own, discrimination will result in the warblers destroying some percent—say, 10 percent of their clutches—with only rare gain. But at 30 percent parasite frequency, they risk harming themselves only 7 percent of the time, while with perfect discrimination, they save themselves a substantial cost (in nurturing other species almost 30 percent of the time). At low frequency, deceivers are hardly worth detecting—only at high frequency are important defenses expected to kick in.
There is one striking peculiarity in the entire system. Birds repeatedly fail to evolve the ability to see that the cuckoo or cowbird chick bears no resemblance to their own chicks beyond mouth color and begging call. In size, a cuckoo chick is often six times or more larger than its host, so that a foster parent may perch on the shoulder of the chick it is about to feed. Since it would seem beneficial to note this absurd size discrepancy and act accordingly, why are birds, in species after species, unable to do so? The answer to the mystery is by no means certain, but there are some interesting possibilities. Failure to make the appropriate discrimination happens preferentially in species in which the brood parasite ejects its foster siblings before they hatch. Thus, if the parent learns the appearance of its own chicks by imprinting on the first ones produced, this will work fine if the first brood is its own, but it will prove fatal if at their first attempt they are parasitized. The host will imprint on the brood parasite and kill its own young whenever it sees them. This will wipe out the host’s entire lifetime reproductive success, since it will now see all of its own chicks as foreign.
More generally, some of the brood parasite’s characteristics are super-optimal from the foster parents’ standpoint. We expect parents often to favor the larger of their chicks as being healthier, stronger, and more likely to provide a good return on investment. This may make foster parents vulnerable to implausibly large chicks that nevertheless release the bias that bigger is better. More to the point, many brood parasites have evolved begging calls that are louder than the host’s and hence presumably harder to resist. Likewise, parasite mouth colors are especially brightly colored. These signals are less costly to magnify than is body size.
~~The Folly of Fools- The Logic of Deceit and Self-Deception in Human Life -by- Robert Trivers
No comments:
Post a Comment