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Thursday, 9 December 2010
One of the advantages of being viviparous is that you know who your children are. Or at least, you do if you are female - if you gave birth to it, then it's yours. You know what your baby looks like; you know its smell.
This is the reason that brood parasitism is much more prevalent among birds than among mammals. Brood parasitism is a very efficient reproductive strategy - you can have cared-for offspring without the trouble of caring for them. But if you are a mammal, it's a lot harder to give live-birth in someone else's burrow and to get away with it than it is for a bird to lay an egg in some other bird's nest.
So why are brood parasites still comparatively rare, even among birds? There is clearly an evolutionary arms race between the parasite and the parasitised. To be effective, the parasite's egg must look as much like the host's egg as possible. To a certain extent, the parasite can take advantage of the superstimulus effect to lay a bigger egg than the host's, so the resulting chick can be stronger and fight its corner better. But there's a limit to this. If parasitism were too common, the superstimulus effect itself would be selected against in the host and cease to work.
Bird eyesight is good anyway, so there's a strong selection pressure on the hosts to be able to recognise the individual speckle patterns on their eggs. It might be thought that the host would also evolve an ability to count, but that is not much use: if you come back to six eggs when you know you laid five, which one do you smash? Also, giving eggs a characteristic smell is not helpful because it nullifies one of the principal advantages of being oviparous in the first place: predators can't smell your young.
However, all this assumes that there are two species competing. But if you are - say - a blackbird, it must also make sense for you to lay an egg in another blackbird's nest. You gain the same advantage for your chick as the cuckoo and the cow bird normally have over you, plus your deception is a lot harder to spot.
This puts a female bird in a similar (but not identical) position to a male mammal: a man (before DNA testing) was never certain that the child he was rearing was his, though a woman was. But the man can father other children that he does not need to care for. By using conspecific brood parasitism both a male and a female bird can have chicks that they don't need to care for, but neither is certain that the chick that they are rearing is their own.
In mammals the Y chromosome is asymmetrically passed exclusively down the male line. Birds have the opposite system: their W chromosome is asymmetrically passed exclusively down the female line. It would be interesting to look for similarities in the behavioural consequences of the possession of these two chromosomes given that egg-laying in the wrong nest puts female birds in a similar evolutionary position to a man fathering children by a woman other than his partner.
Indeed, maybe the Y and W chromosomal asymmetry itself is - in part - an evolutionary consequence of this similarity.