The development of DNA identification has given scientists potent new tools for discovering the genetic relationships between animal parents and their offspring. In recent years this has led to some eye-opening revelations about monogamy and infidelity in the animal world -- particularly in birds, which have traditionally been thought to form monogamous pairs for child-rearing.
Because avian offspring require a lot of parental care -- incubating the eggs and feeding the nestlings -- it seemed to both parents' advantage to be hardworking, faithful partners. Scientists using DNA "fingerprinting" have discovered instead that a surprising number of eggs in birds' nests contain another male's genes. Behind the appearance of monogamy, "sexual fidelity is hard to find," as science author Virginia Morell put it.
"Social" monogamy -- staying together for the sake of the kids -- is one thing. But among birds, scientists are finding, females are sneaking off with other males whose offspring are then raised by the female and her unknowing partner.
Why the deception? A leading theory is that it provides a sort of evolutionary insurance policy for the female bird. By adding some male genes from outside her "family," she's adding some genetic variability that could enable some of her offspring to survive if the environment should change. The female appears to choose "high-quality" males endowed with desirable traits for these "extra pair" matings.
For example, female great reed warblers of Scandinavia select males with a larger repertoire of songs, which have been shown to father healthier offspring.
Studies suggest that monogamy survives when the offspring are so dependent on both parents that it is in their interest not to stray if their offspring are to survive. Such situations, however, are in the great minority.