We think of ourselves as a monogamous species, so it seems strange that among our closest relatives, chimpanzees, bonobos, and gorillas, males and females do not form pairs but rather consort with a variety of mating partners. Few primates know who their father is. The most devoted couples among non-human species, instead, are birds of whom over 90 percent are monogamous (Adkins-Regan and Tomaszycki, 2007) – socially monogamous, that is; recent genetic research into the DNA of nestlings proves that many species have males and females who commonly mate with individuals other than their avowed partner. The necessity for social monogamy in birds is related to reproduction, with the laying of eggs by females rather than the production of live young who nurse from their mother. The eggs have to be protected during incubation and the insatiable nestlings fed until they are able to fend for themselves. This requires a huge amount of parental effort, especially for migratory species that have a limited season in which to rear young able to join in the fall southern migration. In long-lived birds especially, this effort can be best organized if the parents work together efficiently. If pairs of a migratory species are bonded, they can breed earlier in the season because they do not have to hunt for and court a partner; their young have more time to grow strong before they must fly south. A seasoned pair has learned how to raise their young effectively; for example, who will take which shift in incubating the eggs and then in foraging for food for the nestlings: “Each pair must work out its own rhythm, which takes some time to perfect and gives the advantage to established couples” (Burger, 2001). Bonded couples are far more likely to raise young successfully than are non-bonded pairs.

In social mammals when male and female young are born into the same litter, they tend to bond together as they grow up, constantly in each other’s presence. When they reach puberty, however, members of one sex or the other usually disperse from their natal group; this breaking of the brother–sister bond prevents inbreeding. In human beings, there is no such dispersal. An inborn incest taboo prevents a brother and sister from mating, but often the two remain close friends throughout their lives, which are enriched by this relationship.