We've long thought of ourselves as monogamous, and indeed, of monogamy as a distinguishing characteristic of mankind, but Desmond Morris's suggestion in his bestseller The Naked Ape that human beings are a pair-bonding species looks increasingly weak. What's more, other species thought to be monogamous have been found to be rather less faithful than we had imagined:
Female swallows, for whom size is evidently important, have been observed having secret trysts with male swallows in possession of longer tails than their mates.
Geese, when going about the mate-selection business, sample the downy delights of up to six partners, in liaisons lasting from a few days to a few weeks. Around 60 per cent of birds, mostly males, use what is known as a "partner-hold" strategy - still clinging to an old partner while trying out a new one.
Oystercatchers are not always faithful to their long-term partners, indulging occasionally in what observers have called EPCs (extra-pair copulations). For the males, an EPC offers extra reproductive success and the chance of a superior mate for future reproduction; for females, it might bring extra food from courtship feeding, help caring for her own chicks, and potential genetic improvements in her progeny.
Chimpanzee females are highly promiscuous. Gibbons are practically monogamous, and gorillas polygamous, one male dominating a harem of females. This spectrum of sexual behaviour is related to the weight of the testicles (relative to the male's body) in each species: chimps have large testes to ensure their sperm can compete by volume with rival males, while gorillas, who don't have to endure the competition, aren't so well endowed. The human species falls somewhere between these extremes, suggesting that women are naturally more sexually adventurous than society has been willing to allow.