Monogamy is the condition in which a male and female form a close mutually supportive bond, involving a (relatively) exclusive sexual relationship, for a prolonged period.
Many birds are monogamous, but monogamy is very rare in mammals: only an estimated 2-3% of mammalian species practice monogamy. A conspicuous exception is the North American prairie vole, which does form lifelong bonds after mating - though in the case of the prairie vole this first mating is no trivial matter, involving 36-48 hours of more or less continual copulation. The female prairie vole remembers this, perhaps not surprisingly, but reassuringly this memory takes its expression in the fact that the female will subsequently choose to spend her time with the male she mated with rather than with other new males (this is not to say that she won't cheat occasionally; she will). The male after mating exhibits jealousy, in that he will be aggressive towards other males in the neighbourhood. The prairie vole male is a 'good father' in that he assists in maintaining the nest and caring for the young.
This social behaviour of praire voles is very different from the highly promiscuous asocial behaviour of the closely related montane vole. This difference between prairie voles and montane voles has been intensively studied; there are important differences between these species in the brain distribution of the receptors for oxytocin and vasopressin that appear to underlie these differences in behaviour.
It is generally accepted that humans, by their nature, tend to be monogamous, and it is thought that this behaviour is an adaption for ensuring the long post-natal support of dependent offspring. In mammalian species that are naturally polygamous, males tend to be much larger than females, a difference thought to be supported by sexual selection through the demands of competition between males for control of a group of breeding females; men tend to be larger than women, but the difference is less marked than in polygamous species. Humans are unusual in the extent of their sexual activity; in most mammalian species the female is only sexually receptive at the point in their ovarian cycle where they can be impregnated, whereas women are more or less receptive at all times in the menstrual cycle. It is speculated that this continually receptivity may be important in maintaining the pair bond, as sexual activity results in the release of oxytocin within the brain.