Pair bond

In biology, a pair bond is the strong affinity that develops in some species between the males and females in a pair, potentially leading to breeding. Pair-bonding is a term coined in the 1940s that is frequently used in sociobiology and evolutionary psychology circles. The term often implies either a lifelong socially monogamous relationship or a stage of mating interaction in socially monogamous species. It is sometimes used in reference to human relationships.

Monogamous voles, such as prairie voles, have significant differences in the density and distribution of vasopressin receptors in their brain when compared to polygamous voles. These differences are located in the ventral forebrain and the dopamine-mediated reward pathway.

Both vasopressin and dopamine act in this region to coordinate rewarding activities such as mating, and regulate selective affiliation. These species specific differences have shown to correlate with social behaviors, and in monogamous prairie voles are important for facilitation of pair bonding.

Varieties
According to evolutionary psychologists David Barash and Judith Lipton, from their 2001 book The Myth of Monogamy, there are several varieties of pair bonds:
 * Short-term pair-bond: a transient mating or associations
 * Long-term pair-bond: bonded for a significant portion of the life cycle of that pair
 * Lifelong pair-bond: mated for life
 * Social pair-bond: attachments for territorial or social reasons, as in cuckold situations
 * Clandestine pair-bond: quick extra-pair copulations
 * Dynamic pair-bond: e.g. gibbon mating systems being analogous to "swingers"

Examples
When discussing the social life of the bank swallow, Lipton and Barash state:

For about four days immediately prior to egg-laying, when copulations lead to fertilizations, the male bank swallow is very busy, attentively guarding his female. Before this time, as well as after—that is, when her eggs are not ripe, and again after his genes are safely tucked away inside the shells—he goes seeking extra-pair copulations with the mates of other males…who, of course, are busy with defensive mate-guarding of their own.

In various species, males provide parental care and females mate with multiple males. For example, recent studies show that extra-pair copulation frequently occurs in monogamous birds in which a "social" father provides intensive care for its "social" offspring.

A University of Florida scientist reports that male sand gobies work harder at building nests and taking care of eggs when females are present – the first time such "courtship parental care" has been documented in any species.