In biology, a pair bond is the strong affinity that develops in some species between a pair consisting of a male and female, or in some cases as a same-sex pairing, potentially leading to producing offspring and/or a lifelong bond. 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 significantly greater 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.
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"
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.
As noted above, different species of voles vary in their sexual behavior, and these differences correlate with expression levels of vasopressin receptors in reward areas of the brain. Scientists were able to change adult male meadow voles' behavior to resemble that of monogamous prairie voles in experiments in which vasopressin receptors were introduced into the brain of male meadow voles.
See also 
- Affectional bond
- Animal sexuality
- Breeding pair
- Human bonding
- Monogamous pairing in animals
- r/K selection theory
- "Pair-bond". Oxford English Dictionary. 2005.
- Lim MM, Wang Z, Olazábal DE, Ren X, Terwilliger EF, Young LJ (June 2004). "Enhanced partner preference in a promiscuous species by manipulating the expression of a single gene". Nature 429 (6993): 754–7. doi:10.1038/nature02539. PMID 15201909.
- Barash, D.; Lipton, J. (2001). The Myth of Monogamy: Fidelity and Infidelity in Animals and People. New York: Henry Holt and Company. ISBN 0805071369.
- "New Study Explores The Evolution Of Male Parental Care And Female Multiple Mating". ScienceDaily.
- "For A Male Sand Goby, Playing 'Mr. Mom' Is Key To Female's Heart". ScienceDaily.