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 biology 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 peptide argenine vasopressin (AVP), dopamine, and oxytocin 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. When compared to montane voles, which are polygamous, monogamous prairie voles appear to have more of these AVP and oxytocin neurotransmitter receptors. It is important that these receptors are in the reward centers of the brain because that could lead to a conditioned parter in the prairie vole compared to the montane vole which would explain why the prairie vole forms pair bonds and the montane vole does not.
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"
Humans and pair bonding
Humans can experience some or all of the above-mentioned varieties of pair bonds in their lifetime. These bonds can be: temporary or last a lifetime, same-sex or cross-gender, and same age or with different age groups. In a biological sense there are two main types of pair bonds exhibited in humans: social pair bonding and sexual pair bonding. The social pair bond is a strong behavioral and psychological relationship between two individuals that is measurably different in physiological and emotional terms from general friendships or other acquaintance relationships. On the other hand, the sexual pair bond is a behavioral and physiological bond between two individuals with a strong sexual attraction component. In this bond the participants in the sexual pair bond prefer to have sex with each other over other options. Social pair bonds are usually more wide-ranging than their sexual counterparts due to the sexual nature involved in the latter. In humans and other mammals, these pair bonds are created by a combination of social interaction and biological factors including neurotransmitters like oxytocin, vasopressin, and dopamine.
It is important to note the significant difference between pair bonds and marriage in humans. Pair bonds (social and/or sexual) do not equal marriage. Pair bonds are a bio-social creation while marriage is best defined in a purely social context as a way of assuring reproduction, inheriting property, control, and recently in history as the outcome of romantic love. Marriage can be associated with sexual and social pair bonds; however, married couples do not necessarily have to experience both or either of these bonds. Marriage can be a consequence of pair bonding and vice versa; however, neither always creates or leads to the other. Especially, pair-bonding can happen between people of the same sex while marriage in Christianity and other major religions, as well as most countries' legal systems, has usually been restricted to heterosexual couples. Pair bonding in humans helps explain extreme "bonds" that we may share with others but are unable to articulate in terms of contemporary "love". 
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 montane voles' behavior to resemble that of monogamous prairie voles in experiments in which vasopressin receptors were introduced into the brain of male montane voles.
In the cichlid species Tropheus moorii, a male and female will form a temporary monogamous pair bond and spawn, after which the female leaves to mouthbrood the eggs on her own. T. moorii broods exhibit genetic monogamy (all eggs in a brood are fertilized by a single male).
- Affectional bond
- Animal sexuality
- Breeding pair
- Human bonding
- Monogamous pairing in animals
- r/K selection theory
- "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.
- Steinwender, Bernd, Stephan Koblmüller, and Kristina M. Sefc. "Concordant Female Mate Preferences in the Cichlid Fish Tropheus Moorii." Hydrobiologia 682.1 (2011): n. pag. 31 May 2011. Web. 22 Oct. 2013. <http://link.springer.com/article/10.1007/s10750-011-0766-5>.