Animal pair bonding

In biology, a pair bond is the strong affinity that develops in some species between the male and female in a breeding pair. Pair-bonding, from 1940, is a term frequently used in sociobiology and evolutionary psychology circles and is typically meant to imply either a life-long monogamous relationship or a stage of mating interaction in socially monogamous species. It is sometimes used in reference to human relationships.

Pair bonding is also sometimes seen between individuals of the same sex, as demonstrated by behavior similar to that of male-female pair-bonded individuals.

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
 * Life-long pair-bond: mated for the life of that pair
 * 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: