Pair-bonding
Pair-bonding refers to the social and reproductive associations formed between males and females in the animal kingdom, serving various purposes in mating systems. It manifests in different forms, including monogamy, where a pair remains exclusive to each other, and polygamy, where individuals may mate with multiple partners. Pair bonds can last for varying durations, from a single reproductive season to a lifetime commitment, and have been observed across many vertebrate species, with notable prevalence in birds. For example, while most birds engage in some form of pair bonding, species like swans and albatrosses may maintain lifelong bonds, whereas others, such as the American robin, typically bond only during a single breeding season.
In contrast, pair-bonding is less common among fish, with the seahorse being a notable exception that exhibits daily social interactions during the female's visits to the male during pregnancy. In mammals, polygamy dominates, with only a small percentage, such as gibbons and prairie voles, displaying monogamous pair-bonding. The formation and maintenance of these bonds often involve courtship behaviors, physical proximity, and cooperative parenting, all contributing to the strengthening of the relationship. Pair-bonding behaviors are integral to the reproductive success of many species, influencing the social dynamics within animal populations.
Pair-bonding
In the animal world, there is a great variety of ways in which males and females pair or associate for reproductive purposes (called mating systems). While some animals pair with more than one member of the opposite sex (polygamy), others pair exclusively with only one mate (monogamy). While some animals associate only long enough to copulate (promiscuity), other animals form pair bonds that last for varying lengths of time, from one reproductive period (until young leave the nest) to a lifetime. While some pair-bonding has been observed in all vertebrate classes and even in some invertebrates (some crabs and insects), it is particularly common in birds while infrequent in fish and mammals.
Pair-Bonding in Birds
Most bird species are monogamous, although some are polygamous, and a few may be promiscuous. In some birds, such as prairie chickens, male and female pair only for copulation, after which the female is on her own to nest and parent. However, most birds pair bond, remain with their mates, and cooperate in some way until their young can leave the nest and survive independently. Some birds, such as songbirds, pair bond for only a single breeding season, while others, such as swans, geese, penguins, and albatrosses, exhibit pair-bonding for more than one season, sometimes for life.
The American robin, for example, is mostly monogamous while it pair-bonds and shares parenting for one breeding season. Typically, in the spring, male robins are the first to migrate north. When the females arrive about a week later, the males have already selected territories and call or carol as though advertising for a mate. Courtship, involving three types of songs (only the males sing) and feeding, leads to mating and pair-bonding. The female builds a nest and lays her clutch of usually three or four eggs. While the female incubates the eggs, the male stands guard nearby, though he may help with the incubation as well. After the eggs hatch, both parents forage and feed the young. When the young leave the nest, they still need approximately two weeks of feeding and parental care before they are mature enough to survive independently. While the father provides this care, the mother repairs the nest and lays another clutch of eggs. American robins typically have two or three broods per year. In this case, the pair bond usually endures for the breeding season but not through migration south for the winter and the nonbreeding season.
Pair-Bonding in Fish
Most fish do not form lasting pair bonds. An interesting exception, however, is the seahorse. At the beginning of the seahorse breeding season, males and females court for several days. The female produces eggs, and when they reach maturity, she deposits them into an egg pouch on the male seahorse’s trunk. Then she swims away. Male seahorses typically remain in a habitat of approximately one square meter, while females range over an area that may be a hundred times as large. During the male’s pregnancy, the female returns daily for a short, five- to ten-minute morning visit. During this visit, the seahorse pair exhibit social interaction and bonding behaviors reminiscent of courtship. Afterward, the female swims away until the next day, when she returns for another visit. After two to three weeks of pregnancy, the male gives birth to a few dozen or more baby seahorses during the night. When his mate returns in the morning for their daily visit, the male is ready for courtship, and the mating and reproductive cycle starts over again. Females have been observed to refuse to mate with other males during their mate’s pregnancy and so appear to be monogamous for the breeding season. In laboratory experiments, where mates were separated during pregnancy and the female interacted with another male, the original pair bond was broken. Thus, the social interaction and bonding behaviors that occur during the seahorse couple’s short daily visits appear to be important factors in the maintenance and longevity of pair-bonding.
Pair-Bonding in Mammals
Polygamy is the most common mating system among mammals. Within polygamous (or promiscuous) groups, such as baboons and chimpanzees, a male and female may pair temporarily and separate themselves somewhat from the group while engaging in social grooming and sexual activity. Temporary pairings that do not endure to the end of a reproductive cycle are referred to as consort pairs or consortships in the primate literature. When pair-bonds in a polygamous system are more lasting, the social structure is sometimes called a harem. Several species of primates, seals, and deer are known to form harems.
Only a small minority (3 to 5 percent) of mammals exhibit monogamous pair-bonding. Gibbons, the smallest of the apes, are particularly interesting because they are the exceptional case and exhibit not only long-term monogamous pair-bonding but a social organization somewhat analogous to the human nuclear family. Gibbons pair at eight to ten years of age and have five to six offspring, spaced about three years apart, over their ten- to twenty-year reproductive lifetime. Gibbon offspring remain with their family groups until they approach or reach sexual maturity, when they may leave voluntarily or be evicted by the same-sex parent. Gibbons are territorial, and family members cooperate as needed to defend both territory and mate or family. Just before sunrise every morning, mated males sing solo songs that can be heard up to a kilometer away, seemingly identifying their territory as occupied. Later in the morning, mated females sing their own songs and join their mates in singing duets, which appear to publicize both territory and pair-bonding.
Rodents have been used in studies to better understand human pair-bonding and therefore demonstrate their own capabilities to bond. One group, the monogamous prairie vole, is often used as a model system to study pair bonding. They first pair and then mate within an hour. Their bond is stabilized between twelve and twenty-four hours, and they typically cohabitate for life. After one of the partners dies, the other is 80 percent likely to live alone for the remainder of its life.
Pair-Bond Formation and Maintenance
Formation of a pair-bond usually involves the behavior patterns of courtship and mating. Usually, the male initiates pair formation, while the female decides whether a bond is formed or not. How long a pair-bond endures depends upon various factors. Many animal pairs maintain and strengthen their relationship by continuing the bonding behaviors that were initially used in courtship. Some animals maintain close physical proximity, groom each other, communicate with movement (display) or with vocalizations (call or song), or share food, nests, and territory. Reproductive success and dependency of young also appear to maintain pair-bonding.
Principal Terms
Bond, Bonding: the tie or relationship between opposite-sex partners in a pair bond
Bonding Behaviors: behavior patterns that establish, maintain, or strengthen the pair bond
Consort Pair, Consortship: a temporarily bonded pair within a polygamous group
Long-Term Pair bond: pair bonding that continues beyond a single reproductive period
Monogamy: exclusive pair-bonding between one male and one female
Pair, Pairing: may refer to mating, sexual coupling (or copulation), or to the formation of a pair bond, depending upon the context
Polygamy: a mating system in which one male mates with several females (polygamy) or one female mates with several males (polyandry)
Promiscuity: a mating system in which sexual partners do not form lasting pair bonds, where their relationship does not persist beyond the time needed for copulation and its preliminaries
Bibliography
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