Affiliation in Nonhuman Animals

Measuring Affiliation in Other Species

A common measure of affiliation is simply time spent together. If two organisms have a choice of several potential companions and consistently select particular individuals for association, this hints at some affiliative relationship. More specific behavioral measures can include frequency or duration of direct physical contact, such as huddling or grooming behavior (i.e., when one individual combs through the feathers or hair of another using its beak, claws, mouth, or fingers). Nonconceptive sex or mounting behavior may also indicate affiliation. Animals with close social affiliations may mark the relationship through shared songs or calls, as is seen in many birds; through learning the partner's signature whistle, as in dolphins; or converging on a mutual variation of a call type, as seen in bats and some nonhuman primates.

Separation and reunion tests provide information on the emotional component of affiliation. Increased vocalizations or increased agitation when separated from a specific individual and increased affiliative behavior (compared to baseline) following a reunion can index the strength of a relationship. Comparing the reaction of an animal placed in a novel environment with or without its partner illustrates the potential stress-reducing effect of an affiliative relationship. Exposing a young individual to a fear-inducing stimulus and noting the social response can indicate the preferred partner of the young.

Several physiological markers can measure affiliation. Grooming behavior in talapoin monkeys has been shown to increase levels of naturally [Page 55]occurring opiates secreted within the brain. Touching, grooming, and sexual interactions can lead to increased levels of oxytocin, a peptide hormone previously thought to be involved only in uterine contractions and nursing but is now known to be an important hormone in adult pair-bonding for socially monogamous animals (such as prairie voles, small monogamous rodents) and for parent- offspring relationships. Glucocorticoids (cortisol and corticosterone) secreted from the adrenal gland are associated with both physical and psychological stress, and affiliative relationships can modulate these hormones. Pair-bonded monkeys display much lower agitation and cortisol levels when they are with their partner in a novel environment, and hearing recordings of the vocalizations of partners can reduce the physiological response.

Varieties of Affiliative Experience

Affiliation patterns vary as a function of the social structure and kinship structure of different species. The affiliative relationships that are of greatest value are those that will lead to the greatest reproductive success, which can be measured both by the number of offspring of an individual that survive to reproduce and through the number of relatives that are reproductively successful. Thus, affiliation with close kin should lead to higher reproductive output either for the individual and/or any close relatives. Kinship structure varies in different species. In some species, such as most Old World monkeys (baboons, macaques) and many other mammals, males migrate and females remain in the natal group, meaning that there will be female kin in the group. In other species, such as chimpanzees and many New World monkeys, males stay in the natal group and females leave, creating a high potential for male kin relationships. In some cases, related males migrate together, as in lions, so that a lion pride typically has several closely related females and several closely related males.

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