Social grooming behavior presents several evolutionary advantages which come in various forms. Extensive research has been conducted on these advantages, and has aided behavioral ecologists in understanding this behavior through an evolutionary perspective. There are a variety of proposed mechanisms for how social grooming enhances fitness.
Reinforcing social structure and building relationships
One of the most critical functions of social grooming is to establish social networks and relationships. In many species, individuals form close social connections dubbed “friendships.” In primates especially, grooming is known to have major social significance and function in the formation and maintenance of these friendships. Grooming networks in black crested gibbons have been proven to contribute greater social cohesion and stability. Groups of gibbons with more stable social networks formed grooming networks that were significantly more complex, while groups with low stability networks formed far fewer grooming pairs. In meerkats, social grooming has been shown to carry the role of maintaining relationships that increase fitness. In this system, researchers have observed a phenomenon in which dominant males receive more grooming, while grooming others less, thereby demonstrating that less dominant males groom those who are more dominant in order to maintain these valuable relationships. In addition to primates, though far less studied, animals such as deer, cows, horses, vole, mice, meerkats, coati, lions, birds, bats also form social bonds through grooming behavior. Social grooming is critical for vampire bats especially, since it is necessary for them to maintain food-sharing relationships in order to sustain their food regurgitation sharing behavior.
Social grooming relationships have been proven to provide direct fitness benefits to a variety of species. In particular, the Papio cynocephalus baboon species has been studied extensively, with numerous studies showing an increase in fitness as a result of social bonds formed through social grooming behavior. One such study, which collected 16 years of behavioral data on wild baboons, highlights the effects that sociality has on infant survival. A positive relationship is established between infant survival to one year and a composite sociality index, a measure of sociality based on proximity and social grooming. Evidence has also been provided for the effect of sociality on adult survival in wild female baboons. A direct correlation between measures of social connectedness (which focuses on social grooming) and median survival time was modelled. In humans, social connection has been shown to positively influence both health and survival.
Social bonds established by grooming may provide an adaptive advantage in the form of conflict resolution and protection from aggression. In wild savannah baboons, social affiliations are shown to augment fitness by increasing tolerance from more dominant group members and increasing chance of obtaining aid from conspecifics during instances of within-group contest interactions. In this highly studied baboon species, adult females form relationships with their kin, who offer support during times of violent conflict within social groups. In Barbary macaques, social grooming results in the formation of crucial relationships among partners. These social relationships serve to aid cooperation and facilitate protection against combative groups composed of other males, which can often times cause physical harm. Furthermore, social relationships have also been proven to decrease risk of infanticide in several primates.
Social grooming behavior has been shown to elicit an array of health benefits in a variety of species. For example, group member connection has the potential to mitigate the potentially harmful effects of stressors. In macaques, social grooming has been proven to reduce heart rate. Social affiliation during a mild stressor was shown to correlate with lower levels of mammary tumor development and longer lifespan in rats, while lack of this affiliation was demonstrated to be a major risk factor. Grooming has also been shown to play an integral role in reducing tick load in wild baboons (Papio cynocephalus). These ectoparasitic ticks carry the potential to act as vectors for the spreading of disease and infection by common tick-borne parasites such as haemoprotozoan. Baboons with lower tick loads show decreased occurence of such infections and display signs of greater health status, evidence by higher hematocrit (packed red cell volume) levels.
Altruism, in the biological sense, refers to a behavior performed by an individual that increases the fitness of another individual while decreasing the fitness of the one performing the behavior. This differs from the philosophical concept of altruism which requires the conscious intention of helping another. As a behavior, altruism is not evaluated in moral terms, but rather as a consequence of an action for reproductive fitness. It is often questioned why the behavior persists if it is costly to the one performing it, however, Charles Darwin himself proposes group selection as the mechanism behind the clear advantages of altruism in his book The Descent of Man, and Selection in Relation to Sex, (1871).
Social grooming is considered a behavior of facultative altruism- the behavior itself is a temporary loss of direct fitness (with potential for indirect fitness gain), followed by personal reproduction. This tradeoff has been compared to the Prisoner's Dilemma model, and out of this comparison came Robert Trivers reciprocal altruism theory under the title "tit-for-tat". In conjunction with altruism, kin selection bears an emphasis on favoring the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Because of this, kin selection is an instance of inclusive fitness, which combines the number of offspring produced with the number an individual can ensure the production of by supporting others, such as siblings.
Developed by W.D. Hamilton, this rule governs the idea that kin selection causes genes to increase in frequency when the genetic relatedness (r) of a recipient to an actor multiplied by the benefit to the recipient (B) is greater than the reproductive cost to the actor (C). Thus, it is advantageous for an individual to partake in altruistic behaviors, so long as the individual receiving the benefits of the behavior is related to the one providing the behavior.
It was questioned that some animals are instead using altruistic behaviors as a market strategy to trade for something deisrable. In a study on olive baboons, Papio anubis, it was found that individuals perform altruistic behaviors as a form of trade in which a behavior is provided in exchange for benefits, such as reduced aggression. The study found that grooming was evenly balanced across multiple bouts rather than single bouts, suggesting that females are not constrained to complete exchanges with single transactions and use social grooming to solidify long term relationships with those in their social group.
In addition, a study on wild white-handed gibbons (Hylobates lar) confirmed that males were more attentive to social grooming during estrus of the females in their group. Though the behavior of social grooming itself was not beneficial to the one providing the service, the opportunity to mate and subsequent fertilization certainly increases the reproductive fitness of those participating in the behavior. This study was also successful in finding that social grooming performance cycled with that of the females ovarian cycle.
Many social animals groom each other, in an activity known as social grooming, mutual grooming, or allo-grooming. Social grooming also takes the form of stroking, scratching, and massaging. Social grooming animals include primates, insects, birds, ungulates, and bats. While thorough research has still yet to be engaged, much has been learned about social grooming in non-human animals via the study of primates. The driving force behind mammal social grooming is primarily believed to be rooted in either adaptation to consolatory behavior or utilitarian purposes to improve hygiene, however, has also been determined as important in exchanges for food as well as sex.
In mammals, social grooming has been found to correlate to the brain's release of Oxytocin from its Hypothalamus. Known to be significant in humans for both social bonding as well as sexual reproduction, this conserved neural pathway has an influential role in birth, the formation of maternal bonds with newborns in non-human animals, and the establishment of an empathy mechanism throughout the Animal kingdom.
Primates provide perhaps the best example of this activity, also known as monkey-bonding. The trust and bonding built is critical to group cooperation. Among primates, social grooming plays an important role in animal consolation behavior whereby the primates engage in establishing and maintaining alliances as well dominance hierarchies, for building coalitions, and for reconciliation after conflicts. Primates groom socially in moments of boredom as well, and the act has been shown to reduce tension and stress. This reduction in stress is often associated with observed periods of relaxed behavior, and primates have been known to fall asleep while receiving grooming. Recent studies regarding yellow chimpanzee have determined that the release of Oxytocin in Social Grooming can be directly correlated to consolatory behavior. Vervet monkey siblings often have conflict over grooming allocation by their mother. Yet, grooming remains an activity that mediates tension and is low cost for alliance formation and maintenance. This form of social grooming as a consolation mechanism in animals has been found to not only be exclusive to primates. In 2010, researchers determined the existence of a form of social grooming as a consolation behavior within ravens via a form of bystander contact, whereby a raven who observes induced stress in another acts to console the distressed victim via contact sitting, preening, or beak-to-beak touching.
Social grooming in non-human animals has been exchanged for other resources, such as food, sex, and hygeine Animals such as wild babboons have also been found to utilize social grooming as an activity to remove ticks and other insects from others. Overall, primates tend to be better allogroomed in regions where the primates themselves have a more difficult time reaching. Grooming is utilized to remove parasites, dirt, dead skin, as well as tangled fur to help keep an animal's health in good condition despite an inability to reach and clean certain areas. Recent studies have shown that male crab-eating macaques will groom females in order to get sex. One study found that a female has a greater likelihood to engage in sexual activity with a male if he had recently groomed her, compared to males who had not groomed her.
Only a few empirical studies of human social grooming exist. Almost all the studies, except one recent cross-cultural study, rely on self-report survey and experimental methodology of adults, living primarily in the U.S. and other Western cultures. People report grooming romantic partners more than grooming people they have other types of relationships with such as family members, friends, and strangers. Grooming is associated with increased relationship satisfaction, trust, and experience of family affection while growing up. People who groom, as opposed to touching each other without grooming, are perceived to be better potential parents, more in love with the person they have groomed and more caring and committed to them. Women, but not men, tend to think people who have groomed one another are romantically involved. People also think that if people who have groomed one another are romantically involved, they are in a long-term relationship rather than one that has just begun.
A recent empirical study by Seinenu Thein-Lemelson (University of California, Berkeley) was the first to use ethological methods in order to examine cross-cultural differences in human grooming. Naturalistic data was collected through video focal follows with children during routine activities and then analyzed. The study indicates that there are significant cross-cultural differences in rates of caregiver-to-child grooming.
Social grooming has shown to be correlated with changes in endocrine levels within the body of individuals. Specifically, there is a large correlation between the brain's release of oxytocin and social grooming. Oxytocin is hypothesized to promote prosocial behaviours due to its positive emotional response when released. Further, social grooming also releases beta-endorphins which promote physiological responses in stress reduction. These responses can occur from the production of hormones and endorphins, or through the growth or reduction in nerve structures. For example in studies of suckling rats, rats who received warmth and touch when feeding had lower blood pressure levels than rats who did not receive any touch. This was found to be a result of an increased vagal nerve tone, meaning they had had higher parasympathetic nervous response and lower sympathetic nervous response to stimulus, resulting in a lower stress response. Social grooming is a form of innocuous sensory activation. Innocuous sensory activation, characterized by non aggressive contact, stimulates an entirely separate neural pathway from nocuous aggressive sensory activation. Innocuous sensations are transmitted through the dorsal column-medial lemniscal system.
Oxytocin is a peptide hormone known to help express social emotions such as altruism, which in term provides a positive feedback mechanism for social behaviours. For example, studies in vampire bats have shown that intranasal injections of oxytocin have increased the amount of allogrooming done by female bats. The release of oxytocin, found to be stimulated by positive touch( such as allogrooming), positive smells and sounds, can have physiological benefits to the individual. Benefits can include: relaxation, healing, and digestion stimulation. Further, reproductive benefits have been found such as studies in rats have shown that the release of oxytocin can increase male reproductive success. The role of oxytocin is important in maternal pair bonding, and is hypothesized to promote similar bonding in social groups as a result of positive feedback loops from social interactions.
Beta-endorphins are found in neutrons in the hypothalamus and the pituitary gland. Beta-endorphins are found to be opioid agonists. Opioids are molecules that act on receptors to promote feelings of relaxation, and reduce pain. Grooming stimulates the release of beta-endorphin, which is one physiological reason for why grooming appears to be relaxing. A study in monkeys shows the changes in opiate expression in the body, mirroring changes in beta-endorphin levels, influences desire for social grooming. In using opiate receptor blockades, which decrease the level of beta-endorphins, the monkeys responded with an increased desire to be groomed. Differently, when the monkeys were given morphine, the desire to be groomed dropped significantly. Beta- endorphins have been difficult to measure in animal species, differently from oxytocin which can be measured by sampling using cerebrospinal fluid, and therefore have not been linked as strongly with social behaviours.
Additionally, an article published in 1997 concluded that an increase in maternal grooming resulted in a proportionate increase in Glucocorticoid receptors on target tissue in the neonatal rat. Additionally, it was found that the receptor number was altered because of a change in both serotonin and thyroid-stimulating hormone concentrations. An increase in the number of receptors might influence the amount of negative feedback on corticosteroid secretion and prevent the undesirable side effects of an abnormal physiologic stress response.
Further studies have shown that male baboons who participate more in social grooming show lower basal cortisol concentrations.