Social grooming
Social grooming is a behavior in which social animals, including humans, clean or maintain one another's bodies or appearances. A related term, allogrooming, indicates social grooming between members of the same species. Grooming is a major social activity and a means by which animals who live in close proximity may bond, reinforce social structures and family links, and build companionship. Social grooming is also used as a means of conflict resolution, maternal behavior, and reconciliation in some species. Mutual grooming typically describes the act of grooming between two individuals, often as a part of social grooming, pair bonding, or a precoital activity.
Evolutionary advantages
There are a variety of proposed mechanisms by which social grooming behavior has been hypothesized to increase fitness. These evolutionary advantages may come in the form of health benefits including reduction in disease transmission and stress levels, maintenance of social structures, and direct improvement of fitness as a measure of survival.Health benefits
It is often argued as to whether the overarching importance of social grooming is to boost an organism's health and hygiene or whether the social side of social grooming plays an equally or more important role. Traditionally, it is thought that the primary function of social grooming is the upkeep of an animal's hygiene. Evidence to support this statement involves the fact that all grooming concentrates on body parts that are inaccessible by autogrooming, and that the amount of time spent allogrooming regions did not vary significantly even if the body part had a more important social or communicatory function.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. On the other hand, it could be argued that the hygienic aspect of allogrooming does not play as important a role as the social aspect of it. Observational studies performed on 44 different primate species suggest that the number of times a species allogrooms, on average, correlates with its group size rather than with its body size. If allogrooming was required from a purely hygienic standpoint, then the larger an animal, the more and more often it would be groomed by members of its group. However, it has instead been seen that when group size increases, members ensure that they spend an appropriate amount of time grooming everyone. Hence, the fact that animals, particularly primates, groom each other more frequently than necessary from a hygienic standpoint suggests that the social aspect of allogrooming plays an equally, if not more, important role. Another point of evidence for the importance of the social aspect is that in comparison to how and how much a primate grooms itself, allogrooming involves longer periods of time and different techniques, some of which have connotations of being affectionate gestures.
Reinforcing social structure and building relationships
Creation and maintenance of social bonds
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" due to long periods of time spent together doing activities. In primates especially, grooming is known to have major social significance and function in the formation and maintenance of these friendships. Studies performed on rhesus macaques showed that fMRI scans of the monkeys' brains lit up more significantly at the perirhinial cortex and the temporal pole when the monkeys were shown pictures of their friends' faces, compared to when they were shown less familiar faces. Hence, primates recognize familiar and well-liked individuals and spend more time grooming them than less favoured partners. In species with a more tolerant social style, such as Barbary macaques, it is seen that females choose their grooming mates based on whom they know better rather than on social rank. In addition to primates, animals such as deer, cows, horses, voles, mice, meerkats, coatis, lions, birds, and bats also form social bonds through grooming behavior.Social grooming may also serve to establish and recognize mates or amorous partners. For example, in short-nosed fruit bats, the females initiate grooming with the males just before flight at dusk. The male and his close-knit female harem release bodily secretions onto each other, which may allow them to recognize the female's reproductive status. A 2016 study by Kumar et al. chemically analyzed these secretions, concluding that they may be required in chemosensory mediated communication and mate choice. Similarly, in the less aggressive herb-field mouse, males are observed to groom females for longer durations and even allow females to not reciprocate. Since the mating demands of males are greater than those offered by females, the females use social grooming to choose mates and males use it to incite mating.
Finally, kin selection is not as important a factor as friendship or mate preference when choosing a grooming mate as previously thought. In a 2018 study of captive chimpanzees, Phelps et al. found that the animals remembered interactions that were "successful" or "unsuccessful" and used these as a basis to choose grooming mates; they chose grooming mates based on who would reciprocate rather than who would not. More importantly, if the delay between two chimpanzees grooming each other is very little, then the chimpanzees tend to "time match": i.e., the second groomer grooms the first for the same amount of time that he/she was groomed. This "episodic memory" requires a demanding amount of cognitive function and emotional recognition, and has been tested experimentally with respect to food preferences, where apes chose between tasty perishable and non-tasty non-perishable food at shorter and longer delays respectively after trying the food. Hence, apes can distinguish between different events that occurred at different times.
Enforcing hierarchy and social structure
In general, social grooming is an activity that is directed up-hierarchy—i.e., a lower ranking individual grooms a higher ranking individual in the group. In meerkats, social grooming has been shown to carry the role of maintaining relationships that increase fitness. Researchers have observed that in this system, dominant males receive more grooming while grooming others less, thereby indicating that less dominant males groom more dominant individuals to maintain relationships. In a study conducted on rhesus monkeys, it was seen that more dominant group members were "stroked" more than they were "picked at" when being groomed, compared to lower-ranking group members. From a utilitarian standpoint, stroking is a less effective technique for grooming than picking, but it is construed as being a more affectionate gesture. Hence, grooming a higher ranking individual could be done in order to placate a potential aggressor and reduce tension. Moreover, individuals closer in rank tend to groom each other more reciprocally than individuals further apart in rank.Grooming networks in black crested gibbons have been proven to contribute to 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.
Interchange of favours
Grooming is often offered by an individual in exchange for a certain behavioral response or action. 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. In Tibetan macaques, infants are seen as a valuable commodity that can be exchanged for favours; mothers allow non-mothers to handle their infants for short durations in exchange for being groomed. Tibetan macaques measure and perceive the value of the infants by noting the relative ratio of infants in the group; as the number of infants increase, their "value" decreases, and so does the amount of grooming performed by non-mothers for mothers in exchange for infant-handling.It has been suggested that in male bonobos, grooming is exchanged in favour of some emotional component because grooming familiar individuals involves larger time differences and reduced reciprocity. Hence, the presence of some sort of social bond between individuals results in greater "generosity" and tolerance between them.
Direct fitness consequences
Social grooming relationships have been proven to provide direct fitness benefits to a variety of species. In particular, grooming in yellow baboons 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, highlighted 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 baboons. Direct correlations between measures of social connectedness and median survival time for both female and male baboons were modeled.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 were shown to augment fitness by increasing tolerance from more dominant group members and increasing the chance of obtaining aid from conspecifics during instances of within-group contest interactions. In the yellow baboon, 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 oftentimes cause physical harm. Furthermore, social relationships have also been proven to decrease risk of infanticide in several primates.