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Allogrooming is a common affiliative behaviour with hygienic, physiological and social consequences, and has historically been examined in non‐human primates in order to understand the evolution of sociality. In primates, allogrooming is well‐known to have evolved for an antiparasitic function, leading to physiological consequences and further adoption of the behaviour for social functions. This includes exchanging benefits in a biological market, post‐conflict reconciliation and long‐term social bond formation. The relevant importance of these functions is largely undetermined in many non‐primate taxa, including ungulates.We aimed to evaluate the current evidence for allogrooming function in ungulates, asking whether the behaviour serves a social function, and if so, to what magnitude. Ungulates show large differences in their allogrooming behaviour, but the variation of these has not been truly understood.We find that although allogrooming in ungulates has been co‐opted for social purposes such as bond formation, the variety of these has not been fully explored. Identifying species‐specific allogrooming functions can help to clarify the context of behaviour; this allows us to improve our knowledge of how individuals and groups interact within a population, and understand the implications of this behaviour when deducing animal state and sociality.We conclude that there is great potential for additional research into the benefits of affiliative behaviour in ungulates, and we encourage further exploration into the social functions of allogrooming in ungulates. [ABSTRACT FROM AUTHOR]

Social interactions are fundamental properties of gregarious species, helping to establish dominance hierarchies and maintain social bonds within groups. Cattle, Bos taurus , are social ungulates which engage in affiliative and agonistic relationships with other individuals. Although there are approximately 1.5 billion cattle on the planet, the opportunity to research cattle behaviour in free-ranging groups is rare, as there are few feral populations worldwide. Cattle engage in positive social behaviours such as allogrooming, where one individual licks the body of another. The relationship between affiliative behaviours and other individual characteristics (such as sex and dominance status) are frequently studied in other gregarious species, but are largely undetermined in cattle. To investigate the relationships between sex, dominance status and allogrooming, we observed a mixed-sex feral cattle herd in Hong Kong, recording dominance interactions and allogrooming events. The hierarchy was linear and stable, consisting of 22 adult males and 33 adult females. We found that dominant females received more allogrooming than subordinate females, but subordinate females did not perform it more. Males performed allogrooming more towards females than other males, but females groomed both sexes equally. Sex affected dominance position, with males obtaining higher status than females, but not all females were subordinate to males. These preferential allogrooming patterns improve our knowledge of sex-specific interactions and help us to understand the dynamics of agonistic and affiliative behaviours in multimale, multifemale ungulate groups. Studying a free-ranging feral population provides us with a unique insight into ungulate behavioural patterns and the evolution of cattle social behaviours. • Cattle are social ungulates that engage in affiliative and agonistic behaviours. • Feral cattle present a novel opportunity to study free-ranging cattle behaviour. • Sex affected dominance status, and both affected allogrooming performance. • High-ranking animals received more allogrooming and exchanged preferential grooming. • Results aid our understanding of cattle sociality, dominance and positive welfare. [ABSTRACT FROM AUTHOR]

The social grooming model (SGM), which theorizes social media users' social grooming behaviors based on invested costs, is robust, reflecting various and nuanced social grooming styles. However, its core assumptions have not been validated. Using a nationally representative sample of 1,001 Taiwanese social media users, we explored costs and privacy for each social grooming behavior via a survey. Our results supported the hypotheses of the SGM. Users reported greater costs and reputational concerns for private topics than public topics, and higher costs for emotional and controversial topics than for informational and trending topics. With the new five styles identified in this study, social butterflies and meformers reported significantly greater social capital and well-being than lurkers; however, social butterflies reported greater invested costs in social grooming than meformers, indicating that being strategic is most efficient when it comes to social grooming, considering invested costs and the social benefits. SGM is robust and can reflect rich social grooming patterns. [ABSTRACT FROM AUTHOR]

Human joint action seems special, as it is grounded in joint commitment-a sense of mutual obligation participants feel towards each other. Comparative research with humans and non-human great apes has typically investigated joint commitment by experimentally interrupting joint actions to study subjects' resumption strategies. However, such experimental interruptions are human-induced, and thus the question remains of how great apes naturally handle interruptions. Here, we focus on naturally occurring interruptions of joint actions, grooming and play, in bonobos and chimpanzees. Similar to humans, both species frequently resumed interrupted joint actions (and the previous behaviours, like grooming the same body part region or playing the same play type) with their previous partners and at the previous location. Yet, the probability of resumption attempts was unaffected by social bonds or rank. Our data suggest that great apes experience something akin to joint commitment, for which we discuss possible evolutionary origins.

Oxytocin has attracted research attention due to its role in promoting social bonding. In bonobos and chimpanzees, the two species most closely related to humans, urinary oxytocin is known to correlate with key behaviours related to social bonding such as social grooming in chimpanzees and female‐female sexual behaviour in bonobos. However, no study has demonstrated that the administration of oxytocin promotes real‐life social interactions in Pan, leaving it unclear whether oxytocin is merely correlated with social behaviors or does affect them in these species. To test this, we administered nebulized oxytocin or saline placebo to a group of female bonobos and subsequently observed changes in their gross behavior during free interaction. We found an overall effect of more frequent grooming in the oxytocin condition. However, on the individual level this effect remained significant for only one participant in our follow‐up models, suggesting future work should explore interindividual variation. Our results provide some experimental support for the biobehavioural feedback loop hypothesis, which posits that some functions of the oxytocin system support the formation and maintenance of social bonds through a positive feedback loop; however, further tests with a larger number of individuals are required. Our results, at a minimum, demonstrated that oxytocin affects spontaneous, naturalistic social interactions of at least some female bonobos, adding to accumulating evidence that oxytocin modulates complex social behaviors of Pan. Highlights: We administered oxytocin or saline placebo to whole subgroups of bonobos simultaneously. Our overall model finds higher rates of grooming in the oxytocin condition, however, this was only significant in one participant in individual level analysis. This provides some support to the biobehavioural feedback loop hypothesis of oxytocin in bonobo social bonding, while future work should explore individual, group, and species level variation. [ABSTRACT FROM AUTHOR]

In anthropoid primates, social grooming is the principal mechanism (mediated by the central nervous system endorphin system) that underpins social bonding. However, the time available for social grooming is limited, and this imposes an upper limit on the size of group that can be bonded in this way. I suggest that, when hominins needed to increase the size of their groups beyond the limit that could be bonded by grooming, they co-opted laughter (a modified version of the play vocalization found widely among the catarrhine primates) as a form of chorusing to fill the gap. I show, first, that human laughter both upregulates the brain's endorphin system and increases the sense of bonding between those who laugh together. I then use a reverse engineering approach to model group sizes and grooming time requirements for fossil hominin species to search for pinch points where a phase shift in bonding mechanisms might have occurred. The results suggest that the most likely time for the origin of human-like laughter is the appearance of the genus Homoca 2.5 Ma. This article is part of the theme issue 'Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience'. [ABSTRACT FROM AUTHOR]

Bottlenose dolphins perform flipper rubbing in which they rub their partners with their pectoral fins. Considered similar to social grooming in primates, which have social and hygienic functions, flipper rubbing has been suggested to possess certain social functions. However, there are no reports based on quantitative data to support a hygienic function of this behavior. In this study, flipper rubbing and self-rubbing were observed in a group of three captive mother–calf dolphin pairs. In 19.2% of the observed 806 flipper-to-body rubbings and 54% of the 276 self-rubbings, dead skin was shed from the rubbed body part. There was a significant negative correlation between the number of flipper-to-body rubbing episodes the subjects received and their performance of self-rubbing. These results suggest that a function of flipper rubbing is to maintain and care for the body surface. In all mother–calf pairs, the mother rubbed her calf significantly more often than the calf rubbed the mother. Furthermore, each mother became a rubber significantly more often when their own offspring was their rubbing partner than when the partner was any other individual. These findings suggest that flipper rubbing is a mother-to-calf caring behavior. [ABSTRACT FROM AUTHOR]

Proximity and interpersonal contact are prominent components of social connection. Giving affective touch to others is fundamental for human bonding. This brief report presents preliminary results from a pilot study. It explores if exposure to bonding scenes impacts the activity of specific muscles related to physical interaction. Fingers flexion is a very important component when performing most actions of affectionate contact. We explored the visuo-motor affective interplay by priming participants with bonding scenes and assessing the electromyographic activity of the fingers flexor muscle, in the absence of any overt movements. Photographs of dyads in social interaction and of the same dyads not interacting were employed. We examined the effects upon the electromyographical activity: (i) during the passive exposure to pictures, and (ii) during picture offset and when expecting the signal to perform a fingers flexion task. Interacting dyads compared to matched non-interacting dyads increased electromyographic activity of the fingers flexor muscle in both contexts. Specific capture of visual bonding cues at the level of visual cortex had been described in the literature. Here we showed that the neural processing of visual bonding cues reaches the fingers flexor muscle. Besides, previous visualization of bonding cues enhanced background electromyographic activity during motor preparation to perform the fingers flexion task, which might reflect a sustained leakage of central motor activity downstream leading to increase in firing of the respective motor neurons. These data suggest, at the effector level, an implicit visuo-motor connection in which social interaction cues evoke intrinsic dispositions toward affectionate social behavior. [ABSTRACT FROM AUTHOR]

Proximity and interpersonal contact are prominent components of social connection. Giving affective touch to others is fundamental for human bonding. This brief report presents preliminary results from a pilot study. It explores if exposure to bonding scenes impacts the activity of specific muscles related to physical interaction. Fingers flexion is a very important component when performing most actions of affectionate contact. We explored the visuo-motor affective interplay by priming participants with bonding scenes and assessing the electromyographic activity of the fingers flexor muscle, in the absence of any overt movements. Photographs of dyads in social interaction and of the same dyads not interacting were employed. We examined the effects upon the electromyographical activity: (i) during the passive exposure to pictures, and (ii) during picture offset and when expecting the signal to perform a fingers flexion task. Interacting dyads compared to matched non-interacting dyads increased electromyographic activity of the fingers flexor muscle in both contexts. Specific capture of visual bonding cues at the level of visual cortex had been described in the literature. Here we showed that the neural processing of visual bonding cues reaches the fingers flexor muscle. Besides, previous visualization of bonding cues enhanced background electromyographic activity during motor preparation to perform the fingers flexion task, which might reflect a sustained leakage of central motor activity downstream leading to increase in firing of the respective motor neurons. These data suggest, at the effector level, an implicit visuo-motor connection in which social interaction cues evoke intrinsic dispositions toward affectionate social behavior.

When primates groom each other, they tend to concentrate on those parts of the body they cannot efficiently self‐groom (i.e., not visually accessible), and prefer to intensify grooming in areas with high hair density, thus suggesting a hygienic function. However, preferences for some body sites over others during social grooming may also result from different degrees of social bonding and relative dominance. To assess the relative importance of physical (hygienic) and social factors, we examined grooming interactions in two groups of wild black capuchin monkeys (Sapajus nigritus) during 15 nonconsecutive months. We evaluated the distribution of social grooming across body sites according to their accessibility by self‐grooming and hair density. At the same time, we assessed whether the degree of dyadic social bonding affects the relative body orientation between groomer and groomee and the access to vulnerable body sites (e.g., face, throat, groin) during grooming. As expected, capuchins preferentially groomed inaccessible body sites (e.g., back and head), with a disproportionate effort directed to the tufts of their partners. We found that dyadic social bond strength, together with rank distance, significantly affected the proportion of grooming in ventro‐ventral body relative orientation only in dominant‐subordinate groomer‐groomee dyads. This may indicate that, when two individuals differ in rank but are strongly bonded, the level of uncertainty related to the social context is already resolved and thus grooming per se is no longer perceived by the subordinate as an uncertain/risky situation. We found no effect of social bonding on grooming vulnerable body sites for any type of dyad. Our findings suggest that grooming site preferences in black capuchin monkeys simultaneously reflect hygienic and social functions. HIGHLIGHTS: Black capuchin monkeys prefer performing social grooming on inaccessible to self‐grooming body sites, supporting the hygienic function of this behavior.Higher proportions of grooming in frontal body relative orientation according to the degree of social affiliation in dominant‐subordinate grooming dyads reinforces the role of grooming as a behavior aimed at maintaining social bonds in black capuchins. [ABSTRACT FROM AUTHOR]

Human joint action seems special, as it is grounded in joint commitment—a sense of mutual obligation participants feel towards each other. Comparative research with humans and non-human great apes has typically investigated joint commitment by experimentally interrupting joint actions to study subjects’ resumption strategies. However, such experimental interruptions are human-induced, and thus the question remains of how great apes naturally handle interruptions. Here, we focus on naturally occurring interruptions of joint actions, grooming and play, in bonobos and chimpanzees. Similar to humans, both species frequently resumed interrupted joint actions (and the previous behaviours, like grooming the same body part region or playing the same play type) with their previous partners and at the previous location. Yet, the probability of resumption attempts was unaffected by social bonds or rank. Our data suggest that great apes experience something akin to joint commitment, for which we discuss possible evolutionary origins.

To date, the suitability of affiliative behaviours for inclusion in a horse welfare assessment protocol has not been thoroughly studied. This is surprising, given the growing focus on demonstrating the presence of positive welfare rather than just the absence of poor welfare. This shift is driven by the recognition that simply avoiding negative experiences does not necessarily equate to a positive welfare state, and that there is more to good welfare than just the absence of disease, injury or stress. Thus, we critically reviewed the current scientific literature to propose quantitatively assessed animal-based indicators that indicate positive welfare and could therefore be incorporated in a welfare assessment protocol for horses. Amongst the candidate indicators that have previously been associated with positive welfare in horses are socio-positive interactions between horses, such as affiliative activities consisting of social proximity, social play and social grooming. These activities must meet the scientific quality criteria regarding validity, reliability and feasibility to enable an objective assessment of positive animal welfare. However, the current review showed that social play and social grooming amongst adult horses under husbandry conditions are not sufficiently valid indicators. These two affiliative behaviours would meet the requirement for validity only in combination with a differential diagnosis considering the absence of, for example, the stress caused by space restriction. Furthermore, social play and social grooming amongst adult horses may not fulfil the requirement for feasibility, because these behaviours occur rarely and unpredictably throughout the day, hindering their assessment within a limited observation time. As a result of this literature review, we suggest that social proximity may be the only suitable affiliative behaviour indicating positive welfare in adult horses. It best reflects the social bond with preferred partners - a key element for experiencing positive welfare. This affiliative behaviour has been investigated thoroughly in various sport and leisure horse housing systems regarding all three scientific quality criteria. It satisfies the validity criterion because to be counted as a socio-positive interaction, social proximity must be achieved voluntarily whereby the term 'voluntary social proximity' may best fit this requirement. Notably, it also fulfils the reliability and feasibility criteria in terms of good inter-observer agreement given a standardised assessment method including parameters such as distance between horses, duration and context. Thus, social proximity seems to be a promising indicator of positive welfare in horses. [Display omitted] • Social proximity is a reliable and feasible animal-based positive welfare indicator. • Sufficient validity of social proximity can only be achieved by considering environmental factors. • Social play and social grooming are not sufficiently valid as positive welfare indicators. • In a welfare assessment protocol, social play and social grooming are not sufficiently feasible. [ABSTRACT FROM AUTHOR]

During a relatively long period of growth, immature individuals rely on their mothers to obtain nutrition, and a good environment for learning social skills needed to cope with complex environments in adulthood. In this study, we collected the behavioral data of Tibetan macaques (Macaca thibetana) to investigate the effects of females’ social rank on the development of social relationships among their immature offspring from November to June 2021. The results show that there was no difference in the rate/type of social play and grooming among infants. However, among juveniles and adolescents, the higher their mother’s social rank, the higher the rate of social play they participated in, and the more aggressive play they engaged in. Immatures with high-ranking mothers initiated more social play among each other. A similar pattern of playmates was found among juveniles/adolescents with middle-ranking and low-ranking mothers. We also found that immatures preferred immatures with higher-ranking mothers as grooming mates and initiated more grooming with immatures with higher-ranking mothers than with those with lower-ranking mothers. Our study suggests that females’ social ranks affect the development of social relationships among their immature offspring. In despotic nonhuman primates, this indicates that the mother’s dominance hierarchy would directly or indirectly influence the processes of participating in social interactions and choosing partnerships among immature individuals with age (i.e., infancy, juvenile, and adolescent periods).

For gregarious species such as domestic cattle, the social environment is a very important determinant of their welfare and fitness. Understanding the complexity of cows' relationships can assist the development of management practices that are more integrated with the cows' social behavioral processes. The two aims of this study were: (1) to determine the dynamics of affiliative relationships, as indicated by allogrooming, by means of stochastic actor-oriented modeling, in dairy cows during early lactation; (2) to explore the underlying processes and the individual attributes, such as age, social rank and reproductive state, that could shape network pattern changes in grooming contacts between individual. We observed the allogrooming behavior of a dynamic group of 38 dairy cows for 4 h per day for 30 days. Using stochastic actor-oriented models, we modeled the dynamics of weekly contacts and studied how structural processes (e.g., reciprocity, transitivity, or popularity) and individual attributes (i.e., age, social rank, and reproductive state) influence network changes. We found that cows tended to groom individuals that had previously groomed them, implying a possible cooperation. Cows that groomed more actively did not appear to have a preference for specific individuals in the herd, and in return, tended to be groomed by fewer cows over time. Older individuals groomed more cows than younger ones, indicating that allogrooming could be related to seniority. Cows groomed mainly individuals of similar age, suggesting that familiarity and growing up together enhanced social grooming. Over time, cows with higher social rank were groomed by fewer cows and individuals recently reintroduced to the group groomed more herdmates. The study of social network dynamics can be used to better understand the complexity and non-linearity of cow relationships. Our findings, along with further research, can complement and strengthen the design of improved management practices that are more in line with the natural social behavior of cows.

Members of social groups risk infection through contact with those in their social network. Evidence that social organization may protect populations from pathogens in certain circumstances prompts the question as to how social organization affects the spread of ectoparasites. The same grooming behaviors that establish social bonds also play a role in the progression of ectoparasitic outbreaks. In this paper, we model the interactions between social organization and allogrooming efficiency to consider how ectoparasitic threats may have shaped the evolution of social behaviors. To better understand the impacts of social grooming on organizational structure, we consider several dynamic models of social organization using network centrality measures as the basis of neighbor selection. Within this framework, we consider the impact of varying levels of social grooming on both the group structure and the overall ectoparasitic disease burden. Our results demonstrate that allogrooming, along with ongoing dynamic social organization, may be protective with respect to both the timing and the magnitude of ectoparasitic epidemics. These results support the idea that ectoparasitic threat should not be considered a single evolutionary factor in the evolution of host social systems, and may have operated in different ways depending on the broader ecology of the host-ectoparasite interaction.

This study implements a computer-assisted content analysis to identify which social grooming factors reduce social media users' incivility when commenting or posting about the COVID-19 situation in South Korea. In addition, this study conducts semantic network analysis to interpret qualitatively how people express their thoughts. The findings suggest that social network size is a negative predictor of incivility. Moreover, Twitter users who have built larger networks and gained positive responses from others are less likely to use uncivil language. Lastly, linguistic choice among users is different depending on the size of their social network. [ABSTRACT FROM AUTHOR]

Social grooming is assumed to have both social and hygienic functions. In monkey species, body site preferences during social grooming are related to both the dominance ranks of participating individuals and strength of social bonds. It is suggested that low-ranking groomees and individuals with weak social bonds expose less vulnerable sites of their body because doing so allows them to avoid eye contact, thus reducing the risk of aggression. This study aimed to investigate the influence of (1) the levels of affiliation (through analyses of sex, grooming frequency, and group membership during intergroup encounters), (2) dominance, (3) kinship, and (4) age on grooming site preferences (head, front, back, anogenital region, and limbs) in wild bonobos. Subjects were two groups of wild bonobos living in Wamba, DR Congo. Body site preferences were documented from video clips. Our results illustrate that the levels of affiliation between individuals and their dominance status do not seem to influence body site preferences, contrasting with results in monkey species. However, the selection of body sites was mostly influenced by age and kinship. Mothers were found to groom less accessible sites on their immature offspring's body more than when grooming a non-related immature, suggesting that mothers invest in their offspring more to prevent diseases linked to ectoparasites. Finally, mature individuals groomed the inaccessible back more often than immature individuals, while the latter focused their grooming on the head and front. Overall, this study suggests hygienic functions of social grooming in wild bonobos. [ABSTRACT FROM AUTHOR]

Allogrooming provides a window into the social lives of many group-living mammals and birds. The fitness benefits of allogrooming are encouraged by proximate mechanisms that make it physiologically rewarding for both actors and receivers. However, receivers might not always benefit from allogrooming. Some allogrooming decisions might be the actor's response to cues of the recipient's need. Other decisions might only be caused by the actor's motivational state. To test these ideas, we studied what triggers allogrooming in common vampire bats. In test 1, subjects that had experimentally disturbed and wetted fur were more likely to be allogroomed, even when controlling for increased self-grooming. In test 2, allogrooming rates were elevated not only by receiver self-grooming (a cue for receiver need) but also by the actor's previous self-grooming. Both effects were significantly greater than the effect of self-grooming by third parties. Interestingly, we detected a negative interaction: the positive effect of receiver need on allogrooming was smaller when the actor was previously self-grooming. This is consistent with the hypothesis that there are "receiver-driven" allogrooming decisions, which are responses to recipient need, and "actor-driven" decisions, which are not. We predict that receiver-driven allogrooming will bestow greater benefits to recipients compared with actor-driven allogrooming. [ABSTRACT FROM AUTHOR]

Human straight-legged bipedalism represents one of the earliest events in the evolutionary split between humans (Homo spp.) and chimpanzees (Pan spp.), although its selective basis is a mystery. A carrying-related hypothesis has recently been proposed in which hair loss within the hominin lineage resulted in the inability of babies to cling to their mothers, requiring mothers to walk upright to carry their babies. However, a question remains for this model: what drove the hair loss that resulted in upright walking? Observers since Darwin have suggested that hair loss in humans may represent an evolutionary strategy for defence against ticks. The aim of this review is to propose and evaluate a novel tick-based evolutionary hypothesis wherein forest fragmentation in hominin paleoenvironments created conditions that were favourable for tick proliferation, selecting for hair loss in hominins and grooming behaviour in chimpanzees as divergent anti-tick strategies. It is argued that these divergent anti-tick strategies resulted in different methods for carrying babies, driving the locomotor divergence of humans and chimpanzees.

ASMR (Autonomous Sensory Meridian Response) is a physiological sensory reaction usually described as "a tingling, static-like sensation across the scalp, back of the neck and at times further areas in response to specific triggering audio and visual stimuli" (Barratt & Davis, 2015). In the last decade ASMR has also become a growing media cultural phenomenon, especially on YouTube, where ASMRtists create videos that help you overcome stress, loneliness and insomnia while making you feel good. This article follows a question raised by one of these ASMRtists concerning the ability of the videos to generate a feeling of human touch. In a combination of Media and Sound Studies, and supported by an empirical study of user comments on YouTube and Reddit, we argue that ASMR videos offer a social service by affording experiences of telepresence and pseudohaptic social audio-grooming and thereby meeting the 'skin hunger' of modern human beings. [ABSTRACT FROM AUTHOR]

Chimpanzees in zoos with sufficient and appropriate environmental enrichment devices are expected to exhibit behaviors, interactions, and societies similar to those in the wild. In this study, we compared the activity budgets of each observed behavior, characteristics of social grooming, and social networks of captive chimpanzees at Tama Zoological Park (Tama) with those of wild chimpanzees at Mahale Mountains National Park, Tanzania (Mahale), and tested our predictions. We surveyed 16 chimpanzees in both Tama and Mahale and recorded the behaviors and individuals in proximity of each focal individual and social grooming the focal individuals participated in. The proportion of time spent collecting foraging was significantly lower in Tama than in Mahale. Additionally, the percentage of mutual grooming was much higher in Tama than in Mahale. All focal individuals in Mahale performed mutual grooming interactions, including grooming handclasp (GHC) but this was not observed in Tama. The result of a high rate of mutual grooming in chimpanzees in Tama without GHC and the finding that individuals forming the core of their social network are sex independent suggest that chimpanzees placed in an appropriate environmental enrichment have idiosyncratic grooming or social features, even in captivity.

Human social strategies have evolved as an adaption to behave in complex societies. In such societies, humans intensively tend to cooperate with their closer friends, because they have to distribute their limited resources through cooperation (e.g., time, food, etc.). It also makes the situation difficult to have uniform social relationships (social grooming) with all friends. Thus, the social relationship strengths often show a much skewed distribution (a power law distribution). Here we aim to show adaptivity of such social grooming strategies in order to explore the evolution of human social intelligence. We use a model in the framework of evolutionary games where the social grooming strategies evolve via building social relationships with cooperators. Simulation results demonstrate four evolutionary trends. One of the trends is similar to the strategy that humans use. We find that these trends depend on three parameters; individuals' richness, group sizes, and the amount of social grooming. The human-like strategy evolves in large poor groups. Moreover, the increase of the amount of social grooming makes the group size larger. Conversely, this implies that the same strategy evolves when the amount of social grooming is properly adjusted even if the group sizes are different. Our results are important in the sense that, between human and non-human primates, the differences of the group size and the amount of social grooming are significant.

While most primates are tropical animals, a number of species experience markedly cold winters. In a high latitude arid environment, wild female vervet monkeys (Chlorocebus pygerythrus) that are socially integrated experience reduced cold stress. Here, we ask whether sociability is similarly salient for male vervet monkeys, who reside in non-natal groups as adults and who must, therefore, develop social relationships on arrival. We use body temperature and social data from 15 free-ranging male vervet monkeys to determine whether the number of grooming partners is as important for males during winter and whether the length of residency is positively associated with body temperature. We also assess whether larger body size and higher dominance rank mitigate the need for social partnerships. Like females, male vervets respond to lower 24 h ambient temperatures and winter's progression by decreasing minimum and mean 24 h body temperatures and by regulating their temperatures less precisely. Male rank had no effect, while larger body size was associated primarily with reduced temperature fluctuations. Males with more social partners sustained higher minimum and mean body temperatures but, unexpectedly, regulated their temperatures less tightly. Further analysis revealed that higher minimum and mean temperatures were best accounted for by the number of female partners, while increased temperature fluctuation was driven by the number of male partners. As winter and the mating season overlap, we interpret this as indicating that a need to sustain male associations incurs physiological stress that is reflected as a thermoregulatory cost. Lastly, we show that longer residency is associated with higher minimum body temperatures and lower temperature fluctuations independently of social affiliation. [ABSTRACT FROM AUTHOR]