Bonaventura Majolo

Fitness‐related benefits of dominance in primates

Dominance hierarchies are thought to provide various fitness-related benefits to dominant individuals (e.g., preferential access to food or mating partners). Remarkably, however, different studies on this topic have produced contradictory results, with some showing strong positive association between rank and fitness (i.e., dominants gain benefits over subordinates), others weak associations, and some others even revealing negative associations. Here, we investigate dominance-related benefits across primate species while controlling for phylogenetic effects. We extracted data from 94 published studies, representing 25 primate species (2 lemur species, 4 New World monkeys, 16 Old World monkeys, and 3 apes), to assess how dominance affects life-history and behavior. We used standard and phylogenetic meta-analyses to analyze the benefits of dominance in primates. Dominant females had higher infant survival to first year, although we found no significant effect of dominance on female feeding success. Results for female fecundity differed between the two meta-analytical approaches, with no effect of dominance on female fecundity after controlling for phylogeny. Dominant males had a higher fecundity and mating success than subordinate males. Finally, the benefits of dominance for female fecundity were stronger in species with a longer lifespan. Our study supports the view that dominance hierarchies are a key aspect of primate societies as they indeed provide a number of fitness-related benefits to individuals.

Human friendship favours cooperation in the iterated prisoner's dilemma

In the last decades, many studies have attempted to analyse the factors that may favour the evolution of cooperation. Among unrelated individuals, cooperation is expected to occur when partners exchange altruistic acts one another (i.e., reciprocity) or when the donor of an altruistic act may obtain secondary benefits from the act (e.g., increased reputation). The iterated prisoners dilemma (IPD) is frequently used to analyse cooperation between two players. In this game, cooperation is constantly at risk of exploitation. Therefore, previous knowledge of the other players attitude towards cooperation may positively affect an individuals decision to cooperate during the game. In various social species (including humans), group members may form friendly relationships that reduce uncertainty about the partner response and increase the mutual exchange of benefits. In light of this, we analysed whether humans cooperate more when playing an IPD with a friend than with a stranger. Each subject played an IPD twice, with a friend and a stranger, and had £10 per round to donate or not to the other player. Our results evidenced that humans cooperate more with, and donate more money to the other player when playing with a friend than with a stranger. Moreover, in game two subjects playing with unfamiliar players followed a raise the stakes strategy (RTS), i.e. they donated more money as the game progressed. The sense of trust in the other players willingness to reciprocate the altruistic act is probably an important pre-requisite favouring cooperation and it may explain the differences between friends and unfamiliar players. The use of a RTS strategy in game two, however, indicates that subjects were attempting to increase the level of cooperation with unfamiliar players. These findings are consistent with the observation that friendship (and the capacity to build cooperative relationships) may be beneficial to individuals in many social animals and in humans.

Responses to social and environmental stress are attenuated by strong male bonds in wild macaques

Significance Male macaques form social bonds similar to human friendships that buffer them against day-to-day stressors. If male primates live in multimale groups they usually fight fiercely over access to females, but males can develop friendly relationships with a few group mates. The strength of these “friendships” has now been shown to buffer against the negative effects of social and environmental stressors, a phenomenon that was previously only described for females and pair-living animals. Long-term glucocorticoid (stress hormone) elevation can increase susceptibility to disease and mortality. This study shows that variation in everyday stressors such as the amount of aggression received or cold stress can cause such long-term elevated glucocorticoid levels but that keeping a few close male associates will avoid that. In humans and obligatory social animals, individuals with weak social ties experience negative health and fitness consequences. The social buffering hypothesis conceptualizes one possible mediating mechanism: During stressful situations the presence of close social partners buffers against the adverse effects of increased physiological stress levels. We tested this hypothesis using data on social (rate of aggression received) and environmental (low temperatures) stressors in wild male Barbary macaques (Macaca sylvanus) in Morocco. These males form strong, enduring, and equitable affiliative relationships similar to human friendships. We tested the effect of the strength of a male’s top three social bonds on his fecal glucocorticoid metabolite (fGCM) levels as a function of the stressors’ intensity. The attenuating effect of stronger social bonds on physiological stress increased both with increasing rates of aggression received and with decreasing minimum daily temperature. Ruling out thermoregulatory and immediate effects of social interactions on fGCM levels, our results indicate that male Barbary macaques employ a tend-and-befriend coping strategy in the face of increased environmental as well as social day-to-day stressors. This evidence of a stress-ameliorating effect of social bonding among males under natural conditions and beyond the mother–offspring, kin or pair bond broadens the generality of the social buffering hypothesis.

Coping with the cold: predictors of survival in wild Barbary macaques, Macaca sylvanus

We report the death of 30 wild Barbary macaques, living in two groups, during an exceptionally cold and snowy winter in the Middle Atlas Mountains, Morocco. We examined whether an individuals time spent feeding, the quality and number of its social relationships, sex and rank predicted whether it survived the winter or not. The time an individual spent feeding and the number of social relationships that an individual had in the group were positive and significant predictors of survival. This is the first study to show that the degree of sociality affects an individuals chance of survival following extreme environmental conditions. Our findings support the view that sociality is directly related to an individuals fitness, and that factors promoting the establishment and maintenance of social relationships are favoured by natural selection.

Hierarchical steepness and phylogenetic models: phylogenetic signals in Macaca

Phylogenetic models of primate social behaviour posit that core social traits are inherent species characteristics that depend largely on phylogenetic histories of species rather than on adaptation to current socioecological conditions. These models predict that aspects of social structure will vary more between species than within species and that they will display strong phylogenetic signals. We tested these predictions in macaques focusing on dominance gradients, a relatively little studied, yet central, aspect of social structure. We used data from 14 social groups representing nine macaque species living in a variety of conditions. We examined proportions of counteraggression and two recently developed measures of dominance gradients (hierarchical steepness) for phylogenetic signals in nine phylogenetic trees constructed using (1) available genetic data sets and (2) Bayesian Markov Chain Monte Carlo (MCMC) and maximum likelihood algorithms. Hierarchical steepness and counteraggression showed significant variation between species but inconsistent variation within species. Both steepness and counteraggression showed evidence of phylogenetic signals, with results being particularly strong for one steepness measure and for counteraggression. Our results suggest that between-species variation in some core aspects of social structure are shaped by species’ evolutionary relationships, despite differences in living conditions. As such, they provide broad support for the phylogenetic model.

Hierarchical Steepness, Counter-Aggression, and Macaque Social Style Scale

Nonhuman primates show remarkable variation in several aspects of social structure. One way to characterize this variation in the genus Macaca is through the concept of social style, which is based on the observation that several social traits appear to covary with one another in a linear or at least continuous manner. In practice, macaques are more simply characterized as fitting a four‐grade scale in which species range from extremely despotic (grade 1) to extremely tolerant (grade 4). Here, we examine the fit of three core measures of social style—two measures of dominance gradients (hierarchical steepness) and another closely related measure (counter‐aggression)—to this scale, controlling for phylogenetic relationships. Although raw scores for both steepness and counter‐aggression correlated with social scale in predicted directions, the distributions appeared to vary by measure. Counter‐aggression appeared to vary dichotomously with scale, with grade 4 species being distinct from all other grades. Steepness measures appeared more continuous. Species in grades 1 and 4 were distinct from one another on all measures, but those in the intermediate grades varied inconsistently. This confirms previous indications that covariation is more readily observable when comparing species at the extreme ends of the scale than those in intermediate positions. When behavioral measures were mapped onto phylogenetic trees, independent contrasts showed no significant consistent directional changes at nodes below which there were evolutionary changes in scale. Further, contrasts were no greater at these nodes than at neutral nodes. This suggests that correlations with the scale can be attributed largely to species’ phylogenetic relationships. This could be due in turn to a structural linkage of social traits based on adaptation to similar ecological conditions in the distant past, or simply to unlinked phylogenetic closeness. Am. J. Primatol. 74:915‐925, 2012.

The Effect of Climatic Factors on the Activity Budgets of Barbary Macaques (Macaca sylvanus)

Climatic conditions can significantly affect the behavior of animals and constrain their activity or geographic distribution. Barbary macaques (Macaca sylvanus) are one of the few primates that live outside the tropics. Here we analyze if and how the activity budgets of Barbary macaques are affected by climatic variables, i.e., air temperature, relative humidity, rainfall, and snow coverage. We collected scan sampling data on the activity budgets of four groups of macaques living in the Middle Atlas Mountains of Morocco from June 2008 to January 2011. This habitat is characterized by extreme seasonal changes, from cold and snowy winters to hot and dry summers. The activity budgets of the macaques differed across months but not across the time of day (with the exception of time spent feeding). The monkeys spent significantly more time feeding or foraging when there was no snow than when snow coverage was moderate or major. Daily rainfall was positively related to resting time and negatively to time spent moving or in social behavior. Air temperature was negatively related to time spent feeding or foraging. Finally, time spent on social behavior was significantly lower when relative humidity was high. These data indicate that environmental factors significantly affect the time budgets of endangered Barbary macaques, a species that has been little studied in the wild. Our findings support previous studies on temperate primates in showing that snow coverage can have negative consequences on the feeding ecology and survival of these species.

Hierarchical steepness, counter-aggression, and Macaque social style scale

Nonhuman primates show remarkable variation in several aspects of social structure. One way to characterize this variation in the genus Macaca is through the concept of social style, which is based on the observation that several social traits appear to covary with one another in a linear or at least continuous manner. In practice, macaques are more simply characterized as fitting a four‐grade scale in which species range from extremely despotic (grade 1) to extremely tolerant (grade 4). Here, we examine the fit of three core measures of social style—two measures of dominance gradients (hierarchical steepness) and another closely related measure (counter‐aggression)—to this scale, controlling for phylogenetic relationships. Although raw scores for both steepness and counter‐aggression correlated with social scale in predicted directions, the distributions appeared to vary by measure. Counter‐aggression appeared to vary dichotomously with scale, with grade 4 species being distinct from all other grades. Steepness measures appeared more continuous. Species in grades 1 and 4 were distinct from one another on all measures, but those in the intermediate grades varied inconsistently. This confirms previous indications that covariation is more readily observable when comparing species at the extreme ends of the scale than those in intermediate positions. When behavioral measures were mapped onto phylogenetic trees, independent contrasts showed no significant consistent directional changes at nodes below which there were evolutionary changes in scale. Further, contrasts were no greater at these nodes than at neutral nodes. This suggests that correlations with the scale can be attributed largely to species’ phylogenetic relationships. This could be due in turn to a structural linkage of social traits based on adaptation to similar ecological conditions in the distant past, or simply to unlinked phylogenetic closeness. Am. J. Primatol. 74:915‐925, 2012.

Postconflict Behavior Among Male Japanese Macaques

Reconciliation was first described more than 20 years ago. Since then, it has been observed in many mammals (mainly primates) but data on postconflict behavior among males are still scarce because they usually aggressively compete for mating partners, rarely maintain amicable relationships with one another. Accordingly, reconciliation is expected to occur at low rates. Although this is true for Japanese macaque males, the subspecies on Yakushima Island (Macaca fuscata yakui) seems to represent an exception as grooming among males occurs often. We analyzed postconflict behavior among them and discuss the possible factors that may favor the occurrence of grooming and reconciliation. Selective attraction between former opponents—reconciliation—occurred soon after conflicts. Consolation—affiliative interactions between a focal animal and group members other than the former opponents occurring earlier in PCs than in MCs—was absent among males. Conciliatory tendency is higher for Yakushima macaque males (0.31) versus that in studies on the other subspecies Macaca fuscata. We discuss differences in the behavioral ecology of the 2 subspecies, the ecological and social factors that may favor the occurrence of reconciliation, and the possible benefits that males gain from grooming exchange and reconciliation.

Grooming coercion and the post-conflict trading of social services in wild Barbary macaques.

In animal and human societies, social services such as protection from predators are often exchanged between group members. The tactics that individuals display to obtain a service depend on its value and on differences between individuals in their capacity to aggressively obtain it. Here we analysed the exchange of valuable social services (i.e. grooming and relationship repair) in the aftermath of a conflict, in wild Barbary macaques (Macaca sylvanus). The relationship repair function of post-conflict affiliation (i.e. reconciliation) was apparent in the victim but not in the aggressor. Conversely, we found evidence for grooming coercion by the aggressor; when the victim failed to give grooming soon after a conflict they received renewed aggression from the aggressor. We argue that post-conflict affiliation between former opponents can be better described as a trading of social services rather than coercion alone, as both animals obtain some benefits (i.e. grooming for the aggressor and relationship repair for the victim). Our study is the first to test the importance of social coercion in the aftermath of a conflict. Differences in competitive abilities can affect the exchange of services and the occurrence of social coercion in animal societies. This may also help explain the variance between populations and species in their social behaviour and conflict management strategies.