An individual-orientated model of the emergence of despotic and egalitarian societies

Single behavioural differences between egalitarian and despotic animal societies are often assumed to reflect specific adaptations. However, in the present paper, I will show in an individual-orientated model, how many behavioural traits of egalitarian and despotic virtual societies arise as emergent characteristics. The artificial entities live in a homogeneous world and only aggregate, and upon meeting one another and may perform dominance interactions in which the effects of winning and losing are self-reinforcing. The behaviour of these entities is studied in a similar way to that of real animals. It will be shown that by varying the intensity of aggression only, one may switch from egalitarian to despotic virtual societies. Differences between the two types of society appear to correspond closely to those between despotic and egalitarian macaque species in the real world. In addition, artificial despotic societies show a clearer spatial centrality of dominants and, counter-intuitively, more rank overlap between the sexes than the egalitarian ones. Because of the correspondence with patterns in real animals, the model makes it worthwhile comparing despotic and egalitarian species for socio-spatial structure and rank overlap too. Furthermore, it presents us with parsimonious hypotheses which can be tested in real animals for patterns of aggression, spatial structure and the distribution of social positive and sexual behaviour.     

Emergence of complex social networks from spatial structure and rules of thumb: a modelling approach

Individual-based computer models show that simple heuristic governing individuals’ behavior may suffice to generate complex patterns of social behavior at the group level such as those observed in animal societies. ‘GrooFiWorld’ is an example of such kind of computer models. In this model, self-organization and simple behavioral rules generate complex patterns of social behavior like those described in tolerant and intolerant societies of macaques. Social complexity results from the socio-spatial structure of the group, the nature of which is, in turn, a side-effect of intensity of aggression. The model suggests that a similar mechanism may give rise to complex social structures in macaques. It is, however, unknown if the spatial structure of the model and that of macaques are indeed similar. Here we used social networks analysis as a proxy for spatial structure of the group. Our findings show that the social networks of the model share similar qualitative features with those of macaques. As group size increases, the density and the average individual eigenvector centrality decrease and the modularity and centralization of the network increase. In social networks emerging from simulations resembling intolerant societies the density is lower, the modularity and centralization are higher, and the individuals ranking higher in the dominance hierarchy are more central than in the social networks emerging from simulations resembling egalitarian societies. Given the qualitative similarity between the social networks of the model and that of empirical data, our results suggest that the spatial structure of macaques is similar to that of the model. It seems thus plausible that, as in the model, the spatial structure combined with simple behavioral rules plays a role in the emergence of complex social networks and complex social behavior in macaques.     

An individual-oriented model on the emergence of support in fights, its reciprocation and exchange

Complex social behaviour of primates has usually been attributed to the operation of complex cognition. Recently, models have shown that constraints imposed by the socio-spatial structuring of individuals in a group may result in an unexpectedly high number of patterns of complex social behaviour, resembling the dominance styles of egalitarian and despotic species of macaques and the differences between them. This includes affiliative patterns, such as reciprocation of grooming, grooming up the hierarchy, and reconciliation. In the present study, we show that the distribution of support in fights, which is the social behaviour that is potentially most sophisticated in terms of cognitive processes, may emerge in the same way. The model represents the spatial grouping of individuals and their social behaviour, such as their avoidance of risks during attacks, the self-reinforcing effects of winning and losing their fights, their tendency to join in fights of others that are close by (social facilitation), their tendency to groom when they are anxious, the reduction of their anxiety by grooming, and the increase of anxiety when involved in aggression. Further, we represent the difference in intensity of aggression apparent in egalitarian and despotic macaques. The model reproduces many aspects of support in fights, such as its different types, namely, conservative, bridging and revolutionary, patterns of choice of coalition partners attributed to triadic awareness, those of reciprocation of support and 'spiteful acts' and of exchange between support and grooming. This work is important because it suggests that behaviour that seems to result from sophisticated cognition may be a side-effect of spatial structure and dominance interactions and it shows that partial correlations fail to completely omit these effects of spatial structure. Further, the model is falsifiable, since it results in many patterns that can easily be tested in real primates by means of existing data.     

Tolerant food sharing and reciprocity is precluded by despotism among bonobos but not chimpanzees

Tolerant food sharing among human foragers can largely be explained by reciprocity. In contrast, food sharing among chimpanzees and bonobos may not always reflect reciprocity, which could be explained by different dominance styles: in egalitarian societies reciprocity is expressed freely, while in more despotic groups dominants may hinder reciprocity. We tested the degree of reciprocity and the influence of dominance on food sharing among chimpanzees and bonobos in two captive groups. First, we found that chimpanzees shared more frequently, more tolerantly, and more actively than bonobos. Second, among chimpanzees, food received was the best predictor of food shared, indicating reciprocal exchange, whereas among bonobos transfers were mostly unidirectional. Third, chimpanzees had a shallower and less linear dominance hierarchy, indicating that they were less despotic than bonobos. This suggests that the tolerant and reciprocal sharing found in chimpanzees, but not bonobos, was made possible by the absence of despotism. To investigate this further, we tested the relationship between despotism and reciprocity in grooming using data from an additional five groups and five different study periods on the main groups. The results showed that i) all chimpanzee groups were less despotic and groomed more reciprocally than bonobo groups, and ii) there was a general negative correlation between despotism and grooming reciprocity across species. This indicates that an egalitarian hierarchy may be more common in chimpanzees, at least in captivity, thus fostering reciprocal exchange. We conclude that a shallow dominance hierarchy was a necessary precondition for the evolution of human‐like reciprocal food sharing. Am J Phys Anthropol 143:41–51, 2010. © 2010 Wiley‐Liss, Inc.     

The Organization of Collective Group Movements in Wild Barbary Macaques (Macaca sylvanus): Social Structure Drives Processes of Group Coordination in Macaques

Social animals have to coordinate activities and collective movements to benefit from the advantages of group living. Animals in large groups maintain cohesion by self-organization processes whereas in smaller groups consensus decisions can be reached. Where consensus decisions are relevant leadership may emerge. Variation in the organization of collective movements has been linked to variation in female social tolerance among macaque species ranging from despotic to egalitarian. Here we investigated the processes underlying group movements in a wild macaque species characterized by a degree of social tolerance intermediate to previously studied congeneric species. We focused on processes before, during and after the departure of the first individual. To this end, we observed one group of wild Barbary macaques (Macaca sylvanus) in the Middle Atlas, Morocco using all-occurrence behaviour sampling of 199 collective movements. We found that initiators of a collective movement usually chose the direction in which more individuals displayed pre-departure behavior. Dominant individuals contributed to group movements more than subordinates, especially juveniles, measured as frequencies of successful initiations and pre-departure behaviour. Joining was determined by affiliative relationships and the number of individuals that already joined the movement (mimetism). Thus, in our study group partially shared consensus decisions mediated by selective mimetism seemed to be prevalent, overall supporting the suggestion that a species’ social style affects the organization of group movements. As only the most tolerant species show equally shared consensus decisions whereas in others the decision is partially shared with a bias to dominant individuals the type of consensus decisions seems to follow a stepwise relation. Joining order may also follow a stepwise, however opposite, relationship, because dominance only determined joining in highly despotic, but not in intermediate and tolerant species.     

Generous Leaders and Selfish Underdogs: Pro-Sociality in Despotic Macaques

Actively granting food to a companion is called pro-social behavior and is considered to be part of altruism. Recent findings show that some non-human primates behave pro-socially. However, pro-social behavior is not expected in despotic species, since the steep dominance hierarchy will hamper pro-sociality. We show that some despotic long-tailed macaques do grant others access to food. Moreover, their dominance hierarchy determines pro-social behavior in an unexpected way: high-ranking individuals grant, while low-ranking individuals withhold their partner access to food. Surprisingly, pro-social behavior is not used by subordinates to obtain benefits from dominants, but by dominants to emphasize their dominance position. Hence, Machiavellian macaques rule not through “fear above love”, but through “be feared when needed and loved when possible”.     

Distribution of Grooming Among Adult Females in a Large,Free-Ranging Group of Japanese Macaques

We analyzed grooming episodes recorded among adult females in a large, provisioned, free-ranging group of Japanese macaques (Macaca fuscata) at an individual level. Each female groomed on average 10 of the other 84 females, and 54% of them devoted 50% of their grooming to a single female grooming partner, which indicates that most females had grooming interactions with a relatively small subset of available females. Although 65% of the total grooming bouts were between related females, 25% of females disproportionately groomed unrelated females, 22% groomed related and unrelated females equally, and grooming was kin-biased for the remaining 53%. Moreover, 11 of 16 kin-groups included at least one female that groomed unrelated females significantly more often than related females. In 18% of unrelated dyads, grooming was directed down the hierarchy, in 58%, grooming was well-balanced between the two females, and in the remaining 25%, grooming was directed up the hierarchy. The results indicate that although Japanese macaques are considered a despotic species based on their dominance style, this group included some females that showed egalitarian tendencies, i.e., grooming was directed down the hierarchy or was well-balanced, and was directed toward unrelated females as often as or more often than toward related females. The presence of egalitarian individuals might be important to maintain a well-organized, female-bonded group.     

Increased produce enrichment reduces trauma in socially‐housed captive rhesus macaques (Macaca mulatta)

Due to primate adaptations for sociality, captive rhesus macaques have optimal welfare and utility as a biomedical model when they can be maintained in outdoor social groups. As a despotic species; however, aggression can result in costly injuries and may result in temporary or permanent removal of specific individuals from social housing. Enrichment items, such as toys, climbing structures, and foraging material, are employed to keep captive animals occupied. We hypothesized that produce enrichment that requires more processing to extract may reduce socially‐derived injuries by keeping animals occupied. We tested the effects of additional weekly produce (corn‐in‐husk, whole melon, or whole squash) on trauma incidence in an outdoor social group of rhesus macaques across two distinct seasons (mating and birthing seasons) at the California National Primate Research Center. Aggression and status behavioral data, food resource use and proximity, and trauma incidence were collected over two 16‐week periods, with eight control and treatment conditions alternating biweekly. Mixed‐effects regression modeling was used to determine the best predictors of trauma risk and severe aggression at the group level and at an individual level. We found that food resource use was an important predictor of trauma risk at both group and individual levels; greater use of food resources reduced trauma risk. Produce enrichment did not; however, reduce severe aggression. We suggest that other captive social groups of rhesus macaques with high levels of trauma may benefit from supplemental produce enrichment that increases animal engagement with food resources.     

Dominance hierarchy and social grooming in female lion- tailed macaques (Macaca silenus) in the Western Ghats, India

This article reports the structure of dominance and its relationship with social grooming in wild lion-tailed macaque females. The strength of dominance hierarchy was 0.79 on a scale of 0 to 1 indicating a moderate linearity in the ranking system. Dominance scores were converted into an ordinal as well as an interval scale. Grooming scores were also converted into interval scales using standard scores. Grooming received and grooming given correlated positively and negatively respectively with dominance ranks indicating that high ranking females received more and gave less grooming. Grooming was also positively related to encounter rates for dyads of females. More grooming among adjacent ranks, and grooming being more reciprocal, occurred only in the case of dominant females. The grooming patterns, therefore, appeared to be more of despotic than egalitarian nature. While ranking macaques into different Grades of social systems ranging from despotic to egalitarian, Thierry (2004) has placed lion-tailed macaques in Grade 3 corresponding to the ‘relaxed’ social system. Our results indicate that the grooming and dominance relationships in this species are more despotic, and hence, the Grade for this species requires to be shifted toward 2 or 1.     

Counter aggression and reconciliation in Assamese macaques (Macaca assamensis)

Patterns of aggressive and affiliative behavior, such as counter aggression and reconciliation, are said to covary in the genus Macaca; this is referred to as the systematic variation hypothesis. These behavior patterns constitute a species dominance style. Van Schaik's [1989] socioecological model explains dominance style in macaques in terms of within- and between-group contest competition. Dominance style is also said to correlate with phylogeny in macaques. The present study was undertaken to examine phylogenetic and socioecological explanations of dominance style, as well as the systematic variation hypothesis. We collected data on counter aggression and reconciliation from a habituated group of Assamese macaques (Macaca assamensis) at the Tukeswari Temple in Assam, India. The proportion of agonistic episodes that involved counter aggression was relatively low. Counter aggression, however, occurred more often among males than among females, and it was most common when females initiated aggression against males. The conciliatory tendency for this group of Assamese macaques was 11.2%. The frequency of reconciliation was low for fights among males and for fights among females, but reconciliation was particularly rare for opposite-sexed opponents. Female social relationships were consistent with the systematic variation hypothesis, and suggest a despotic dominance style. A despotic dominance style in Assamese macaques weakens the correlation between dominance style and phylogeny in macaques, but it is not inconsistent with the socioecological model. Male-female relationships were not well explained by the despotic-egalitarian framework, and males may well have more tolerant social relationships than do females. Sex differences need to be considered when categorizing species according to dominance style.     

Male Social Behavior and Dominance Hierarchy in the Sulawesi Crested Black Macaque (Macaca nigra)

In a 6-week study of the social behavior of wild Sulawesi crested black macaques (Macaca nigra), we found a linear and transitive dominance hierarchy among the six adult males in one social group. Dominance rank, as determined by the direction of supplantations, correlated strongly with percentage of time near more than four neighbors, frequency of grooming received from adult females, and percentage of time with an adult female as nearest neighbor. These results suggest that high-ranking males are socially attractive. Adult females sexually solicited high-ranking males more often than low-ranking males, but frequency of copulation was not correlated with dominance rank. Frequency and intensity of aggression between males are strongly correlated with rank distance, but aggression toward females was greatest for mid-ranking males. Males of all rank displayed significantly more aggression toward sexually receptive females than toward females in other estrous states. These data indicate that male Sulawesi crested black macaques display a social organization similar to that reported for multimale groups in other macaque species rather than the egalitarian social organization described for female Sulawesi macaques.     

Dominance style of Japanese macaques compared with rhesus and stumptail macaques

In the present study, we seek to relate dominance style with group cohesion in a captive group of Japanese macaques (Macaca fuscata). Social data were gathered on approach rate, result, and direction, aggression rate and intensity, grooming rate and direction, and conciliatory tendency. Data were collected using focal animal sampling and instantaneous scan sampling. Reconciliation data were collected using ad libitum observations of aggression with ten-minute post-conflict and matched-control focal observations. Data were compared to prior studies on rhesus (M. mulatta) and stumptail macaques (M. arctoides) living in similar environments. Each species demonstrated the presence of a formalized dominance hierarchy based on the teeth-baring display. The Japanese macaque group showed a lower rate of approach with a higher proportion of negative outcomes than either of the other species. Rates of aggression and reconciliation were also lower in the study troop, suggesting a strict hierarchy while maintaining an optimal nearest-neighbor distance. Overall, this group of Japanese macaques was less sociable than other groups of the same species, perhaps due to a history of individual removals. © 1995 Wiley-Liss, Inc.     

Extra-large cluster formation by Japanese macaques (Macaca fuscata) on Shodoshima Island, central Japan, and related factors

Japanese macaques on Shodoshima Island habitually form very large rest clusters, in which 50+ or even 100+ individuals huddle together. This behavior is not seen in any other populations of the species. Mean cluster sizes of two groups of Shodoshima monkeys are three and four in summer and 17 and 16 in winter, respectively. A maximum of 137 individuals have been seen to huddle in one cluster. It is difficult to explain the extra large clusters on Shodoshima only as an adaptive behavior against cold, since Shodoshima is relatively warm in the range of habitats for Japanese macaques. Compared with other groups of Japanese macaques, Shodoshima monkeys show: more frequent affinitive interactions, shorter inter-individual distance, more frequent ignoring of exclusion, more frequent aggression, less intense aggression, and more frequent counter-aggression. These characteristics suggest that the Japanese macaques on Shodoshima have relaxed dominant relations. The specific social organization of Shodoshima monkeys may sustain the formation of extra large clusters. Inter-group comparisons suggest that the social structure of Japanese macaques might be highly plastic, and that Shodoshima monkeys have less despotic, more tolerant social relations than typically reported for this species.     

The evolution of “egalitarian” and “despotic” social systems among macaques

Recent studies of captive macaques have revealed considerable inter-species differences in dominance styles among females. In “egalitarian” species such as stumptail (Macaca arctoides) or tonkean macaques (M. tonkeana), social interactions are more symmetrical and less kin-biased than in “despotic” species such as Japanese (M. fuscata) or rhesus macaques (M. mulatta). Field observations of moor macaques (M. maurus), close relatives of tonkean macaques, suggest that tolerance during feeding characterizes their egalitarian dominance style in the natural habitat. Although it has been proposed that communal defense against other groups may be the main selective force in the evolution of egalitarian dominance style among females, few field data support this prediction. A game theory analysis showed that both an “egalitarian” strategy and a “despotic” strategy are possible evolutionarily stable strategies (ESS) under certain conditions. The difference in dominance styles might reflect the difference in ESS. This means that an egalitarian dominance style can emerge without strong between-group contest competition. A phylogenetic comparison among macaques suggests that despotic dominance styles very likely evolved from egalitarian dominance styles. In the future, primate socioecological studies should pay more attention to the evolutionary history of each species.     

Despotic wild patas monkeys (Erythrocebus patas) in Kala Maloue, Cameroon

The socio-ecological model predicts that the quality, distribution, and patch size of food resources determines the dominance hierarchy of female monkeys based on the type of food competition they experience. Comparative studies of closely related species have evaluated the socio-ecological model and confirmed its validity. For example, female patas monkeys in Laikipia, Kenya, form a nonlinear and unstable dominance hierarchy (i.e., egalitarian), whereas females of sympatric, closely related savannah monkeys form a linear and stable dominance hierarchy (i.e., despotic), in accordance with the model's predictions of the characteristics of food resources. I compared agonistic interactions involving food between patas monkeys (Erythrocebus patas) and sympatric savannah monkeys (Cercopithecus aethiops) in Kala Maloue, Cameroon. I found linear dominance hierarchies not only in savannah monkeys, but also in patas monkeys in Kala Maloue. The rates of agonistic interactions during feeding between patas monkeys were equivalent to those between savannah monkeys in Kala Maloue; further, these rates were significantly higher than those of both Laikipia patas and savannah monkeys. The results imply that patas monkeys in Kala Maloue are not egalitarian, but are despotic, similar to savannah monkeys. Disparity in the dominance hierarchies of patas monkeys between Kala Maloue and Laikipia were attributable to the differences in the characteristics of food resources. Although patas monkeys in Laikipia subsist on small and dispersed food resources within a high-density area, those in Kala Maloue subsisted on food resources that were clumped in intermediate-sized patches within a low-density area. This study shows that the socio-ecological model is applicable not only for interspecific comparisons but also for intraspecific comparisons.     

Dominance Style Among Macaca thibetana on Mt. Huangshan, China

The dominance style concept has proven useful for understanding covariation patterns in relationship qualities, particularly among macaques. However, the dominance styles of many macaques, including Tibetan macaques (Macaca thibetana), have not been examined in detail. We describe patterns of bidirectionality of aggression, postconflict affiliation and kin bias in a group of wild, but provisioned Tibetan macaques over a 2-yr period in order make an initial assessment of their dominance style. Bidirectional aggression, including percentage of counteraggression (1.9%), and conciliatory tendencies (6.4%) were consistently low across partner combinations, seasons and locations (forest vs. provisioning area). In addition, females consistently displayed high levels of kin bias in affiliation and tolerance. Compared with macaque species with better known dominance styles, the Tibetan data generally fell within the range for despotic species and outside the range for relaxed species. Although other researchers have tentatively classified them as tolerant or relaxed, we conclude that Tibetan macaques display a despotic dominance style. This conclusion poses complications to explanations based both on phylogenetic inertia and socio-ecological models.     

Dyadic and triadic reconciliation in pigtail macaques (Macaca nemestrina)

The tendency in primates for former antagonists to approach and affiliate following aggression has been termed reconciliation because the response is thought to resolve social conflicts produced by aggression. In primate societies, however, an aggressive interaction between two individuals often spreads to include other group members, especially the kin of the combatants. If post conflict affiliation resolves aggressive conflicts in a group, then affiliative increases might occur between combatants and the kin of their opponents following aggression as well as between former opponents. This hypothesis was tested in a captive group of 39 pigtail macaques (Macaca nemestrina) by comparing affiliative response frequencies of combatants during the 5 minute period following aggression to affiliative response frequencies during 5 minute baseline periods not preceded by aggressive activity. Following aggression, affiliation rates increased between combatants and their opponents, aggressors and the kin of their opponents, and aggressors and their own kin. Additional analyses showed that aggression among kin was reconciled more often than aggression among nonkin. Recipients of aggression reconciled with their attackers more often than aggressors reconciled with their victims. Animals with similar dominance ranks reconciled proportionately more often than those with large rank disparities and aggressive infractions of a calculated dominance hierarchy were reconciled more often than attacks consistent with the hierarchy. Results suggest that both dyadic and triadic reconciliations occur in M. nemestrina and that compared to other primate species M. nemestrina exhibit a moderate-to-high conciliatory tendency.     

Conflict and reconciliation in two groups of crab-eating monkeys differing in social status by birth

Two groups of captive macaques (M. fascicularis) were studied at Kassel University, Germany. One included animals whose mothers were high-ranking, another, those whose mothers were low-ranking. The first group was a despotic community in which conflicts were severe and occurred mainly between single individuals; the reconciliation tendency was weak, the male leader was the controlling animal, and the affiliative preferences were marked. The second group was an egalitarian community split into two mutually hostile conalitions; the conflicts were less severe, the tendency for reconciliation was strong, the male leader could control only his own bloc and had no strong affiliative ties with other group members.     

Visual signals of individual identity in the wasp Polistes fuscatus

Individual recognition is an essential component of interactions in many social systems, but insects are often thought incapable of the sophistication necessary to recognize individuals. If this were true, it would impose limits on the societies that insects could form. For example, queens and workers of the paper wasp Polistes fuscatus form a linear dominance hierarchy that determines how food, work and reproduction are divided within the colony. Such a stable hierarchy would be facilitated if individuals of different ranks have some degree of recognition. P. fuscatus wasps have, to our knowledge, previously undocumented variability in their yellow facial and abdominal markings that are intriguing candidates for signals of individual identity. Here, I describe these highly variable markings and experimentally test whether P. fuscatus queens and workers use these markings to identify individual nest-mates visually. I demonstrate that individuals whose yellow markings are experimentally altered with paint receive more aggression than control wasps who are painted in a way that does not alter their markings. Further, aggression declines towards wasps with experimentally altered markings as these novel markings become familiar to their nestmates. This evidence for individual recognition in P. fuscatus indicates that interactions between insects may be even more complex than previously anticipated.     

Feedback loop between kinship and dominance: the macaque model

There is growing evidence that macaque social systems represent sets of coadapted traits in which strength of hierarchies and degree of nepotism covary. A framework is developed to explain the link between dominance and kinship phenomena, assuming that power brought by alliances among non-kin is allometrically related to those involving relatives. This can account for the type of social relationships observed in "despotic" systems vs. "egalitarian" ones. When social bonds are mostly founded on kinship, lineages are closed and social power generated by coalitions among relatives may reach high levels; social power frequently outweighs the fighting abilities of single individuals, and asymmetry of dominance between group members may be marked. When lineages are more open, social bonds and alliances are less kin-biased, social relationships are more equal, and as the influence of coalitions is less important, the individual retains a certain degree of freedom in relation to the power of kin-networks. Acknowledging that the balance between individual and social power is not set at the same level across different species can explain a number of variations in rules of rank inheritance and relative dominance of males and females among macaques. The framework illustrates how epigenetic processes may shape complex features of primate social systems, and offers opportunities for testing.