A Proximity-Based Social Network of a Group of Sichuan Snub-Nosed Monkeys (Rhinopithecus roxellana)

Examining postmounting grooming in male golden snub-nosed monkeys to investigate the functions of same-sex mounts

Same-sex mounting behavior has been documented across primate species and several biological functions, that are often difficult to differentiate, have been proposed. Postmounting affiliative behaviors (e.g., embracing and grooming) have been partly overlooked and their performance may more clearly reveal the function of same-sex mounts for different age-sex classes. Here, considering postmounting grooming behaviors (PMG), we tested the affiliation hypothesis for same-sex mounts in an all-male unit (AMU) of captive golden snub-nosed monkeys (Rhinopithecus roxellana) (n = 7-8) and the needing to learn hypothesis in the juveniles of the AMU (n = 4) and the juvenile male from the neighboring one-male unit in Shanghai Wild Animal Park, China from November 2014 to June 2015. A total of 1986 same-sex mounts were recorded from the AMU individuals and the juvenile of the neighboring one-male unit. We found that neither dyadic proximity-based association nor grooming-based affiliation predicted the occurrence of mounts in the AMU, and PMG was more likely to occur in nontense than tense social contexts, providing no support for the affiliation hypothesis. Although dyadic grooming-based affiliation predicted the occurrence of PMG, it exerted no influence on the occurrence of mounts, which was necessary to support the hypothesis. However, consistent with the needing to learn hypothesis, from ages two to four, juveniles' mounting duration increased and they performed pelvic thrusting during mounting more and more often as they grew older. Reciprocated series mounts were much more common among juveniles than other mounting dyads, providing learning opportunities for both participants. The mounter was more likely to groom the mountee at the end of a mount among juveniles than other mounting dyads, suggesting that juvenile mounters may profit from mounting and groom mountees in return for providing them with opportunities to learn copulatory skills. A hypothesis that merits further investigation.

A Shared Intentionality Account of Uniquely Human Social Bonding

Many mechanisms of social bonding are common to all primates, but humans seemingly have developed some that are unique to the species. These involve various kinds of interactive experiences-from taking a walk together to having a conversation-whose common feature is the triadic sharing of experience. Current theories of social bonding have no explanation for why humans should have these unique bonding mechanisms. Here we propose a shared intentionality account of uniquely human social bonding. Humans evolved to participate with others in unique forms of cooperative and communicative activities that both depend on and create shared experience. Sharing experience in these activities causes partners to feel closer because it allows them to assess their partner's cooperative competence and motivation toward them and because the shared representations created during such interactions make subsequent cooperative interactions easier and more effective.

Drone Observation for the Quantitative Study of Complex Multilevel Societies

Unmanned aerial vehicles (drones) have recently been used in various behavioral ecology studies. However, their application has been limited to single groups, and most studies have not implemented individual identification. A multilevel society refers to a social structure in which small stable "core units" gather and make a larger, multiple-unit group. Here, we introduce recent applications of drone technology and individual identification to complex social structures involving multiple groups, such as multilevel societies. Drones made it possible to obtain the identification, accurate positioning, or movement of more than a hundred individuals in a multilevel social group. In addition, in multilevel social groups, drones facilitate the observation of heterogeneous spatial positioning patterns and mechanisms of behavioral propagation, which are different from those in a single-level group. Such findings may contribute to the quantitative definition and assessment of multilevel societies and enhance our understanding of mechanisms of multiple group aggregation. The application of drones to various species may resolve various questions related to multilevel societies.

Patent Value Promotion Based on the Technology Proximity Network: An Empirical Analysis of Artificial Intelligence for Healthcare

Patent value plays an important role in the open innovation system, which can stimulate enthusiasm for research and development investment and innovation collaboration. High-value patents on artificial intelligence for healthcare can attract venture capital and encourage more enterprises to conduct research and development, thus driving the development of related industrial clusters for a smart medical system. This study focused on the mechanism of impacting the patent value from the perspective of technological proximity and made an empirical analysis by using patent data and multiple ordinary least squares regression models. The results showed that the technological proximity network extracted from patent data significantly affected patent value through the centrality measurements of patent nodes in the proximity network. Finally, the study also provided some implications for the formulation of research and development planning and technological innovation policies.

Do golden snub-nosed monkeys use deceptive alarm calls during competition for food?

Tactical deception can be beneficial for social animals during intra-specific competition. However, the use of tactical deception in wild mammals is predicted to be rare. We tested whether a food-provisioned free-ranging band of golden snub-nosed monkeys (Rhinopithecus roxellana) use alarm calls in a functionally deceptive manner to gain access to food resources, whether the rate of deceptive alarm calls varies among individuals, and whether there are any counter-deception behaviors. We used a hexagonal camera array consisting of 10 cameras to record videos during feeding, which allowed us to identify individual alarm callers. We found evidence that these monkeys use deceptive alarms and that adult females were more likely to use such calls than other individuals. The monkeys increased their rates of response to alarm calls when competition for food was high. However, we found no direct evidence of any counter-deception strategies.

Maternal caretaking behavior towards a dead juvenile in a wild, multi-level primate society

Maternal caretaking and transport of dead infants are widespread among nonhuman primates, having been reported in numerous species of monkeys and apes. By contrast, accounts of such behaviors toward dead juveniles are scarce. Here, we describe responses by the mother and other group members to the death of a juvenile in a wild, multi-level group of Sichuan snub-nosed monkeys (Rhinopithecus roxellana). Following the juvenile’s fatal accident, his mother transported and cared for the corpse for four days. Immature monkeys belonging to the same one-male unit, and some individuals from other social units also showed interest in and tended the corpse. Comparisons of this case with those involving the deaths of infants and an adult female in the same population highlight possible effects of physiological, psychological and emotional factors in primate thanatological responses, and provide an additional perspective on the origin and evolution of compassionate acts.

Repeatability and heritability of social reaction norms in a wild agamid lizard

In the evolutionary transition from solitary to group living, it should be adaptive for animals to respond to the environment and choose when to socialize to reduce conflict and maximize access to resources. Due to the associated proximate mechanisms (e.g. neural network, endocrine system), it is likely that this behavior varies between individuals according to genetic and non-genetic factors. We used long-term behavioral and genetic data from a population of eastern water dragons (Intellagama lesueurii) to explore variation in plasticity of social behavior, in response to sex ratio and density. To do so, we modeled individual variation in social reaction norms, which describe individuals' mean behavior and behavioral responses to changes in their environment, and partitioned variance into genetic and non-genetic components. We found that reaction norms were repeatable over multiple years, suggesting that individuals consistently differed in their behavioral responses to changes in the social environment. Despite high repeatability of reaction norm components, trait heritability was below our limit of detection based on power analyses (h² < 0.12), leading to very little power to detect heritability of plasticity. This was in contrast to a relatively greater amount of variance associated with environmental effects. This could suggest that mechanisms such as social learning and frequency-dependence may shape variance in reaction norms, which will be testable as the dataset grows.

Aerial drone observations identified a multilevel society in feral horses

The study of non-human multilevel societies can give us insights into how group-level relationships function and are maintained in a social system, but their mechanisms are still poorly understood. The aim of this study was to apply spatial association data obtained from drones to verify the presence of a multilevel structure in a feral horse society. We took aerial photos of individuals that appeared in pre-fixed areas and collected positional data. The threshold distance of the association was defined based on the distribution pattern of the inter-individual distance. The association rates of individuals showed bimodality, suggesting the presence of small social organizations or "units". Inter-unit distances were significantly smaller than those in randomly replaced data, which showed that units associate to form a higher-level social organization or "herd". Moreover, this herd had a structure where large mixed-sex units were more likely to occupy the center than small mixed-sex units and all-male-units, which were instead on the periphery. These three pieces of evidence regarding the existence of units, unit association, and stable positioning among units strongly indicated a multilevel structure in horse society. The present study contributes to understanding the functions and mechanisms of multilevel societies through comparisons with other social indices and models as well as cross-species comparisons in future studies.

Spatiotemporal association patterns in a supergroup of Rwenzori black-and-white colobus (Colobus angolensis ruwenzorii) are consistent with a multilevel society

Primates display broad diversity in their social organization. The social groups of a few primate species are organized in a multilevel fashion, with large groups composed of multiple, core one-male units (OMUs). A characteristic of multilevel societies is that the higher levels can include hundreds of individuals. The Rwenzori black-and-white colobus (Colobus angolensis ruwenzorii) in the montane forests of Rwanda form supergroups and have been suspected to exhibit multilevel social organization. Here we present the first data on the "anatomy" of a supergroup numbering 500+ individuals. We identified subgroups within the supergroup based on progression data, extracting the social network structure from the time-stamped spatiotemporal distribution of passing individuals identified to age-sex class, and selecting an optimal time window for each network using the two-step approach developed by Uddin, Choudhury, Farhad, and Rahman (2017). We detail the existence of core units-multi-male units (MMUs) with a mean of 1.7 adult males and 3.1 adult females, as well as OMUs, all-female units and bachelor units composed of adult and sub-adult males. More than two-thirds of units are MMUs. These grouping patterns conform to a multilevel society with predominantly multi-male core units, a social system that has recently also been described for a population of the same taxon in Uganda. Individual identification will be required to corroborate these interpretations.

A multi-level society comprised of one-male and multi-male core units in an African colobine (Colobus angolensis ruwenzorii)

Several mammalian species exhibit complex, nested social organizations, termed multi-level or modular societies. Multi-level societies comprise stable core units that fission and fuse with one another in a hierarchical manner, forming groups that vary in size over time. Among nonhuman primates, these social systems have been confirmed in several African papionin and Asian colobine species. We use data from August 2017 to July 2018 on individually-recognized Rwenzori Angolan colobus living near Lake Nabugabo, Uganda to document the first multi-level society in an African colobine. The study band comprised up to 135 individuals organized into 12 socially and spatially distinct core units that ranged in size from 4 to 23 individuals. These core units showed a strong affinity to one another, spending roughly 75% of their time together. Core units fissioned and fused non-randomly with one another throughout the day, leading to different combinations of core units being observed. Using association indices between core units, we employed hierarchical cluster analyses and permutation tests to show that some core units associated preferentially into clans. Thus, we confirm three tiers of social organization for Rwenzori Angolan colobus: core unit, clan, and band. The social organization of this subspecies is unlike any reported previously in a nonhuman primate, with about half the core units containing a single adult male and the others containing multiple reproductive adult males. The discovery of a unique primate multi-level society in a novel lineage could allow for a better understanding of the evolution of these complex social systems across the Animal Kingdom. Preliminary data show males transfer within the band and females transfer outside of the band, which is proposed for hominin multi-level societies. This subspecies could thus also provide insight into the selective pressures underlying multi-level societies in our own lineage.

Presence of kin-biased social associations in a lizard with no parental care: the eastern water dragon (Intellagama lesueurii)

Numerous studies have observed kin-biased social associations in a variety of species. Many of these studies have focused on species exhibiting parental care, which may facilitate the transmission of the social environment from parents to offspring. This becomes problematic when disentangling whether kin-biased associations are driven by kin recognition, or are a product of transmission of the social environment during ontogeny, or a combination of both. Studying kin-biased associations in systems that lack parental care may aid in addressing this issue. Furthermore, when studying kin-biased social associations, it is important to differentiate whether these originate from preferential choice or occur randomly as a result of habitat use or limited dispersal. Here, we combined high-resolution single-nucleotide polymorphism data with a long-term behavioral data set of a reptile with no parental care to demonstrate that eastern water dragons (Intellagama lesueurii) bias their nonrandom social associations toward their kin. In particular, we found that although the overall social network was not linked to genetic relatedness, individuals associated with kin more than expected given availability in space and also biased social preferences toward kin. This result opens important opportunities for the study of kinship-driven associations without the confounding effect of vertical transmission of social environments. Furthermore, we present a robust multiple-step approach for determining whether kin-biased social associations are a result of active social decisions or random encounters resulting from habitat use and dispersal patterns.

Individuality in coo calls of adult male golden snub-nosed monkeys (Rhinopithecus roxellana) living in a multilevel society

Vocal individuality is a prerequisite for individual recognition, especially when visual and chemical cues are not available or effective. Vocalizations encoding information of individual identity have been reported in many social animals and should be particularly adaptive for species living in large and complexly organized societies. Here, we examined the individuality in coo calls of adult male golden snub-nosed monkeys (Rhinopithecus roxellana) living in a large and multilevel society. Coo calls are one of the most frequently occurring call types in R. roxellana and likely serve as the signals for contact maintenance or advertisement in various contexts including group movement, foraging, and resting. From April to October 2016, April to July 2017, and September to October 2017, we recorded a total of 721 coo calls from six adult males in a provisioned, free-ranging group and one adult male in captivity in Shennongjia National Park, China. We selected 162 high-quality recordings to extract 14 acoustic parameters based on the source-filter theory. Results showed that each of all parameters significantly differed among individuals, while pairwise comparisons failed to detect any parameter that was different between all pairs. Furthermore, a discriminant function analysis indicated that the correct assignment rate was 80.2% (cross-validation: 67.3%), greater than expected by chance (14.3%). In conclusion, we found evidence that coo calls of adult male R. roxellana allowed the reliable accuracy of individual discrimination complementarily enhanced by multiple acoustic parameters. The results of our study point to the selective pressures acting on individual discrimination via vocal signals in a highly gregarious forest-living primate.

Sleeping site and tree selection by Sichuan snub-nosed monkeys (Rhinopithecus roxellana) in Baihe Nature Reserve, Sichuan, China

For non-human primates to optimize their survival chances, sleeping site selection is crucial as they spend much of their time sleeping. We studied sleeping site and tree selection by a group of wild Sichuan snub-nosed monkeys (Rhinopithecus roxellana) in the temperate forests of Baihe Nature Reserve, Sichuan, China, to assess if certain site and tree characteristics were selected. We identified a total of 39 sleeping sites and 111 sleeping trees over a period of 1 year. We compared nine sleeping site and six sleeping tree variables related to the environment, habitat, and vegetation structure. We found that certain characteristics of sleeping sites and sleeping trees predicted their selection by R. roxellana. On a larger spatial scale, sleeping sites were selected based mainly on four factors: canopy height, slope direction, slope gradient, and vegetation type. They also selected sites with trees that were taller and larger, had larger crown diameters and higher bole branches. On a smaller spatial scale, they selected larger trees with larger crown diameters. The selection of these characteristics could be explained in terms of predator avoidance and thermoregulation efficiency, although it was difficult to delineate which was more important. This could be due to some characteristics of R. roxellana, that is, large groups living in temperate climates, which required a balanced strategy for sleeping site and tree selection in order to optimize their survival chances.

Social connectivity among female Tibetan macaques (Macaca thibetana) increases the speed of collective movements

Social network analysis provides insights into patterns of group movements in primates, but fewer studies to date have focused on the dynamics of how such movements occur. In this study, we proposed and tested two hypotheses about the influence of sex on social connectivity and group movement in Tibetan macaques (Macaca thibetana): (1) adult females are socially more connected than are adult males and (2) social connectivity facilitates the speed of collective decision-making. We collected data from 128 successful collective movements (≥ 2 individuals followed an initiator within 5 min) over a 2-month period in a group of adult Tibetan macaques at Mt. Huangshan, China. Although high-ranking individuals of both sexes in our dataset were more central in their social network than were low-ranking individuals, our results show that affiliations between females were stronger, with more preferred associations than those between males. Groups with more females reached collective decisions faster than groups with fewer females. We conclude that female Tibetan macaques use their social networks to enhance the speed of collective decision-making, which may have associated fitness benefits.

The influence of a demographic change on social relationships among male golden snub-nosed monkeys (Rhinopithecus roxellana)

It has been suggested that social relationships are more likely to be prone to variation in the dispersing sex than the philopatric sex. However, we know less about the dynamics of all-male groups in male-dispersing species than we do about other types of primate groups. We studied male sociality in a captive group of golden snub-nosed monkeys (Rhinopithecus roxellana), which was composed of a one-male unit (OMU, N = 7) and an all-male unit (AMU, N = 7 or 8), in Shanghai Wild Animal Park, China. Using data collected for 6 months, during which there was a demographic change in the AMU and the alpha male was replaced by a newcomer, we found that a dramatic change in social ranks occurred accompanied by elevated aggression following this social upheaval. A proximity-based social network analysis revealed that members did not associate randomly any more but formed differentiated relationships post-upheaval, resulting in three distinct sub-units in the AMU. In terms of inter-unit interactions, significant changes were found in the affiliations between the male juvenile of OMU and AMU individuals. He interacted with AMU individuals randomly and frequently pre-upheaval, but cut down his affiliations and had a preferred partner post-upheaval, who was a member of the dominant male's sub-unit. Our findings suggest that social networks in the dispersing sex are dynamic structures and vary by some demographic change (e.g., individual immigration) in the studied species. We also put forward that individual dominance could be a criterion when the male juvenile chooses partners before he immigrates into a group. In conclusion, the high level of behavioral flexibility of the dispersing sex could be an evolutional strategy and good for individuals' future dispersing life.

Vocal repertoire of free-ranging adult golden snub-nosed monkeys (Rhinopithecus roxellana)

Vocal signaling represents a primary mode of communication for most nonhuman primates. A quantitative description of the vocal repertoire is a critical step in in-depth studies of the vocal communication of particular species, and provides the foundation for comparative studies to investigate the selective pressures in the evolution of vocal communication systems. The present study was the first attempt to establish the vocal repertoire of free-ranging adult golden snub-nosed monkeys (Rhinopithecus roxellana) based on quantitative methods. During 8 months in Shennongjia National Park, China, we digitally recorded the vocalizations of adult individuals from a provisioned, free-ranging group of R. roxellana across a variety of social-ecological contexts. We identified 18 call types, which were easily distinguishable by ear, visual inspection of spectrograms, and quantitative analysis of acoustic parameters measured from recording samples. We found a great sexual asymmetry in the vocal repertoire size (females produced many more call types than males), likely due to the sex differences in body size and social role. We found a variety of call types that occurred during various forms of agonistic and affiliative interactions at close range. We made inference about the functions of particular call types based on the contexts in which they were produced. Studies on the vocal communication in R. roxellana are particularly valuable since they provide a case about how nonhuman primates, inhabiting forest habitats and forming complex social systems, use their vocalizations to interact with their social and ecological environments.

The influence of age on wild rhesus macaques' affiliative social interactions

The social relationships that individuals experience at different life stages have a non-negligible influence on their lives, and this is particularly true for group living animals. The long lifespan of many primates makes it likely that these animals have various tactics of social interaction to adapt to complex changes in environmental or physical conditions. The different strategies used in social interaction by individuals at different life stages, and whether the position (central or peripheral) or role (initiator or recipient) of an individual in the group social network changes with age, are intriguing questions that remain to be investigated. We used social network analysis to examine age-related differences in social interaction patterns, social roles, and social positions in three affiliative social networks (approach, allogrooming, and social play) in a group of wild rhesus macaques (Macaca mulatta). Our results showed that social interaction patterns of rhesus macaques differ between age classes in the following ways: i) young individuals tend to allocate social time to a high number of groupmates, older individuals prefer to focus on fewer, specific partners; ii) as they grow older, individuals tend to be recipients in approach interactions and initiators in grooming interactions; and iii) regardless of the different social interaction strategies, individuals of all ages occupy a central position in the group. These results reveal a possible key role played by immature individuals in group social communication, a little-explored issue which deserves closer investigation in future research.

A framework for the identification of long-term social avoidance in longitudinal datasets

Animal sociality is of significant interest to evolutionary and behavioural ecologists, with efforts focused on the patterns, causes and fitness outcomes of social preference. However, individual social patterns are the consequence of both attraction to (preference for) and avoidance of conspecifics. Despite this, social avoidance has received far less attention than social preference. Here, we detail the necessary steps to generate a spatially explicit, iterative null model which can be used to identify non-random social avoidance in longitudinal studies of social animals. We specifically identify and detail parameters which will influence the validity of the model. To test the usability of this model, we applied it to two longitudinal studies of social animals (Eastern water dragons (Intellegama lesueurii) and bottlenose dolphins (Tursiops aduncus)) to identify the presence of social avoidances. Using this model allowed us to identify the presence of social avoidances in both species. We hope that the framework presented here inspires interest in addressing this critical gap in our understanding of animal sociality, in turn allowing for a more holistic understanding of social interactions, relationships and structure.

Postconflict behavior among Rhinopithecus roxellana leader males in the Qinling Mountains, China

Since reconciliation was first described more than 20 years ago, a large number of postconflict behaviors have been observed among females in many polygynous primate species. However, few studies have been conducted among males, perhaps due to the rarity with which they maintain friendly relationships with one another and their aggressive competition for resources. Although this is true for many primate males, Sichuan snub-nosed monkeys Rhinopithecus roxellana represent a potential exception as male cooperation has been known to occur. In this study, using postconflict/matched-control(PC-MC) and time-rule methods, we analyzed postconflict behavior among males and the possible occurrence of bystander affiliation or reconciliation. A total of 246 PC-MC pairs among leader males were obtained. On average, each leader male exhibited only 0.04 aggressive behaviors per observation hour, and conciliation among leader males occurred at a low rate (2.03%) relative to other primate species. The occurrence of consolation-affiliation interactions between focal males and group members other than former opponents differed significantly between PCs and MCs, which is the first time this has been confirmed among R. roxellana males. We discuss the results in light of recent theories concerning consolation in primates. The patterns of postconflict contact demonstrated that R. roxellana may be a unique species among colobines.

Affiliative Subgroups in Preschool Classrooms: Integrating Constructs and Methods from Social Ethology and Sociometric Traditions

Recent studies of school-age children and adolescents have used social network analyses to characterize selection and socialization aspects of peer groups. Fewer network studies have been reported for preschool classrooms and many of those have focused on structural descriptions of peer networks, and/or, on selection processes rather than on social functions of subgroup membership. In this study we started by identifying and describing different types of affiliative subgroups (HMP- high mutual proximity, LMP- low mutual proximity, and ungrouped children) in a sample of 240 Portuguese preschool children using nearest neighbor observations. Next, we used additional behavioral observations and sociometric data to show that HMP and LMP subgroups are functionally distinct: HMP subgroups appear to reflect friendship relations, whereas LMP subgroups appear to reflect common social goals, but without strong, within-subgroup dyadic ties. Finally, we examined the longitudinal implications of subgroup membership and show that children classified as HMP in consecutive years had more reciprocated friendships than did children whose subgroup classification changed from LMP or ungrouped to HMP. These results extend previous findings reported for North American peer groups.

Nearest-neighbour clusters as a novel technique for assessing group associations

When all the individuals in a social group can be easily identified, one of the simplest measures of social interaction that can be recorded is nearest-neighbour identity. Many field studies use sequential scan samples of groups to build up association metrics using these nearest-neighbour identities. Here, I describe a simple technique for identifying clusters of associated individuals within groups that uses nearest-neighbour identity data. Using computer-generated datasets with known associations, I demonstrate that this clustering technique can be used to build data suitable for association metrics, and that it can generate comparable metrics to raw nearest-neighbour data, but with much less initial data. This technique could therefore be of use where it is difficult to generate large datasets. Other situations where the technique would be useful are discussed.

Satellite telemetry and social modeling offer new insights into the origin of primate multilevel societies

Multilevel societies (MLS), in which polygynous reproductive units are nested in a larger social matrix, represent a highly complex social system documented only in a small number of mammalian species. Using long-term behavioural data, satellite telemetry and social network analysis, we present a new framework for understanding the function and social dynamics of the golden snub-nosed monkey MLS. Here we show that several one-male units form a cohesive breeding band that associates with one or more all-male units to form a herd. Herds seasonally aggregate and exchange members, thus facilitating gene flow and inbreeding avoidance. This MLS evolved from the aggregation of independent one-male, multifemale units that characterize ancestral Asian colobines; the evolutionary pathway leading to this MLS contrasts with that proposed for African papionins, which appear to have undergone internal fissioning of multimale-multifemale groups. The results suggest that both environmental and phylogenetic factors are important in the evolution of a primate MLS.

Males collectively defend their one-male units against bachelor males in a multi-level primate society

Group-level male-male co-operation, which has been documented in several primate and non-primate societies, may be mutualistically advantageous to the participants when confronted with threats such as takeovers and cuckoldry by external males. Co-operation among members of distinct social units-while universal among humans-is extremely rare in non-human primates. We present the first observations of collective action or co-operation among males of different one-male units (OMU) in a multi-level society of Rhinopithecus roxellana. A total of 59 instances of male co-operation were recorded. Male co-operation included coordinated chasing, joint vigilance, and patrolling behavior directed at lone adult males trying to enter an OMU. Male co-operation was significantly more frequent during the mating season when the risk of incursions and extra-group paternity was higher. Paternity of infants born in the subsequent birth season and kin relationships among resident males were identified using microsatellite genotype. All infants were sired by OMU males, which we interpret as possible evidence for their success at thwarting mating attempts by satellite males. OMU males were principally unrelated suggesting that male co-operation is best understood in terms of the mutual direct benefits individuals obtain through collective action. Our findings lend support to the bachelor threat hypothesis in which the cooperative behavior of several individuals is more effective than the lone action of a single individual in providing mate defense. Our research has implications for understanding male bonding, higher-level collective action, and the evolution of social co-operation in human societies.

Evolution of Multilevel Social Systems in Nonhuman Primates and Humans

Multilevel (or modular) societies are a distinct type of primate social system whose key features are single-male-multifemale, core units nested within larger social bands. They are not equivalent to fission-fusion societies, with the latter referring to routine variability in associations, either on an individual or subunit level. The purpose of this review is to characterize and operationalize multilevel societies and to outline their putative evolutionary origins. Multilevel societies are prevalent in three primate clades: papionins, Asian colobines, and hominins. For each clade, we portray the most parsimonious phylogenetic pathway leading to a modular system and then review and discuss likely socioecological conditions promoting the establishment and maintenance of these societies. The multilevel system in colobines (most notably Rhinopithecus and Nasalis) has likely evolved as single-male harem systems coalesced, whereas the multilevel system of papionins (Papio hamadryas, Theropithecus gelada) and hominins most likely arose as multimale-multifemale groups split into smaller units. We hypothesize that, although ecological conditions acted as preconditions for the origin of multilevel systems in all three clades, a potentially important catalyst was intraspecific social threat, predominantly bachelor threat in colobines and female coercion/infanticide in papionins and humans. We emphasize that female transfers within bands or genetic relationships among leader males help to maintain modular societies by facilitating interunit tolerance. We still lack a good or even basic understanding of many facets of multilevel sociality. Key remaining questions are how the genetic structure of a multilevel society matches the observed social effort of its members, to what degree cooperation of males of different units is manifest and contributes to band cohesion, and how group coordination, communication, and decision making are achieved. Affiliative and cooperative interunit relations are a hallmark of human societies, and studying the precursors of intergroup pacification in other multilevel primates may provide insights into the evolution of human uniqueness.