Multilevel Societies Research Articles

Bechstein’s bats maintain individual social links despite a complete reorganisation of their colony structure

Several social mammals, including elephants and some primates, whales and bats, live in multilevel societies that form temporary subgroups. Despite these fission-fusion dynamics, group members often maintain long-term bonds. However, it is unclear whether such individual links and the resulting stable social subunits continue to exist after a complete reorganisation of a society, e.g. following a population crash. Here, we employed a weighted network analysis on 7,109 individual roosting records collected over 4 years in a wild Bechstein's bat colony. We show that, in response to a strong population decline, the colony's two stable social subunits fused into a non-modular social network. Nevertheless, in the first year after the crash, long-term bonds were still detectable, suggesting that the bats remembered previous individual relationships. Our findings are important for understanding the flexibility of animal societies in the face of dramatic changes and for the conservation of social mammals with declining populations.

Vocal communication in a complex multi-level society: constrained acoustic structure and flexible call usage in Guinea baboons.

BackgroundTo understand the evolution of acoustic communication in animals, it is important to distinguish between the structure and the usage of vocal signals, since both aspects are subject to different constraints. In terrestrial mammals, the structure of calls is largely innate, while individuals have a greater ability to actively initiate or withhold calls. In closely related taxa, one would therefore predict a higher flexibility in call usage compared to call structure. In the present study, we investigated the vocal repertoire of free living Guinea baboons (Papio papio) and examined the structure and usage of the animals’ vocal signals. Guinea baboons live in a complex multi-level social organization and exhibit a largely tolerant and affiliative social style, contrary to most other baboon taxa. To classify the vocal repertoire of male and female Guinea baboons, cluster analyses were used and focal observations were conducted to assess the usage of vocal signals in the particular contexts.ResultsIn general, the vocal repertoire of Guinea baboons largely corresponded to the vocal repertoire other baboon taxa. The usage of calls, however, differed considerably from other baboon taxa and corresponded with the specific characteristics of the Guinea baboons’ social behaviour. While Guinea baboons showed a diminished usage of contest and display vocalizations (a common pattern observed in chacma baboons), they frequently used vocal signals during affiliative and greeting interactions.ConclusionsOur study shows that the call structure of primates is largely unaffected by the species’ social system (including grouping patterns and social interactions), while the usage of calls can be more flexibly adjusted, reflecting the quality of social interactions of the individuals. Our results support the view that the primary function of social signals is to regulate social interactions, and therefore the degree of competition and cooperation may be more important to explain variation in call usage than grouping patterns or group size.

Social monitoring in a multilevel society: a playback study with male Guinea baboons

Keeping track of social interactions among conspecifics is a driving force for the evolution of social cognition. How social cognition, such as social knowledge, ties in with a species' social organization is, however, largely unexplored. We investigated the social knowledge of wild Guinea baboons (Papio papio) ranging in Senegal, a species that lives in a fluid multilevel society with overlapping habitat use. Using playback experiments, we tested how adult males differentiate between subjects from their own vs. a neighboring or a stranger social unit (“gang”) and assessed ranging patterns with Global Positioning System (GPS) data. While territorial species usually differentiate between group and nongroup members and often respond more strongly to strangers than neighbors (the “dear enemy” effect), subjects in this highly tolerant species should largely ignore other unit members and mainly attend to subjects from their own unit. Males responded strongly after playback of calls recorded from members of their own gang, while they attended only briefly to neighbor or stranger calls. Apparently, males benefit from monitoring the social maneuvers in their own social unit, while it remains to be resolved whether they are unmotivated or unable to keep track of the identities and actions of individuals outside their own gang. The study highlights how the allocation of social attention is tuned to the specifics of a species' social organization, while a complex social organization does not necessarily translate into the need for more elaborate social knowledge.

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.

Comparisons of Intraunit Relationships in Nonhuman Primates Living in Multilevel Social Systems

Multilevel social systems have evolved in several species of cercopithecoid primates and appear to be an effective means of changing group size amid variation in environmental conditions. Larger groupings of these species fission and fuse, making intraunit relationships essential to maintain the integrity of the smallest social units. We examine these intraunit relationships in four primates with multilevel social systems: proboscis monkeys (Nasalis larvatus), snub-nosed monkeys (Rhinopithecus roxellana), hamadryas baboons (Papio hamadryas), and geladas (Theropithecus gelada), using social network analysis. The proboscis monkeys and hamadryas baboons were wild and unprovisioned, whereas the snub-nosed monkeys and geladas were partly provisioned. Comparison of eigenvector centrality coefficients revealed a phylogenetic difference in the key individuals maintaining social networks between the colobines and the cercopithecines: females were more central in proboscis and snub-nosed monkeys, with males generally peripheral to social interaction, whereas males were more central than females in geladas and hamadryas. A comparison of sex differences in clustering coefficients, however, revealed a significant difference only in geladas, suggesting that one-male–multifemale units in this species become more unstable when females, but not males, are removed from social networks. Taken together, our results reveal the strongest differences between geladas, characterized by female philopatry and male dispersal, and the three species with bisexual dispersal. These results demonstrate the potential for social network analysis to reveal the social bonds most important for maintaining cohesion of the smallest units of primate multilevel societies. This, in turn, can serve as a proxy, in the absence of long-term data, for underlying patterns of sex-biased dispersal and philopatry.

Multilevel Societies in Primates and Other Mammals: Introduction to the Special Issue.

Murdoch (1981) remarked that nowhere on Earth do people live regularly in isolated families. The habitual formation of superfamily level groupings is one of the unmistakable universals of human sociality. Males and females within these higher level groupings are connected via kinship and affinity ties, and affiliative and cooperative bonds reach far beyond the nuclear family unit (Rodseth et al. 1991; Wiessner 1977). Although interunit encounters are usually circumvented or characterized by animosity because of mating or resource competition in many nonhuman primates (Cheney 1987; Fashing 2001), some primates such as hamadryas baboons (Papio hamadryas), geladas (Theropithecus gelada), snub-nosed monkeys (Rhinopithecus spp.), and proboscis monkeys (Nasalis larvatus) (for an exhaustive list, see Grueter et al. 2012) exhibit a social arrangement in which regular or constant proximity as well as coordinated activity among subunits is the norm. This type of social organization has been termed multilevel, nested, or modular. The phenomenon of social modularity is not restricted to the primate order. Similarly structured societies can be observed in other mammals, most notably African elephants (Loxodonta africana: de Silva and Wittemyer 2012; Moss and Poole 1983; Wittemyer et al. 2005), Asian elephants (Elephas maximus: de Silva and Wittemyer 2012), plains zebras (Equus burchelli: Rubenstein and Hack 2004), khulans (Equus hemionus: Feh et al. 2001), prairie dogs (Cynomys ludovicianus: Hoogland 1995), sperm whales (Physeter macrocephalus: Whitehead et al. 1991, 2012), and killer whales (Orcinus orca: Baird 2000).

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

Studies of spatial proximity between individuals are important for an understanding of social structure because animals are more likely to interact with individuals in close spatial proximity. Here, we apply social network analysis to proximity data collected between 2001 and 2003 from an individually identified, provisioned, free-ranging band of Sichuan snub-nosed monkeys (Rhinopithecus roxellana) in the Qinling Mountains, central China. We aimed to quantify the social network structure and to gain insight into each individual’s position within the social network. The overall network connectivity of the study band was sparse, with a low group density of 0.17. We identified nine one-male-multifemale units (OMUs) in the study band using hierarchical cluster analysis, which confirms that this species forms a multilevel society in its natural habitat. Based on sex differences in eigenvector and betweenness centralities, adult females have more important social roles than males. Among females, lactating females scored higher betweenness and eigenvector centralities than other females. However, our results do not suggest the existence of key individual(s) in the social network of the study band. The global clustering coefficient characteristic of the band was 0.3 ± 0.1, with little variation between individuals, suggesting that the removal or death of any specific individual would not significantly disrupt its general network structure. Our results also show that proximity commonly occurs among unit members, but can also occur between females of different OMUs. These observations suggest that snub-nosed monkeys have a loose-knit or fluid rather than a rigid female-bonded social system, which may be a common trend for species living in multilevel societies.

Multilevel Societies in New World Primates? Flexibility May Characterize the Organization of Peruvian Red Uakaris (Cacajao calvus ucayalii)

Researchers have described multilevel societies with one-male, multifemale units (OMUs) forming within a larger group in several catarrhine species, but not in platyrhines. OMUs in multilevel societies are associated with extremely large group sizes, often with >100 individuals, and the only platyrhine genus that forms groups of this size is Cacajao. We review available evidence for multilevel organization and the formation of OMUs in groups of Cacajao, and test predictions for the frequency distribution patterns of male–male and male–female interindividual distances within groups of red-faced uakaris (Cacajao calvus ucayalii), comparing year-round data with those collected at the peak of the breeding season, when group cohesion may be more pronounced. Groups of Cacajao fission and fuse, forming subgroup sizes at frequencies consistent with an OMU organization. In Cacajao calvus ucayalii and Cacajao calvus calvus, bachelor groups are also observed, a characteristic of several catarrhine species that form OMUs. However, researchers have observed both multimale–multifemale groups and groups with a single male and multiple females in Cacajao calvus. The frequency distributions of interindividual distances for male–male and male–female dyads are consistent with an OMU-based organization, but alternative interpretations of these data are possible. The distribution of interindividual distances collected during the peak breeding season differed from those collected year-round, indicating seasonal changes in the spatial organization of Cacajao calvus ucayalii. We suggest a high degree of flexibility may characterize the social organization of Cacajao calvus ucayalii, which may form OMUs under certain conditions. Further studies with identifiable individuals, thus far not possible in Cacajao, are required to confirm the social organization.

A Papionin Multilevel Society as a Model for Hominin Social Evolution

Multilevel societies are unique in their ability to facilitate the maintenance of strong and consistent social bonds among some individuals while allowing separation among others, which may be especially important when social and sexual bonds carry significant and reliable benefits to individuals within social groups. Here we examine the importance of social and sexual bonds in the multilevel society of hamadryas baboons (Papio hamadryas) and apply these principles to social evolution in Plio-Pleistocene hominins. The behavior, adaptations, and socioecology of baboons (Papio spp.) have long been recognized as providing an important comparative sample to elucidate the processes of human evolution, and the social system of hamadryas baboons in particular shares even more similarities with humans than that of other baboons. Here we draw parallels between processes during the evolution of hamadryas social organization and those characterizing late Pliocene or early Pleistocene hominins, most likely Homo erectus. The higher costs of reproduction faced by female Homo erectus, exacerbated by an increased reliance on difficult to acquire, nutrient-dense foods, are commonly thought to have been alleviated by a strengthening of male–female bonds (via male provisioning and the evolution of monogamy) or by the assistance of older, postreproductive females (via grandmothering). We suggest that both of these social arrangements could have been present in Plio-Pleistocene hominins if we assume the development of a multilevel society such as that in hamadryas baboons. The evolution of a multilevel society thus underlies the adaptive potential for the complexity that we see in modern human social organization.

Ecology and sociality in a multilevel society: Ecological determinants of spatial cohesion in hamadryas baboons

The multilevel society of hamadryas baboons, consisting of troops, bands, clans, and one-male units (OMUs), is commonly perceived to be an effective means of adapting to variable food availability while allowing spatial cohesion in response to predator pressure. The relationship between these variables, however, has never been tested quantitatively. The Filoha site in Awash National Park, Ethiopia is ideally suited to such an investigation as it contains nutrient-dense palm forests in addition to the Acacia scrublands typical of hamadryas distribution elsewhere, allowing comparisons of spatial cohesion across habitat types. Here, we use observations over a 1-year period to examine the relationship between resource availability, perceived predator pressure, and spatial cohesion in a band of wild hamadryas baboons at Filoha. Our results demonstrate that the band was more likely to break into OMUs when foraging in habitats with lower food availability, and that the band fissioned into independent clans more often when preferred resources were not available. Furthermore, the baboons remained in larger aggregations for longer periods of time (i.e., prior to embarking on their daily foraging route) on mornings after predators were heard in the vicinity, and increased cohesion in response to encounters with people who may have been perceived as predators. These results support the notion that hamadryas baboons change their social groupings in response to both food availability and predation risk and that the ability of hamadryas bands to cleave and coalesce in response to changes in these factors underlies the evolution of the hamadryas modular social structure.

Multilevel Societies of Female Sperm Whales (Physeter macrocephalus) in the Atlantic and Pacific: Why Are They So Different?

We can examine the evolution of multilevel societies using comparative studies. Intraspecific comparisons are valuable because confounding factors are reduced. Female sperm whales live in multilevel societies. However, studies at several locations have found substantial and consistent differences in social structure between the eastern Pacific and North Atlantic Oceans, even though nuclear DNA shows no significant differentiation between the populations. In the Pacific, female sperm whales live in nearly permanent social units that typically contain about 11 females and immatures of multiple unrelated matrilines. These units form temporary groups with other units for periods of days, apparently exclusively with other units from the same cultural clan. Clans contain thousands of females, are not distinct in nuclear DNA, but are sympatric and have distinctive culturally determined vocalizations and movement patterns. In the North Atlantic social units rarely group with other units, and there is no evidence for sympatric cultural clans. Possible drivers of these contrasts include oceanographic differences, predation, the effects of whaling, and culture. We suggest that protection against predation by killer whales is the primary reason for grouping in the Pacific, and as killer whales do not seem such a threat in the Atlantic, social units there rarely form groups, and have not evolved the clans that primarily function to structure interunit interactions. This analysis highlighted several factors that may influence the evolution of multilevel societies, ranging from the attributes of resources, to predation, anthropogenic effects, culture, and even the cultures of other species.

Defining Higher Levels in the Multilevel Societies of Geladas (Theropithecus gelada)

Multilevel societies, identified by two or more nested levels (or modules) of organization, have been touted as some of the most complex social systems. However, few empirical studies have effectively quantified the association patterns that delineate the various levels in such societies. In particular, the multiple levels of gelada society were first described >3 decades ago, yet no operational definitions exist for the higher levels, i.e., levels above the one-male unit. In geladas, multiple units form aggregations that fission and fuse throughout the day, and throughout the year, blurring the distinctions between previously described higher social levels: teams, bands, and communities. Here we use 5 yr of data on the daily composition of a population of geladas living in the Simien Mountains National Park, Ethiopia to test the hypothesis that higher levels of gelada organization are discrete entities. If gelada aggregations are nothing more than a group of units that share a home range, then we expect a continuous distribution of unit association. If, however, gelada aggregations are indeed discrete organizational levels, then we expect discontinuity in the patterns of association. We found significant discontinuity at the 50% association level, indicating a sharp distinction between members of the same band (>50% association) and members of the same community (<50% association). We also found evidence that recently fissioned units form teams that associate significantly more than other band members. Thus, despite their extremely fluid social organization, gelada social levels are nevertheless clearly identifiable and quantifiable. Based on these results, we suggest that gelada society is an extremely flexible, multilevel society with fission–fusion dynamics, and as such gelada society presents an unusual example for understanding the evolution of modular societies.

Fission–Fusion Behavior in Yunnan Snub-Nosed Monkeys (Rhinopithecus bieti) in Yunnan, China

Several species of mammals live in relatively large social groups that temporarily fission or subgroup in response to changes in food availability, predation risk, foraging strategies, and mating competition. Although the dynamics of subgrouping are not well understood, evidence of facultative fission–fusion behavior in species that generally exhibit a highly cohesive group structure may help to clarify the set of ecological and social factors that constrain or affect group size in primates. We here examined patterns of subgrouping in Yunnan snub-nosed monkeys (Rhinopithecus bieti) inhabiting the Baimaxueshan National Nature Reserve in Yunnan, China. Rhinopithecus bieti lives in a large multilevel society consisting of 6–41 one-male, multifemale units (OMUs) and one all-male unit (AMU). Over a 5-yr period from 2003 through 2008, we tracked a group of >450 Rhinopithecus bieti. We recorded the location in the group’s home range where fission and fusion events occurred, the size and number of subgroups, days spent in subgroups, and food availability, and monitored predator sightings. The results indicate that the focal group underwent12 group fission events, all of which occurred between mid-June and July, and all in the same area of the group’s range. During these fissioning events, the AMU also showed subgrouping behavior. In all cases, the presence of bamboo shoots, an important seasonal component of the snub-nosed monkey diet, appeared to trigger subgrouping behavior. The subgroups reunited in other parts of their range after a period of 2–11 d. We found no evidence that the presence of predators had a direct affect on subgrouping behavior in Rhinopithecus bieti, as proposed in previous studies.

The Socioecology of Network Scaling Ratios in the Multilevel Society of Hamadryas Baboons (Papio hamadryas hamadryas)

Multilevel or modular societies characterize a range of mammalian taxa, allowing social groups to fission and fuse in response to ecological factors. The modular society of hamadryas baboons has previously been shown to consist of 4 levels: troop, band, clan, and one-male unit (OMU). A recent study by Hill et al. (Biology Letters 4:748–751, 2008) revealed a mean scaling ratio across successive levels of multilevel societies of ca. 3; this was consistent across elephants, orca, geladas, and hamadryas baboons. Here we reanalyze the scaling ratio for hamadryas baboons with previously unavailable data from Filoha. Our analysis revealed a mean scaling ratio for hamadryas of 3.28 without data on the hamadryas clan layer of organization at Filoha, but a ratio of 6.17 with these data included. This discrepancy is due to the large clan and band sizes at Filoha yielding a larger than average gap between the OMU and the clan. Further analysis revealed subsets of OMUs within clans, suggesting a 5th level of society in this population. When this 5th layer of social structure is included in the analysis, the scaling ratio at Filoha is consistent with that of other hamadryas populations and other taxa. These results suggest that a consistent mammalian scaling ratio can be used to detect previously hidden levels of organization within societies and to predict their sizes in taxa for which detailed behavioral data are not available.

Male dispersal in a provisioned multilevel group ofRhinopithecus roxellanain Shennongjia Nature Reserve, China

Most Old World monkeys show male-biased dispersal. We present the first systematic data on male dispersal in a provisioned multilevel group of Rhinopithecus roxellana, based on 4.5 years of field observations in Shennongjia National Nature Reserve, China. We evaluated both ultimate (inbreeding avoidance and male mating competition) and proximate (food availability and predation risk) factors influencing male dispersal. The focal group contained 34-53 individuals, in 3-4 one-male units (OMUs) and 1 all-male unit (AMU). We observed 37 dispersal events involving 10 of 11 adults, 7 of 8 subadults, and 7 of 15 juveniles. Most interunit transfers within the focal group occurred around the months of mating season. Adult males competed for the leader positions of OMUs mainly through aggressive takeovers, and young males transferred from the OMUs to the AMU at the median age of 41 months, forced out by leader males. No young males older than 4 years remained in natal or non-natal OMUs. The male mating competition hypothesis was supported. The young males emigrated voluntarily from the focal group at the average age of 58.6 months, and no young emigrating male was observed to return, suggesting inbreeding avoidance also played a role in the dispersal of young males. Most emigration/immigration events were parallel dispersal and occurred during intergroup encounters, suggesting increased predation risk during the dispersal period. Males were more likely to emigrate/immigrate during the months when preferred foods were most available. We compared the dispersal patterns in R. roxellana with those in gelada baboons and hamadryas baboons, both living in multilevel societies. Similar to R. roxellana, young male geladas disperse at puberty, but they may return and breed in their natal groups. Males in hamadryas also disperse, but much less commonly than in R. roxellana. Provisioning may have influenced results, and confirming studies on unprovisioned groups would be valuable.

Seasonality of group size, feeding, and breeding in wild red‐shanked douc langurs (Lao PDR)

In Asian colobines, small one-male groups (OMG) seem to predominate alongside all-male groups (AMG), while larger multimale groups (MMG) are rare, but are reported for Hanuman langurs and red-shanked douc langurs. Recently, however, it has been speculated that the genus Pygathrix could have multilevel societies based on (1) a theoretical extension of the multilevel societies found in Rhinopithecus to all odd-nosed colobines and (2) first data for black-shanked douc langurs. This assumes bands composed of small OMG with a skewed adult sex ratio. Band size may vary with seasonal food availability resulting in smaller bands when feeding competition is increased. To investigate the social organization of red-shanked douc langurs and potential seasonal influences, we observed 2 unhabituated groups at Hin Namno National Protected Area, Lao PDR from March 2007 to August 2008 for 803 hr. We recorded births and performed group counts and scan sampling of feeding behavior. Most births (79% of N = 15) occurred from June-September, indicating a 4-month peak conception season from November to February. Group size averaged 24.5 individuals (range 17-45) with 2.45 adult males (range 1-4). Although the smaller group remained at a stable size (about 18 individuals), the larger group reduced from about 45 to 25 individuals during the 7-months long lean season, when less than 50% of the feeding time was spent on fruits. This suggests feeding competition as a potential cause of seasonal variation in group size. With 1.9 females per male the skew in adult sex ratio was much lower compared with Rhinopithecus, indicating MMG rather than multilevel societies. However, data on the spacing and interaction patterns between recognized individuals need to be collected and analyzed before the social organization can be determined. Detailed ecological data are furthermore required to investigate the basis for the seasonal changes in group size found.

Impact of Group Size on Female Reproductive Success of Free-Ranging Rhinopithecus roxellana in the Qinling Mountains, China

Group size influences female reproductive success through scramble/contest feeding competition, predation pressures and infanticide risks in primates. The Sichuan snub-nosed monkey (Rhinopithecus roxellana) is an endangered folivorous colobine species living in a multilevel society. From 2002 to 2008, we studied a free-ranging band of R. roxellana in the Qinling Mountains of China to examine the effect of group size on female reproductive success. During our observation period, the number of monkeys in the study band fluctuated from 61 to 108, and the number of one-male/multi-female groups within it varied from 7 to 10. A significant negative linear relationship was found between group size and birth rate, but group size was not significantly correlated with infant survival rate or interbirth interval. These results suggest that group size influences female reproductive success via within-group scramble competition in this folivorous species.

Female “dispersal” in hamadryas baboons: Transfer among social units in a multilevel society

Unlike most cercopithecines, hamadryas baboons (Papio hamadryas hamadryas) are characterized by female-biased dispersal. To clarify this pattern within the context of their hierarchical social system (comprising one-male units, clans, bands, and troops), we report here 7 years of data on female transfers among social units in wild hamadryas baboons in Ethiopia. Female tenure in one-male units (OMUs) ranged from 1 to 2,556 days (N = 208) and survival analysis revealed a median tenure length of 1,217 days (40 months). Changes in OMU membership consisted almost exclusively of takeovers by males, not voluntary transfer. Of 130 takeovers, 67% occurred within the band and 33% across bands, and, of the 22 takeovers for which we have clan membership data, 77% occurred within, not between, clans. These results reinforce the notion that hamadryas female dispersal is not analogous to sex-biased dispersal in other taxa, because (1) at least in Ethiopian populations, females do not disperse voluntarily but are transferred, often forcibly, by males; (2) only dispersal between bands will promote gene flow, whereas females are most often rearranged within bands; (3) hamadryas females undergo social dispersal but not usually locational dispersal; and (4) while male hamadryas are far more philopatric than females, they have been observed to disperse. It thus appears that the ancestral baboon pattern of female philopatry and male dispersal has evolved into a system in which neither sex is motivated to disperse, but females are forcibly transferred by males, leading to female-mediated gene flow, and males more rarely disperse to find females.

Inter‐unit contests within a provisioned troop of Sichuan snub‐nosed monkeys (Rhinopithecus roxellana) in the Qinling Mountains, China

Numerical superiority does not always ensure victory in intergroup contests. Although group size is likely to determine the maximum resource holding potential (RHP) of a group, the realized RHP is the collective outcome of individual group members' choices about participation in any given contest. For any group member, the choice about participation should be based on the assessment of costs and benefits that are affected by both ecological and social factors. In this study, we studied inter-unit contests in a provisioned troop of Sichuan snub-nosed monkeys (Rhinopithecus roxellana). We spent 368 hr in contact with 9 one-male units sharing the same home range, during which we recorded 148 inter-unit contests at a provisioning site. Inter-unit contests always started as inter-individual contests. Contests escalated only when the two individuals were of different age-sex classes and one was an adult male. When a contest escalated, additional individuals were likely to get involved, and the outcome of the contest depended on unit members' choices about participation. The superiority in the number of participants rather than the superiority in unit size led to victory in inter-unit contests, given that the difference in unit size did not predict a difference in the number of participants. Unit members were more likely to support others in inter-unit contests in winter when food was sparse than in spring when food was abundant. In addition, unit members were more likely to support others in escalated contests than in those resulting in displacement, indicating that they tended to alter the outcome of a contest to gain immediate benefit. Although males initiated most inter-unit contests, a clear win-loss was most likely when females joined the fight. This sex difference may reflect the benefits to males vs. females of living in a multi-level society.

Chinese Sichuan snub-nosed monkey and its multi-level societies

Chinese Sichuan snub-nosed monkey and its multi-level societies