Group Fission Research Articles

Long-term Site Fidelity and Individual Home Range Shifts in Lophocebus albigena.

We investigated long-term site fidelity of gray-cheeked mangabey (Lophocebus albigena) groups in Kibale National Park, Uganda. Concurrently, we monitored shifts in home range by individual females and subadult and adult males. We documented home range stability by calculating the area of overlap in successive years, and by recording the drift of each group’s monthly centroid from its initial location. Home ranges remained stable for 3 of our 4 groups (overlap over 10 yr >60%). Core areas were more labile, but group centroids drifted an average of only 530 m over the entire decade. Deviations from site fidelity were associated with dispersal or group fission. During natal dispersal, subadult males expanded their home ranges over many months, settling ≤4 home ranges away. Adult males, in contrast, typically dispersed within a few days to an adjacent group in an area of home range overlap. Adult males made solitary forays, but nearly always into areas used by their current group or by a group to which they had previously belonged. After secondary dispersal, they expanded their ranging in the company of their new group, apparently without prior solitary exploration of the new area. Some females also participated in home range shifts. Females shifted home ranges only within social groups, in association with temporary or permanent group splits. Our observations raise the possibility that male mangabeys use a finder-joiner mechanism when moving into new home ranges during secondary dispersal. Similarly, females might learn new resource locations from male immigrants before or during group fission.

Dominance and Affiliation Mediate Despotism in a Social Primate

Group-living animals routinely have to reach a consensus decision and choose between mutually exclusive actions in order to coordinate their activities and benefit from sociality. Theoretical models predict "democratic" rather than "despotic" decisions to be widespread in social vertebrates, because they result in lower "consensus costs"-the costs of an individual foregoing its optimal action to comply with the decision-for the group as a whole. Yet, quantification of consensus costs is entirely lacking, and empirical observations provide strong support for the occurrence of both democratic and despotic decisions in nature. We conducted a foraging experiment on a wild social primate (chacma baboons, Papio ursinus) in order to gain new insights into despotic group decision making. The results show that group foraging decisions were consistently led by the individual who acquired the greatest benefits from those decisions, namely the dominant male. Subordinate group members followed the leader despite considerable consensus costs. Follower behavior was mediated by social ties to the leader, and where these ties were weaker, group fission was more likely to occur. Our findings highlight the importance of leader incentives and social relationships in group decision-making processes and the emergence of despotism.

A genetic analysis of group movement in an isolated population of tree-roosting bats

Group fission is an important dispersal mechanism for philopatric adults. In Cypress Hills Interprovincial Park, Saskatchewan, tree-roosting big brown bats (Eptesicus fuscus) exhibit fission-fusion roosting behaviour. During 2004-2007, the majority of females previously resident to roosting area 1 (RA1) moved to a new roosting area (RA4). We examined how genetic relationships, inferred from data for microsatellite loci and mitochondrial DNA, influenced new roost area (RA) selection during 2006 when colony members were split between the RAs. We found that females who moved to RA4 had higher average relatedness than those that remained in RA1. We found that nearly all females belonging to matrilines with high average relatedness moved to RA4 while females from matrilines with low average relatedness were split between the two RAs. These results suggest that closely related maternal kin preferentially move to new RAs. However, daily roosting preferences within a RA are not based on genetic relationships probably because daily roosting associations between kin and non-kin are used to ensure adequate roost group size. Studying the effects of kinship on the fission and movements of groups not only enhances our understanding of social behaviour and population genetics but also informs conservation decisions.

Population demography and social structure changes in Eulemur fulvus rufus from 1988 to 2003

Eulemur fulvus rufus has been described as having stable multi-male/multi-female groups, a male-biased sex ratio, and female philopatry. However, in a 16-year study of this subspecies we documented a great deal of demographic change as several groups permanently fissioned, some groups disappeared, and new groups formed. We split the dataset into two periods, 1988 to 1993 and 1994 to 2003, which coincided with the first disappearance of a study group (in August 1994) and the first permanent group fission (in December 1994). The average group size decreased by nearly half between the study periods (10.5-5.6), while the frequency of group membership changes increased (2.0-8.3 times/year), and the birth rate decreased (0.56-0.38). Females, as well as males, immigrated into study groups and transferred between groups, something that has been rarely seen in this subspecies. We also found a significant decline in the amount of fruit from the earliest part of the study to the latter part of the study. Study groups did not switch to other types of foods during periods of fruit shortage, but traveled outside of their home range areas more often over the study period. Finally, the density E. f. rufus decreased in the study area while the densities of their main food competitors, Varecia variegata and Eulemur rubriventer, increased. Although few primate populations are numerically stable over time, we suggest that female behavioral responses to decreases in fruit availability may have influenced some of the demographic changes we witnessed in this study.

Patterns of paternity and group fission in wild multimale mountain gorilla groups

To understand variation among social systems, it is essential to know the relative reproductive success of individuals in group-living species. Particularly interesting for such studies are taxa such as mountain gorillas in which both one-male and multimale groups are common, because of the opportunity to estimate the costs and benefits to males of pursuing different reproductive strategies. We genotyped 68 individuals from two groups of multimale mountain gorilla groups in Bwindi Impenetrable National Park, Uganda to determine the distribution of paternity among the males. In both groups, the dominant male sired the majority of offspring. One group underwent a fission, and we found that the eight offspring assigned to the dominant silverback (and their mothers) remained with their father, while the two offspring of unknown paternity ended up in the small group headed by the formerly subordinate silverback. This is consistent with the proposal that the outcome of group fission in primates is not only influenced by maternal relationships among individuals, but also by patrilineal relationships. Results of this study show that subordinate males may gain reproductive benefits even while queuing for dominance status. Despite ecological differences between Bwindi and the Virunga Volcanoes, male mountain gorillas living in both populations benefit from remaining in multimale groups.

Habitat-specific chemical cues influence association preferences and shoal cohesion in fish

The structure of social animal groups can be dynamic, characterized by high rates of group fission and fusion. Despite this, group composition is often well ordered by factors such as species, body size and by numerous other phenotypic traits. Research in shoaling fishes has revealed that individuals refine group membership decisions still further and are capable of assimilating chemical cues pertaining to recent habitat and prey use by prospective group mates, preferring to associate with others whose recent resource use history closely matches their own. In this study, we firstly examined the dynamics of the formation and breakdown of these preferences, revealing that they can be acquired and replaced in a matter of just a few hours. Using such cues enables individuals to accurately assess the resource use of conspecifics, allowing them to indirectly sample the local environment while reducing the chances of acquiring outdated information that can precipitate maladaptive behaviors. Secondly, we found that shoals composed of individuals with shared recent habitat use history were more cohesive compared to those where the constituent individuals differed in recent habitat use. Increased shoal cohesion may reduce predation risk, and could enhance the ability of individuals to detect and use social information.

Divided destinies: group choice by female savannah baboons during social group fission

Group living provides benefits to individuals while imposing costs on them. In species that live in permanent social groups, group division provides the only opportunity for nondispersing individuals to change their group membership and improve their benefit to cost ratio. We examined group choice by 81 adult female savannah baboons (Papio cynocephalus) during four fission events. We measured how each female’s group choice was affected by several factors: the presence of her maternal kin, paternal kin, age peers, and close social partners, her average kinship to groupmates, and her potential for improved dominance rank. Maternal kin, paternal kin, and close social partners influenced group choice by some females, but the relative importance of these factors varied across fissions. Age peers other than paternal kin had no effect on group choice, and average kinship to all groupmates had the same effect on group choice as did maternal kin alone. Most females were subordinate to fewer females after fissions than before, but status improvement did not drive female group choice; females often preferred to remain with social superiors who were their close maternal kin, rather than improving their own social ranks. We suggest that during permanent group fissions, female baboons prefer to remain with close maternal kin if those are abundant enough to influence their fitness; if they have too few close maternal kin then females prefer to remain with close paternal kin, and social bonds with nonkin might also become influential.

Consequences of group fission for the patterns of relatedness among rhesus macaques

When mammalian social groups exceed their optimal size, they often tend to split. In view of the potential evolutionary benefits, it should be more advantageous for animals to stay with kin, rather than nonkin, during such fission events. In the present study, the spontaneous fission of two social groups, R and S, of rhesus macaques living on Cayo Santiago, Puerto Rico, provided the opportunity to compare the kinship structure of the corresponding parent and daughter groups, using information on both maternal and paternal relatedness. In both instances, maternal half-siblings and pairs of animals from the same family were significantly more prevalent in the fission products than in the parent group. During the split of group R, significantly more paternal half-siblings stayed in the remnants of the parent group than joined the seceding group. Our findings are compatible with previous behavioural studies demonstrating that female primates bias their social behaviour more to maternal than to paternal kin, but that both types of half-siblings prefer each other more than unrelated animals. It remains to be clarified by future research, however, whether the observed co-segregation of paternal half-sibs in our study reflects active choice or is a by-product of the group-specific kin structures, prior to fission.

From Fault Line to Group Fission: Understanding Membership Changes in Small Groups

Group fissions occur when two or more members leave a parent group to either form a new group or join an existing group. This article investigates the interplay between two factors: the presence of an intragroup conflict and subgroup boundaries on the group fission process. It is hypothesized that subgroup divisions act as potential fault lines along which groups split after they experience conflict. The results of three experiments, one scenario study and two laboratory studies involving small task groups, support the group fault line hypothesis. The authors discuss the implications of these findings for theory and research on membership changes in small groups.

Bone and Flesh, Seed and Soil: Patriliny by Father's Brother's Daughter Marriage

Behind patrilineal descent is an asymmetrical descent structure based on sex, and father's brother's daughter marriage. Because it is means of constructing the patrilineage, patrilateral cousin marriages continue to exist. The Kurds eastern and southeastern Turkey illustrate this apparent paradox with the position of women the patrilineage and their structural relationship with the mother's brother. (FBD marriage, patriliny, Turkey, Kurds) ********** Patrilateral cousin marriages occupy special place the study of kinship and marriage. Such marriages are characteristic of Middle East peoples and are referred to as preferred. Thus, in some social contexts Middle Easterners assert that if woman and her family choose not to marry father's brother's son, his consent and that of his family must be obtained (Eickelman 1998:169). Such consent implies that the father's brother's son has priority of marriage with the father's brother's daughter. The fact that tribal societies (e.g., Arabs and Kurds) where such practices are followed are also patrilineal makes father's brother's daughter (FBD) marriage problematic. Bourdieu (1991:32), for example, points out that structuralism either ignores or brackets off this problem, but structural-functionalist theory holds this marriage type provides stability family and kinship relations. While Barth (1986:396) emphasizes that these marriages enhance in-group solidarity and prevent corporate group fission, Murphy and Kasdan (1959:18), contrast, claim that the factor underlying segmentation is FBD marriage. These positions are opposition, yet both attribute to FBD marriage the function of providing homeostasis for social and political organization. By contributing to maintaining harmony within the family (Khuri 1970:597), FBD marriage is credited with having positive function for social and psychological stability, and economic factors reveal similar stability. Through FBD marriage, property remains intact within the family (Rosenfeld 1958:1138), thus preserving established property relations. Underlying these arguments is the presumption that parallel cousin marriage is the only structurally pertinent form of marriage: other forms of alliance do not constitute' normative unions' (Atran 1985:667). Atran (1985:686) criticizes this presumption because it reduces kinship and marriage forms to a single mechanical model, when FBD marriage should be understood as a matter of social strategy. Bourdieu (1991:49) emphasizes that FBD marriage can never be fully defined genealogical terms, and separates practical kin, relatives that are present relation set by the individual, from official kin, those relatives that are included the genealogy (Bourdieu 1991:33-35). For him, matrimonial strategies are material and symbolic capital, managed by relatives practical groups who rest on community of dispositions (habitus) and interests which is also the basis of undivided ownership of the material and symbolic patrimony(Bourdieu 1991:35). He points out that FBD marriage assumes various meanings and functions that differ with context. Marriages which are identical as regards genealogy ... may thus have different, even opposite, meanings and functions, depending on the strategies which they are involved [and that] any two marriages between cousins may have nothing common (Bourdieu 1991:48). However, patrilineal descent is accepted as given these discussions and analyses are based on it. Although researchers (e.g., Eickelman 1998:171; Atran 1985:665) have pointed out that kin relations with both the father's and the mother's sides are equally important with tribal societies the Middle East, these analyses are based on presumed patrilineal descent system. That descent system is determined by indications such as to whom property is passed and from whom descent is traced (Patai 1965; Fortes 1971; Keesing 1975). …

The ties that bind: genetic relatedness predicts the fission and fusion of social groups in wild African elephants.

Many social animals live in stable groups. In contrast, African savannah elephants (Loxodonta africana) live in unusually fluid, fission-fusion societies. That is, 'core' social groups are composed of predictable sets of individuals; however, over the course of hours or days, these groups may temporarily divide and reunite, or they may fuse with other social groups to form much larger social units. Here, we test the hypothesis that genetic relatedness predicts patterns of group fission and fusion among wild, female African elephants. Our study of a single Kenyan population spans 236 individuals in 45 core social groups, genotyped at 11 microsatellite and one mitochondrial DNA (mtDNA) locus. We found that genetic relatedness predicted group fission; adult females remained with their first order maternal relatives when core groups fissioned temporarily. Relatedness also predicted temporary fusion between social groups; core groups were more likely to fuse with each other when the oldest females in each group were genetic relatives. Groups that shared mtDNA haplotypes were also significantly more likely to fuse than groups that did not share mtDNA. Our results suggest that associations between core social groups persist for decades after the original maternal kin have died. We discuss these results in the context of kin selection and its possible role in the evolution of elephant sociality.

Demography, range use, and behavior in black lemurs (Eulemur macaco macaco) at Ampasikely, northwest Madagascar

We studied a black lemur population over a 2-year period (1992-1993) and 8 years later (2000) in a 50-ha secondary forest in northwest Madagascar. All of the animals were marked to investigate population dynamics and seasonal variation in ranging and behavior, and new data on black lemurs were obtained. Our data on demographic characteristics were expanded to include other forest sites and contrasted with those collected in other Eulemur macaco macaco field studies, in relation to human activity and the presence of introduced and cultivated plant species. Density is affected by deforestation and hunting. Group size and home range depend on the composition of the forest and probably food patches. Sex ratio at birth varies according to the number of females per group, a result that fits the local resource competition model. Groups are multimale-multifemale, and adult females form the core of the groups. Reproductive parameters indicate sharply defined seasonal breeding, a high female reproductive rate, and birth synchrony. Changes in group composition reveal male and female juvenile dispersal, male transfer between groups at the time of mating, and adult female transfer and group fission when groups exceed a critical size. At mating and birth, intergroup agonistic encounters occurred at home-range boundaries, and larger groups were dominant over smaller groups. Patterns of intragroup interactions suggest that males compete for access to groups of females during the mating season, and that females may compete for food resources during the birth season. Our study also reports female social dominance and lack of sexual weight dimorphism in this species.

Modelling the influence of demographic parameters on group structure in social species with dispersal asymmetry and group fission

Female philopatry characterizes many mammal populations subdivided into social groups. Fission of these social groups is a relatively discrete event in the life of groups or of individuals, leading to the distribution of females among several newly formed groups. Fission is an important event because it can be a way for females to disperse. Group fissions have rarely been observed and their modalities generally remain poorly known, the best-documented species being primates. Most group fissions occur along lines of maternal relatedness, but the death of a matriarch may disrupt the cohesion within a matriline, inducing separation of sisters, accompanied by their descendants, when a group splits. Our model shows that the numbers and sizes of matrilines within groups depend on the precise demographic parameters and age structure of a population and not only on its rate of increase. For comparable population-growth periods, high survival rates of adult females induce an increase in the sizes of matrilines, whereas high survival rates of immature individuals induce an increase in the numbers of matrilines. Following fission, groups of a given size included, in the first case, only a few large matrilines, whereas in the second case, they consisted mainly of many small matrilines. The present study constitutes a preliminary stage, before modelling consequences of demographic structure of groups or populations on their genetic structure.

Demographic and life-history patterns in a population of ring-tailed lemurs (Lemur catta) at Beza Mahafaly Reserve, Madagascar: a 15-year perspective.

Over 15 field seasons (1987-2001), we collected census and life-history data on a population of individually identified ring-tailed lemurs at the Beza Mahafaly Reserve, Madagascar. No significant difference was found in population size over the study period, though a marked decline in the population occurred following a 2-year drought. The population rebounded rapidly after the immediate postdrought period. There was nearly a complete replacement of individuals over the study period. Average group size is 11.5 animals, and adult male to female sex ratio is 0.92. Most females reproduce annually, and the average fecundity rate is 84.3%. The greatest variability in fecundity is found among old females. We suggest that ring-tailed lemur females follow an "income breeding" strategy, i.e., females use maximum resources during reproduction rather than relying on fat stores, as do "capital breeders." Infant mortality to 1 year of age in a nondrought year is 52%, higher than infant mortality in small to medium-sized anthropoids. The oldest known female was 18 years old in 2001. We suggest that 18-20 years may represent the maximum life-span for wild ring-tailed lemurs. Because males regularly emigrate from the population, we have no data regarding male life-span; however, there is some indication that males do not survive as long as females. Group fission has occurred three times: twice from one parent group living in the driest area of the reserve, with the most dispersed food resources. We suggest that the reproductive strategy that has evolved in this species, wherein females reproduce early in life and annually until old age, is a response to the unusual climate and environmental conditions under which Lemur catta has evolved.

Delayed maturity, group fission and the limits of group size in female Cape ground squirrels (Sciuridae: Xerus inauris)

AbstractFemale Cape ground squirrels Xerus inauris live in matrilineal kin groups in which the number of reproductive positions is limited to three females. Field observations on a population in Namibia revealed that the mechanisms by which group size was limited were reproductive suppression and group fission. Sub‐adult females attained sexual maturity at 8 months of age when only a single adult female was present in their social group, whereas sub‐adult females in groups with more than one adult female matured at 12 months of age. Social groups with more than three breeding females subsequently split into smaller groups. Several hypotheses for limiting group size were considered. There was no evidence of higher ectoparasite loads on females in larger groups, nor was survival of adult females affected by group size. Although adult females have lower body masses and higher mortality during the winter when food is more scarce, feeding competition alone does not seem to limit group size. Groups that split continued to overlap in their feeding ranges and there was no seasonal difference in juvenile survival. However, juvenile survival was affected by group size. The greatest cost of being in a larger group was lower juvenile survival. Larger groups use larger feeding ranges, and because squirrels return to their sleeping burrow when threatened, not just to the nearest refuge, slower juveniles have greater exposure to predators. Thus, although competition for food resources may be important, predation seems to be a major constraint on female group size as well as the primary selective force leading to female group formation.

Group fission in Moor macaques (Macaca maurus)

This is the first report of group fission in a wild group of Moor macaques (Macaca maurus) in South Sulawesi, Indonesia. The subject group, which has been observed on the basis of individual identification since 1988, showed no sign of fission in April 1999. In August 1999, the group had split in two, with the same number of mature females in each new group. For the most part, mothers and their offspring joined the same groups. Dominance relations and association patterns established during the previous year among adult females did not strongly affect new group membership. The difference in female reproductive state between the two branch groups was a prominent characteristic. The α-male of the original group visited both groups at the first stage of group fission, even though otherwise the compositions of the new groups were stable. After the division, six adult males from outside the original group immigrated exclusively into the group that did not contain the α-male of the original group. Severe intergroup encounters occurred between the two groups. We discuss the process of the fission and the resultant pattern in relation to the egalitarian dominance style among females, lack of seasonality in reproduction, and resemblance to one-male type social organization.

Group histories and offspring sex ratios in ringtailed lemurs ( Lemur catta )

Birth sex ratios were examined for ringtailed lemurs (Lemur catta) at the Duke University Primate Center. This population provides a long-term database of births under a variety of demographic and management conditions, including two semi-freeranging groups between which males transfer freely and females defend stable territorial boundaries. We examined three hypotheses usually considered in studies of primate sex ratio bias. The Trivers-Willard hypothesis predicts that dominant females produce males, local resource competition at the population level (LRC-population) predicts that the dispersing sex (males) will be overproduced in dense populations, and local resource competition among individuals (LRC-individual) predicts that dominant females overproduce the philopatric sex (females). We also examined a fourth hypothesis, local resource enhancement (LRE), which is usually subsumed under LRC-individual in studies of primate sex ratio evolution. LRE predicts that under certain conditions, females will produce the sex that provides later cooperative benefits, such as alliance support for within- or between-group competition. Our data provide support for LRE: females overproduce daughters given prospects of new group formation, either through group fission or threatened expulsion of young mothers. Behavioral data from Duke and also wild populations show that daughters serve mothers as important allies in this context and LRE effects also have been documented in other mammals that experience similar group histories. Nonsignificant trends in the data supported the LRC-population hypothesis, and we suggest that LRC interacts with LRE to explain offspring sex ratios in ringtailed lemurs.

Geographic Distribution of Mitochondrial DNA Variations among Grivet (Cercopithecus aethiops aethiops) Populations in Central Ethiopia

Since mitochondrial DNA (mtDNA) are maternally inherited without recombination, geographic distribution of mtDNA in semiterrestrial cercopithecines is considered to be influenced by female philopatry. I examined the effect of sex difference in migration patterns on geographic distribution in a habitat whose environment has changed frequently. I investigated ten groups (n = 77) of grivets (Cercopithecus aethiops aethiops) along a 600-km stretch of the Awash River, Ethiopia. I examined the mtDNA distribution among natural local populations whose nuclear variation was already shown to have a widely homogeneous distribution. RFLP analysis of whole mtDNA genome using 17 enzymes identified ten haplotypes in five clusters (haplogroups). Sequence divergence within haplogroups ranged from 0.17%–0.38%, while divergence between haplogroups ranged between 1.0%–2.5%. Haplogroups were distributed in blocks which ranged from 120–250 km along the Awash River. The haplotype distribution pattern of males indicated that they migrate between the boundaries of these blocks. Moreover, a clumped distribution pattern suggests the history of matrilineal distribution by group fission and geographic expansion.

Resource structure, scalar stress, and the development of Inuit social organization

This paper contrasts Inuit socio‐economic organization in two regions: coastal north Alaska and the Mackenzie Delta, Northwest Territories. Peoples of both regions relied on rich and highly seasonal marine mammal resources, and both of their settlement patterns incorporated relatively large and long‐term aggregations. However, their respective patterns of social organization contrasted in a number of ways, most notably in terms of the organization of labour and the scale and physical structure of the household. I argue that these differences can be attributed in large part to the interplay of two factors: scalar stress and resource structure. Scalar stress refers to the stresses inherent in large population aggregations, which must be reduced through strategies as diverse as increasingly hierarchical social organization, group fission, and increasing incidence of group ritual (Johnson 1982). Resource structure refers to aspects of the physiology and behaviour of exploited species which affect the way in which they are utilized by humans. It is my contention that the structure of the focal resource in each region, bowhead whales in north Alaska and beluga whales in the Mackenzie Delta, constrained the nature of responses to scalar stress in the large settlements which existed in each region. In north Alaska, the hunting of bowhead whales was associated with organized whaling crews led by an umialik who owned the hunting equipment and a large share of the whale meat which he and his wife redistributed to others. This redistribution reinforced a degree of social hierarchy which contributed to a reduction in scalar stress. In the Mackenzie Delta, belugas were associated with drive hunting methods in which each adult male owned his own equipment, and the products of the hunt. Social hierarchies were less easily enforced, with a result that people aggregated themselves into much larger households, as represented by the ethnographically and archaeologically well‐known ‘cruciform’ houses which held an average of six families.

Social Relationships and Intergroup Transfer of Young Adult Male Japanese Macaques Pre- and Post-Fission of the Katsuyama A Group

The social relationships of 10 young adult males in a free-ranging group of Japanese macaques at Katsuyama, Okayama Prefecture, were examined pre- and post-fission. Before group fission, higher-ranking young adult males interacted less frequently with not only their mothers, but also with other matrilineally-related females than did lower-ranking males. Six-year-old males clearly formed two subgroups according to their relative rank, even though they did not form any clear subgroups at the age of two.Higher-ranking males spent more time away from females of the group than did lower-ranking males. However, when it was time for artificial feeding, higher-ranking males obtained scattered food more frequently than lower-ranking males. After group fission, four of the five higher-ranking males and one of the five lower-ranking males remained in the main group, while three of the five lower-ranking males were in the fission group.These differences in the social relationships of young adult males might reflect the female subgroups which appeared prior to group fission.