Matrilineal Rank Research Articles

Reduced injury risk links sociality to survival in a group-living primate

Sociality has been linked to a longer lifespan in many mammals, including humans. Yet, how sociality results in survival benefits remains unclear. Using 10 years of data and over 1,000 recorded injuries in rhesus macaques (Macaca mulatta), we tested two injury-related mechanisms by which social status and affiliative partners might influence survival. Injuries increased individual risk of death by 3-fold in this dataset. We found that sociality can affect individuals' survival by reducing their risk of injury but had no effect on the probability of injured individuals dying. Both males and females of high social status (measured as female matrilineal rank and male group tenure) and females with more affiliative partners (estimated using the number of female relatives) experienced fewer injuries and thus were less likely to die. Collectively, our results offer rare insights into one mechanism that can mediate the well-known benefits of sociality on an individual's fitness.

Maternal rank acquisition in a primate with low aggression and coalition rates

For many group-living animals, acquiring adult dominance rank is an important social transition. In species with maternal rank “inheritance,” juveniles appear to acquire their adult rank by receiving coalitionary support from kin and by altering patterns of aggressive behavior as they age. Previous studies of rank acquisition, however, have focused on species with high rates of aggression and coalitionary support. We studied rank acquisition in blue monkeys, a species with relatively low aggression and coalition rates, hypothesizing that individualistic changes in agonistic behavior would accompany rank acquisition while the presence of potential coalition partners would be relatively unimportant. Focusing on juvenile females in a wild population, we evaluated latency to reach maternal status and assessed how agonistic behavior changed with age. We also compared rank acquisition in orphaned vs. non-orphaned juveniles. Females from higher ranking matrilines and those in larger groups took longer to reach their maternal status and having more kin did not accelerate rank acquisition. As they aged, subjects were more likely to show aggression to individuals that they were expected to outrank as adults. Female juveniles also appeared to work their way up the hierarchy by overturning ranks with groupmates sequentially. Most orphans reached their matrilineal rank, though orphans were less likely to reach their matrilineal rank than non-orphans, suggesting that in some cases mothers do play an important role in rank acquisition. Orphans with more or high-ranking kin were not more likely to reach their matrilineal rank. These results support the hypothesis that age-related changes in agonistic behavior accompany rank acquisition in blue monkeys while the availability of kin as coalition partners often has little effect on the process. Behavioral mechanisms underlying maternal rank acquisition may vary between closely related species in conjunction with other patterns of social behavior, such as aggression and coalition rates. In group-living animals with matrilineal hierarchies, individuals are born low-ranking, but eventually acquire a rank similar to their mother’s. In species with high rates of aggression and coalition formation, juveniles reach their mother’s rank by receiving support from kin in aggressive interactions, and by selectively directing aggression to groupmates from lower ranking families. We studied maternal rank acquisition in blue monkeys, hypothesizing that the process might occur differently in a species with low rates of aggression and coalitions. Indeed, the availability of kin appeared to have limited effects on rank acquisition. Maternal rank acquisition was, however, associated with age-related changes in juvenile behavior. We concluded that in some species with matrilineal hierarchies, rank acquisition might occur relatively independent of support from kin.

Matrilineal rank inheritance varies with absolute rank in Japanese macaques

Matrilineal rank inheritance varies with absolute rank in Japanese macaques

Dominance among female white-faced capuchin monkeys (Cebus capucinus): hierarchical linearity, nepotism, strength and stability

Research on Old World primates provided the foundation for understanding competitive strategies resulting from social and ecological pressures. The neotropical primate, Cebus capucinus shares many social patterns with Old World cercopithecines (e.g., female philopatry, male dispersal), which may contribute to similar expression of competitive strategies. To clarify the nature of dominance patterns among female white-faced capuchins we examined hierarchical linearity, rank acquisition, matrilineal rank inheritance, hierarchical strength and stability. We collected focal data on 22 adult females (2008) and long-term dominance data (1986–2008) on 33 adult females in Sector Santa Rosa, Costa Rica. Females displayed linear hierarchies based on the direction of dyadic submission. At sexual maturity females quickly acquired rank positions beneath their mother and older sisters. Hierarchies were considered strong based on high proportions of food-related agonism, short latency to detection of hierarchies (21 h/female) and low directional inconsistency scores (<5%). Hierarchies were considered stable based on lack of tied submissive interactions (indicative of uncontested rank positions), low rates of rank change (0.510 changes/year), and long-term stability in matrilineal rank order. These findings enhance our understanding of capuchin social systems and how the competitive strategies of white-faced capuchins compare to those of Old World primates.

Rates of Agonism by Diurnal Lemuroids: Implications for Female Social Relationships

Sterck and colleagues (Behaviour 134:749–774, 1997) focused attention on the evolutionary ecology of female social relationships within and between groups and proposed a model that distinguishes 4 categories of female relationships, which correspond to particular types of intra- and intergroup competition. They emphasized literature on haplorhines in their model because of numerous, detailed studies conducted on a range of species in the wild; in contrast, strepsirrhines such as the lemuroids are poorly represented. We evaluate more closely their classification of lemuroids as Dispersal-Egalitarian using a greater number of species of Lemur, Eulemur, Varecia, Hapalemur, Indri, and Propithecus. For the focal species we found that female philopatry occurs rarely, agonistic rates are relatively low, female dominance hierarchies are not stable and do not exist year-round, and intra- and intergroup female-female competition is infrequent. Therefore, our results support the suggestion that a majority of lemuroid taxa we surveyed correspond to the Dispersal-Egalitarian category with 2 probable exceptions: Lemur catta and Propithecus edwardsi. Because female Lemur catta are philopatric, have year-round dominance hierarchies with female matrilines, exhibit the highest rates of agonism in studied lemuroids, and have frequent intra- and intergroup female-female competition, it would seem that they more closely correspond to the category Resident-Nepotistic. However, maternal Lemur catta rarely support their offspring in agonistic contests and matrilineal rank is not inherited, which leads us to state that the species does not fit into any existing category that explains the nature of female social relationships. The relationships of female Propithecus edwardsi are also a challenge to categorize under the current model because some of their characteristics —typical female dispersal and low agonistic rates— fall into the Dispersal-Egalitarian category, yet other behaviors —intense targeted aggression and stable and year-round female dominance hierarchies— do not.

Matrilineal Cohesion and Social Networks in Macaca fuscata

In a provisioned troop of Japanese macaques (Macaca fuscata) in Arashiyama, Japan, greater adherence to Kawamura's rules of matrilineal rank inheritance and youngest ascendancy occurred among high-ranking females versus low-ranking females. Accordingly, high-ranking females formed more clustered hierarchies and low-ranking females had more dispersed hierarchies. A proximate explanation for this finding may be related to differences in how females maintain their social networks. To determine whether the clustering in the hierarchy was reflected in patterns of social cohesiveness, I compared network sizes of coalition and grooming partners for females in each third of the hierarchy. I calculated the proportion of available partners that were coalition and grooming partners within each category of relatedness (0.5 ≥ r ≤ 0.004 and r = 0). High-ranking females formed coalitions with a large proportion of their close relatives and a small proportion of their distant relatives; middle-ranking females supported an intermediate proportion of their close relatives and a small proportion of their distant relatives; and, low-ranking females formed coalitions with very few available close and distant relatives. High-ranking females groomed nearly all available close relatives and an intermediate proportion of distant relatives, whilst middle- and low-ranking females groomed a large proportion of available close relatives and a very small proportion of distant relatives. Thus, levels of clustering within the hierarchy appeared to reflect levels of social cohesion, in terms of grooming and coalition formation.

Matrilineal rank Inheritance varies with absolute rank in Japanese macaques.

In many cercopithecine primates, females form linear dominance hierarchies based on kinship. It is known that female rank follows the rules of matrilineal rank inheritance (MIR): (1) maternal rank inheritance, (2) maternal dominance, and (3) youngest ascendancy among sisters. Although, several determining such variation remain largely unknown. In this paper, I investigate the dominance relation-ships of 69 adult (>6 yr old) female Japanese macaques (Macaca fuscata fuscata) in a free-ranging provisioned troop living in Shiga-Heights (Nagano Prefecture, Japan) and report new evidence of intra-group variation. Dominance relationships among high-ranking females followed MRI within kin units, those among low-ranking females did not. Maternal rank inheritance and youngest ascendancy operated between mother/daughter dyads and sister dyads of high-rank, but not in the dyads of low-rank. The dominance ranks of females from low-ranking kin units were dispersed and less predictable. These findings suggest that MRI varies with absolute dominance rank, and are discussed in relation to other asymmetries between high-and low-rank.

Kinship bonds are not necessary for maintaining matrilineal rank in captive Japanese macaques

Although kinship is of central importance in matrilineal (nepotistic) dominance systems, various lines of evidence indicate that its role has probably been overestimated. For example, alliances among nonkin, patterned on the basis of dominance per se, contribute to the maintenance of rank and to the remarkable stability of matrilineal rank orders. But because kin and nonkin alliances exert their stabilizing effects on the rank order simultaneously, the effect of nonkin alliances per se is unknown. We tested whether nonkin alliances are sufficient to ensure the stability of matrilineal rank relations in a laboratory-housed group of Japanese macaques (Macaca fuscata). We first created a group composed of two female subgroups: (1) a subordinate subgroup of close kin known to constitute an effective revolutionary alliance and (2) a dominant subgroup composed mostly of nonkin and distant kin known to support each other against lower-ranking females. In the course of 12 consecutive experimental manipulations, we reduced progressively the relative power of the dominant subgroup by manipulating its size and age composition. The members of the dominant subgroup maintained their rank in all experimental situations except when alone. Thus, nonkin alliances remained effective up to extremely pronounced levels of power imbalance. These results suggest that the remarkable stability of matrilineal rank orders is not conditional upon the existence of nepotistic alliances and strong matrilines of close kin.

Group fission in Barbary macaques (Macaca sylvanus) at affenberg salem

We analyzed eight group fissions occurring during a 20-year period in three groups of a free-ranging provisioned Barbary macaque population. The founder group fissioned four times within 3.5 years after transfer to the enclosure, indicating that external factors—new environment, more space, absence of other groups—facilitated group fissions. Two groups resulting from these fissions, split twice within 2.5 and 1 years, respectively, many years later. The process of fissioning lasted from a few months to almost 2 years. Fissions were preceded by peripheralization/subgrouping of mainly young adult males (8-10 years old), suggesting that male competition was the primary force for the fissions. The males were joined by middle- to low-ranking but not the lowest-ranking females. The resulting new groups were usually smaller than the groups in which the former α-matriline—old groups—stayed, and they were also more variable in size and sex ratio, suggesting that variable numbers of surplus individuals were expelled during fission. Mean adult sex ratios were similar in both groups after fission, indicating that the competitively superior males in the old groups (groups + α-matriline) could not increase their breeding opportunities. Female kin, even of large matrilinies, almost always stayed together during fission. Natal males strongly preferred to join the old groups, and this preference was most pronounced in juveniles and subadults. Hence, most natal males stayed with maternally related females, i.e., remained true natal males, if the females stayed in old groups. They were separated from female kin, i.e., became seminatal, if the females joined the new groups. These seminatal males did not differ from natal males with respect to matrilineal rank, but they had more female relatives, above all more close relatives (sisters), indicating that avoidance of mating with maternal kin was important for group choice. Despite joining the same group as female kin during fissioning, breeding opportunities of natal males (ratio of unrelated females/male) were not less than that of their seminatal peers, because natal males had fewer female relatives. Only a minority of both groups of males would have done better by joining the alternative group. Paternal relatives were distributed during fission by chance, and loss of patrilinies was therefore much less pronounced. We conclude that the rules governing social relationships among Barbary macaque males are less apt to cope with the high number of males resulting from provisioning, whereas the rules regulating social relationships of females living in a nepotistic, female-bonded society are very robust in this respect.

Acquisition of matrilineal rank in captive spotted hyaenas: emergence of a natural social system in peer-reared animals and their offspring

Spotted hyaenas, Crocuta crocuta, live in clans characterized by a matrilineal social system, with female dominance over males. As part of an ongoing study of the development of hyaena social behaviour, the ‘de novo’ emergence of a natural social system was documented in captive groups of peer-reared hyaenas that had not acquired their mother's rank prior to removal to a captive situation. The acquisition of rank in their descendants was examined. Dominance relationships were studied in six breeding groups using competitive feeding tests administered in different social contexts. Founding members of these groups had been collected in Africa as infants (<2 months of age) and reared in peer groups from the time of collection. Although female dominance was not evident in groups of peer-reared animals prior to sexual maturity, a natural matrilineal social system emerged in groups of peer-reared animals with their offspring. Juvenile hyaenas acquired ranks adjacent to those of their mothers, dominating much larger adults that were lower in rank that the juveniles' mothers, including the adult males. Group dominance relationships persisted during competitive tests with mothers removed, or with all adults removed, and during dyadic tests. Despite differences in group composition and in the social experience of mothers, a matrilineal social organization clealy emerged in the captive breeding groups of spotted hyaenas and their cubs. The acquisition of matrilineal rank among the offspring of peer-reared hyaenas, as well as among the offspring reared with mothers who had themselves acquired matrilineal rank as juveniles, suggests that matrilineal rank acquisition may be an inherent outcome of relationships generated between mother-offspring units and other group members, independent of a mother's degree of experience in a matrilineal social environment.

Induction of matrilineal rank instability by the alpha male in a group of Japanese macaques.

Destabilizing alliances and ensuing rank changes occur infrequently in matrilineal dominance systems, and consequently very little is known about their dynamics. In one such type of alliance, called bridging, dominant individuals are ultimately responsible for subordinate monkeys out-ranking intermediate-ranking animals. Previously reported bridging alliances were based on sexual attraction, kinship ties, or special affinitive bonds between partners. In this paper we describe bridging alliances involving the alpha male in a group of Japanese macaques (Macaca fuscata) housed at the University of Montreal Laboratory of Behavioral Primatology. The comparison of rank relations in the presence and absence of the alpha male indicates that the latter was responsible for the destabilization of more than half of rank relations between members of the first and second matrilines. Another experiment revealed that the alpha male also induced extensive intermatriline reversals in priority of access to concentrated and highly prized food. These effects could be attributed to the male's consistent pattern of agonistic interventions in favor of the second matriline. However, the alpha male was not engaged in preferential affinitive relationships or reciprocally supportive interactions with the members of the second matriline. On the other hand, various lines of evidence indicate that the alpha male competed for dominance with the matriarch of the first matriline, suggesting that he might have consistently joined other individuals against that female and her kin in this context. The present data constitute further evidence for the nonaltruistic nature of aggressive interventions and the effectiveness of bridging alliances. © 1995 Wiley-Liss, Inc.

Dominance competition among siblings in Japanese macaques: constraints on nepotism

Abstract. Matrilineal rank orders, common in cercopithecine monkeys, are often called nepotistic because female kin occupy adjacent ranks and support each other preferentially. But alliances among non-kin also play a central role in these dominance systems, enabling females to maintain their rank above lower-ranking individuals. The aim of this study was to investigate the extent to which nepotism (among siblings) might conflict with a female's need to form alliances with non-kin. The study was conducted at the University of Montreal Laboratory of Behavioural Primatology housing a group of 35 Japanese macaques, Macaca fuscata. In this species females outrank their older sisters after a period of aggressive challenge. Younger siblings that were individually unable to maintain their rank above an older sister were found to be able to do so in the presence of dominant non-kin, indicating that the latter are effective allies for younger siblings. Thus, in conflicts between non-kin allies and their older sisters, younger siblings may face a conflict of interest between siding with the non-kin ally to increase their competitiveness against their sister, or help their sister. One thousand and seventy-seven aggressive interventions involving 22 sibling dyads over a 56-month period were analysed. In accordance with the experimental results, dominant non-kin supported preferentially the younger siblings. The latter appeared divided between siding with their older sister and the non-kin ally. However, in conflicts between non-kin individuals and kin that they did not target for dominance, younger siblings acted nepotistically more often. Thus, nepotism appeared constrained by a female's need for non-kin allies. These data are best interpreted in terms of individual advantages rather than in terms of kin selection or reciprocal altruism.

Male aggression: a cost of female mate choice in Cayo Santiago rhesus macaques

Sociality has been linked to a longer lifespan in many mammals, including humans. Yet, how sociality results in survival benefits remains unclear. Using 10 years of data and over 1,000 recorded injuries in rhesus macaques (Macaca mulatta), we tested two injury-related mechanisms by which social status and affiliative partners might influence survival. Injuries increased individual risk of death by 3-fold in this dataset. We found that sociality can affect individuals’ survival by reducing their risk of injury but had no effect on the probability of injured individuals dying. Both males and females of high social status (measured as female matrilineal rank and male group tenure) and females with more affiliative partners (estimated using the number of female relatives) experienced fewer injuries and thus were less likely to die. Collectively, our results offer rare insights into one mechanism that can mediate the well-known benefits of sociality on an individual’s fitness.

Aggressive interventions and matrilineal dominance relations in semifree-ranging barbary macaques (Macaca sylvanus)

Female Barbary macaques (Macaca sylvanus) form matrilineal hierarchies, i.e. they come to rank below their mother in relation to non-kin females in the course of maturation. The stability of such hierarchies and the acquisition of the matrilineal rank are achieved through dyadic aggressions and third party interventions in conflicts. This study examines the dynamics of interventions in non-kin conflicts in a semifree-ranging group of 109 Barbary macaques at “La Montagne des Singes,” Kintzheim, France. Focal sampling on 13 females aged 3 and 4 years not yet dominant over all older females from lower ranking kin groups (lower born females) was carried out during 16 months in 1987 and 1988. Results on the direction of support in non-kin female conflicts (all samples pooled) show that interventions were generally provided on behalf of the female from the higher ranking kin group (higher born female). Rates of interventions (derived from focal samples) given and received were correlated with the hierarchy; higher born females received more support and intervened more often than lower born females. A subset of interventions (based on the age of the females involved) was analyzed according to the rank distance between the opponents and the type of support provided (spontaneous or solicited). On the basis of their representation, intermediate-ranking supporters (i.e. females ranking between the opponents) intervened more often than above-ranking supporters (i.e. females dominant to both opponents), and they intervened more often spontaneously than following a solicitation. The results on interventions are discussed in the perspective of benefits to supporters. Twenty-one instances of outranking of older females (matrilineal rank acquisition) were observed. By the end of the study, the number of older lower born females not yet outranked by the focal females was negatively correlated to the rank distance between the two sets of females. However no such correlation was found between these two groups when compared according to their age difference in years.

Rank relations among sisters in semi-free-ranging Barbary macaques (Macaca sylvanus)

Although semi-free-ranging Barbary macaque females are able to outrank older females from lower-ranking matrilines (matrilineal rank acquisition), they do not systematically outrank their older sisters, as is known to be the case for semi-free-ranging rhesus monkeys (Macaca mulatta) and Japanese macaques (Macaca fuscata). We test the hypothesis that differences in the support received by younger sisters against their older sisters and against older lower-ranking females might account for this interspecific difference. Thirty-one sister dyads, members of a group of 109 Barbary macaques living at La Montagne des Singes, France, were observed during 16 months. The results indicate that (1) all females were dominant to their younger sisters, and the latter were never observed to challenge their older sisters; (2) younger sisters received as much kin support against their older sisters as against older lower-ranking females; (3) only very young females received support from their kin against their older sisters; (4) younger sisters received much more support from nonkin females against lower-ranking females than against their older sisters; and (5) Barbary macaque females appear to be supported against their older sisters less frequently than rhesus macaque females are. We conclude that the lack of nonkin support is the main factor accounting for the failure of younger sisters to outrank their older sisters in Barbary macaques. Initially this might result from kin support not being sufficient to induce younger sisters to challenge and to solicit support against their older sisters.

Agonistic Interactions of Juvenile Savanna Baboons

Abstract19 juvenile members of known genealogies in two wild baboon groups were studied over a 16‐month period. Males and females in two age classes were compared regarding third‐party aggression and support given and received during agonistic interaction. Young juvenile males and females (1–2.5 years‐old) were supported predominantly by their probable fathers. The adult males supported without regard to the family ranks of juveniles and their opponents. Old juveniles (3–5.5 years‐old) no longer had probable fathers in their groups and rarely or never received support from adult males. Females received considerable maternal support and, with age, most often received support from unrelated adult females. Males rarely received support from either their mothers or unrelated adult females. 100% of adult female support for and against 11 of the 12 females reinforced existing rank relations among matrilines. In contrast, only about half of adult female support of males reinforced matrilineal rank relations. Juvenile females exploited aggression among adult females during rank acquisition: they supported high‐ranking adults during attacks on females they had targeted for rank reversal. In contrast, males attacked adult females in support of peers or young juveniles. Females also solicited support during fights more often than males. They most often solicited adult females, whereas males directed their rare solicitations toward other males. Despite low maternal rank, one female targeted all older females for rank reversal except the adults of her group's top‐ranking family. She was the only female to receive support from adult females that contradicted existing rank relations among matrilines, and the only female ultimately to achieve stable adult rank above families outranking her own. Adult female baboons appear to structure the development of rank relations for juveniles in their groups. Juvenile females are typically limited to acquisition of familial status, while males are free to achieve rank over whichever group members they can independently intimidate.

Experimental matrilineal inheritance of rank in female Japanese macaques

In many species of cercopithecines a female inherits her mother's (or genealogical) rank. Matrilineal rank inheritance may be defined in general terms as the process whereby a female (termed ‘dependent’) acquires the rank of an ‘ally’ above another female (‘target’). An attempt was made to reproduce this process in time-lapse form in a group of 17 Japanese macaques, Macaca fuscata, comprised of three families with similar age-sex compositions. Experimental female subgroups were formed such that in each of them a female (dependent) was given more alliance power than a dominant target. The results indicate that in each of the subgroups that was tested the dependent female inherited the rank of her ally (mother or older sister) above same-age or older targets. Four processes of rank inheritance were observed. The fact that females who were given more alliance power did not solicit their ally, or challenge the target females, before they were aided by their ally suggests that aggressive interventions are the primary mechanism of rank inheritance. The results also account for the reported rarity of aggressive interventions in natural groups and for the observation that orphans may nevertheless inherit the rank of their family. The role and dynamics of the recognition of alliances in the establishment of rank relationships are discussed.

Experimental rank reversals among peers in Macaca fuscata: Rank is maintained after the removal of kin support.

In many species of cercopithecines female rank is inherited matrilineally so that maternal kin occupy adjacent ranks. A female is said to have acquired her matrilineal rank in relation to a given subordinate female when the submission she receives from that female and the aggression she directs to her are both unidirectional. Kin support appears to be important in that process (rank acquisition). Matrilineally dominant females also appear to need kin support for maintaining their rank once they have acquired it. This is inferred from observations indicating that subordinates can form kin-based coalitions and outrank single dominant females. Therefore, if subordinates did not, or could not, form coalitions, the maintenance of rank among similar-sized females might be independent of kin support. One test of this hypothesis would be to remove all kin support shortly after rank acquisition has taken place. We report such a test performed in a group of captive Macaca fuscata composed of three families with similar age-sex compositions. Experimental subgroups were formed by placing together peers from different families and a single kin (ally) of one subordinate peer (mother or older sister). These experiments induced a series of rank reversals whereby the subordinate peer, whose ally was present, outranked the dominant peers. Following the rank reversals, the ally of the newly dominant peer was removed and the peers left together. All five experimentally inverted rank orders of peers remained stable. The finding that experimentally subordinate individuals abstained from challenging experimentally dominant peers despite their long ontogenetic experience of reversed dominance roles suggests (1) that matrilineal rank acquisition is a punctuate (versus ontogenetically cumulative) process, (2) that the maintenance of rank among peers does not necessitate kin support if subordinates cannot form coalitions, and (3) that the "minimal risk constraint on competition" helps account for the stability of matrilineal hierarchies.

Continuity and change in dominance relations among female baboons

Female baboons, Papio cynocephalus, in Amboseli National Park establish linear dominance hierarchies in which maternal kin usually occupy adjacent ranks. Previous work had shown that few changes in the relative rank order of matrilines had occurred between 1971 and 1981 (Hausfater et al. 1982). During a 9-month period beginning in December 1982, the rate and magnitude of changes in matrilineal rank order accelerated. Major changes in the relative ranks of members of a few matrilines resulted in changes in the absolute ranks of females of all but one matriline. However, the ordering between many pairs of matrilines did not change and the genealogical structure of dominance relations was generally maintained. This brief period of rapid change was succeeded by a period of slow change and relative stability in dominance relations, lasting at least 27 months. Data from the 15-year period suggest that the rates of change in female dominance relations are variable: long periods of stability are sometimes punctuated by short periods of instability and change. No single explanation accounted for this variability.

Secondary sex ratio variation in the Cayo Santiago macaque population.

Secondary sex ratios (SSR) were calculated from 1,385 offspring delivered by 372 females in the Cayo Santiago population of free-ranging rhesus monkeys (Macaca mulatta) from 1976 through 1984. The SSR for the entire colony ranged from 0.86 to 1.46 males per female (combined total: 1.08), but no significant difference was observed (P > .05). SSR values were compared among the troops for each year. The SSR differed significantly among the six social groups (P < .05) only in 1978. The annual SSR of each troop was compared over 9 years. Significant variation was found only in group O. The annual SSR was significantly skewed (P < .05, males > females) for three troops in 3 separate years. The SSR did not vary according to troop rank. No significant difference was found among the 17 matrilines of the population, but comparison of matrilines within each social group revealed a significant difference in the SSR (P < .02) for the three matrilines in group I. This was due to the significantly skewed SSR (P = .0080, females > males) of the DM genealogy in that troop. SSR values were not related to matrilineal rank. Individual dominance rank did not bias the SSR. Complete reproductive histories for 266 females showed no evidence of significantly skewed SSR values. Age-related effects on the SSR were examined by using cross-sectional and cohort-based analyses. The SSR did not vary significantly (P > .05) with maternal age, but it was significantly skewed (P < .05) toward males at the ages of 5 and 9 years. Parity had no significant effect (P > .05) on SSR values. Wide variation occurred in the SSR of the Cayo Santiago population. Rank-related adjustment of the SSR at the level of the troop, matriline, or individual, as reported in short-term studies of other primate social groups, may reflect normal annual variation in the SSR evident only from longitudinal observations of large multigroup primate populations.