Gorilla Groups Research Articles

Behavioral effects of changing group membership among captive lowland gorillas

Group membership was modified among 4 gorilla groups at Zoo Atlanta in an attempt to increase compatibility and sexual behavior. The modifications ultimately moved 7 adult females among the groups. The moves were made by individually introducing the new group members to the old members, resulting in 18 total introductions. Several individual and social behaviors differed in the two conditions. The most obvious change was a temporary increase in aggressive display behaviors, as well as an increase in contact and social examination, in the postmovement condition. Additionally, the individual behaviors of travelling and eating increased after the introductions, while resting decreased. Other self-maintenance behaviors did not change. Finally, there was an increase in time mothers and offspring spent closer to one another following the moves. Previous research and anecdotal evidence suggest that gorillas respond to changes in group membership with aggression; these data support and extend those data to different age/sex groups, postmovement trends in behavior, and other aspects of gorilla behavior. © 1996 Wiley-Liss, Inc.

Behavioral effects of changing group membership among captive lowland gorillas

Group membership was modified among 4 gorilla groups at Zoo Atlanta in an attempt to increase compatibility and sexual behavior. The modifications ultimately moved 7 adult females among the groups. The moves were made by individually introducing the new group members to the old members, resulting in 18 total introductions. Several individual and social behaviors differed in the two conditions. The most obvious change was a temporary increase in aggressive display behaviors, as well as an increase in contact and social examination, in the postmovement condition. Additionally, the individual behaviors of travelling and eating increased after the introductions, while resting decreased. Other self-maintenance behaviors did not change. Finally, there was an increase in time mothers and offspring spent closer to one another following the moves. Previous research and anecdotal evidence suggest that gorillas respond to changes in group membership with aggression; these data support and extend those data to different age/sex groups, postmovement trends in behavior, and other aspects of gorilla behavior. © 1996 Wiley-Liss, Inc.

Food Density and Ranging Patterns of Gorillas and Chimpanzees in the Kahuzi-Biega National Park, Zaire.

Diet and ranging patterns of gorillas (Gorilla gorilla graueri) and chimpanzees (Pan troglodytes schweinfurthii)were studied during the 1994 dry season in the montane forest of KahuziBiega National Park. A transect census was also made to estimate the diversity and density of trees in primary forest, secondary forest and swamp. Although the diversity of trees in this area was lower than those estimated in lowland tropical forest, more than half of the total 50 tree species were used for foods either by gorillas or by chimpanzees during the study period. A semi-habituated group of gorillas and a unit group of chimpanzees extensively overlapped their diet and ranging areas, but gorillas tended to consume fibrous foods qualitatively and quantitatively and to range in wider area than did chimpanzees. Gorillas daily traveled in both primary and secondary forest to avoid reuse of the previous ranging areas, while chimpanzees persistently range in a small patch of primary forest. The diversity and density of food trees with bearing fruits were higher in primary forest than in secondary forest, and both apes preferred the fruit species which trees appeared at low densities in the transect. The low density of chimpanzees may be owed to the small size and fragmented distribution pattern of primary forest in this area. The differences in foraging patterns between gorillas and chimpanzees may reduce ecological competition between them.

Function and Meaning of Wild Gorilla 'Close' Calls

We investigate the social significance of the within group ('close') calls of gorillas by examining correlates of calling with dominance rank and with relatedness of adults, and by examining whether the outcomes of interactions between adults differ depending on the calls given during the interactions. In two wild gorilla groups, the majority of adults give most of their calls when near another adult and fully in sight of them. Thus gorillas' use 'close' calls as more than mere contact calls. An adult gorilla's use of 'close' calls correlates with its own and with its partner's dominance rank, with effects being most obvious for the most dominant animals, the fully adult males. Thus the proportion of 'double grunts', the most common 'close' call, in an individual's repertoire correlated consistently with dominance rank; all non-silverback adults gave a higher percent of double grunts in the presence of subordinates than they did near dominant animals; individuals were most likely to give 'non-syllabled' grunts in the adult male's presence; and they consistently exchanged calls at a higher rate with the adult males than with other group members. An adult's type of calls did not obviously differ depending on whether their neighbour was kin or non-kin, but kin were overall more likely to give calls in the presence of kin, and to exchange calls with kin than with non-kin, although the association was not consistent throughout the two years of the study. With regard to the consequences of calling, subordinates were less likely to be feeding one minute after an approach by a fully adult male during which calls were exchanged than during silent approaches. In contrast to findings from studies of some other species, calling did not correlate with duration of grooming. We suggest that, among other functions, gorillas' 'close' calls mediate social interactions. One form of calls, 'non-syllabled' calls, are interpretable as appeasement signals. The broadest interpretation of the 'double grunt' is that it is an exaggerated announcement of presence, whose function is to attract attention to the caller, and to signal conditional future activity.

Interpositions in conflicts between males in bimale groups of mountain gorillas.

In bimale groups of mountain gorillas, conflicts between the silverbacks sometimes induce infants and females to interpose between the opponents. This paper describes a set of 40 such interpositions, using data from 380 h of focal observation of the silverbacks in 2 groups at the Karisoke Research Center. Aggressive interactions between males can be disruptive for females and infants: they might prevent them from feeding and resting normally. Interpositions occurred in 10-25% of the conflicts between silverbacks. None of the males seemed to receive a greater proportion of interpositions. Interpositions increased the interval between conflicts in one of the research groups. As a mechanism for resolving conflicts, interpositions could be advantageous when control interventions are not possible, i.e. when the individuals in conflict are the most dominant of the group. Because they might reduce the rate of aggression between males, these interpositions could also be one of the mechanisms allowing male coexistence in gorillas.

Post‐conflict Social Events in Wild Mountain Gorillas (Mammalia, Hominoidea). I. Social Interactions between Opponents

AbstractResearchers have documented elevated rates of affinitive interaction between opponents shortly after aggressive conflicts, or reconciliation, in many primate species. Reconciliation may ameliorate the immediate negative effects of aggression by reducing the chance of further aggression between opponents and thus reducing tension, and may avert or repair damage to long‐term social relationships important to the animals' fitness. Data on post‐conflict interactions between opponents in two groups of wild mountain gorillas(Gorilla gorilla beringei)fail to show reconciliation after conflicts between females, between males, or between immatures, but females seem to commonly reconcile themselves with males after receipt of male aggression. Females and subordinate males often avoid same‐sex opponents after conflicts. Females commonly retaliate against female aggressors, and post‐conflict rates of aggression between females are higher than baseline levels. Females may not need to achieve reconciliation with each other because relationships between co‐resident relatives are resiliant, while those between non‐relatives are mostly neutral to antagonistic. Males are, however, important social partners and protectors of females, and female transfer is common. Thus, the results strongly support the ‘important relationships’ hypothesis for the function of reconciliation.

Post‐conflict Social Events in Wild Mountain Gorillas. II. Redirection, Side Direction, and Consolation

AbstractInevitably, members of primate groups sometimes have aggressive contests with each other. Targets of aggression can engage in several types of interaction with third parties to ameliorate its adverse effects. They can redirect aggression, which may reduce the risk of further aggression from the initial opponents and reduce tension, or they can initiate affinitive interactions with third parties, in order to seek protection or reassurance. They can also receive reassuring acts from them (‘consolation’). Researchers have documented high levels of redirection in many primate species, but consolation is thus far known only in chimpanzees. Data on post‐conflict social interactions between targets of aggression and third parties in two groups of wild mountain gorillas (Gorilla gorilla beringei) show that immatures and subordinate males, but not females, redirect aggression at high rates. Furthermore, immatures seek affinitive interactions with their mothers, and adult females seek them with adult males, at elevated rates. Affinitive interactions between females and males seem mainly to have a protective function and are associated with decreased risks of further aggression between female opponents. Redirection by females after female‐female conflicts may be uncommon because targets commonly retaliate against aggressors. Males may offer females protection and consolation as services, and the results support the argument that such services are important when dominance relationships between females are often undecided and retaliation between opponents is common.

A Demographic Analysis of Male Life History and Social Structure of Mountain Gorillas

AbstractTwenty-five years of demographic data on free-ranging mountain gorillas (Gorilla gorilla beringei) from the Karisoke Research Center, Rwanda were used to assess male life histories and the variation within the age-graded social structure. Group types include lone silver-backs, one-male, multimale, and all-male groups. Although 60% of gorilla groups in the Virunga population are one-male, a multimale structure may increase chances of survival and future reproductive success for males at three stages of their lives. Infants born in one-male groups appear more likely to die from infanticide than those in multimale groups. Immature males in one-male groups may face decreased future reproductive opportunities compared to males in multimale groups. Adult males in one-male groups lack possible partners for coalition formation during intergroup encounters. Demographic constraints, such as length of time to male maturation, coupled with intense male-male competition for mates may limit the number and duration of groups with a multimale structure. Individuals are not restricted to one group type for their entire adult lives and males that attain maturity in each group type may eventually reproduce. Variation in male reproductive success is based both on length of reproductive tenure and on the number of mates.

Agonistic relationships between female mountain gorillas (Gorilla gorilla beringei)

Females in some mountain gorilla groups can be ranked on the basis of decided, non-aggressive approach-retreat interactions. However, data on four gorilla groups show that females do not form clear agonistic dominance hierarchies. Most aggressive interactions between females were undecided, and most dyads had undecided dominance relationships (Fig. 2). Females ignored aggression or responded to it aggressively more often than they responded submissively (Fig. 3). Moreover, directional inconsistency in aggression (including escalated contests) was high, and in some groups females showed significant bidirectionality in initiation of aggression (i.e., the more often they received aggression from partners, the more often they directed aggression at those partners; Fig. 4). Assigning ranks on the basis of aggressive interactions has limited power to explain variation in rates of initiating aggression and in responses and outcomes to aggression. Aggression was most common during feeding, but usually did not interrupt feeding bouts. It was proportionately more common in contexts that seem to involve competition for access to males (Figs 5, 6). This is consistent with the argument that females depend crucially on male services. As predicted by current models of female primate social relationships, agonistic relationships between gorilla females are unlike those typical in cercopithecine primates. They resemble those of some other primates and some equids in which contest competition for food is relatively unimportant and ecological costs of female transfer are low.

Functions of Wild Gorilla 'Close' Calls. I. Repertoire, Context, and Interspecific Comparison

Abstract Individuals in social groups of a number of species produce and exchange among themselves frequent, quiet vocalisations. The function of most such vocalisations, here termed 'close' calls, remains obscure, because of the lack of any obvious context of behaviour associated with their production. In this first of two papers that attempt to determine the function of these calls in wild girilla groups, we describe the call repertoire, the age-sex distribution of frequencies of call-types, the contexts in which the calls are given, compare the above measures with available descriptions for the other African great apes, the common and bonobo chimpanzees, and briefly speculate on possible functions. We provide sonograms and 21 acoustical measures of 15 calls in total, including eight 'close' calls, recorded from identified individuals of two habituated heterosexual groups of wild gorillas (Gorilla gorilla) of average composition for the region. In so doing, we provide the most detailed description yet of any great ape's within-group vocalisations. The age-sex classes differed in their use of 'close' calls: adult males called the most in total, immatures the least; and while most classes gave all calls, they gave them at differing frequencies. The differences were consistent across the two-year study period. We suggest two main contexts of production, namely situations of potential separation, and potentially agonistic situations. Thus animals called when far from others and before changes in group activity, and they called when unusually close to one another, especially while feeding. In comparison to the closely related common and bonobo chimpanzees, gorillas apparently had no calls specific to subordinate individuals behaving submissively: subordinate gorillas gave calls in such a situation, but they and dominants gave the same calls (at our current level of analysis) in other situations also. Acoustically and functionally, the gorilla's 'close' calls can be separated into 'syllabled' grunts and non-syllabled' longer calls, we suggest. 'Syllabled' calls might function to maintain contact between animals, to coordinate activity, and to act as mild threats; 'non-syllabled' calls might function as appeasement signals, and to cohere and coordinate group movement when given at good feeding sites, both by attracting animals to the site, and by inhibiting competition at it.

Nesting Behavior of Lowland Gorillas in the Dzanga-Sangha Reserve, Central African Republic: Implications for Population Estimates and Understandings of Group Dynamics.

An analysis of 1231 nests from 163 nest-sites recorded during a twenty-seven month long study (August 1990-NovemberI992) on the feeding ecology oflowland gorillas at the Bai Hokou Study Site, Dzanga-Sangha Dense Forest Reserve, Central African Republic is presented and compared with results from a concurrent nest-count census at the same site as well as results from an earlier census of the regional population and those from other gorilla populations. Nest counts are commonly used 1) to estimate population sizes and densities (Tutin & Fernandez, 1984; Carroll, 1986, 1988; Fay, 1988; Fay & Agnagna, 1992) and 2) to supplement direct observations and thus aid in understanding fluctuations in the size and composition of gorilla groups (Kuroda, 1992; Mitani et. ai., 1993). At the Bai Hokou Study Site, 44 % of gorilla nests recorded during the longitudinal study had no construction. At 15% of nest sites none of the nests were constructed. During a fine-grained census of the same study area, 30% of the nests identified were bare ground nests. These results contrast with reports from other sites of 0-10% bare ground nests (Tutin & Fernandez, 1984; Williamson 1988; Carroll, 1986; Fay, 1988; Schaller 1963). Nest construction type is probably related to the availability of suitable nesting materials at the nest site and at 8ai Hokou is shown to be influenced by micro-habitat, season and group size. These factors are also likely to affect interpopulation variability in nest construction. Bare ground nests and associated dung generally deteriorate to become unrecognizable after 4 days, while other nest construction types remain recognizable for more than 50 days (Tutin & Fernandez 1984, this study). Therefore, nest-counts will generally underestimate the numbers of nests at sites older than 4 days and may even fail to record entire nest sites. The accuracy of nest-count surveys may thus vary with location, habitats sampled and season.

Functional analysis of social staring behavior in an all-male group of mountain gorillas

A total 202 social staring episodes (prolonged gazing by one individual toward another within a short distance) were observed in various social contexts among six unrelated, adult, and subadult male mountain gorillas. Staring was not accompanied by distinct facial expressions by actors or recipients, irrespective of their age or dominance rank. Younger, subordinate animals tended to stare at elder, dominant animals more frequently than vice versa. Staring may have multiple functions depending on the social context. In the initiation of non-agonistic interactions, staring rarely occurred, but was very successful for younger males in eliciting play or homosexual interactions from older animals. Staring was also directed by younger males to older males for greeting or appeasement. It may possibly play a role in reducing the increased social tension that occurs during or after conflict and in averting the potential conflict among older males. Younger males occasionally supplanted older males by staring at feeding spots. This slow supplantation is similar to the phenomenon of food sharing achieved through begging behavior in chimpanzees and bonobos. A prolonged gaze including staring by subordinates towards dominants may characterize the frequent and prolonged face-to-face interactions in the African great apes, and contrasts with a frequent gaze aversion by subordinates towards dominants in macaques or baboons. A difference between gorillas and other apes is that, chimpanzees and bonobos make positive contact with each other through eye contact, while gorillas simply stare at another without physical contact in greeting or appeasement process. Staring may serve an effective strategy for younger male gorillas to intervene safely in olders' conflict and sometimes to suppress or inhibit olders' performance, but their non-agonistic contacts or supporting attacks may not contribute to the establishment or support of social bonds between them. It is possible that staring may be common among the African great apes and man, and that it has evolved as a tactics to mask the dominant/subordinate relationships between individuals with multiple functions.

Deception and hiding in captive lowland gorillas (Gorilla gorilla gorilla)

Deception and hiding are described for a group of captive lowland gorillas comprising two adult males, one female, and her infant. The males repeatedly interacted with the infant, and their interest was repeatedly thwarted by the mother. As in other social situations in which one animal controls access to an object desired by another, deceit was used to thwart the other's control. Either one or both males at times concealed their actions toward the infant, inhibited their attention to the infant, created a neutral image to get the infant, or interacted with the infant to tease the mother.

Strategies of Habitat Use by Mountain Gorillas

Patterns of home range use by a mountain gorilla group are examined here in relation to variation in food abundance and quality, diet quality and rainfall, to interactions between different social units and to the distribution of hazards (poaching risk). Variation in habitat quality influenced both long-term area occupation densities and shorter-term movement parameters (day journey length and the distance moved between consecutive feeding sites). Observed home range use patterns increased the gorillas' foraging efficiency. Rainfall influenced the evenness of home range use slightly, but this was the only seasonal variability. Interactions with other gorillas and poaching risk had proximate effects superimposed on the fundamental influence of food distribution patterns. Data from a larger sample of groups show a positive influence of group size on day journey length, but this was apparent only at very small and relatively large group sizes. The absolute distance travelled per day tends to be short even for large groups. The data support arguments that the costs of social foraging are low for mountain gorillas.

Further developments in the breeding population of lowland gorillas at Apenheul

AbstractIn 1987 a second group of lowland gorillas was formed at Apenheul. Aspects of the zoo‐technical management (diet, accommodation, etc.) are described. The management and the development of the population, including mother‐rearing of all offspring, are described. Special attention is given to the formation of a second group by introducing two zoo‐born females and a non‐breeding female from one group to a nonrelated male to form a second group. These transfers of group members resulted rather quickly in pregnancies. The two groups are housed on adjacent islands, and the advantages and disadvantages of such a facility are discussed.

Gorilla conservation in Zaire

The Zaire Gorilla Conservation Project was launched in 1984 after the 1981 census of mountain gorillas in the Virungas showed how the Mountain Gorilla Project in Rwanda, in which FFPS was closely involved, had helped to swell numbers. Over the past few years the Zaire project has made encouraging progress. The 1986 census counted the highest number of gorillas since the early 1970s and some of that increase can be attributed to improved protection in Zaire. In addition, the park is benefiting from income from tourists visiting habituated gorilla groups. The project continues to extend, taking in eastern lowland gorillas and chimpanzees, as well as developing an overall education programme for the region around the park.

Functions and social consequences of infant–adult male interaction in a captive group of lowland gorillas (Gorilla gorilla gorilla)

AbstractThe functions and social consequences of infant‐adult male interaction in a captive group of lowland gorillas were evaluated. The two males repeatedly attempted to interact with the infant, the infant sometimes attempted to interact with the males, and the mother usually interrupted interactions between male and infant. Infant‐directed actions by the two males frequently showed their interest in the infant and infrequently showed care giving toward her; their other infant‐directed actions occurred near the time of excitement or of playful actions between adults. Male‐directed actions by the infant frequently showed its interest in one male and infrequently showed care seeking from him. All infant‐adult male interactions but one occurred in the less stimulating of the gorillas' two enclosures. Boredom and the mother's frequent thwarting of contact between a male and infant are suggested as influences on the males' interest in the infant. The data suggest that availability of interesting objects alleviated the boredom of captivity for males and thus dissuaded them from seeking stimulation which sometimes resulted in harmful behaviors toward the infant.

Functions of Alliances in Contests Within Wild Gorilla Groups

AbstractPrimates frequently support other individuals in aggressive contests among group members. Several functions of such alliances have been suggested, some of them dependent on continued residence of females in the group of their birth. The distribution of alliances in the gorilla (Gorilla gorilla), in which both sexes emigrate, was analysed to investigate constraints on the use of alliances by primates. Functions were inferred on the basis of the identity of participants, it being assumed, for example, that a mother supported its infant against an adolescent male for a different reason than did an adolescent female who supported a dominant female against a subordinate female. Gorillas ally, we suggest, to protect relatives from harm, to provide them with access to otherwise unobtainable resources and, perhaps, to emphasise their dominance over rivals; it seems that individuals might also ally to obtain resources for themselves, and to establish supportive relationships with other group members. The first two inferred functions were the most common, involved in over 80% of alliances. The major contrast with female-resident primates lay not so much in the range of functions for which alliances were used, but in the lack of evidence that gorillas attempted to use alliances to manipulate the dominance status of their kin. Possible reasons for the contrast are that i) the widespread distribution and low quality of the gorilla's main food plants meant that ability in defence competition was not sufficiently advantageous for the benefits of raising the rank of kin to outweigh the costs; ii) the large difference in age among immature gorillas in the study area, which was mostly a consequence of the small group size, made it too costly to use support in contests to raise an individual's rank above that of older peers; iii) the leading male prevented the unequal investment among his offspring that would be entailed if individuals used alliances in contests to raise or reinforce the rank of kin. Currently, these possibilities cannot be distinguished.

The influence of help in contests on dominance rank in primates: hints from gorillas

In a number of primate species, daughters have a dominance rank adjacent to their mother's, and younger sisters dominate older ones. The acquisition of a dominance rank largely independent of body size has been related to the reception of help in contests. This paper concentrates on a three-stage process. (1) Vulnerable individuals are helped in contests. (2) The more successfully helped individuals challenge and, with help, eventually dominate less successfully helped ones who are relatively close to them in size. (3) The rank so acquired is reinforced by continued help after dominance is achieved. This hypothesis is substantiated by data from two groups of wild gorillas. In them, only the first stage of help occurred: 94% of incidents were to protect vulnerable kin. Lacking the second two stages, dominance rank remained closely proportional to body size. Help in contests increases the complexity of calculations of asymmetry in competitive ability. Since such help is far more commonly seen among primates than nonprimates, it is possible that it was a main selection pressure in the evolution of primate intelligence.

Socialization processes in a female lowland gorilla

AbstractA low‐interactive, captive, female lowland gorilla, Molly, was studied following the introduction into her enclosure of three gorillas, two males and a female, raised from birth in captive gorilla groups. Observations were made 6 mo after the new gorillas were introduced. Throughout the period of observation, Molly interacted in an affiliative manner with one of the males, playing or sitting quietly with him in a tree (where Molly spent most of her time) and occasionally on the ground. Agonistic displays between Molly and the new female decreased after they were released in the enclosure without the males for a series of days. Molly, however, continued to react to the other male, the most dominant, in an agonistic manner, and usually retreated from his reach, climbed the tree, and/or grimaced and piloerected whenever he approached. Although Molly's continued avoidance of the dominant male impeded her complete socialization, we propose that the interventions employed in this study—introduction of new younger gorillas into and an enclosure, and a series of dyadic separations between the noninteractive gorilla and each of the new group members—are possible strategies that can be used to facilitate socialization of captive, noninteractive gorillas.