Agonistic Interactions and Matrifocal Dominance Rank of Wild Bonobos (Pan paniscus) at Wamba

I studied dominance relations in a wild group of bonobos at Wamba, Democratic Republic of Congo. Although agonistic interactions between males occurred frequently, most of them consisted only of display, and physical attacks were infrequent. Dominance rank order seemed to exist among males, but its linearity is unclear. Dominant males rarely disturbed copulatory behavior by subordinate males. However, high-ranking males usually stayed in the central position of the mixed party and, so, would have more chance of access to estrous females. Among females, older individuals tended to be dominant over younger individuals. However, agonistic interactions between females occurred rather infrequently, and most consisted of displacement without any overt aggressive behavior. Dominance between males and females is unclear, but females tended to have priority of access to food. The close social status between males and females may be related to the prolonged estrus of females and their close aggregation during ranging. Existence of a male's mother in the group and her dominance status among females seemed to influence his dominance rank among males. Young adult males whose mothers were alive in the group tended to have high status. In some cases, change in dominance between high-ranking males was preceded by a corresponding change in dominance between their mothers. As the dominance status of females is similar to that of males, mothers may be able to support their sons to achieve high status, stay in the center of the mixed party, and so have greater access to females, which may maximize the number of descendants of the mothers.     

Allogrooming Behavior and Grooming Site Preferences in Captive Bonobos (Pan paniscus): Association with Female Dominance

I tested the utility of Seyfarth's (1977) model of rank-related attractiveness to explain the distribution of allogrooming behavior among captive bonobos (Pan paniscus). Adult female bonobos generally have high social status and may be dominant over males. As predicted by the model, I found that high-ranking adult females received most allogrooming within each of the four investigated groups. Among adult female-adult female dyads, however, allogrooming was not clearly associated with dominance rank. Contradictory to predictions of the model, the highest-ranking females were responsible for most displacements over allogrooming, and grooming competition is positively correlated with dominance rank. In the second part of this study, I investigated the social significance of allogrooming body site preferences. Bonobos direct significantly most allogrooming to the face of conspecifics, and high- and low-ranking individuals, as well as males and females, differ significantly in their preferences for certain allogrooming sites. Subordinates and males tended to avoid facial grooming and preferred the back and anogenital region, while high-ranking individuals and females directed most allogrooming to the face and head of grooming partners. Data from this study support the hypothesis that high-ranking females are the most attractive grooming partners within a female-centered bonobo society. Many other aspects of allogrooming behavior, however, are not consistent with the model of rank-related attractiveness.     

Population Dynamics of Wild Bonobos (Pan paniscus) at Wamba

We analyzed population dynamics and birth seasonality of wild bonobos at Wamba, Democratic Republic of the Congo, based on 20 years of observations (1976–1996). Wamba Bonobo infant mortality is much lower than that reported for chimpanzees. This seemes to be related to several socioecological characteristics of bonobos: the use of abundant fruit and herbaceous foods, larger food patch size, female feeding priority, and the absence of infanticide. The mean interval between live births of 4.8 years is shorter than those reported for chimpanzees, and some females simultaneously carried and nursed two successive offspring. Mother–offspring conflicts, such as refusal of suckling attempts and interference with mothers' copulation, which are common in chimpanzees, are rare in Wamba bonobos. A birth peak seems to occur during the light rainy season from March to May, just after the season with the least rainfall. This timing of births is similar to those reported for chimpanzee populations, and might benefit both mother and offspring by maximizing the amount of time before the next dry season.     

Context and Development of Sexual Behavior of Wild Bonobos (Pan paniscus) at Wamba, Zaire

I studied sexual behavior of immature bonobos (Pan paniscus) in a wild group living at Wamba, Zaire, with special reference to its development. Even immature individuals under 1 year old performed sexual behavior. Sexual behavior occurred in almost all age–sex combinations, except between immature and mature females. Based on analyses of behavioral pattern and context, I classified sexual behavior involving immature individuals into three categories. (1) Genital contact between immature individuals was observed during play, and was performed by males more frequently than by females. This sexual behavior shared many traits with that of other great apes. (2) Copulation-like genital contact was observed between immature males and mature females. Its frequency increased with the immature male's age; it developed into copulation in adulthood. (3) Genital contact used to regulate interindividual relationships. This behavior, which is unique to bonobos, was absent among infants. It developed between late juvenile and early adolescent periods in association with changes in social circumstances.     

Habitat Use and Ranging of Wild Bonobos (Pan paniscus) at Wamba

The relationship between vegetation and ranging patterns of wild bonobos at Wamba, Democratic Republic of the Congo, was examined. Via Landsat data, we distinguished three types of vegetation—dry forest, swamp forest, and disturbed forest—at Wamba. The home ranges of the study groups changed considerably from year to year, due mainly to intergroup relationships. The population density of each group varied between 1.4 and 2.5 individuals per km ² and was lowest during a period of population increase. Home ranges consisted mainly of dry forest. The bonobos used dry forest more frequently than the other forest types, though they also used swamp and disturbed forest almost every day. The latter types of forest seemed to be important resources for the bonobos, owing to the abundant herbaceous plants that are rich in protein and constantly available. The bonobos tended to use dry forest more frequently in the rainy season than in the relatively dry season, probably because the favored fruits in the dry forest were mostly available in the rainy season. There was no seasonal difference in the size of the daily ranging area.     

Sexual competition in a group of captive bonobos (Pan paniscus)

We describe the occurrence of sexual competition, expressed as harassment of sexual interactions in a captive group of bonobos. We monitored all aggressive and pestering interventions during sexual interactions of three captive adult females, one adolescent, and three adult males. The study period covered two complete menstrual cycles for each female, with continuous daily observations. There was relatively little overt sexual competition by the males, in analogy with other studies. Most male interventions occurred towards interactions with the alfa female. The alfa female performed the most intense and the highest number of interventions towards the sexual interactions of the other females. The data provide evidence for female intra-sexual competition in this female dominant species.     

Sex Differences in the Steepness of Dominance Hierarchies in Captive Bonobo Groups

Bonobos have a reputation as a female-dominated and egalitarian species. We examined the 2 aspects of dominance in 6 captive bonobo groups. Females do not consistently evoke submission from all males in all contexts. Though females occupy the highest-ranking positions in the dominance hierarchy, there are in each group males that obtain rather high ranks and are able to dominate ≥1 female. Thus female dominance is not complete and hierarchies can be better described as nonexclusive female dominance. We studied egalitarianism by measuring linearity and steepness of dominance hierarchies. The hierarchies of all groups are highly linear. Hierarchies among males are steeper than among females. On average, male bonobos are more despotic than females, but females too can have despotic relations, both with other females and with males. Hence one can call bonobos in captivity semidespotic rather than egalitarian.     

Comparing infant and juvenile behavior in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): a preliminary study

The dichotomy between the two Pan species, the bonobo (Pan paniscus) and chimpanzee (Pan troglodytes) has been strongly emphasized until very recently. Given that most studies were primarily based on adult individuals, we shifted the “continuity versus discontinuity” discussion to the infant and juvenile stage. Our aim was to test quantitatively, some conflicting statements made in literature considering species differences between immature bonobos and chimpanzees. On one hand it is suggested that infant bonobos show retardation in motor and social development when compared with chimpanzees. Additionally it is expected that the weaning process is more traumatic to chimpanzee than bonobo infants. But on the other hand the development of behaviors is expected to be very similar in both species. We observed eight mother–infant pairs of each species in several European zoos. Our preliminary research partially confirms that immature chimpanzees seem spatially more independent, spending more time at a larger distance from their mother than immature bonobos. However, the other data do not seem to support the hypothesis that bonobo infants show retardation of motor or social development. The development of solitary play, environmental exploration, social play, non-copulatory mounts and aggressive interactions do not differ between the species. Bonobo infants in general even groom other group members more than chimpanzee infants. We also found that older bonobo infants have more nipple contact than same aged chimpanzees and that the weaning process seems to end later for bonobos than for immature chimpanzee. Additionally, although immature bonobos show in general more signs of distress, our data suggest that the weaning period itself is more traumatic for chimpanzees.     

Structure and use of distance calls in wild bonobos (Pan paniscus)

We report the physical structure and use of a distance call (high-hoot) by wild bonobos (Pan paniscus).Although spectrographic analyses reveal high structural variability, the total sample can be subdivided according to the composition of units—the presence or absence of an initial segment—and the range of the lowest harmonic. Analyses of samples from male—female pairs,vocalizing simultaneously and in close proximity, reveal that both animals utter calls in more or less precise temporal alternation but with different spectral ranges. Whether these differences are gender-specific or related to other factors, such as age or the social relations between particular individuals, is not clear. We suggest that (a) individuals of the same party may coordinate their vocal activity on both the temporal and the spectral level and (b) high hootings stimulate emission of equal vocalizations by members of other parties and may increase cohesion among community members. Comparison of a restricted number of spectrograms from known individuals indicates that bonobos may be able to adjust spectral parameters of one type of distance calls (high- hoot) according to corresponding calls of conspecifics.     

Function and distribution of coalitions in captive bonobos (Pan paniscus)

We examined the distribution of support behaviour within a captive group of bonobos. Most support was evoked by inter-sexual conflicts with the two highest ranking females. Within a dyad, the usual winner was most often supported. Individuals that challenged the rank order by aggressions and pestering were aggressed more often by their targets in the company of an ally. The two lowest ranking males served as scapegoats, receiving 80% of the contra-support. In coalitions, inviduals did not aggress victims they would not dare to attack without supporters. However, the victims of coalitions reacted more strongly with fear and rarely counteraggressed than when being attacked alone, indicative of the high impact of aggression in support. The alpha female showed some control behaviour when intervening in conflicts. The data fitted with several functional hypotheses: coalitions functioned to maintain existing ranks, to acquire ranks, to reduce tension, and to test or strengthen the bond. We suggest that support behaviour fulfilled a crucial role in the maintenance of the power of the two highest ranking females over the males. Among the females themselves the dominance relationships were not based on coalitions, but on individual attributes.     

Peering in mature, captive bonobos (Pan paniscus)

“Peering”—close-proximity staring at the mouth of another—was observed in ten (three males and seven females) mature (at least 7 years old) bonobos (Pan paniscus) living in three social groups at the San Diego Zoo and Wild Animal Park. Instantaneous scan samples, taken at 2-min intervals, over a three-and-a-half year period, yielded 617 observations of peering (1.4 per observation hour). Food was exchanged in only 15 of these scans. Peering was most often performed by younger animals and was primarily directed toward older females (“matrons”). In a given dyad, the animal more likely to peer at the other was also more like to both peer and be peered at if they frequently groomed and infrequently displayed aggression at a given female. An adolescent male showed the highest frequency of peering when living with two older females, but dropped to adult male levels when later housed with two younger (albeit mature) females. A reversal in which animal was more likely to peer, follow, and groom occurred in one female dyad, after the birth of the younger animal's first infant. After a similar birth in the other group, no such changes were observed. We discuss how these and related findings, in conjunction with what is known of the social structure of this species, suggest that one possible function of peering in bonobos may be as a signal acknowledging female status.     

Association and social interactions between strangers and residents in bonobos (Pan paniscus)

This study reports on close spatial association and repeated behavioural interactions between two strange adult male bonobos with residents of another community. Over a period of 12 months one of the two males developed friendly social relations to some of the females and other residents, which were indistinguishable from those existing between co-residents. Aggression by resident males against the strangers decreased but the former remained intolerant. The strange males appeared at a time when the number of adult resident males was lower as in the years before and when the adult sex ratio (number of adult females per male) was higher as in the years before. Using definitions from studies on dispersal patterns of male gorillas (Harcourt, 1978) and female bonobos (Furuichi, 1989) the spatial association between the two strange males and residents could be described as male transfer.     

Bonobo (Pan paniscus) Spatial Memory and Communication in a 20-hectare Forest

We used an artificial language as a tool for the study of spatial memory organization in a young Pan paniscus. In the first experiment, we showed the bonobo a road sign just outside its indoor sleeping area. The sign indicated, by means of arbitrarily designated geometrical shapes (lexigrams), where food was hidden. Only 2 of the 15 locations were visible from the sign. Distances ranged up to 170 m from the sign. In 99 of 127 test trials the bonobo went directly to the designated location on its first move. In a second experiment, we presented the road sign at varied points in the woods rather than at the original fixed place. In these trials the goal was a preferred toy. The bonobo's human companions were never told the location of the goal and distances were up to 650 m. In all 12 trials the bonobo led its companions to the designated place via an efficient path. The bonobo appeared to be able to move, based on the information provided by a lexigram, from almost any arbitrary starting location in its 20-ha environment to any one of the numerous goal locations.     

Reactions of a group of pygmy chimpanzees (Pan paniscus) to their mirror-images: Evidence of self-recognition

A group of seven pygmy chimpanzees (Pan paniscus) was tested for their mirror-image reactions during a ten-day experiment. The time spent viewing the mirror waned quickly. Little social responses directed towards the mirror were observed. Self-directed behaviors were shown from testday one on. It was concluded that four out of seven animals could correctly identify their mirror-image, one infant was not (yet) able to do so, and for two other individuals the results were inconclusive.     

Vocal Production by a Language-Competent Pan paniscus

Human spoken language and nonhuman primate vocalization systems have traditionally been regarded as qualitatively different from one another with respect to their semanticity and the way in which individuals acquire and utilize these signals. However, recent studies of the vocal behaviors of both captive and free-ranging monkeys and apes suggest that this dichotomy may not be unequivocal. We examined the vocalizations produced by a linguistically-competent adult male bonobo (Pan paniscus) named Kanzi. We analyzed his vocalizations during communicative interactions with humans in order to determine whether they vary systematically according to the semantic context in which they are produced. We determined semantic contexts based upon a vocalization's co-occurrence with predefined behavioral correlates. Spectrographic and statistical analyses revealed that acoustic structure is similar among the vocalizations that occurred within a specific semantic context and structural differences are evident between the vocalizations produced in different contexts. The results provide evidence that, during communicative interactions with humans, Kanzi modulates his vocal output on both the temporal and spectral levels.     

Food calling by captive bonobos (Pan paniscus): An experiment

We examined (i) whether bonobos display a specific food-calling behavior when discovering a hidden food resource, (ii) whether the presence of competitors affects this behavior, and (iii) whether food quantity or gender influences its appearance. We carried out experiments (n = 108) within a captive group of eight bonobos at the Animal Park Planckendael (Mechelen,Belgium). We hid highly preferred food items (n = 7 or 25) in their enclosure and recorded vocal behavior and interactions between discoverer and group members. As a control, we gave the same number of items to the individuals when isolated from the group, a situation without potential food competition (n = 38). The only vocalization frequently uttered by the discoverer was the food peep. They uttered food peeps significantly more often when no food competition was possible. The amount of food had no significant influence on whether food peeps were uttered. The same applies to the individuals’ identity or gender. Although the costs of food calling behavior seemed much higher for males, both sexes uttered food calls to the same extent. We hypothesize thai males signal food presence in order to attract potential mates and are willing to give up the discovered food resource in return for sex: sex for food exchange. In contrast, females may vocalize to attract coalition partners. Through these coalitions, they can monopolize food resources vis-à-vis males. It is also possible that females have less reason to suppress food calk, since they are dominant to males. This study suggests that bonobos are able to give shaded signals about their environment and have the potential to communicate this information in order to promote their sexual strategy.     

Interspecific interactions between wild pygmy chimpanzees (Pan paniscus) and red colobus (Colobus badius)

Interspecific interactions accompanied by physical contacts between wild pygmy chimpanzees (Pan paniscus) and red colobus (Colobus badius) were observed on three occasions at Wamba, Republic of Zaire. In all cases, the red colobus initiated the interactions by approaching the pygmy chimpanzees. Most of the pygmy chimpanzees, which were within 5 m of the red colobus, were juveniles or infants but the adult male pygmy chimpanzees never showed any interest in the red colobus. The red colobus groomed the chimpanzees in two cases, but the latter never groomed the former. No true aggressive interactions were observed between the two species. The lack of any evidence of hunting of red colobus through longitudinal studies of the pygmy chimpanzees of Wamba, together with the present observations, suggests that red colobus are probably not targets of hunting by the pygmy chimpanzees.     

Short-Term Benefits of Play Behavior and Conflict Prevention in Pan paniscus

Most theories on the function of play have focused on ultimate rather than proximate benefits. Play peaks during juvenility but, in some species, it is present in adulthood as well. In primates, social play and grooming often show a matched pattern because they bring individuals into close contact and favor social cohesion. In Pan, researchers have widely documented anticipation of competition at feeding time. Chimpanzees limit aggression over food by grooming (celebration), whereas bonobos use sociosexuality as a reassurance mechanism. We examined the function of play in the context of conflict prevention in the Apenheul bonobo colony. We analyzed the distribution of social play, grooming, and sexual contacts in periods around feeding and in a control condition. Adult-adult and adult-immature play frequencies were significantly higher during prefeeding than in any other condition, thus not supporting the commonly held view that social stress suppresses play. Further, there is a significant positive correlation between adult-adult play and rates of cofeeding. During feeding, adults engaged in their highest levels of sociosexual behaviors, whereas an increase in grooming rates occurred in prefeeding, though not significantly compared to the control rates. In conclusion, bonobos apparently cope with competition and social tension via 2 different mechanisms of conflict management: play to prevent tension, e.g., prefeeding, and sociosexual behaviors as appeasement and reassurance mechanisms once a tense situation emerges.     

Seed dispersal by pygmy chimpanzees (Pan paniscus): A preliminary report

The role in seed dispersal played by the pygmy chimpanzees (Pan paniscus) inhabiting Wamba, Republic of Zaïre, was studied. Germination was tested for seeds of 17 plant species recovered from the feces of pygmy chimpanzees at Wamba. The fecal seeds of 13 species germinated, and in six of the species the germination rate for the fecal seeds was higher than that of control seeds. Although five other species showed a higher germination rate in the control seeds than in the fecal seeds, the remaining two species revealed no difference in germination rate between the fecal and control seeds. There was no great difference in germination velocity between the fecal and control seeds of the same species. For comparison, seeds of four plant species collected from the feces of common chimpanzees (Pan troglodytes) and gibbons (Hylobates lar) in captivity in Okinawa were tested for their germinability. In this test, although the seeds had passed through the digestive tract, their germinability demonstrated little change. Based on the behavioral characteristics of the pygmy chimpanzee at Wamba and observations of the captive primates on Okinawa, it seems that pygmy chimpanzees may play an important role in the seed dispersal of fruit plant species at Wamba.