Functional analysis of chimpanzee (Pan troglodytes) private signing

The chimpanzee's use of American Sign Language (ASL) to communicate with humans and with each other has been empirically demonstrated in several reports, but this is the first research to experimentally examine their use of sign language in a nonsocial fashion: private signing. This experiment examined the private signing behavior of five signing chimpanzees, using a remote videotaping technique with no human present. It was found that all five chimpanzees signed to themselves for a total of 368 instances. These instances of private signing were classified into nine different functional categories as has been done in the analysis of private speech and signing in hearing and deaf human children. Similar to humans, a few of the categories accounted for the majority of the instances of private signing. These findings empirically demonstrate a behavior similar to private speech and signing in humans.     

Social play in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): Implications for natural social systems and interindividual relationships

This study compares adult play behavior in the two Pan species in order to test the effects of phylogenetic closeness and the nature of social systems on play distribution. The social play (both with fertile and immature subjects) performed by adults did not differ between the two species. In contrast, in bonobos, play levels among fertile subjects were higher than in chimpanzees. Findings regarding levels of undecided conflicts (more frequent in bonobos) and formal submission displays (lacking in bonobos) confirm, in the two colonies under study, that bonobos exhibit "egalitarianism" more than chimpanzees. Some authors emphasized the importance of play-fighting for social assessment when relationships among individuals are not codified and structured according to rank-rules. Indeed, adult bonobos played more roughly than chimpanzees. Moreover, adult bonobos displayed the full play-face at a high frequency especially during rough play sessions, whereas in chimpanzees, the frequency of play signals was not affected by roughness of play. The frequency of social play among bonobo females was higher than in any other sex combinations, whereas no difference was found for chimpanzees. As a matter of fact, social play can be viewed as a balance between cooperation and competition. Among bonobo females, characterized by social competence and affiliation, social play might enhance their behavioral flexibility and increase their socially symmetrical relationships which, after all, are the basis for their egalitarian society.     

Social play of juvenile and adolescent chimpanzees in the Mahale Mountains National Park,Tanzania

Social play of juvenile and adolescent chimpanzees (Pan troglodytes schweinfurthii) in the Mahale Mountains National Park, Tanzania, was studied, by analyzing processes of play and interindividual relationships in play. The results are discussed in relation to communication mechanisms. Play was initiated in several ways. Communication about play seems to depend on the receiver's interpretation: They can interpret the sender's behavior as play, referring to (1) play signals accompanied by the behavior, (2) transformation of the behavior in timing, strength, or rhythm, and (3) situation of the occurrence. Initiation attempts sometimes failed because one hesitated in playing with the other. Although the stronger often reduced his/her activity during play, play tended to escalate in activity. Players may enjoy such escalation. Play also had a mechanism not to escalate into fighting. Play was influenced by individuals other than the players. The third party's movement often affected the players' interaction. The term play does not indicate a behavior itself but the context of the behavior: The players interpret their behaviors in their play context.     

Age differences in spatial positioning of males in a chimpanzee unit-group at the mahale mountains National Park,Tanzania

Proximity partner choice by male chimpanzees of various age classes was analyzed in relation to their spatial positioning. Field work was carried out twice at the Mahale Mountains National Park, Tanzania. Proximity data were recorded at 3 and 10m from the focal animal. The data for the proximity between the focal male and other individuals allowed the males to be classified into two categories according to both criteria: early adolescence to young adult, and prime to old age. Between the males, the 3m proximity data permitted a classification into two categories as above, but those for 10m did not. These two spatial distances thus probably have different meanings for the males. The numbers of male proximity partners and proximity with the alpha male also allowed the males to be classified into two categories: early and late adolescence, and young adult to old age. Together, the above results support the classification of males into three age-graded categories: (1) early and late adolescence, (2) young adult, and (3) prime to old age. This does not arise because the males of each category form an age group. Prime or older males are most frequently in proximity, while their juniors consistently attempt to approach them. However, even prime or older males are not equally in proximity with one another. Their proximity partners change as time passes. Probably recognizing such changes, they form coalitions or are in rivalry. The sexual interest of adolescent males is probably a factor stimulating them to separate from their mothers, and to approach older males. Young adult males, even though physically mature, do not have equal proximity relations with older males. They are not yet sufficiently qualified to join the coalitions formed by their seniors.     

Sniffing behaviors in Mahale chimpanzees

Although it is difficult for observers to determine how non-human primates use olfaction in a natural environment, sniffing is one clue. In this study, the sniffing behaviors of wild chimpanzees were divided into six categories, and sex differences were found in most categories. Males sniffed more frequently than females in sexual and social situations, while females sniffed more often during feeding and self-checking. Chimpanzees sniffed more frequently during the dry season than during the wet season, presumably due to the low humidity. This suggests that the environment affects olfactory use by chimpanzees and that chimpanzees easily gather new information from the ground via sniffing.     

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.     

Sex differences in play behavior in juvenile tufted capuchin monkeys (Cebus apella)

According to the motor training hypothesis, play behavior in juvenile primates improves motor skills that are required in later adult life. Sex differences in juvenile play behavior can therefore be expected when adult animals assume distinct sexually dimorphic roles. Tufted capuchin monkeys show sexually dimorphic levels of physical antagonism in both inter- and intra-group encounters. Accordingly, it can be predicted that juvenile capuchins also show sex differences in social play behavior. To test this hypothesis, the play behavior of nine juvenile and two infant capuchins was examined. As predicted, juvenile males showed significantly higher levels of social play (wrestle, chase) than juvenile females, but no differences were found in nonsocial play (arboreal, object). Levels of infant play behavior were comparable to that of juveniles. These results lend support to the motor training hypothesis and highlight the need for more detailed investigations of individual differences in play behavior. An erratum to this article can be found at     

Meat eating and ant dipping by wild chimpanzees in Sierra Leone

This report presents the first records of meat-eating and ant dipping by wild chimpanzees,Pan troglodytes, from Sierra Leone. The study was conducted in the proposed Outamba-Kilimi National Park, Northern Sierra Leone. Measurements of tools used to dip for driver ants, are compared with those from four other study sites in Africa. The results reveal some fundamentally common characteristics. From both faecal analysis and direct observation, evidence was found that the chimpanzees eat meat. These recordings indicate a varied choice of prey and add new species to those preyed upon by wild chimpanzees. These findings preliminarily support the idea that despite a wide geographical distribution throughout Africa, chimpanzees share some essential conventional behavioural patterns.     

Leaf-pile pulling: an unusual play pattern in wild chimpanzees

Play in nonhuman animals has generally been viewed as being uniform among study sites. No studies have examined whether there are local variations in play. In this work we report an apparently locality-specific form of play that is basically solo locomotor play, but also has aspects of object play and social play. We describe this unusual "leaf-pile pulling" (LPL) pattern based on video footage of the chimpanzees of Mahale, Tanzania. Typically, when a party of chimpanzees moves in a procession down a slope in the dry season, a youngster will turn around and walk backward while raking many dry leaves with both hands. This activity accumulates many dry leaves while producing a lot of sound. After the player walks 1-15 m, he/she either turns around and walks forward or moves in a somersaulting fashion. The performer usually faces an individual that is immediately following him/her in the procession. The age of the performers ranges from 2 to 22 years, but 3-10 years are most typical. Compared to younger (< 8 years) individuals, older (> or = 8 years) individuals tend to cover longer distances during play, and to be more likely to play only on sloped surfaces and during travel. One of the authors (W.W.) has only seen the behavior exhibited by five different individuals on 10-15 occasions at Gombe. The behavior is exhibited less often at Gombe than at Mahale. To date, LPL has not been reported elsewhere. The absence of LPL at other sites may or may not be explained by environmental differences, such as differences in the availability of dead leaves and sloped terrain. Although the possibility that LPL is acquired by individual learning cannot be completely ruled out, the hypothesis that it is a tradition of the Mahale study group is more likely. This suggests that play may warrant more careful consideration in studies of nonhuman primate culture.     

Behavioral Ecology of Sympatric Chimpanzees and Gorillas in Bwindi Impenetrable National Park,Uganda: Diet

Via a field study of chimpanzees (Pan troglodytes schweinfurthii) and gorillas (Gorilla gorilla beringei) in Bwindi Impenetrable National Park, Uganda, we found that their diets are seasonally similar, but diverge during lean seasons. Bwindi chimpanzees fed heavily on fruits of Ficus sp., which were largely ignored by the gorillas. Bwindi gorilla diet was overall more folivorous than chimpanzee diet, but was markedly more frugivorous than that of gorillas in the nearby Virunga Volcanoes. During 4 mo of the year Bwindi gorilla diet included more food species than that of the chimpanzees. Three factors in particular—seasonal consumption of fibrous foods by gorillas, interspecific differences in preferred fruit species, and meat consumption by chimpanzees—contributed to dietary divergence between the two species. When feeding on fruits, gorillas ate Myrianthus holstii more frequently than chimpanzees did, while chimpanzees included more figs in their annual diet. Chimpanzee diet included meat of duikers and monkeys; gorilla frequently consumed decaying wood.     

Do chimpanzees cooperate in a learning task?

The aim of this study was to assess the ability of chimpanzees (Pan troglodytes) to cooperate in an instrumental task. A specially constructed fruit distributor was presented to a group of six captive chimpanzees. A cooperative response required two chimpanzees: both had to pull a handle simultaneously to make a fruit fall into the cage. The dominant male of the group and an infant produced most of the operant responses, and the male got nearly all the fruits. Other conspecifics avoided the dominant male at the apparatus. Social influences appear to limit the possibility of co-operation between individuals because a certain level of interindividual tolerance is required. The results revealed a significant increase in the number of pulls each time both chimpanzees were together at the apparatus. Operant chimpanzees learn to coordinate their actions in time and space.     

When does play panting occur during social play in wild chimpanzees?

To clarify the social functions of play panting in chimpanzees, I investigated when they emitted play panting in social play and how the interactions were affected by the occurrence of play panting. The subjects were the M-group chimpanzees living in Mahale, Tanzania. The following observations were made: (1) chimpanzees emitted play panting when they were tickled or chased but rarely did so when they tickled or chased others. Chimpanzee play panting does not have the function of a play signal communicating that these aggressive actions are performed not as aggression but as play. (2) Chimpanzees emitted play panting more often when they received aggressive actions that supposedly elicited higher arousal. (3) A chimpanzee tended to continue to perform aggressive actions when the target emitted play panting. Play panting activates the interaction of social play by encouraging the performer to continue tickling or chasing. These results can be summarized as showing that chimpanzee play panting serves as positive feedback to the play partner for continuing somewhat fragile interactions, which may contain the risk of excessive arousal and the risk of confusing defensive actions by the target of the aggressive actions with real efforts to escape the situation.     

A note on the chimpanzee ecology of the Ugalla Area,Tanzania

Results of a nine-day survey of the chimpanzee habitat in Ugalla area of western Tanzania in 1975 is reported. The most dominant vegetation of the area wasJulbernardia globiflora woodland. Evidence of chimpanzees, both indirect (such as beds, faeces, and food remnants) and direct (calls and actual observation), suggests that chimpanzees are sparsely distributed throughout the area, but that they disproportionately useBrachystegia bussei woodland along escarpments andCynometra-Albizzia riverrine forests. The hilly country of Sisegwa/Mnyangwa was the only area where chimpanzees appeared to be comparatively abundant. Mammal hair found in faeces indicates mammal-predation by the chimpanzees in this area. Hard-shelled fruits ofStrychnos were apparently bitten open without the use of extra-somatic objects. The food repertoire included fruits of the generaAzanza, Canthium, Cordia, andTamarindus.     

Ant dipping and meat eating by wild chimpanzees in the kalinzu forest, uganda

New evidence of ant dipping and meat eating by chimpanzees was recorded in the Kalinzu Forest, Uganda. We found stems and branches at the nests of driver ants,Dorylus molestus, just after chimpanzees had left the spot. Fecal samples also revealed that chimpanzees sometimes ate driver ants. The configuration of stems and branches and the condition of holes at the driver ant's nests suggested that chimpanzees used them as wands to dip for ants. The frequency of ant dipping and length of wands may be more related to culturel rather than ecological factors. Although hunting was not seen, we found chim-panzees eating a blue monkey and a redtail monkey. In both cases, they ate meat and leaves alternatively, and shared meat with each other.     

Behavioral Adaptation of Pan troglodytes troglodytes

As wild primate populations decline, numbers of orphaned primates, sanctuaries, and attempts to release primates back to the natural environment increase. Release projects frequently are poorly documented despite IUCN guidelines recommending post-release monitoring and systematic data collection as central to the process. Since 1996, Habitat Ecologique et Liberté des Primates (HELP) has been releasing wild-born orphaned chimpanzees into natural habitat in the Conkouati-Douli National Park, Republic of Congo. HELP developed a post-release monitoring system as an integral component. We present activity budgets and diet of released chimpanzees, and compared them to those of wild chimpanzee, as primary indicators of successful release. Feeding, moving, and resting dominated activity budgets, reflecting the overall patterns in wild populations. Diet was diverse and dominated by fruit, and the released chimpanzees showed specialization on a smaller number of species, as in many wild communities. The high survival rates of the chimpanzees and overall success of the release program are attributed to careful planning and post-release support facilitated via the monitoring process. Systematic post-release data collection monitoring has confirmed that wild-born chimpanzees can adjust behaviorally and nutritionally to the wild. Survival statistics of the reintroduced chimpanzees—confirmed 56%, possible 88%— reflect the behavioral adaptability.2nd revision March 11, 2005An erratum to this article can be found at     

Play in adult bonobos (Pan paniscus): modality and potential meaning

The aim of this study was to thoroughly investigate social play and its modalities among adult bonobos. We evaluated how play intensity varies according to the sex-class combination of the playmates and we also performed an analysis on social locomotor-rotational movements (L-R play) and contact interactions (C play). Rough and gentle play sessions were performed with comparable frequencies by male-female and female-female adult dyads, with play signals unlikely when the playmates strongly differed in age and in rank position. L-R play rates did not differ according to the sex-combination of the players; in contrast, C play sessions were particularly frequent among females. Play faces (play signals) were significantly higher during C play than L-R play sessions, thus suggesting that playmates assess reciprocally yet safely their relationships by using facial displays to avoid any kind of misunderstanding. Play was positively correlated with grooming and contact sitting interactions, suggesting that it may be used as a social enhancer. Finally, we found no correlation between both play contexts (L-R and C play) and age, size and rank differences of the players. In conclusion, we suggest that bonobos with their egalitarian society, peculiar social structure, and playful tendency represent an attractive testing subject to examine empirically many emerging hypotheses on adult play behavior.     

Hormones and behavior of prepubertal and peripubertal chimpanzees

Twenty-one chimpanzees ranging in age from 2.9 to 9.2 years at the midpoint of a study consisting of five 4-week blocks were studied behaviorally in four groups of five or six animals per group, balanced for age and sex. Blood samples for radioimmunoassay of follicle-stimulating hormone (FSH), luteinizing hormone, 17 beta-estradiol, testosterone, dehydroepiandrosterone (DHA), DHA sulfate (DHAS), and cortisol were obtained once each 4-week block. Sex differences were found only in the categories of play duration and initiative and genital inspection, all of which were greater for the males. Several categories (6) of play and other affiliative behaviors were negatively correlated with age and/or body weight for the males, whereas fewer of those categories (2) were so correlated in the females. Hierarchical behavior, genital inspection, solitary behavior other than play, and autogrooming were all positively correlated with age and/or body weight for the males, and only autogrooming for the females. FSH and testosterone levels and testicular volume were positively correlated with age and body weight in the males, whereas for the females cortisol was negatively correlated with body weight and only FSH and the ratios of DHA and DHAS to cortisol were positively correlated with age and/or body weight. Most of the behaviors that were significantly correlated with age and body weight for the males were also correlated in the same direction with FSH and testosterone levels and testicular volume, but not with DHA or DHAS levels. The data are consistent with the view that testosterone, but not the adrenal androgens DHA and DHAS, contributed to the behavioral development of the males. There were few significant correlations between hormones and behavior for the females and interpretation is not clear. The absence of age-related increases in DHA and DHAS of both the males and females, in contrast to the pattern of FSH (and testosterone for the males), supports the growing consensus that adrenarche and puberty are independent developmental processes. The absence of any strong correlations between behavior and levels of the adrenal androgens in either the males or females suggests that adrenarche per se is not a significant event in the behavioral development of chimpanzees.     

Predation on mammals by chimpanzees in the montane forest of Kahuzi, Zaire

The rate of predation on mammals by chimpanzees was determined from carcasses and from fecal specimens found on fresh trails during a 16-month period in the montane forest of Kahuzi-Biega National Park, Zaire. A unit-group of semi-habituated chimpanzees, composed of 22 – 23 individuals including 8 adult or adolescent males, appeared to kill about 18 – 30 mammalian prey (16 – 28Cercopithecus monkeys) per year, if the multiple kills by chimpanzees were not considered. A juvenile l'Hoest's monkey was recorded for the first time as the prey of chimpanzees in this study. Predation occurred in the late dry and the early rainy seasons, when the diversity of ripe fruits was the highest during the year. The Kahuzi chimpanzees tended to kill mammals less frequently but to killCercopithecus monkeys more frequently than chimpanzees in other habitats. The absence of red colobus monkeys, which are the most frequent prey in Gombe, Mahale, and Tai, might be responsible for the low predation rate. However, the estimated rate of predation onCercopithecus monkeys is the highest record among various chimpanzee habitats. At least 11 – 18% of theCercopithecus population seemed to be lost annually as a result of being killed by chimpanzees. Chimpanzees may be the most important predators on these monkeys in the absence of leopards at Kahuzi. The examination of fecal samples and carcasses suggested that adult (probably male) or adolescent chimpanzees tended to eat juvenile or subadult monkeys most frequently, as is also seen for chimpanzees in Gombe, Mahale, and Tai.     

The Right Time to Happen: Play Developmental Divergence in the Two Pan Species

Bonobos, compared to chimpanzees, are highly motivated to play as adults. Therefore, it is interesting to compare the two species at earlier developmental stages to determine how and when these differences arise. We measured and compared some play parameters between the two species including frequency, number of partners (solitary, dyadic, and polyadic play), session length, and escalation into overt aggression. Since solitary play has a role in developing cognitive and physical skills, it is not surprising that chimpanzees and bonobos share similar developmental trajectories in the motivation to engage in this activity. The striking divergence in play developmental pathways emerged for social play. Infants of the two species showed comparable social play levels, which began to diverge during the juvenile period, a ‘timing hotspot’ for play development. Compared to chimpanzees, social play sessions in juvenile bonobos escalated less frequently into overt aggression, lasted longer, and frequently involved more than two partners concurrently (polyadic play). In this view, play fighting in juvenile bonobos seems to maintain a cooperative mood, whereas in juvenile chimpanzees it acquires more competitive elements. The retention of juvenile traits into adulthood typical of bonobos can be due to a developmental delay in social inhibition. Our findings show that the divergence of play ontogenetic pathways between the two Pan species and the relative emergence of play neotenic traits in bonobos can be detected before individuals reach sexual maturity. The high play motivation showed by adult bonobos compared to chimpanzees is probably the result of a long developmental process, rooted in the delicate transitional phase, which leads subjects from infancy to juvenility.     

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.