Mechanical implications of chimpanzee positional behavior.

Mechanical hypotheses concerning the function of chimpanzee anatomical specializations are examined in light of recent positional behavior data. Arm-hanging was the only common chimpanzee positional behavior that required full abduction of the humerus, and vertical climbing was the only distinctive chimpanzee positional behavior that required forceful retraction of the humerus and flexion of the elbow. Some elements of the chimpanzee anatomy, including an abductible humerus, a broad thorax, a cone-shaped torso, and a long, narrow scapula, are hypothesized to be a coadapted functional complex that reduces muscle action and structural fatigue during arm-hanging. Large muscles that retract the humerus (latissimus dorsi and probably sternocostal pectoralis major and posterior deltoid) and flex the elbow (biceps brachii, probably brachialis and brachioradialis) are argued to be adaptations to vertical climbing alone. A large ulnar excursion of the manus and long, curved metacarpals and phalanges are interpreted as adaptations to gripping vertical weight-bearing structures during vertical climbing and arm-hanging. A short torso, an iliac origin of the latissimus dorsi, and large muscles for arm-raising (caudal serratus, teres minor, cranial trapezius, and probably anterior deltoid and clavicular pectoralis major) are interpreted as adaptations to both climbing and unimanual suspension.     

Body weights of wild chimpanzees (Pan troglodytes schweinfurthii) of the Mahale Mountains National Park, Tanzania

Ten male and nine female habituated chimpanzees (Pan troglodytes schweinfurthii) in the Kasoje area of the Mahale Mountains National Park, Tanzania, were weighed intermittently between December 1973 and March 1980 by luring them up a rope hung on a spring balance: six adult males averaged 42.0 kg and eight adult females 35.2 kg. Seasonal change in body weight was recognized at least partially; body weights tended to decrease in the later part of the wet season presumably because of food shortage in the middle of the wet season. Comparison of body weight among three populations of the same subspecies suggests that adult female chimpanzees of Mahale appear to be heavier than those of the Gombe National Park, Tanzania, and that they seem to be similar to the forest-living counterparts of eastern Zaire. On the other hand, body weights of adult male chimpanzees from the three populations do not show significant differences. Perhaps feeding competition among adult females in a small, isolated habitat is more severe than that among adult males, which may result in the body weight reduction among adult female chimpanzees at Gombe.     

Anxiety-related behavior of orphan chimpanzees (Pan troglodytes schweinfurthii) at Gombe National Park, Tanzania

This study examined the anxiety levels and social interactions of two orphan and four mother-reared adolescent chimpanzees (Pan troglodytes schweinfurthii) in the Kasekela community at Gombe National Park, Tanzania. We used focal sampling in the field at Gombe to observe these adolescent individuals. Their social interactions and anxious behavior, measured as rough scratching, were recorded. The two orphans differed from others of a similar age by exhibiting higher levels of anxiety and lower levels of play. These results suggest that a mother’s absence, even in naturalistic conditions in which other members of the community are available to the orphan, may have long-lasting impact on an adolescent’s anxiety and its ability to engage in complex social interactions, such as play.     

Severe Aggression Among Female Pan troglodytes schweinfurthii at Gombe National Park, Tanzania

Aggression is generally more severe between males than between females because males gain greater payoffs from escalated aggression. Males that successfully defeat rivals may greatly increase their access to fertile females. Because female reproductive success depends on long-term access to resources, competition between females is often sustained but low key because no single interaction leads to a high payoff. Nonetheless, escalated aggression can sometimes immediately improve a female’s reproductive success. Resisting new immigrants can reduce feeding competition, and infanticide of other females’ young can increase a female’s access to resources. East African chimpanzees live in fission-fusion communities in which females occupy overlapping core areas. Growing evidence indicates that reproductive success correlates with core area quality, and that females compete for long-term access to core areas. Here we document 5 new cases of severe female aggression in the context of such competition: 2 attacks by resident females on an immigrant female, a probable intracommunity infanticide, and 2 attacks on a female and her successive newborn infants by females whose core areas overlapped hers. The cases provide further evidence that females are occasionally as aggressive as males. Factors influencing the likelihood and severity of such attacks include rank and size differences and the presence of dependable allies. Counterstrategies to the threat of female aggression include withdrawing from others around the time of parturition and seeking male protection. We also discuss an unusual case of a female taking the newborn infant of another, possibly to protect it from a potentially infanticidal female.     

The foraging adaptation of chimpanzees,and the recent behaviors of the provisioned apes in Gombe and Mahale National Parks,Tanzania

Current understandings of chimpanzee behavior and social organization are based largely on twenty years of studies of provisioned chimpanzees in Gombe and Mahale National Parks. These data indicate that chimpanzees are aggressive, dominance-seeking and fiercely territorial. Reports from a number of naturalistic (nonprovisioning, unobtrusive) field studies contrast sharply. All report open groups of nonaggressive, nonhierarchical chimpanzees, which repeatedly fission for foraging and reunite as a larger social group. Because of the authority accorded the Mahale and Gombe reports, the naturalistic studies are often discounted. In this paper foraging theory is used to show that the recent behaviors of the artificially fed apes are maladaptive, while those of the nonprovisioned, wild chimpanzees are excellent foraging strategy. It is suggested, but not concluded, that the recent behaviors of the Gombe and Mahale chimpanzees may be frustration-induced responses to feeding methods which introduce a blockage between the apes and the desirable bait foods.     

An observed birth in a free-living chimpanzee (Pan troglodytes schweinfurthii) in Gombe National Park,Tanzania

During 19 years of study of chimpanzees in the Gombe National Park only one birth has been observed; this is probably the first such observation for any chimpanzee in the natural habitat. The birth took place in a nest in a tall tree during the morning. Details were recorded by Tanzanian field staff from a neighbouring tree. Labour and parturition are described as well as the mother’s care of the infant immediately after birth. The mother consumed the placenta as she lay in another nest. Throughout the birth process the mother’s juvenile son remained close by and watched with apparent interest. Another mother and her offspring were present during the birth and an adult male approached the mother while she was feeding on the placenta. Their behaviour is described.     

Fecal bacterial diversity of human‐habituated wild chimpanzees (Pan troglodytes schweinfurthii) at Mahale Mountains National Park,Western Tanzania

Although the intestinal flora of chimpanzees has not been studied, insight into this dynamic environment can be obtained through studies on their feces. We analyzed fecal samples from human‐habituated, wild chimpanzees at Mahale Mountains National Park, Tanzania, and compared microbial community profiles to determine if members of the same social group were similar. Between July and December 2007, we collected fresh fecal samples from 12 individuals: four juveniles, four adolescents, and four adults, including three parent–offspring pairs. Each sample was analyzed using Terminal‐Restriction Fragment Length Polymorphism of amplified 16S rRNA genes. Twelve different profiles were generated, having between 1 and 15 Terminal‐Restriction Fragments (T‐RFs). Overall, a total of 23 different T‐RFs were produced. Putative assignments of T‐RFs corresponded to the phyla Firmicutes (Clostridia, Bacilli, and Lactobacilli), Bacteroidetes, Tenericutes (Mollicutes Class), Actinobacteria, and Proteobacteria, as well as to uncultured or unidentified organisms. Firmicutes and Bacteroidetes phyla and Mollicutes Class were the most commonly assigned in 11, 8, and 8 of the samples, respectively, with this being the first report of Mollicutes in wild chimpanzees. Principal Components Analysis (PCA) revealed clustering of nine samples, and 80.5% of the diversity was accounted for by three samples. Morisita indices of community similarity ranged between 0.00 and 0.89, with dissimiliarity (<0.5) between most samples when compared two at a time. Our findings suggest that, although phylotypes are common among individuals, profiles among members of the same social group are host‐specific. We conclude that factors other than social group, such as kinship and age, may influence fecal bacterial profiles of wild chimpanzees, and recommend that additional studies be conducted. Am. J. Primatol. 72:566–574, 2010. © 2010 Wiley‐Liss, Inc.     

Troop fission in free-ranging baboons in the Gombe Stream National Park, Tanzania.

During a 13 month field study of baboons in the Gombe Stream National Park, Tanzania, the troop under observation divided into two groups. One group left the main troop to become ultimately an independent troop with a separate range. The troop fission followed a period of instability in the male status hierarchy. The events prior to and during the troop fission are described in detail and the membership of the group which left is analyzed. Social bonds prior to the fission, including kinship, determined membership in the group which left. Aggression from members of the main troop played a major role in causing the group which left to establish a new range. This fission is compared with troop fissions observed in rhesus and Japances macaques.     

Prevalence of muzzle-rubbing and hand-rubbing behavior in wild chimpanzees in Mahale Mountains National Park, Tanzania

In 1998, four chimpanzees in the Mahale Mountains National Park, Tanzania, were observed wiping their mouths with non-detached leaves or stalks of grass, or rubbing their mouths with a tree trunk or branch, especially while eating lemons. The number of mouth-wiping/rubbing individuals increased to 18 in 1999. By 2005, 29 chimpanzees were documented wiping/rubbing their muzzles in this way. Although it is difficult to determine whether the chimpanzees acquired this behavior as a result of trial and error or social learning, the fact that chimpanzees at other sites perform this behavior with detached leaves or leafy twigs much more often than with intact items suggests the possibility that cleaning with intact plant parts at Mahale spread via social learning.     

Parasitology of five primates in Mahale Mountains National Park, Tanzania

Parasitological surveillance in primates has been performed using coprological observation and identification of specimens from chimpanzees (Pan troglodytes schweinfurthii) in Mahale Mountains National Park, Tanzania (Mahale). In this study, we conducted coprological surveillance to identify the fauna of parasite infection in five primate species in Mahale: red colobus (Procolobus badius tephrosceles), red-tailed monkeys (Cercopithecus ascanius schmidti), vervet monkeys (Cercopithecus aethiops pygerythrus), yellow baboons (Papio cynocephalus), and chimpanzees. Fecal samples were examined microscopically, and parasite identification was based on the morphology of cysts, eggs, larvae, and adult worms. Three nematodes (Oesophagostomum spp., Strongyloides sp., and Trichuris sp.), Entamoeba coli, and Entamoeba spp. were found in all five primate species. The following infections were identified: Bertiella studeri was found in chimpanzees and yellow baboons; Balantidium coli was found in yellow baboons; three nematodes (Streptopharagus, Primasubulura, an undetermined genus of Spirurina) and Dicrocoeliidae gen. sp. were found in red-tailed monkeys, vervet monkeys, and yellow baboons; Chitwoodspirura sp. was newly identified in red colobus and red-tailed monkeys; Probstmayria gombensis and Troglocorys cava were newly identified in chimpanzees, together with Troglodytella abrassarti; and Enterobius sp. was newly identified in red colobus. The parasitological data reported for red colobus, vervet monkeys, and yellow baboons in Mahale are the first reports for these species.     

Tool use by wild chimpanzees (Pan troglodytes) to obtain termites (Macrotermes herus) in the Mahale Mountains,Tanzania

Wild chimpanzees (Pan troglodytes) used probes made of vegetation to fish for termites (Macrotermes herus) in the Mahale Mountains of western Tanzania. Data on both the artefacts and behavior associated with their use were recorded over a 5-month period. The chimpanzees of one unit group, B Group, were seen four times to use the tools to extract the insects from their mounds. A sample of 290 tools was collected and analyzed in terms of age, seasonality, length, width, class of raw materials, species, methods of making, extent of use, and damage incurred through use. There were differences across groups in tool use in feeding on termites; some could be related to biotic factors, but others appeared to be cultural differences.     

Association partners of young chimpanzees in the Mahale Mountains National Park,Tanzania

Association partners of young chimpanzees at the Mahale Mountains National Park were analyzed. Juvenile and adolescent chimpanzees associated frequently with their mothers, although mother-offspring association decreased as the offspring grew up. Males tended to leave their mothers and associate with adult males, while females remained frequently associating with their mothers in early adolescence. In late adolescence and young adulthood, males usually associated with adult males and cycling adult females. Females may transfer into neighboring unit-groups in this stage. Although an immigrant female tended to be alone when her estrous cycle stopped, she associated with many individuals, in particular with adult males, when she resumed cycling. Some orphans were observed to associate frequently with particular adults. The findings were discussed in relation to the unique characteristics of chimpanzee social system.     

A Retrospective Analysis of Factors Correlated to Chimpanzee (Pan troglodytes schweinfurthii) Respiratory Health at Gombe National Park, Tanzania

Infectious disease and other health hazards have been hypothesized to pose serious threats to the persistence of wild ape populations. Respiratory disease outbreaks have been shown to be of particular concern for several wild chimpanzee study sites, leading managers, and researchers to hypothesize that diseases originating from and/or spread by humans pose a substantial risk to the long-term survival of chimpanzee populations. The total chimpanzee population in Gombe National Park, Tanzania, has declined from 120–150 in the 1960s to about 100 by the end of 2007, with death associated with observable signs of disease as the leading cause of mortality. We used a historical data set collected from 1979 to 1987 to investigate the baseline rates of respiratory illness in chimpanzees at Gombe National Park, Tanzania, and to analyze the impact of human-related factors (e.g., banana feeding, visits to staff quarters) and non-human-related factors (e.g., sociality, season) on chimpanzee respiratory illness rates. We found that season and banana feeding were the most significant predictors of respiratory health clinical signs during this time period. We discuss these results in the context of management options for the reduction of disease risk and the importance of long-term observational data for conservation.     

Chimpanzee grouping patterns and food availability in Mahale Mountains National Park,Tanzania

The aim of this study was to test for a correlation between party size and food (fruit) availability among the M group chimpanzees (Pan troglodytes) in the Mahale Mountains, Tanzania. Chimpanzee unit groups (or communities) show fission–fusion grouping patterns and form temporal parties. Fruit availability is assumed to be one of the important limiting factors in relation to the size of these parties. Different methods have been proposed to measure party size, but they all appear to focus mainly on two aspects of grouping phenomena. In “face-to-face parties”, party size is measured by scan sampling, whereas in “nomadic parties”, all members observed during a specific time period are counted. The mean monthly group size resulting from these two measures was compared with fruit availability, i.e. fruiting plant density and mean potential patch size. Nomadic party size was correlated with both values. Thus, party formation at this level was considered to be sensitive to overall fruit availability in the habitat. On the other hand, face-to-face party size remained stable and showed weak or no correlations with density and potential patch size. Although large patches are available during the peak fruiting season, Mahale chimpanzees depend on the liana species Saba comorensis, which, when fruiting, encourages individuals to spread out to eat. Thus, the lack of correlation between face-to-face-party size and fruit availability was attributed to the influence of physical limitations countervailing the fluctuation in fruit availability. Maximum face-to-face party size relative to unit-group size, regarded as the cohesiveness of a unit group, was compared among sites. The values differed largely: Mahale groups M and K, Bossou, and, in some years, Budongo, showed high cohesiveness, while others remained low. Thus, the distribution of the most important food during the fruiting season in each study site may be a crucial factor in the grouping phenomena of chimpanzees.     

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.     

Sexual behavior of adult male chimpanzees of the Mahale Mountains national park,Tanzania

This study, based on 687 hr of focal observations, aims to describe overall patterns of the sexual behavior of the adult male chimpanzees of the Mahale Mountains, to compare the results with previous reports, and to explain the variations between studies. Genital inspection of cycling females by adult males was eight times as frequent as that of lactating females, and twice as frequent as that of pregnant females. Inspection of the genitals of cycling females increased dramatically 7–10 days before the onset of maximal swelling and gradually decreased as the day of ovulation approached. Adult males likely obtained information on the attractivity of females by inspecting their genitals. Mating was usually initiated by male courtship and followed by pelvic thrusts in a dorsoventral posture, performed on, rather than above, the ground, which continued for 7 s. on average, and was typically followed by female squeaking and darting from the male, or by the male grooming the female. Higher-ranking males mated with females in the peri-ovulatory period more frequently than did lower-ranking males. In particular, two alpha males mated with such females more often than did any other adult males. A male who interfered with a mating pair was dominant over the mating male in other agonistic contexts. The duration of intromission was correlated with neither dominance rank nor age. However, when an adult male declined in rank from alpha in 1991 to third in 1992, he showed a significantly shorter duration of intromission. This indicates that for a particular male, the alpha rank guaranteed longer duration of intromission. Allies of alpha males tended to mate with peri-ovulatory females more frequently than expected from their low dominance ranks. The number of mating partners was not correlated with male dominance rank, but was sometimes negatively correlated with male age. Females were significantly more likely to emit a copulatory squeak when mating with younger, rather than older, adult males. Male dominance rank and the rate of female copulatory squeaking were not correlated. Weaning infants regularly interfered with their mothers' mating. Occasionally, unrelated adolescent males and rarely females pushed themselves in between copulating adults. Female choice was indicated when they performed a “penis erection check” or took the initiative in courtship, or on the other hand showed strong reluctance to mate with particular males. Young adult males more often received erection checks than did prime males, while none of the three old adult males did. Courtship initiated by estrous females was not directed to two of the oldest males, the exception of which was the alpha male. The oldest males, except for the alpha, were consistently avoided by many estrous females, both young and old. In response to female reluctance, males behaved violently, however, this was not effective, because other more dominant males came to rescue the female. Neither courtship nor mating was seen between mature sons and their mothers, nor between brothers and sisters.     

Food transfer between mother and infant chimpanzees of the Mahale Mountains National Park,Tanzania

We studied food transfer between chimpanzee mothers and infants in the Mahale Mountains National Park, Tanzania. The rate of infant solicitation for food dramatically increased in the second year of life, then gradually decreased and, in the seventh year, virtually disappeared. The pattern of the ontogeny of food sharing precisely followed that of solicitation because mothers shared food only when requested to do so by their infants. The success rate of solicitation, however, did not display extreme changes across ages. Food that was difficult to process was shared more frequently because it was more likely to be demanded by infants. We defined food retrieval as an infant’s recovery of leftovers discarded by its mother. Food types retrieved were often those that are difficult to process and were also likely to be shared by mothers. However, infants tended to solicit small, difficult food types for sharing while they often retrieved the remains of large, difficult food types. The function of food sharing and food retrieval lies in an infant’s learning food types that it cannot easily obtain or process by itself. The level of competition for food between mothers and infants remained low throughout infancy. We noted no particular characteristic about foods from which infants were displaced by mothers. As infants grew older, they increased the distance between themselves and mothers that became more aggressive.