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.     

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.     

Opportunistic and restrictive matings among wild chimpanzees in the Mahale Mountains,Tanzania

The mating patterns of free-ranging chimpanzees (Pan troglodytes) of the Mahale Mountains, Tanzania, were studied. Opportunistic mating (non-competitive and temporary mating) was frequently observed in a large-sized unit-group, among young, low-ranking males, and among young, newcomer, non-ovulating females. Restrictive mating (a continuous sexual relationship between a particular pair which includes possessiveness and consortship) was frequently observed in a small-sized unit-group, among middle- and old-aged, high-ranking males, and among old, resident, ovulating females. Relations between those characteristics, such as group size and composition, ages of the individuals of both sexes, female estrous stages, and life history, and the 2 mating patterns are discussed.     

Reconciliation and post-conflict third-party affiliation among wild chimpanzees in the Mahale Mountains,Tanzania

This study investigated post-conflict (PC) behavior among wild chimpanzees (Pan troglodytes) of the M-group in the Mahale Mountains, Tanzania, and examined what types of behavior characterize the PC situation in this group, and the factors that influence the occurrence of PC affiliation between opponents soon after the end of an aggressive conflict (i.e., reconciliation). We found that the opponents affiliated selectively soon after the end of aggression, suggesting that reconciliation occurred in this group. The mean individual corrected conciliatory tendency (CCT) (Veenema et al. 1994 in Behav Proc 31:29–38) was 14.4%, which is similar to or lower than frequencies observed in studies of captive and wild chimpanzees. The valuable relationship hypothesis predicts that the CCT is higher among individuals who share valuable relationships (e.g., males or affiliative dyads) than among individuals who do not (e.g., females or less-associative dyads). However, the analysis based on data for aggression between unrelated individuals (including one incident between an adult and non-adult) and aggression between unrelated adults, did not uncover this difference. Affiliation by a previously uninvolved individual with the victim (consolation) and with the aggressor (appeasement) occurred more frequently following aggression than in the control condition. The results are compared with previous studies of captive and wild chimpanzees.     

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.     

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.     

Grips and hand movements of chimpanzees during feeding in Mahale Mountains National Park,Tanzania

It has long been assumed that stone tool making was a major factor in the evolution of derived hominin hand morphology. However, stresses on the hand associated with food retrieval and processing also have been recognized as relevant early hominin behaviors that should be investigated. To this end, chimpanzee food manipulation was videotaped in the Mahale Mountains National Park, Tanzania. Grips and hand movements by 39 chimpanzees were analyzed for arboreal and terrestrial feeding involving 10 food‐types and associated vegetation. It was predicted that (1) new grips would be found that had not been observed in captivity, (2) forceful precision grips would be absent from the repertoire, as in captivity, and (3) precision handling would be observed. New grips involving the full thumb and buttressed index finger, and a new integrated pattern of grips and forceful hand movements were discovered, associated with feeding on large fruits and meat. Participation of the full thumb in these grips, rather than the distal thumb and fingers, throws light on feeding behaviors that may have become increasingly significant factors in the evolution of derived hominin thumb morphology. The proximal thumb stabilizes food with the flexed index finger against the pull of the teeth and provides leverage in breaking food into portions. Isolated qualitative observations of possibly forceful pinch by the thumb and side of the index finger highlight the need for comparative quantitative data to test whether humans are unique in forceful precision gripping capability. Precision handling was not seen. Am J Phys Anthropol 156:317–326, 2015. © 2014 Wiley Periodicals, Inc.     

Newly acquired feeding habits among the chimpanzees of the Mahale Mountains National Park,Tanzania

The M group chimpanzees (Pan troglodytes schweinfurthii) of the Mahale Mountains National Park, Tanzania, began to feed on three agricultural fruit species, guava, mango and lemon. It took them 7–8 years until they began to taste these fruits since the villagers left the park area in 1974. Although adult chimpanzees are conservative in their feeding habits, they are capable of rapidly acquiring new feeding habits, or new traditions, once they notice that the food is suitable.     

A preliminary note on the intestinal parasites of wild chimpanzees in the Mahale Mountains,Tanzania

Feces of wild chimpanzees in the Mahale Mountains, Tanzania, were inspected for intestinal parasites under a compound microscope. Eggs or larvae ofOesophagostomum, Strongyloides, Trichuris, Prosthenorchis, andBertiella were found. Intestinal nematodes significantly increased in the mid-rainy season. This finding supports (or, at least, is not in conflict with) the hypothesis thatAspilia leaves which are occasionally swallowed by chimpanzees may function as a vermicide, since ingestion of such leaves also increases significantly in the mid-rainy season.     

Aggression toward large carnivores by wild chimpanzees of Mahale Mountains National Park, Tanzania

In the Mahale Mountains National Park of Tanzania, a group of about 33 chimpanzees were observed to surround a leopard den containing a mother and at least one cub and to drag out and kill the cub. This is the first report of chimpanzees or any other primate species killing their potential predator's offspring. The incident suggests that chimpanzees, without any weapons, can manage to defend themselves against a carnivore of at least up to leopard size, and implicates how the early hominids may have reacted against their potential predators.     

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.     

Dominance among male chimpanzees in the Mahale Mountains National Park,Tanzania: A preliminary study

Dominance relationships among male chimpanzees in the Mahale Mountains National Park, Tanzania, were analyzed. Although all adolescent males were unequivocally subordinate to all adult males, dominance relationships within the age classes were much less clear. Especially among adolescent males, few pant-grunts or agonistic interactions occurred. While adolescent males frequently pant-grunted at adult males, these latter males, except the alpha and the youngest, rarely pant-grunted to one another. This suggests that a difference of social status exists between adolescent and adult males. Adult males rarely display overt dominance to one another probably because the presence of other males affects their interactions. Moreover, they seem to try to keep their dominance relationship ambiguous when making it overt is not advantageous to them. This may be a political way for males to coexist with one another in a unit-group.     

Demography,female life history,and reproductive profiles among the chimpanzees of Mahale

Demography provides critical data to increase our understanding of the evolution, ecology, and conservation of primate populations. The chimpanzees of the Mahale Mountains National Park, Tanzania, have been studied for more than 34 yr on the basis of individual identification and standardized attendance records. From this long-term study, we derived the following demographic data: The major cause of death was disease (48%), followed by senescence (24%) and within-species aggression (16%). Fifty percent of Mahale chimpanzees died before weaning. The median ages of female life history variables were: first maximal swelling, 10.0 yr (n = 5); emigration, 11.0 yr (n = 11); and first birth, 13.1 yr (n = 5). The median period of adolescent infertility was 2.8 yr (n = 4) when calculated from the age at immigration to that at first birth. Female fecundity was highest between 20 and 35 yr of age, with an annual birth rate of 0.2. Twenty-six females that were observed from a young age (10-13 yr) to death at various ages (15-40 yr) gave birth to an average of 3.9 and weaned an average of 1.4 offspring. Twenty-five females that were observed from middle age (18-33 yr) to death in older age (31-48) gave birth to an average of 2.7 and weaned an average of 2.0 offspring. The post-reproductive lifespan for female chimpanzees was defined as the number of years that passed from the year when the last offspring was born to the year when the female died, minus 5. Twenty-five percent of old females had a post-reproductive lifespan. The interbirth interval after the birth of a son (x = 72 mo) tended to be longer than that after the birth of a daughter (x = 66 mo). The extent of female transfer, which is a rule in chimpanzees, is influenced by the size and composition of the unit group and size of the overall local community.     

Seasonal trends in intestinal nematode infection and medicinal plant use among chimpanzees in the Mahale Mountains, Tanzania

A longitudinal study of nematode infection in chimpanzees was conducted between 1989 and 1994 on the M group chimpanzees of the Mahale Mountains National Park, Tanzania during two annual dry and rainy season periods and a third rainy season. Chemical and physical antiparasite properties of medicinal plant use against the strongyle nematodeOesophagostomum stephanostomum have recently been reported at Mahale. Here, the incidence of nematode infections were analyzed for seasonal trends to elucidate the possible influence of parasite infection on previously reported seasonality of medicinal plant use and to test the hypothesis that the use of these plants is stimulated byO. stephanostomum. The number of chimpanzees infected byO. stephanostomum was significantly higher in the rainy season than in the dry season of both 1989–1990 and 1991–1992. However, the incidence ofTrichuris trichura andStrongyloides fuelleborni showed no seasonality. Reinfection of individuals byO. stephanostomum occurred in synchrony with annual variation in rainfall: there was a sharp rise in the occurrence of new infections per individual within one to two months after the beginning of the first heavy rains of the season. This pattern coincides with the reproductive cycle of this nematode species.O. stephanostomum (95%) infections were associated significantly more frequently with medicinal plant use than eitherT. trichiura (50%) orS. fuelleborni (40%) infections. These observations are consistent with previous reports for the increased use of these plants during the rainy season and are consistent the hypothesis that medicinal plant use is stimulated byO. stephanostomum infection.     

Sex and group differences in feeding on animals by wild chimpanzees in the Mahale Mountains National Park,Tanzania

Sex differences in animal prey intake were revealed by fecal analysis among wild chimpanzees of the large-sized M-group (ca. 100 members) in the Mahale Mountains National Park, Tanzania: prime adult or old males feed more on vertebrates, while adult females more onCamponotus ants. By contrast, such differences were not obvious in the neighboring, small-sized K-group (ca. 20–30 members), despite the similar environment in which the two unit-groups lived. Such sex and group differences may be explained in terms of various factors, either ecological or social, or both, but social factors seem most responsible in particular for the group differences. It seems likely that increased capture rate of vertebrates per unit-group in the larger-sized M-group results in increased per capita intake of meat among prime adult or old males. Also, the more frequent interactions among prime adult or old males of M-group appear to reduce the frequency of theirCamponotus ant-fishing behavior.     

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.     

Geochemistry and clay mineralogy of termite mound soil and the role of geophagy in chimpanzees of the Mahale Mountains,Tanzania

Earth from a termite mound in the Mahale Mountains National Park, Tanzania, eaten by chimpanzees, was analyzed to determine the possible stimulus, or stimuli, for geophagy. The termite mound sample contains relatively high aluminum (10.0%), iron (3.0%), and sodium (0.5%). This correlates well with the mineralogy of the clay (<2 μm) fraction, which is high in metahalloysite, a 1:1 (Si:Al=1:1) clay mineral similar in chemical composition to the clay mineral kaolinite, and smectite (montmorillonite), which is a 2:1 expandable clay mineral. The combination of metahalloysite and smectite produces a substance much like the pharmaceutical Kaopectate^™ widely used by humans as an anti-diarrheal agent. These analyses and preliminary observations linking geophagy with instances of severe diarrhea, and other signs of gastrointestinal upset in the Mahale chimpanzees, suggest that one function for the ingestion of this substance by chimpanzees may be to help provide temporary relief from gastrointestinal ailments. Further detailed investigations into the relationship between health and geophagy should provide important insights into the diverse roles of this behavior as a form of self-medication.