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.
ABSTRACT. 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 nematode Oesophagostomum 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 by O. stephanostomum. The number of chimpanzees infected by O. stephanostornum was significantly higher in the rainy season than in the dry season of both 1989-1990 and 1991-1992. However, the incidence of Trichuris trichiura and Strongyloides fuelleborni showed no seasonality. Reinfection of individuals by O. 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 either T. trichiura (50~ or S. 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 by O. stephanostomum infection.
ABSTRACT. Two cases of within-group infanticide and cannibalism were observed among the M Group chimpanzees of the Mahale Mountains, Tanzania. In both cases, victimized infants were male, 5 -6 months of age, and in good health when killed. Four to five years have passed since the mothers of the victims immigrated into M Group as nulliparous immigrants. In one case the 2nd-ranking male was observed to detach the infant from the mother's belly. Both infants were finally killed by the alpha male after several adult males scrambled for the bodies. There was no evidence that the mothers had mated with males other than those of M Group. Nor was there evidence that the mothers had restrictive mating relationships with some of the M Group adult males. What little evidence is available shows that the mothers had mated mostly with adolescent and other immature males during their conception cycles. However, at least in one case, the mother began to mate more with adult males rather than with immature males after the infanticide. It is proposed that the function of withingroup male infanticide can be explained by the male-male competition hypothesis developed for hanuman langurs and other nonhuman primates.
ABSTRACT. During a long-term study of wild chimpanzees in Tanzania, an unusual gang attack on a young adult male by the alpha male and seven other members of the same unit group (community) was observed and videotaped. Before the gang attack, the victim had not pant-grunted to the alpha male or to other adult males except for the second-ranked male, but had often bullied adult females without apparent reason. The alpha male's violent behavior, therefore, might have been punishment of or revenge against the ill-mannered young chimpanzee. This is the first record of an adult male other than an ex-alpha male becoming the object of ostracism by wild chimpanzees, although the victimized male did manage to return to the group after more than three months, when an ex-alpha male succeeded in reestablishing his former position.
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