Mountain gorillas Gorilla beringei beringei are Critically Endangered, with just two small populations: in Bwindi Impenetrable National Park in south-western Uganda and the nearby Virunga Volcanoes on the borders with Rwanda and Uganda. A survey of the Bwindi population was carried out in 2002 and results were compared with the previous census in 1997. Our estimate of total population size increased over that period by c. 7% to 320 individuals and the structure and distribution of the population were largely unchanged. Signs of human disturbance were more common in 2002 than 1997, and gorillas tended to be found in areas of relative low disturbance. This suggested that disturbance could be a constraint on population growth and distribution but demographic stochasticity may also be responsible for the observed level of population change over a short time period. Other potential limiting factors, including habitat availability and disease, are discussed. While conservation activities in Bwindi have probably contributed to the stability of the population, strengthening of law enforcement and continued vigilance are needed to ensure the population's long-term growth and survival.

We investigated long-term site fidelity of gray-cheeked mangabey (Lophocebus albigena) groups in Kibale National Park, Uganda. Concurrently, we monitored shifts in home range by individual females and subadult and adult males. We documented home range stability by calculating the area of overlap in successive years, and by recording the drift of each group’s monthly centroid from its initial location. Home ranges remained stable for 3 of our 4 groups (overlap over 10 yr >60%). Core areas were more labile, but group centroids drifted an average of only 530 m over the entire decade. Deviations from site fidelity were associated with dispersal or group fission. During natal dispersal, subadult males expanded their home ranges over many months, settling ≤4 home ranges away. Adult males, in contrast, typically dispersed within a few days to an adjacent group in an area of home range overlap. Adult males made solitary forays, but nearly always into areas used by their current group or by a group to which they had previously belonged. After secondary dispersal, they expanded their ranging in the company of their new group, apparently without prior solitary exploration of the new area. Some females also participated in home range shifts. Females shifted home ranges only within social groups, in association with temporary or permanent group splits. Our observations raise the possibility that male mangabeys use a finder-joiner mechanism when moving into new home ranges during secondary dispersal. Similarly, females might learn new resource locations from male immigrants before or during group fission.

Mangabey groups studied in the Kibale Forest Reserve, Uganda, in 1971 were studied again in 1991 using similar data collection protocols. The results were used to assess the effect of group size on activity budgets and travel costs, and to document the effects of habitat changes on mangabey density and demography. Larger mangabey groups traveled longer distances per day than smaller groups. Time budgets were less clearly influenced by group size. Mangabey population density increased over the 20 year period. This increase in population density paralleled habitat changes, particularly an increase in tree density, and was accompanied by increased use of regenerating forest. o

Effective management of anthropogenic threats is key to sustaining biological diversity in protected areas. Types and distribution of threats to Bwindi Impenetrable National Park, Uganda were investigated to assess the Park's status 12 years after it was upgraded from a forest reserve to a national park. Bwindi, like many tropical forested parks, is surrounded by dense human populations. Threats were quantified in 104 1-km edge-interior transects set around the Park. The distribution of threats was patchy and was most common within 300-350 m of the edge. The commonest threat was harvesting of wood and poles. Other threats included occurrence of exotic species, degradation of adjacent habitat fragments and high impact of problem animals on some of the neighbouring communities. The fact that threats were primarily associated with the edges of the Park, when previously they were widespread throughout the Park, suggests that illegal resource harvesting has been reduced since the forest was upgraded to a national park. Park legislation, enforcement and related conservation efforts have been effective, and there should be increased effort to manage the peoplepark interface. Edge-based assessments appear to be useful for quantifying threats to protected areas and identifying areas in which they are concentrated.

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Two opposing hypotheses concerning determinants of mangabey (Cercocebus albigena) ranging patterns have been advocated. One hypothesis suggests that ranging patterns of mangabeys are largely a response to fruit availability, while the other hypothesis advocates that concerns of fruit availability are supplemented or overridden by concerns of fecal contamination and that the risk of parasite infection, especially during dry weather, determines their pattern of range use. In this 9 month study of mangabeys in the Kanyawara study area of Kibale National Park, mangabeys moved longer distances during the wet season than during the dry season. There were no seasonal differences in group spread, number of 50 by 50 m quadrats used, or in quadrat overlap between sequential sample periods. Intensity of quadrat use was closely related to the number of fruiting trees/lianas in the quadrats, irrespective of season. These findings are consistent with the hypothesis that fruit availability is a main factor influencing mangabey ranging patterns. The results are not consistent with the hypothesis that mangabey ranging patterns largely reflect differential seasonal risk of parasite infection. Am. J. Primatol. 43:65–78, 1997. © 1997 Wiley‐Liss, Inc.

We describe the movements and fates of 36 collared gray-cheeked male mangabeys (Lophocebus albigena) that resided in seven social groups in Kibale National Park, Uganda. The male mangabeys were captured, radiotagged, and then contacted regularly over a period of up to 8 years. Individuals varied considerably in how tightly they were associated with their groups. "Central" males were usually found in close spatial association with their own groups, but they sometimes visited others. "Peripheral" males were often found several hundred meters away from their own groups. Most adult males were characterized by repeated intergroup transfers, manifested as "visits" and "dispersals." Visits were transfers that did not result in mating. The visits were brief, and ranged from a few minutes to a few days before the male returned to his previous group, moved on to another group, or returned to solitary life. The term "dispersal" refers to a transfer that results in mating, after which the male remains in the new group. Young males began to drift away from their natal groups in early subadulthood, but the timing of first associations with estrous females in other groups was highly variable. Natal dispersers were generally solitary for a month or more, and at least half moved into nonadjacent groups. Secondary dispersal was common. After they immigrated, adult males remained in a group for a median of 19 months before they emigrated again, with the probability of departure being approximately constant in time. Secondary dispersal was usually made into an adjacent group and lasted less than 7 days. Emigration and immigration were not seasonal, and males emigrated singly and apparently independently; however, immigrations were clumped in time, and male dispersal contributed to considerable heterogeneity of group composition. L. albigena dispersal patterns are intermediate between those described for Papio baboons and forest Cercopithecus, and models that predict demographic consequences of dispersal for baboons can be refined to apply to mangabeys.

Remedies for reduction in primate numbers following rainforest habitat disturbance must incorporate understanding of how disturbance affects population biology. Between July 1996 and July 1998, I captured 31 adult male grey cheeked mangabeys (  Lophocebus albigena) from groups occupying logged and unlogged forest in Kibale National Park, Uganda. Captured males were weighed, measured, fitted with radiocollars, and radiotracked three to five times a week. Males from groups in the unlogged forest were significantly heavier than males from the logged forest. Males that immigrated into the study groups were significantly lighter than resident males in unlogged forest, but they did not differ in mass from males in logged forest. I tested hypotheses that these differences were due to differences in age, skeletal size, and tissue mass. Results support the hypothesis that differences are due to differences in tissue mass. Results of further tests showed that differences in tissue mass were explained more by differences in habitat type than by rates of male turnover. There was no evidence to suggest that differences in tissue mass were related to differences in costs of travel, crowding, or seasonal variation in habitat quality. My results suggest that selective logging may affect primate biomass, although these changes may or may not be reflected in estimates of primate density.