Group Size Dynamics over 15+ Years in an African Forest Primate Community

Group size affects many aspects of the ecology and social organization of animals. We investigated group size stability for five primate species in Kibale National Park, Uganda from 1996 to 2011 at three nested spatial scales. Survey data indicated that group sizes did not change for most species, with the exception of red colobus monkeys (Procolobus rufomitratus), in which group size increased at all spatial scales. Mangabey (Lophocebus albigena) group size increased in old‐growth forest, but the sample size and increase were small. To augment this survey data, we collected several years of demographic data on three habituated groups of redtail monkeys (Cercopithecus ascanius), eight groups of black‐and‐white colobus (Colobus guereza), and one red colobus group. The red colobus group increased from 59 to 104 individuals, while redtail monkey and black‐and‐white colobus group sizes were stable, mirroring our survey results. To understand mechanisms behind group size changes in red colobus versus stability in other primates, we monitored forest dynamics at two spatial scales between 1990 and 2013, considered changes in predator population, and explored evidence of disease dynamics. The cumulative size of all trees and red colobus food trees increased over 24 yr, suggesting that changing food availability was driving group size changes for red colobus, while predation and disease played lesser roles. Overall, our results and evidence of changing primate densities suggest that the Kibale primate community is in a non‐equilibrium state. We suggest future conservation and management efforts take this into consideration.     

Monkey Abundance and Social Structure in Two High-Elevation Forest Reserves in the Udzungwa Mountains of Tanzania

The effects of human activity on population and social structure are a pantropical concern for primate conservation. We compare census data and social group counts from two forests in the Udzungwa Mountains, Tanzania. The main aim is to relate differences within and between the forests to current theory on the effect of human disturbance on primate abundance and group size. The survey reveals the presence of the restricted-range red colobus, Procolobus gordonorum, in New Dabaga/Ulangambi Forest Reserve (NDUFR). The primate community of NDUFR is impoverished compared to that in Ndundulu forest. Red colobus and black-and-white colobus (Colobus angolensis palliatus) abundance and group size are lowest in NDUFR. Fission-fusion of red colobus social groups may be occurring in previously logged areas of both forests. Our observations are consistent with current theory on the effect of habitat degradation and hunting on primates, but the relative effects of the 2 factors could not be differentiated. We pooled the results with previous data to show that abundance of red colobus in the Udzungwa Mountains is lowest at high elevations. Low red colobus group sizes appear to be related to human activity rather than elevation. Black-and-white colobus and Sykes monkeys (Cercopithecus mitis) show no relationship with elevation. Future studies will require more detailed information on vegetation, diet and ranging patterns to interpret fully intraspecific variation in population demography and social structure in the Udzungwa Mountains.     

Group size in folivorous primates: ecological constraints and the possible influence of social factors

The ecological-constraints model assumes that food items occur in depletable patches and proposes that an increase in group size leads to increased day range due to more rapid patch depletion. Smaller groups become advantageous when an increase in travel costs is not repaid by an increase in energy gained or some other fitness advantage. On the other hand, we also know that group size can be influenced by social factors. Here we contrast the diet and group size of red colobus (Procolobus badius) and black-and-white colobus (Colobus guereza) in Kibale National Park, Uganda to consider how ecological and social factors are affecting their group sizes. Subsequently, we examine whether the insights gained from this detailed comparison can provide an understanding of why the social organization and group size of mantled howlers (Alouatta palliata) and black howlers (A. pigra) differ. Two groups of red colobus and two groups of black-and-white colobus were studied over 10 months. Red colobus groups were larger (48 and 24) than black-and-white colobus groups (9 and 6). The two groups of red colobus overlap home ranges with the two groups of black-and-white colobus; 75% and 95% of their home ranges were within red colobuss home range. There was a great deal of similarity in the plant parts eaten by the two species and both species fed primarily on young leaves (red colobus 70%, black-and-white colobus 76%). In terms of the actual species consumed, again there was a great deal of similarity between species. The average dietary overlap among months for the two neighboring groups of red colobus was 37.3%, while the dietary overlap between the red colobus and the black-and-white colobus group that had its home range almost entirely within the home range of the red colobus groups averaged 43.2% among months. If ecological conditions were responsible for the difference in group size between the two colobine species, one would expect the density of food trees to be lower in the home ranges of the black-and-white colobus monkeys, since they have the smaller group size. We found the opposite to be true. Both black-and-white colobus groups had more food trees and the cumulative size of those trees was greater than those in the red colobuss home ranges. We quantify how these differences parallel differences in mantled and black howlers. The average group size for mantled howlers was 12.9 individuals, and for black howlers it was 5.3 individuals. We explore possible social constraints, such as infanticide, that prevent black-and-white colobus and black howlers from living in large groups.This revised version was published online in April 2005 with corrections to the cover date of the issue.     

Spatial distribution of primates in a mosaic of colonizing and old growth forest at Ngogo, Kibale National Park, Uganda

Primate censuses were conducted in a mosaic of colonizing (two locations) and old-growth forests using line transect methods at the Ngogo study site, Kibale National Park, Uganda. Black and white colobus monkeys (Colobus guereza) were encountered more frequently in the colonizing forests than in the old growth forest, while chimpanzees (Pan troglodytes) were encountered more frequently in the old growth forest than in colonizing forests. Although not significant, results suggest that blue monkeys (Cercopithecus mitis) frequented colonizing forests more often than old growth forest. The encounter rates of mangabey (Lophocebus albigena), and redtail (Cercopithecus ascanius) groups were ambiguous with their density being higher in some colonizing forests but not others as compared to old-growth forest. No significant differences were detected for baboons (Papio anubis), Lhoests (Cercopithecus lhoesti), and red colobus monkeys (Piliocolobus tephroscales). The conversion of forests to farmland is one of the major problems encountered in primate conservation. This study shows that secondary forests replacing anthropogenic grasslands have the potential of supporting some primate species such as black and white colobus, redtail monkeys, and possibly blue monkeys. Therefore, such areas should not be given up but should be conserved for the benefit of primates that can survive in secondary forests; as the forests mature further, primate species that are adapted to old growth forest will colonize the area provided there is a nearby source.     

Effects of Forest Fragmentation on the Abundance of <Emphasis Type="Italic">Colobus angolensis palliatus</Emphasis> in Kenya’s Coastal Forests

We documented the occurrence and abundance patterns of Angola black-and-white colobus (Colobus angolensis palliatus) in 46 coastal forest fragments ranging from 1 ha to >1400 ha in the Kwale District, Kenya. In field surveys conducted in 2001, we also recorded forest spatial, structural, resource, and disturbance characteristics to determine the effects of habitat quality and fragmentation and the factors most critical to the continued survival of the little-known species. We tested 13 hypotheses to explain variation in patch occupancy and abundance patterns of Colobus angolensis palliatus in relation to habitat attributes. Minimal adequate models indicated that the occurrence of colobus in forest fragments is positively associated with fragment area and canopy cover, whereas the density of colobus in occupied fragments is attributable to forest area, the proportion of forest change over the previous 12 yr, and the basal area of 14 major food trees. Large-scale illegal extraction of major colobus food trees in the District for human resource use, in both protected and unprotected forests, together with ongoing forest clearance and modification, are the major threats to Colobus angolensis palliatus in Kenya.     

Population Declines of Colobus in Western Uganda and Conservation Value of Forest Fragments

The processes of habitat loss and fragmentation are probably the most important threats to biodiversity. It is critical that we understand the conservation value of fragments, because they may represent opportunities to make important conservation gains, particularly for species whose ranges are not in a protected area. However, our ability to understand the value of fragments for primates is limited by the fact that researchers have conducted many studies in protected areas, which do not represent most fragments, and studies are typically short term. Here we determine the long-term survival probability of red (Procolobus pennantii) and black-and-white colobus (Colobus guereza) inhabiting forest fragments outside of Kibale National Park, Uganda. Local communities use the fragments primarily for subsistence agriculture and fuelwood. We surveyed primate populations 3 times over 8 yr, made a total inventory of all trees 2 times, contrasted behavior of groups inhabiting 1 fragment with groups in the continuous forest, and judged the conservation value of the fragments by quantifying patterns of forest use by local people. Of the 20 fragments surveyed, 16 supported resident populations of colobus in 1995, 2 were cleared in 2000, and an additional 2 fragments were cleared by 2003. In 1995 we counted 165 black-and-white colobus, whereas in 2000 and 2003, we counted 119 and 75 individuals, respectively. Seven fragments supported red colobus in 1995, 11 in 2000, and 9 in 2003. In 2000 we counted 159 red colobus, while in 2003, we saw 145 individuals. For both species, activity patterns in continuous forest were similar to those in a fragment, with the exception that individuals in the fragment rested more. Colobus in the fragment ate more mature leaves than colobus in the continuous forest did. Fragments supported all the fuelwood needs of an average of 32 people who lived immediately adjacent to them, and partially supported families up to 3 farms away (ca. 400 m), representing 576 people. Intensive harvesting for fuelwood occurred when neighboring households engaged in beer brewing (an average of 9.6% of the households), gin distilling (8.8%), or charcoal production (14.5%). Overall, between 2000 and 2003, the average density of trees declined by 14 trees/ha (range = 0–60 trees/ha). If current rates of clearing continue, the probability that the fragments will continue to support colobus populations is low.     

Primate abundance along five transect lines at Ngogo, Kibale National Park, Uganda

Using the line transect methods, I studied the primate density at Ngogo, Kibale National Park, Uganda for 18 months. Comparisons with other studies show that the population of red colobus monkeys (Procolobus rufomitratus) and blue monkeys (Cercopithecus mitis) is declining, whereas the populations of black-and-white colobus (Colobus guereza), red-tailed monkeys (Cercopithecus ascanius), grey-cheeked mangabeys (Lophocebus albigena), baboons (Papio anubis), and chimpanzees (Pan troglodytes) remain constant or slightly increase. In this paper, I compare data on density from this study to data from previous and recent censuses at Ngogo and with data from other sites in the Kibale forest to examine the stability of primate populations. Furthermore, I test the hypothesis that the changes in red colobus and blue monkey density are due to changes in the forest structure and abundance of their most selected feeding trees, and show that changes in forest composition cannot account for changes in their red colobus abundance, but that hunting by chimpanzees provides a reasonable explanation.     

Primate population dynamics: variation in abundance over space and time

The rapid disappearance of tropical forests, the potential impacts of climate change, and the increasing threats of bushmeat hunting to wildlife, makes it imperative that we understand wildlife population dynamics. With long-lived animals this requires extensive, long-term data, but such data is often lacking. Here we present longitudinal data documenting changes in primate abundance over 45 years at eight sites in Kibale National Park, Uganda. Complex patterns of change in primate abundance were dependent on site, sampling year, and species, but all species, except blue monkeys, colonized regenerating forest, indicating that park-wide populations are increasing. At two paired sites, we found that while the primate populations in the regenerating forests had increased from nothing to a substantial size, there was little evidence of a decline in the source populations in old-growth forest, with the possible exception of mangabeys at one of the paired sites. Censuses conducted in logged forest since 1970 demonstrated that for all species, except black-and-white colobus, the encounter rate was higher in the old-growth and lightly-logged forest than in heavily-logged forest. Black-and-white colobus generally showed the opposite trend and were most common in the heavily-logged forest in all but the first year of monitoring after logging, when they were most common in the lightly-logged forest. Overall, except for blue monkey populations which are declining, primate populations in Kibale National Park are growing; in fact the endangered red colobus populations have an annual growth rate of 3%. These finding present a positive conservation message and indicate that the Uganda Wildlife Authority is being effective in managing its biodiversity; however, with constant poaching pressure and changes such as the exponential growth of elephant populations that could cause forest degradation, continued monitoring and modification of conservation plans are needed.     

Giardia sp. and Cryptosporidium sp. infections in primates in fragmented and undisturbed forest in western Uganda

In June 2005, we collected 115 fecal samples from wild primates in western Uganda and examined them for Cryptosporidium sp. and Giardia sp. with the use of immunofluorescent antibody (IFA) detection. We sampled primates from an undisturbed forest in Kibale National Park and from 3 highly disturbed forest fragments outside the park. Of disturbed forest samples, red colobus (Pilocolobus tephrosceles) and red-tailed guenons (Cercopithecus ascanius) harbored species of Cryptosporidium or Giardia, but black-and-white colobus (Colobus guereza) did not. All primate samples from undisturbed forest were negative for both parasites. Seven of 35 (20%) red colobus and 1 of 20 red-tailed guenons (5%) from forest fragments were infected with either Cryptosporidium sp. or Giardia sp. The presence of Cryptosporidium and Giardia species in primates living in forest fragments, but not in primates in undisturbed forest, suggests that habitat disturbance may play a role in transmission or persistence of these pathogens.     

Association of Terrestrial Mangabeys (Cercocebus atys) with Arboreal Monkeys: Experimental Evidence for the Effects of Reduced Ground Predator Pressure on Habitat Use

We examined the behavior of two arboreal monkeys—Piliocolobus badius (western red colobus) and Cercopithecus diana (Diana monkey)—in the presence and absence of a third, predominately terrestrial monkey, Cercocebus atys (sooty mangabey) in the Ivory Coast's Taï Forest. Via experimental data, we show that sooty mangabeys are effective sentinels for ground predators. Then we compared strata use of red colobus and Diana monkeys in the presence and absence of mangabeys. Our data indicate that red colobus and Diana monkeys descend to low forest levels and to the forest floor significantly more often when mangabeys are spatially intermingled with them, i.e. in polyspecific associations. The niche extension may provide some foraging advantage, especially for red colobus. We identified the specific causal agent—relaxed ground predator pressure—of a temporary shift in habitat use. In this instance, the presence of one species (Cercocebus atys) leads to the temporary expansion of a niche normally avoided by others Piliocolobus badius and Cercopithecus diana.     

Thirty Years of Research in Kibale National Park,Uganda, Reveals a Complex Picture for Conservation

Kibale National Park, Uganda, has a rich and abundant primate community and a complicated history of anthropogenic disturbance. Moreover, it has been the focus of over 30 yr of research and has received considerable attention from nongovernmental and governmental conservation organizations. As a result, Kibale serves as a valuable case study with which to evaluate the factors that regulate primate population density and the challenges of deriving generalizations for conservation. We review the impact of logging and forest fragmentation on primate population density and trace the efficacy of various conservation strategies. A 28-yr comparison of primate abundance in logged and unlogged forests and a 10-yr study of forest dynamics showed that primate recovery in logged areas is generally slow or not occurring at all for some species, which is likely driven by the fact that the forest is not recovering as expected. No primate species characteristic predicted their ability to live in forest fragments around Kibale. While a nutritional model was useful to predict the abundance of colobus in forest fragments outside of Kibale, a 5-yr study revealed that human land-use practices are more fundamentally shaping population dynamics. We evaluate data on primate abundance in light of Milton’s protein/fiber model to predict colobine biomass. We demonstrate that while the model can predict colobus biomass in pristine habitats, the 2 colobus species respond differently to disturbance. We offer suggestions for future conservation research and consider strategies to conserve forested national parks based on experiences gained over 30 yr.     

Densities of Two Frugivorous Primates with Respect to Forest and Fragment Tree Species Composition and Fruit Availability

Conservation of wildlife populations requires extensive knowledge of their habitat requirements, efficient methods to evaluate habitat quality, and an understanding of the value of fragments and edges. Kibale National Park, Uganda has areas that differ in the densities of 2 species of frugivorous monkeys—Cercopithecus mitis and Lophocebus albigena—including one on an edge and forest fragments outside the park that lack both species. We compared the basal area densities of important food trees with primate densities. The density of Cercopithecus mitis correlates most strongly with the basal area density of all types of food trees combined. The density of Lophocebus albigena does not correlate with the basal area densities of any category of food trees or with fruit availability. An index of their density—number of groups seen per km walked—correlates to fruit availability but with marginal significance. Lack of a relationship between the basal area densities of food trees and density of Lophocebus albigena may be the result of a mismatch in scale between the forest area measured and their large home ranges. We compared the unused area of forest to the other areas of the forest and the fragments and found it had higher basal area densities in all food tree categories for both species than the fragments and lower basal area densities of most categories than the other parts of the forest, indicating that the fragments are poor quality and would probably be unused even if dispersal were likely.     

Does the tropical agricultural matrix bear potential for primate conservation? A baseline study from Western Uganda

As for most large mammals, conservation research on primates usually focuses on protected areas, and yet not much is known about primate communities in land-use systems in the absence of hunting. Using line transects, we estimated population densities for four primate species in a mixed agroforest landscape up to a distance of 4000 m from a forest reserve, in a region where no primate hunting takes place. We then modelled encounter rates and cluster size in relation to landscape parameters by means of bivariate analysis and Generalised Linear Models (GLMs). Black-and-white colobus (Colobus guereza) were most common, with confidence intervals for density estimates of 31.1–62.8 individuals/km². Red-tailed monkeys (Cercopithecus ascanius) and blue monkeys (Cercopithecus mitis) occurred at 15.2–37.9 ind/km² and 13.9–36.7 ind/km² respectively, and chimpanzees (Pan troglodytes) at 1.0–2.8 ind/km². Chimpanzee nest numbers and distribution appeared to be significantly constrained by transect forest coverage, forest coverage within 500 m and distance from the main forest, whereas monkey sightings were generally less restricted by landscape variables. The considerable population density of monkeys suggests that, in the absence of hunting, mixed agroforest systems may play a relevant role in primate conservation and highlight that it is useful to consider primate ecology in land sharing approaches to conservation.     

Constraints on Group Size in Red Colobus and Red-tailed Guenons: Examining the Generality of the Ecological Constraints Model

The ecological constraints model proposes that an increase in group size will increase intragroup feeding competition and thereby constrain group size. Although this model has received wide acceptance, tests of it are based only on a few studies of species that have similar ecological requirements and social organizations, and there are reasons to question the widespread acceptance of the assumptions underpinning it. Via a 2-year study, we explored determinants of group size in species that feed on markedly different types of foods: the folivorous red colobus (Procolobus pennantii) and the frugivorous/insectivorous red-tailed guenon (Cercopithecus ascanius). We established 4 study sites approximately 15 km apart in Kibale National Park, Uganda, to examine the relationship between average group size and food availability. In both species, we quantified interdemic variation in diet, density of food trees, rate of travel, and group size. Red colobus at all sites relied heavily on leaf resources (75.5%–86.9%), but fruit (6.4%–13.9%) and flowers (2.0%–13.9%) were important in some populations. In general, red-tailed guenons fed on fruit (35.7%–59.7%), insects (14.5%–17.6%), and young leaves (12.2%–32.8%), but the amount of time allocated to these foods varied among sites. Average monthly density of trees bearing food items ranged among sites from 45 to 79 trees/ha for red colobus and from 19.6 to 67.3 trees/ha for red-tailed guenons. For both species, rate of travel was similar among sites, with one exception for red colobus. Average red colobus group size varied among sites from 14 to 40 (28 groups counted). Red-tailed guenon group size varied among sites from 11 to 24 (16 groups counted). As predicted by the ecological constraints model, group size increased with food tree density across sites for both species.     

Interspecific interaction of the primate groups in Kibale Forest,Uganda

An ecological survey on the influence of interspecific interaction of the primates upon the distribution of their group ranges was carried out in 100 ha of the isolated forest northern outskirts of Kibale Forest in western Uganda, Africa. The study period of 105 days was from the 12th of November, 1970 to the 24th of February, 1971, including a preliminary survey of about two months. The subjects of this study are five species of primates, i.e., black and white colobus (Colobus polykomos), red colobus (Colobus badius), red tailed monkey (Cercopithecus ascanius), blue monkey (Cercopithecus mitis), and vervet monkey (Cercopithecus aethiops), which inhabited the study area. The red colobus group is thought to be the most influential of the five in the interspecific interaction.     

Abundance of Diurnal Primates in Mwanihana Forest, Udzungwa Mountains, Tanzania

Many individual researchers have used line transect counts to estimate forest primate abundance. They have devoted less attention to the interpretation of line transect data obtained by several observers, as is often the case in long-term monitoring programs. We present primate relative abundance data that 5 observers collected over 6 yr (not continuous) along 4 different transects each 4 km long in the Mwanihana Forest, Udzungwa Mountains National Park, Tanzania. Total distance walked during transect repetitions is ca. 700 km. The species we saw most frequently was the endemic Udzungwa red colobus Procolobus gordonorum (mean 0.59 groups/km walked), followed by the Angolan black-and-white colobus Colobus angolensis (0.43 groups/km) and Sykes’s monkey Cercopithecus mitis (0.35 groups/km). We sighted the endemic Sanje mangabey Cercocebus galeritus sanjei and the yellow baboon Papio cynocephalus infrequently, the latter being confined to the deciduous forest parts of the transects. We analyzed sighting frequency by gross habitat type, transect, season, and observer. Interobserver differences in the relative abundance of each species were moderate and the few cases of significant variations were due to discordance of only 1 observer from the others. Estimated distances of primate group sightings differ significantly among observers, thus preventing us from deriving estimates of absolute density. Frequency distributions of distance-class intervals are not significantly different among observers, which may indicate gross interobserver consistency in the width of the area sampled. We conclude that unless consistency in data collection is checked, as we did for 2 observers who collected data simultaneously, potential interobserver differences remain an underlying source of variance in the results that cannot be separated from other sources of variance.     

Parasite prevalence and richness in sympatric colobines: effects of host density

Factors that influence proximity and the number and duration of contacts among individuals can influence parasite transmission among hosts, and thus parasite prevalence and species richness are expected to increase with increasing host density. To examine this prediction we took advantage of a unique situation. Following the clearing of a forest fragment that supported red colobus (Piliocolobus tephrosceles) and black-and-white colobus (Colobus guereza), the animals moved into a neighboring fragment that we had been monitoring for a number of years and for which we had described the primate parasite community. After the animals immigrated into the fragment, the colobus populations more than doubled and colobus density became almost twice that found in Kibale National Park, Uganda. Despite this increase in host density, the richness of the parasite community did not increase. However, in both colobus species the prevalence of Trichuris sp., the only commonly occurring gastrointestinal parasite, increased. Over the next 5 years the prevalence and intensity of infection of Trichuris sp. in red colobus declined and their population numbers slowly increased. In contrast, the prevalence and intensity of infection of Trichuris sp. increased in black-and-white colobus and remained high following the immigration, and their population size declined. While Trichuris sp. infections are typically asymptomatic, we consider it a possibility that they contributed to the decline of the black-and-white colobus, and that the red colobus may be serving as a reservoir for Trichuris, thereby increasing the infection risk for black-and-white colobus.     

Tana river red colobus and crested mangabey: Results of recent censuses

Censuses of the Tana River red colobus (Colobus badius rufomitratus) and crested mangabey (Cercocebus galeritus galeritus), primate species endemic to riverine forests of the lower Tana River in eastern Kenya, showed significant population declines for both species between the mid-1970s and 1980s. Red colobus declined in terms of group size and number of groups per forest; the mangabey population declined only in numbers of groups per forest. There was no significant change in mean group age/sex composition between the two time periods for either species. Differences in the feeding and ranging ecology of the two species may explain why the mangabey population suffered a less severe decline than the red colobus. Similarities in census results over 3 years in the late 1980s suggest that the primate populations are no longer decreasing.     

Density Estimation of the Endangered Udzungwa Red Colobus (Procolobus gordonorum) and Other Arboreal Primates in the Udzungwa Mountains Using Systematic Distance Sampling

Estimates of population density and abundance are essential for the assessment of nonhuman primate conservation status, especially in view of increasing threats. We undertook the most extensive systematic primate survey yet of the Udzungwa Mountains of Tanzania, an outstanding region for primate endemism and conservation in Africa. We used distance sampling to survey three arboreal monkey species, including the endangered and endemic Udzungwa red colobus (Procolobus gordonorum). Overall, we encountered 306 primate clusters over 287 km walked along 162 line transects. We found the lowest cluster densities for both red colobus and Angolan colobus (Colobus angolensis; 0.8 clusters/km²) in the least protected forest (Uzungwa Scarp Forest Reserve, US), while we found the highest densities (3.2 and 2.6 clusters/km² for red colobus; 3.2 and 2.7 clusters/km²for Angolan colobus) in two large and protected forests in the national park. Unexpectedly, Magombera, a small forest surrounded by plantations, had the highest densities of red colobus (5.0 clusters/km²), most likely a saturation effect due to the rapid shrinking of the forest. In contrast, Sykes’ monkey (Cercopithecus mitis monoides/moloneyi) had more similar densities across forests (3.1–6.6 clusters/km²), including US, suggesting greater resilience to disturbance in this species. For the endemic red colobus monkey, we estimated an abundance of 45–50,000 individuals across all forests, representing ca. 80% of the global population. Though this is a relatively high abundance, the increasing threats in some of the Udzungwa forests are of continued concern for the long-term survival of red colobus and other primates in the area.     

Population Sizes and Distribution of Primates in the Lower Tana River Forests,Kenya

We studied the population size and distribution of diurnal primates in the lower Tana River forests, Kenya. They are the only remaining habitats for 2 threatened primates: the Tana River red colobus (Procolobus rufomitratus) and the Tana River crested mangabey (Cercocebus galeritus galeritus). We conducted censuses in 73 forest patches from January through March 2001. We estimate population size of the red colobus to be 788 individuals in 82 groups and that of the crested mangabeys to be 2,070 individuals in 59 groups. The data suggest that over a 7-year period (1994-2001), there was an 18% increase in the crested mangabey population and a 5% decline in red colobus numbers. Further, the red colobus range has expanded both north and south, whereas that of crested mangabeys has only expanded south. Fifty-six percent of crested mangabeys and 46% of red colobus groups were inside the Tana River Primate National Reserve (TRPNR). Other primates encountered included 170 groups of Sykes' monkeys (Cercopithecus mitis), 70 groups of yellow baboons (Papio cynocephalus) and 4 groups of grivets [Chlorocebus (Cercopithecus) aethiops]. Mean group densities of the 2 endangered primates and of baboons were higher inside than outside the TRPNR, reinforcing the importance of TRPNR for their conservation. An intervention program is required to stem further decline in the red colobus population and to protect small isolated groups in forest patches outside TRPNR.