Implications of small scale variation in ecological conditions for the diet and density of red colobus monkeys

In this 3-year investigation we documented patterns of density, diet, and activity of red colobus monkeys (Procolobus tephrosceles) in six areas in or near Kibale National Park, Uganda and related these patterns to availability of food resources. There were large differences in the density and behavior of the red colobus among the sites. For example, the red colobus at one site with a diverse plant community of more than 61 tree species, had a diet that included at least 42 species. In contrast, at a second site red colobus spent 92% of their feeding time eating from one species that dominated the tree community. The density of important red colobus food trees varied among sites from 32 trees/ha to 204 trees/ha, and red colobus density ranged from 0.70 groups/km² to 7.41 groups/km². Among sites, red colobus density was related to the cumulative DBH of important food trees, when one apparently anomalous site was excluded, and populations with more plant species in their diets tended to be those that were found at higher densities. Activity budgets of the red colobus populations varied markedly among sites. For example, feeding time ranged among sites from 29 to 55%, and traveling varied from 5 to 20%. When faced with increased foraging demands, red colobus reduced the time spent resting, while the time spent socializing remained fairly constant. Comparative socioecological studies typically contrast species separated by large geographical distances to ensure there is sufficient variation in the environment to detect behavioral responses. The marked differences in ecological conditions and red colobus behavior we documented over short geographical distances, suggests that small-scale contrasts are a useful tool to examine ecological determinants of behavior and community structure.     

Memory Effects on Movement Behavior in Animal Foraging

An individual’s choices are shaped by its experience, a fundamental property of behavior important to understanding complex processes. Learning and memory are observed across many taxa and can drive behaviors, including foraging behavior. To explore the conditions under which memory provides an advantage, we present a continuous-space, continuous-time model of animal movement that incorporates learning and memory. Using simulation models, we evaluate the benefit memory provides across several types of landscapes with variable-quality resources and compare the memory model within a nested hierarchy of simpler models (behavioral switching and random walk). We find that memory almost always leads to improved foraging success, but that this effect is most marked in landscapes containing sparse, contiguous patches of high-value resources that regenerate relatively fast and are located in an otherwise devoid landscape. In these cases, there is a large payoff for finding a resource patch, due to size, value, or locational difficulty. While memory-informed search is difficult to differentiate from other factors using solely movement data, our results suggest that disproportionate spatial use of higher value areas, higher consumption rates, and consumption variability all point to memory influencing the movement direction of animals in certain ecosystems.     

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.     

Patch depletion behavior differs between sympatric folivorous primates

Food competition in group-living animals is commonly accepted as a critical determinant of foraging strategies and social organization. Here we examine food patch depletion behavior in a leaf-eating (folivorous) primate, the guereza (Colobus guereza). Snaith and Chapman (2005) studied the sympatric folivorous red colobus (Procolobus rufomitratus), which shares many food resources with the guereza. They determined that red colobus deplete the patches (feeding trees) they use, while we found contrary evidence for guerezas using the same methods. We found that the time guerezas spent feeding in a patch was affected by neither tree size, an indicator of food abundance, nor the size of the feeding group, an indicator of feeding competition. For their principal food item (young leaves), intake rate remained constant and coincided with a decrease in the distance moved to find food within a patch, implying that guerezas do not deplete patches. This points to a fundamental difference in the use of food by guerezas and red colobus, which may be linked to the large difference in their group sizes and/or to a disparity in their digestive physiologies. However, further analyses revealed that the number of feeders within a patch did not affect patch depletion patterns in either species, leaving the potential for a physiological basis as the most plausible explanation. Our research highlights the need for a more critical examination of folivorous primate feeding ecology and social behavior, as all folivorous primates are typically lumped into a single category in socioecological models, which may account for conflicting evidence in the literature.     

A test of spatial memory and movement patterns of bumblebees at multiple spatial and temporal scales

Naive bumblebee foragers appear to use movement rules at smallspatial and temporal scales, but it is not clear whether theserules determine movement patterns as the scales increase. Onestrategy for efficient foraging used by bumblebees is near-farsearch, involving short flights when in good patches of flowersand longer flights when in poor patches. Bumblebees also demonstratethe use of a spatial memory strategy by returning repeatedlyto patches of flowers, and even following the same route betweenflowers, over periods of days. We attempted to determine atwhat spatial scales bumblebees use spatial memory while foragingwithin a patch and after how many flower visits spatial memoryoutweighs near-far search. Bumblebees in the laboratory foragedon a 4 x 4 array of artificial flowers with distances rangingfrom 10 to 80 cm between flowers in two simple spatial patterns.The proportion of visits to flowers containing a sucrose rewardwas monitored for either 100 or 400 flower visits in two separateexperiments, after which the locations of the rewarding andnonrewarding flowers were interchanged, producing a mirror image.A drop in accuracy after the mirror image switch would indicatethat the bees had memorized the location of rewarding flowers.Mirror image tests, and comparisons to a simulation model ofnear-far search based on actual flight distances, indicate thatnaive bumblebees used near-far search on flowers 10 cm apartbut increasingly used spatial memory as experience and spatialseparation increased. Bumblebees thus have multiple tacticsavailable to forage efficiently in different environments.     

Energy benefits and emergent space use patterns of an empirically parameterized model of memory‐based patch selection

Many species frequently return to previously visited foraging sites. This bias towards familiar areas suggests that remembering information from past experience is beneficial. Such a memory‐based foraging strategy has also been hypothesized to give rise to restricted space use (i.e. a home range). Nonetheless, the benefits of empirically derived memory‐based foraging tactics and the extent to which they give rise to restricted space use patterns are still relatively unknown. Using a combination of stochastic agent‐based simulations and deterministic integro‐difference equations, we developed an adaptive link (based on energy gains as a foraging currency) between memory‐based patch selection and its resulting spatial distribution. We used a memory‐based foraging model developed and parameterized with patch selection data of free‐ranging bison Bison bison in Prince Albert National Park, Canada. Relative to random use of food patches, simulated foragers using both spatial and attribute memory are more efficient, particularly in landscapes with clumped resources. However, a certain amount of random patch use is necessary to avoid frequent returns to relatively poor‐quality patches, or avoid being caught in a relatively poor quality area of the landscape. Notably, in landscapes with clumped resources, simulated foragers that kept a reference point of the quality of recently visited patches, and returned to previously visited patches when local patch quality was poorer than the reference point, experienced higher energy gains compared to random patch use. Furthermore, the model of memory‐based foraging resulted in restricted space use in simulated landscapes and replicated the restricted space use observed in free‐ranging bison reasonably well. Our work demonstrates the adaptive value of spatial and attribute memory in heterogeneous landscapes, and how home ranges can be a byproduct of non‐omniscient foragers using past experience to minimize temporal variation in energy gains.     

Step selection techniques uncover the environmental predictors of space use patterns in flocks of Amazonian birds

Understanding the behavioral decisions behind animal movement and space use patterns is a key challenge for behavioral ecology. Tools to quantify these patterns from movement and animal–habitat interactions are vital for transforming ecology into a predictive science. This is particularly important in environments undergoing rapid anthropogenic changes, such as the Amazon rainforest, where animals face novel landscapes. Insectivorous bird flocks are key elements of avian biodiversity in the Amazonian ecosystem. Therefore, disentangling and quantifying the drivers behind their movement and space use patterns is of great importance for Amazonian conservation. We use a step selection function (SSF) approach to uncover environmental drivers behind movement choices. This is used to construct a mechanistic model, from which we derive predicted utilization distributions (home ranges) of flocks. We show that movement decisions are significantly influenced by canopy height and topography, but depletion and renewal of resources do not appear to affect movement significantly. We quantify the magnitude of these effects and demonstrate that they are helpful for understanding various heterogeneous aspects of space use. We compare our results to recent analytic derivations of space use, demonstrating that the analytic approximation is only accurate when assuming that there is no persistence in the animals' movement. Our model can be translated into other environments or hypothetical scenarios, such as those given by proposed future anthropogenic actions, to make predictions of spatial patterns in bird flocks. Furthermore, our approach is quite general, so could potentially be used to understand the drivers of movement and spatial patterns for a wide variety of animal communities.     

Genetic evidence for spatio‐temporal changes in the dispersal patterns of two sympatric African colobine monkeys

Western black‐and‐white colobus and Temmink's red colobus are two forest‐dependent African primates with similar ecological requirements, often found in sympatry. Their most striking difference lies in their social system: black‐and‐white colobus live in small groups with mainly male‐mediated dispersal but where females can also disperse, whereas red colobus live in larger groups with males described as philopatric. To investigate whether genetic evidence supports the reported patterns of dispersal based on observational data, we examined eight black‐and‐white and six red colobus social groups from Cantanhez National Park, Guinea‐Bissau. Microsatellite markers revealed a lack of sex‐biased dispersal for black‐and‐white colobus. Gene flow, mainly mediated by females, better explained the genetic patterns found in red colobus, with some evidence for less extensive male dispersal. In contrast to the microsatellite data, low mitochondrial diversity for the black‐and‐white colobus suggests that historical and/or long‐range male‐mediated gene flow might have been favored. In red colobus, the co‐existence of three divergent mitochondrial haplogroups suggests that the Cantanhez population contains a secondary contact zone between divergent lineages that evolved in allopatry. Female‐biased dispersal in this species may be a major factor contributing to the colonization by such differentiated mitochondrial lineages in the region. Overall, we find evidence for a spatio‐temporal change in the dispersal patterns of the colobus monkeys of Cantanhez, with mitochondrial DNA indicating dispersal by mainly a single sex and microsatellite data suggesting that recently both sexes appear to be dispersing within the population. Am J Phys Anthropol, 2013. © 2013 Wiley Periodicals, Inc.     

Modeling monkeys: A comparison of computer-generated and naturally occurring foraging patterns in two species of neotropical primates

We present a series of computer-generated foraging models (random movement, olfactory navigation, and spatial memory) designed to examine the manner in which sensory cues and cognitive skills might be used by rainforest monkeys to locate patchily distributed feeding sites. These simulations are compared with data collected in the Amazon Basin of northeastern Peru on the foraging patterns of two species of neotropical primates, the moustached tamarin monkey (Saguinus mystax) and the saddle-back tamarin monkey (Saguinus fuscicollis). The results indicate that, although tamarins may rely on olfactory cues to locate nearby feeding sites, their foraging patterns are better explained by an ability to maintain a detailed spatial map of the location and distribution of hundreds of feeding trees in their home range. There is evidence that such informationis retained for a period of at least several weeks and is used to minimize the distance traveled between widely scattered feeding sites. The use of computer simulations provides a powerful research tool for generating predictive models regarding the role of memory and sensory cues in animal foraging patterns.     

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.     

Spatial memory shapes migration and its benefits: evidence from a large herbivore

From fine‐scale foraging to broad‐scale migration, animal movement is shaped by the distribution of resources. There is mounting evidence, however, that learning and memory also guide movement. Although migratory mammals commonly track resource waves, how resource tracking and memory guide long‐distance migration has not been reconciled. We examined these hypotheses using movement data from four populations of migratory mule deer (n = 91). Spatial memory had an extraordinary influence on migration, affecting movement 2–28 times more strongly than tracking spring green‐up or autumn snow depth. Importantly, with only an ability to track resources, simulated deer were unable to recreate empirical migratory routes. In contrast, simulated deer with memory of empirical routes used those routes and obtained higher foraging benefits. For migratory terrestrial mammals, spatial memory provides knowledge of where seasonal ranges and migratory routes exist, whereas resource tracking determines when to beneficially move within those areas.     

Modern termites inherited the potential of collective construction from their common ancestor

Animal collective behaviors give rise to various spatial patterns, such as the nests of social insects. These structures are built by individuals following a simple set of rules, slightly varying within and among species, to produce a large diversity of shapes. However, little is known about the origin and evolution of the behavioral mechanisms regulating nest structures. In this study, we discuss the perspective of inferring the evolution of collective behaviors behind pattern formations using a phylogenetic framework. We review the collective behaviors that can be described by a single set of behavioral rules, and for which variations of the environmental and behavioral parameter values produce diverse patterns. We propose that this mechanism could be at the origin of the pattern diversity observed among related species, and that, when they are placed in the proper conditions, species have the behavioral potential to form patterns observed in related species. The comparative analysis of shelter tube construction by lower termites is consistent with this hypothesis. Although the use of shelter tubes in natural conditions is variable among species, most modern species have the potential to build them, suggesting that the behavioral rules for shelter tube construction evolved once in the common ancestor of modern termites. Our study emphasizes that comparative studies of behavioral rules have the potential to shed light on the evolution of collective behaviors.     

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.     

Optimal foraging: movement patterns of bumblebees between inflorescences

Nectar-collecting bumblebees are hypothesized to employ rules of movement which result in the maximum net rate of energy gain (i.e., are optimal). The optimal movement rules are derived from a mathematical model and are used to generate predicted patterns of movement. The predicted patterns are compared with field observations. These observations support the hypothesis. An important component of the mathematical model is the memory of the foraging animal. The field data have implications concerning the memory capabilities of the bumblebees.     

Where to catch a fish? The influence of foraging tactics on the ecology of bottlenose dolphins (Tursiops truncatus) in Florida Bay,Florida

Observations of bottlenose dolphins ( Tursiops truncatus ) in Florida Bay, Florida, between 2002 and 2005 revealed the use of three distinct foraging tactics. The goal of this study was to identify ecological correlates with tactic use and describe the impact of foraging specializations on the overall habitat use and distribution patterns of this dolphin population. Foraging tactics showed strong association with contrasting environmental characteristics, primarily depth. Locations of two of these tactic groups were spatially repulsed. Analyses of sighting histories of individual dolphins observed at foraging events determined that dolphins which employed one tactic never employed the other, and vice versa . Although bottlenose dolphins have plastic foraging behaviors, dolphins in Florida Bay appear to specialize in one tactic and subsequently limit their overall distribution patterns to coincide with habitats that facilitate success using that foraging tactic. This study demonstrates how foraging behavior can be an ecological determinant of overall dolphin habitat use patterns and works to create spatial structure within a population due to consistent mapping of tactics onto environmental variation. These foraging specializations potentially impact the social and demographic patterns of this dolphin population. The possible evolutionary mechanisms behind this intraspecific variation, including resource limitation and social learning, are considered.     

Modeling Behavior by Coastal River Otter (Lontra Canadensis) in Response to Prey Availability in Prince William Sound,Alaska: A Spatially-Explicit Individual-Based Approach

Effects of climate change on animal behavior and cascading ecosystem responses are rarely evaluated. In coastal Alaska, social river otters (Lontra Canadensis), largely males, cooperatively forage on schooling fish and use latrine sites to communicate group associations and dominance. Conversely, solitary otters, mainly females, feed on intertidal-demersal fish and display mutual avoidance via scent marking. This behavioral variability creates “hotspots” of nutrient deposition and affects plant productivity and diversity on the terrestrial landscape. Because the abundance of schooling pelagic fish is predicted to decline with climate change, we developed a spatially-explicit individual-based model (IBM) of otter behavior and tested six scenarios based on potential shifts to distribution patterns of schooling fish. Emergent patterns from the IBM closely mimicked observed otter behavior and landscape use in the absence of explicit rules of intraspecific attraction or repulsion. Model results were most sensitive to rules regarding spatial memory and activity state following an encounter with a fish school. With declining availability of schooling fish, the number of social groups and the time simulated otters spent in the company of conspecifics declined. Concurrently, model results suggested an elevation of defecation rate, a 25% increase in nitrogen transport to the terrestrial landscape, and significant changes to the spatial distribution of “hotspots” with declines in schooling fish availability. However, reductions in availability of schooling fish could lead to declines in otter density over time.     

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.     

From random walks to informed movement

The analysis of animal movement is a large and continuously growing field of research. Detailed knowledge about movement strategies is of crucial importance for understanding eco‐evolutionary dynamics at all scales – from individuals to (meta‐)populations. This and the availability of detailed movement and dispersal data motivated Nathan and colleagues to published their much appreciated call to base movement ecology on a more thorough mechanistic basis. So far, most movement models are based on random walks. However, even if a random walk might describe real movement patterns acceptably well, there is no reason to assume that animals move randomly. Therefore, mechanistic models of foraging strategies should be based on information use and memory in order to increase our understanding of the processes that lead to animal movement decisions. We present a mechanistic movement model of an animal with a limited perceptual range and basic information storage capacities. This ‘spatially informed forager’ constructs an internal map of its environment by using perception, memory and learned or evolutionarily acquired assumptions about landscape attributes. We analyse resulting movement patterns and search efficiencies and compare them to area restricted search strategies (ARS) and biased correlated random walks (BCRW) of omniscient individuals. We show that, in spite of their limited perceptual range, spatially informed individuals boost their foraging success and may perform much better than the best ARS. The construction of an internal map and the use of spatial information results in the emergence of a highly correlated walk between patches and a rather systematic search within resource clusters. Furthermore, the resulting movement patterns may include foray search behaviour. Our work highlights the strength of mechanistic modelling approaches and sets the stage for the development of more sophisticated models of memory use for movement decisions and dispersal.     

Individual-level personality influences social foraging and collective behaviour in wild birds

There is increasing evidence that animal groups can maintain coordinated behaviour and make collective decisions based on simple interaction rules. Effective collective action may be further facilitated by individual variation within groups, particularly through leader–follower polymorphisms. Recent studies have suggested that individual-level personality traits influence the degree to which individuals use social information, are attracted to conspecifics, or act as leaders/followers. However, evidence is equivocal and largely limited to laboratory studies. We use an automated data-collection system to conduct an experiment testing the relationship between personality and collective decision-making in the wild. First, we report that foraging flocks of great tits (Parus major) show strikingly synchronous behaviour. A predictive model of collective decision-making replicates patterns well, suggesting simple interaction rules are sufficient to explain the observed social behaviour. Second, within groups, individuals with more reactive personalities behave more collectively, moving to within-flock areas of higher density. By contrast, proactive individuals tend to move to and feed at spatial periphery of flocks. Finally, comparing alternative simulations of flocking with empirical data, we demonstrate that variation in personality promotes within-patch movement while maintaining group cohesion. Our results illustrate the importance of incorporating individual variability in models of social behaviour.     

Ecology of the Zanzibar Red Colobus Monkey: Demographic Variability and Habitat Stability

We examined the Zanzibar red colobus' (Procolobus kirkii) social structure and population dynamics in relation to the density, diversity and dispersion of food resources in ground-water forest and agricultural land, which we characterized in terms of red colobus food species density, diversity, basal area and dispersion. We used transect sampling and group follows to describe population dynamics and social systems. Two agricultural areas, SJF Shamba and Pete Village, had higher densities and more uniformly dispersed red colobus food tree species than those of the ground-water forest. Red colobus at these two sites had greater population densities and natality, and smaller home ranges than red colobus in the ground-water forest. However, these findings apply to a very small area of agricultural land (approximately 18 ha) that is contiguous with an area of the forest reserve having a high density of red colobus. It is not representative of agricultural areas elsewhere on Zanzibar which support much lower densities or no red colobus. Although agricultural areas contiguous with the forest reserve had high densities of red colobus, they appear to be very unstable. Within the agricultural areas, we observed higher intergroup variation in group size and composition, study groups that decreased dramatically in size and disappeared from the study site, significantly lower levels of juvenile recruitment, and red colobus food trees that exhibited definite signs of overbrowsing. This apparent instability in the subpopulation of red colobus utilizing agricultural systems probably reflects the lower basal area of food trees and the greater fragmentation of suitable habitat and floristic dynamics due to human activities in these areas. A fusion-fission social system occurred only in the ground-water forest subpopulation, which we hypothesize to be due to highly clumped food resources.