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The golden monkey, Cercopithecus mitis kandti, an Endangered subspecies of blue monkey, is found in only two small populations in the central part of the Albertine Rift region in Central/East Africa. Since the 1950s, the habitat of golden monkeys has been lost or degraded in the Virunga massif (Democratic Republic of the Congo, Rwanda, Uganda), including a reduction of approximately 50% in the Volcanoes National Park (VNP), the Rwandan part of the Virunga massif, and by a staggering 98% in the Gishwati forest located in Rwanda. As human disturbance continues within the golden monkey habitat, pressure on the remaining small and fragmented populations may result in the extinction of this subspecies. Consequently, a regional conservation action plan was developed in collaboration with conservationists, park managers, researchers, and local communities from the three countries where golden monkeys occur to address threats to the species.

Group-living animals face a number of threats from extragroup conspecifics: from individuals seeking mating opportunities to rival groups attempting to access limited resources. The consequences of intergroup interactions can therefore include loss of mates, increased energy expenditure, and injury or death. There is increasing evidence that aggressive intergroup interactions can affect subsequent intragroup behavior, and that such post-conflict behavior may be directly related to the threats posed by different opponent groups (e.g., familiar vs. unfamiliar). However, empirical evidence for changes in intragroup behavior following intergroup conflict in social animals is limited. We compared the proximity and behavior of group members before and after 84 aggressive intergroup interactions involving 14 study groups of wild, habituated mountain gorillas (Gorilla beringei beringei) in Volcanoes National Park, Rwanda. Intergroup conflict affected subsequent intragroup behavior in several ways. We found that immediate movement away from the opponent following intergroup conflict was less likely between familiar groups compared to unfamiliar groups. However, both winning and losing groups spent twice as much time moving post-conflict, and losing groups spent less time resting. There was an increase in female intragroup affiliative interactions and a decrease in male intragroup agonistic interactions post-conflict. These results demonstrate that such intergroup contests can influence intragroup dynamics beyond the immediate period of interaction.

As mountain gorilla density increases in an isolated forest, violent encounters between groups cause infant deaths., Density-dependent processes such as competition for resources, migration, predation, and disease outbreaks limit the growth of natural populations. The analysis of 50 years of mountain gorilla data reveals that social behavior changes observed at high group density may also affect population growth in social species. A sudden increase in social group density observed in 2007 caused a threefold increase in the rate of violent encounters between social units (groups and solitary males). A fivefold increase in the rate of infanticide and seven cases of lethal fights among mature males were subsequently recorded, and the annual subpopulation growth rate declined by half between 2000 and 2017. The increase in infanticide alone explains 57% of this decline. These findings highlight the complex relationship between population density and growth in social species and hold important implications for the management of island populations.

Conservation efforts have led to the recovery of the endangered mountain gorilla populations. Due to their limited potential for spatial expansion, population densities increased, which may alter the epidemiology of infectious diseases. Recently, clinical gastrointestinal illnesses linked to helminth infections have been recorded in both gorilla populations. To understand drivers and patterns of helminth infections we quantified strongylid and tapeworm infections across both Virunga Massif and Bwindi populations using fecal egg counts. We assessed the impact of age, sex, group size, season and spatial differences used as a proxy, which reflects observed variation in the occurrence of gastrointestinal problems, vegetation types, gorilla subpopulation growth and associated social structure on helminth infections. We revealed striking geographic differences in strongylid infections with higher egg counts mostly in areas with high occurrences of gastrointestinal disease. Increased helminth egg counts were also associated with decreasing group size in some areas. Observed spatial differences may reflect mutual effects of variations in subpopulation growth rates, gorilla social structure, and vegetation associated with altitude across mountain gorilla habitat. Helminth infection intensities in Virunga gorillas were lowest in the youngest and the oldest animals. Elucidating parasite infection patterns of endangered species with low genetic diversity is crucial for their conservation management.

The future of primates depends on conservationists in primate range countries having the education and opportunities needed to facilitate conservation efforts. However, most primates are found in countries where conservation is underfunded. Rwanda is home to 14 primate species, with three being listed as endangered on the International Union for Conservation of Nature (IUCN) Red List. Their remaining native habitats are scarce, highly fragmented, and surrounded by high human densities. Lacking education opportunities and the loss of qualified individuals following decades of political instability created a need for well-trained conservationists to tackle conservation challenges in Rwanda. The Memoirs Program, a science capacity-building partnership between Dian Fossey Gorilla Fund (DFGF), University of Rwanda (UR), the Rwanda Development Board, and Cleveland Metroparks Zoo, aims to train the next generation of Rwandan conservationists. Annually, the Memoirs Program provides 10-12 top students from the UR Conservation Biology undergraduate program the opportunity to conduct senior research projects (Memoirs) at DFGF addressing conservation issues in and around Rwanda's protected areas. Students receive one-on-one supervision by DFGF scientists and affiliates to supplement UR mentorship, along with targeted teaching modules that reinforce and complement UR curricula. In continuation of the Memoirs Program, participants are offered professional development, employment opportunities, and support to publish their research in peer-reviewed journals. Since inception of the Program in 2003, 123 students have participated with 83% being employed or pursuing higher education degrees in conservation-related fields in Rwanda and the region. Pre- and post-training comparison of oral presentation skills, one of the teaching modules, involving 22 students demonstrated increased post-training performance. The Memoirs Program represents an effective partnership between the government, an international conservation non-governmental organization, and a public higher-learning institution in a developing country to support high-performing students to become tomorrow's conservation leaders. This model could be replicated within Rwanda and in other regions of the world.

In group-living species, encounters with extragroup rivals can be one of the riskiest actions in which individuals participate. Different group members often have different incentives to participate during intergroup interactions, and individuals with fewer payoffs of competition, including those of the smaller sex and/or lower rank, may ‘free-ride’ to avoid the costs of conflict. However, there is little evidence for how different types of intergroup interactions (e.g. interactions that do not involve conflict) can influence the participation of individuals. We examined the ecological, demographic and social predictors of individual participation in interactions between 14 fully habituated mountain gorilla groups in Volcanoes National Park, Rwanda from 2003 to 2015. The probability of an individual participating decreased with group size but remained relatively high in aggressive interactions and in multimale groups, illustrating the potential for ‘load lightening’ among group members. Males with fewer mating opportunities participated less often than males with more mating opportunities; however, male participation was significantly higher than female participation across all types of intergroup interactions. Females were more likely to be involved in aggressive interactions with solitary males, possibly to avoid the potential cost of infanticide if a resident male is killed or injured. Both sexes demonstrated more affiliative behaviours towards familiar groups, indicating a benefit of maintaining social relationships with familiar groups. Individuals show considerable variation in behaviour during intergroup interactions, and our results suggest that this variation is primarily driven by intergroup familiarity and individual reproductive benefits, both of which may have long-term consequences for individual fitness.

Eastern gorillas ( Gorilla beringei ) are among the few mammal species that seasonally consume large quantities of young bamboo shoots, which are a rich source of energy. Here, we document how the consumption of bamboo shoots coincides with changes in behavior of adult mountain gorillas ( Gorilla beringei beringei ) monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center in Volcanoes National Park, Rwanda. We offer a preliminary analysis of possible mechanisms underlying this behavioral change by measuring energy intake rates and the presence of cyanide and alcohol—ingredients potentially associated with increased activity levels—in fresh bamboo shoots. The percentage of bamboo shoots in the diet of gorillas was correlated positively with the rate of play behavior shown by adults in 2 of the 3 study groups. Play behavior was not the result of better weather conditions and also did not reflect the availability of spare time. Rather, there is some, but not consistent, evidence for a link between energy intake rates and play behavior of adults. Cyanide was not detected in young bamboo shoots, and the presence of alcohol remains inconclusive, albeit unlikely. In sum, our results show that consumption of a high-quality food can have a direct influence on the activity budget (and by extrapolation energy expenditure) of mountain gorillas through increased rates of play behavior. However, the physiological aspects underlying this elevated activity warrant further investigation.

The ability to accurately measure morphological characteristics of wild primates in the field is challenging, yet critical for understanding fundamental aspects of their biology and behavior. Recent studies have shown that digital photogrammetry can be used to non-invasively measure morphological traits of wild primates, as it allows for the determination of geometric properties of objects remotely from photographic images. We report here on a rare opportunity to test this methodology by comparing measurements obtained directly from living great apes to those obtained from photographs. We test the accuracy and precision of two independent photogrammetric techniques, employing the use of parallel lasers and a distance meter, respectively, for obtaining measurements of static objects and captive western lowland gorillas (Gorilla gorilla gorilla) (n = 4) at Zoo Atlanta. For static objects, the mean percent error between corresponding measurements collected by the same observer directly versus using photogrammetry was 0.49-0.74% for the parallel laser method and 0.62-0.76% for the distance meter method. For gorillas, mean percent error between corresponding direct and remote measurements was 2.72-5.20% for the parallel laser method and 2.20-7.51% for the distance meter method. Correlations between direct measurements and corresponding parallel laser and distance meter measurements of gorillas were highly significant with R2 values and slopes approaching 1.0 (parallel lasers: R2 = 0.9989, P

Socioecological models predict that contest competition will arise when high quality foods can be usurped or monopolized, leading to more favorable energy balances and higher reproductive success for high-ranking females. Gorillas are interesting species for studying such predictions due to the variety of ecological conditions that they experience in different locations. Using data from 23 female mountain gorillas in 3 social groups in the Virunga Massif, we examined food characteristics that may influence contest competition (food site residence times [FSRT]), proximate mechanisms of such competition (aggression and avoidance), and potential consequences of competition (rank-related differences in energy intake rates, travel expenditures, and activity budgets). The average FSRT of each female was significantly correlated with dominance rank, which suggests that high-ranking females may have greater access to foods that are easier to usurp (as predicted with contest competition). High-ranking females were significantly more aggressive than low-ranking females, and both aggression and avoidance were significantly higher while feeding than during other activities. Contrary to predictions for contest competition, however, rank was not significantly correlated with energy intake rates nor with the proportion of time spent traveling versus feeding. Thus, we did not find any energetic benefits to explain why high-ranking females had significantly higher reproductive success in earlier studies. We propose several alternative explanations and discuss the potential complications of assessing contest competition in species with weak dominance relationships.

The effect of feeding competition on foraging efficiency is an important link between ecological factors and the social organization of gregarious species. We examined the effects of group size on daily travel distances, activity budgets, and energy intake of mountain gorillas in Rwanda. We measured daily travel distances of five groups, activity budgets of 79 gorillas in nine groups, and energy intake data for 23 adult females in three groups over a 16-month period. Travel distances and the proportion of time spent traveling increased with size for most groups, which would be expected if their foraging efficiency is limited by intragroup feeding competition. However, travel distances and times decreased for the largest group, which also had higher energy intake rates than intermediate sized groups. The improved foraging efficiency of the largest group may be explained by advantages in intergroup contest competition. The largest group had much lower home range overlap than the other study groups which may be due to groups avoiding one another as a result of male mating competition. Collectively, our results indicate that intermediate sized groups had the lowest foraging efficiency and provide a new twist on the growing evidence of non-linear relationships between group size and foraging efficiency in primates.

Intergroup interactions in social animals can vary from hostile to affiliative and may be influenced by factors such as competitive ability, resource values and existing intergroup relationships. Despite the potential for intergroup interactions to affect individual fitness and group stability, few studies have comprehensively tested how social, demographic and ecological factors may simultaneously influence intergroup interactions. Using 13 years of continuous data on intergroup interactions (n = 464), group composition, range use and diet, we investigated the factors that influenced the initiation and escalation of intergroup interactions in a fully habituated subpopulation of mountain gorillas in Volcanoes National Park, Rwanda. The majority of interactions were non-physically agonistic (57%), while peaceful exchanges and physical aggression were less common (18% and 25% of interactions respectively). Solitary males and young dominant silverbacks were the most likely to initiate an interaction, presumably because these males have the highest incentive to attract mates. Aggressive interactions between a group and a solitary male involved a high number of participating group members, reflecting the incentive to avoid injury and infanticide associated with solitary male encounters. Aggression between social groups escalated when groups were similarly sized, perhaps because these groups have similar competitive abilities. Peaceful intergroup interactions most commonly involved opponents that contained familiar and related individuals, suggesting that the short dispersal distance of gorillas may facilitate kin-selected intergroup tolerance. Variation and plasticity in gorilla behaviour during intergroup interactions are therefore dependent on the opponent’s familiarity and threat level. Intergroup interactions can vary from aggressive physical disputes to tolerant intermingling among extra-group individuals. Competition for access to limiting resources can often influence the occurrence of aggressive intergroup competition, but may not be sufficient to describe peaceful mingling between groups in social species. We found that although the main driver of intergroup interactions may be mating competition between males, interactions between familiar social groups were significantly more peaceful than interactions involving unfamiliar groups. These findings suggest that maintaining social and kin relationships with neighbouring groups may act as a strategy for reducing conflict in group-structured social species.

Since the 1980s, the Virunga mountain gorilla population has almost doubled, now reaching 480 individuals living in a 430-km2 protected area. Analysis of the gorillas' ranging patterns can provide critical information on the extent and possible effects of competition for food and space. We analyzed 12 years of daily ranging data and inter-group encounter data collected on 11 gorilla groups monitored by the Karisoke Research Center in Rwanda. During that period, the study population increased in size by almost 50% and the number of groups tripled. Groups had small yearly home ranges compared to other known gorilla populations, with an average 90% kernel density estimate of 8.07 km2 and large between-group variations (3.17–23.59 km2). Most groups had consistent home range location over the course of the study but for some, we observed gradual range shifts of up to 4 km. Neighboring groups displayed high home range overlap, which increased dramatically after the formation of new groups. On average, each group used only 28.6% of its 90% kernel home range exclusively, and in some areas up to six different groups had overlapping home ranges with little or no exclusive areas. We found a significant intra-group positive relationship between the number of weaned individuals in a group and the home range size, but the fitted models only explained 17.5% and 13.7% of the variance in 50% and 90% kernel home range size estimates, respectively. This suggests that despite the increase in size, the study population is not yet experiencing marked effects of feeding competition. However, the increase in home range overlap resulting from the formation of new groups led to a sixfold increase in the frequency of inter-group encounters, which exposes the population to elevated risks of fight-related injuries and infanticide. Am. J. Primatol. 76:730–746, 2014. © 2014 Wiley Periodicals, Inc.

Objectives Great apes show considerable diversity in socioecology and life history, but knowledge of their physical growth in natural settings is scarce. We characterized linear body size growth in wild mountain gorillas from Volcanoes National Park, Rwanda, a population distinguished by its extreme folivory and accelerated life histories. Methods In 131 individuals (0.09–35.26 years), we used non-invasive parallel laser photogrammetry to measure body length, back width, arm length and two head dimensions. Nonparametric LOESS regression was used to characterize cross-sectional distance and velocity growth curves for males and females, and consider links with key life history milestones. Results Sex differences became evident between 8.5 and 10.0 years of age. Thereafter, female growth velocities declined, while males showed increased growth velocities until 10.0–14.5 years across dimensions. Body dimensions varied in growth; females and males reached 98% of maximum body length at 11.7 and 13.1 years, respectively. Females attained 95.3% of maximum body length by mean age at first birth. Neonates were 31% of maternal size, and doubled in size by mean weaning age. Males reached maximum body and arm length and back width before emigration, but experienced continued growth in head dimensions. Conclusions While comparable data are scarce, our findings provide preliminary support for the prediction that mountain gorillas reach maximum body size at earlier ages compared to more frugivorous western gorillas. Data from other wild populations are needed to better understand comparative great ape development, and investigate links between trajectories of physical, behavioral, and reproductive maturation.

The robust masticatory system of mountain gorillas is thought to have evolved for the comminution of tough vegetation, yet, compared to other primates, the toughness of the mountain gorilla diet is unremarkable. This may be a result of low plant toughness in the mountain gorilla environment or of mountain gorillas feeding selectively on low-toughness foods. The goal of this paper is to determine how the toughness of the mountain gorilla diet varies across their habitat, which spans a large altitudinal range, and whether there is a relationship between toughness and food selection by mountain gorillas. We collected data on the following variables to determine whether, and if so how, they change with altitude: leaf toughness of two plant species consumed by mountain gorillas, at every 100 m increase in altitude (2,600-3,700 m); toughness of consumed foods comprising over 85% of the gorilla diet across five vegetation zones; and toughness of unconsumed/infrequently consumed plant parts of those foods. Although leaf toughness increased with altitude, the toughness of the gorilla diet remained similar. There was a negative relationship between toughness and consumption frequency, and toughness was a better predictor of consumption frequency than plant frequency, biomass, and density. Consumed plant parts were less tough than unconsumed/infrequently consumed parts and toughness of the latter increased with altitude. Although it is unclear whether gorillas select food based on toughness or use toughness as a sensory cue to impart other plant properties (e.g., macronutrients, chemicals), our results that gorillas maintain a consistent low-toughness dietary profile across altitude, despite toughness increasing with altitude, suggest that the robust gorilla masticatory apparatus evolved for repetitive mastication of foods that are not high in toughness.

We examined demographic and behavioural influences on the dispersal decisions of 31 male mountain gorillas living in mixed-sex groups. Approximately half (45%) of silverback males dispersed from the groups in which they matured. Mean dispersal age corresponded with the age of physical maturity. For two-thirds (64%) of males, dispersal was a one time decision. The remaining individuals made repeated visits to their natal groups before permanently dispersing. Of dispersing males, 43% left with females. The remaining individuals dispersed alone, despite the presence of similar-aged male dispersal partners in their group. Dispersal decisions appeared voluntary; there was no evidence of increased aggression by the dominant silverback or peripheralization in the months prior to dispersal. Instead, dispersal decisions were influenced by limited opportunities to copulate within the group and the absence of a male's mother. Dispersing did not provide increased access to females, at least in the short term; only three dispersing males were observed to form groups whereas the remaining eight for whom data were available remained solitary (range 1–112 months) at the end of the study. The question remains why males choose to disperse rather than remain philopatric. Inbreeding avoidance seems unlikely, as all groups contained unrelated females. It may be that dispersal decisions are the result of several factors acting in combination, and thus dispersal is a more appropriate option for some males. Alternatively, dispersal decisions may actually be disadvantageous and reflect an incomplete adaptation to the ecological extremes of the Virunga region.

We mapped and measured all of the Adansonia rubrostipa trees in 40 ha of dry deciduous forest in Kirindy Forest, Menabe, western Madagascar. Survey effort was split between three compartments which had been heavily logged or selectively logged for timber trees, not including A.rubrostipa,inthe1980s,orwhichremainedunlogged.We recorded304treesranginginstemdiameteratbreastheight from 5 to 221 cm. Trees were most abundant and the majority of regeneration was recorded in the unlogged compartment. Regeneration was generally poor in the two logged compartments. We suggest that seed predation and poor seed dispersal contribute to a generally ageing population, and that the dense shrub cover resulting from disturbance during logging impedes seedling establishment.

The ability to accurately measure morphological characteristics of wild primates in the field is challenging, yet critical for understanding fundamental aspects of their biology and behavior. Recent studies have shown that digital photogrammetry can be used to non-invasively measure morphological traits of wild primates, as it allows for the determination of geometric properties of objects remotely from photographic images. We report here on a rare opportunity to test this methodology by comparing measurements obtained directly from living great apes to those obtained from photographs. We test the accuracy and precision of two independent photogrammetric techniques, employing the use of parallel lasers and a distance meter, respectively, for obtaining measurements of static objects and captive western lowland gorillas (Gorilla gorilla gorilla) (n = 4) at Zoo Atlanta. For static objects, the mean percent error between corresponding measurements collected by the same observer directly versus using photogrammetry was 0.49-0.74% for the parallel laser method and 0.62-0.76% for the distance meter method. For gorillas, mean percent error between corresponding direct and remote measurements was 2.72-5.20% for the parallel laser method and 2.20-7.51% for the distance meter method. Correlations between direct measurements and corresponding parallel laser and distance meter measurements of gorillas were highly significant with R

Here we report a description of putative tool use in wild mountain gorillas. At the Dian Fossey Gorilla Fund's Karisoke Research Center in Volcanoes National Park (Rwanda), an adult female was observed using a bamboo culm as a ladder for her offspring. This is one of only a few documented cases of possible tool use in wild gorillas, although tool use behavior is commonly observed in captive gorillas. Although the behavior observed may have been incidental, the present report in combination with previous evidence (Breuer et al., 2005) suggests that tool use in gorillas occurs outside the context of food acquisition and may be directed at overcoming physical obstacles in complex rugged microhabitats such as bamboo forest and swamps.

Since the 1980s, the Virunga mountain gorilla population has almost doubled, now reaching 480 individuals living in a 430-km(2) protected area. Analysis of the gorillas' ranging patterns can provide critical information on the extent and possible effects of competition for food and space. We analyzed 12 years of daily ranging data and inter-group encounter data collected on 11 gorilla groups monitored by the Karisoke Research Center in Rwanda. During that period, the study population increased in size by almost 50% and the number of groups tripled. Groups had small yearly home ranges compared to other known gorilla populations, with an average 90% kernel density estimate of 8.07 km2 and large between-group variations (3.17-23.59 km2). Most groups had consistent home range location over the course of the study but for some, we observed gradual range shifts of up to 4 km. Neighboring groups displayed high home range overlap, which increased dramatically after the formation of new groups. On average, each group used only 28.6% of its 90% kernel home range exclusively, and in some areas up to six different groups had overlapping home ranges with little or no exclusive areas. We found a significant intra-group positive relationship between the number of weaned individuals in a group and the home range size, but the fitted models only explained 17.5% and 13.7% of the variance in 50% and 90% kernel home range size estimates, respectively. This suggests that despite the increase in size, the study population is not yet experiencing marked effects of feeding competition. However, the increase in home range overlap resulting from the formation of new groups led to a sixfold increase in the frequency of inter-group encounters, which exposes the population to elevated risks of fight-related injuries and infanticide.