Feeding ecology of western lowland gorillas in the Nouabalé-Ndoki National Park,Congo

The feeding ecology of western lowland gorillas (Gorilla gorilla gorilla) living in the Nouabalé-Ndoki National Park, northern Congo, was surveyed for one full year. This is the first record to make clear the seasonal changes in the feeding habits of gorillas in a whole year, living in the primary lowland forest almost completely undisturbed. Fecal contents, feeding traces, and direct observation were analyzed with reference to a fruit availability survey. Although the gorillas fed largely on fruits in the forest, their basic diet was fibrous parts of plants, including shoots, young leaves, and bark. Terrestrial herbaceous vegetation, such as monocotyledons of the Marantaceae and aquatic herbs having much protein content and minerals, were frequently eaten even in the fruiting season. As these highly nutritious fibrous foods were superabundant all year, the major foods of the Ndoki gorillas seemed to be those plants. However, they selected fruits as their alternative food resources in the fruiting season. Gorillas foraged on many fruit species, while showing strong preferences for some particular species. The swamp forest, including marshy grasslands, was an important and regular habitat for the Ndoki gorillas.     

Observations on the ontogeny of feeding behavior in mountain gorillas (Gorilla gorilla beringei)

This report describes observations on the ontogeny of food choice in mountain gorillas (Gorilla gorilla beringei), made during a 17-month field study of mountain gorilla feeding ecology in the Parc National des Volcans, Rwanda. Data are presented on the feeding behavior of two infants observed from birth to the age of 8 months and on older infants and juveniles. This information is compared with data on the composition and diversity of the diets of young adults and adults in the same social group. Initial feeding by infants is usually synchronized with the mother's behavior: infants ingest the same food, or a different part of the same plant species, currently being eaten by the mother or just eaten by her. This suggests that observational learning is largely responsible for the transmission of food preferences. Most feeding by young infants, whether or not synchronized with the mother's, is on those foods eaten most frequently by adults. Infants also independently sample potential foods, some of which are apparently not consumed by adults. The frequency of sampling declines with age, although even adults occasionally ingest foods not observed to be eaten by other adults. By the age of 3 years, young mountain gorillas have developed the basic dietary patterns of adults, in terms of the number of foods eaten, the proportions in which specific foods are consumed, and diet diversity and equitability. There is a strong possibility that chemical cues influence food choice, but their role remains unclear.     

Relative growth,ontogeny, and sexual dimorphism in gorilla(Gorilla gorilla gorilla and G. g. beringei): Evolutionary and ecological considerations

Gorillas are the largest and among the most sexually dimorphic of all extant primates. While gorillas have been incorporated in broad-level comparisons among large-bodied hominoids or in studies of the African apes, comparisons between gorilla subspecies have been rare. During the past decade, however, behavioral, morphological, and molecular data from a number of studies have indicated that the western lowland (Gorilla gorilla gorilla) and eastern mountain (Gorilla gorilla beringei) subspecies differ to a greater extent than has been previously believed. In this study I compare patterns of relative growth of the postcranial skeleton to evaluate whether differences between subspecies result from the differential extension of common patterns of relative growth. In addition, patterns of ontogeny and sexual dimorphism are also examined. Linear skeletal dimensions and skeletal weight were obtained for ontogenetic series of male and female G.g. gorilla (n = 315) and G.g. beringei (n = 38). Bivariate and multivariate methods of analysis were used to test for differences in patterns of relative growth, ontogeny, and sexual dimorphism between sexes of each subspecies and in same-sex comparisons between subspecies. Results indicate males and females of both subspecies are ontogenetically scaled for postcranial proportions and that females undergo an earlier skeletal growth spurt compared to males. However, results also indicate that the onset of the female growth spurt occurs at different dental stages in lowland and mountain gorillas and that mountain gorillas may be characterized by higher rates of growth. Finally, data demonstrate lowland and mountain gorilla females do not differ significantly in adult body size, but mountain gorilla males are significantly larger than lowland gorilla males, suggesting mountain gorillas are characterized by a higher degree of sexual dimorphism in body size. Thus, although lowland and mountain gorillas do not appear to have evolved novel adaptations of the postcranium which correlate with differences in locomotor behavior, the present investigation establishes subspecies differences in ontogeny and sexual dimorphism which may be linked with ecological variation. Specifically, these findings are evaluated in the context of risk aversion models which predict higher growth rates and increased levels of sexual dimorphism in extreme folivores. Am. J. Primatol. 43:1–31, 1997. © 1997 Wiley-Liss, Inc.     

Further developments in the breeding population of lowland gorillas at Apenheul

In 1987 a second group of lowland gorillas was formed at Apenheul. Aspects of the zoo-technical management (diet, accommodation, etc.) are described. The management and the development of the population, including mother-rearing of all offspring, are described. Special attention is given to the formation of a second group by introducing two zoo-born females and a non-breeding female from one group to a nonrelated male to form a second group. These transfers of group members resulted rather quickly in pregnancies. The two groups are housed on adjacent islands, and the advantages and disadvantages of such a facility are discussed.     

Occurrence of hepatitis viruses in wild-born non-human primates: a 3 year (1998-2001) epidemiological survey in Gabon

Hepatitis B and C infections are endemic in human population in central Africa, particularly in Gabon. The aim of this study was to determine the prevalence of hepatitis B virus (HBV) and eventual occurrence of hepatitis C virus (HBC)-related strains in a variety of wild-born non-human primates living in Gabon and Congo. Plasma samples were screened for HBV and HCV markers. A non-invasive method of DNA extraction from faeces followed by specific HBV-DNA amplification was developed to study this infection in wild troops of chimpanzees and gorillas. No HCV infection in non-human primates, wild-born or captive, was detected among 596 samples tested. No HBV infection could be detected in samples tested and obtained from Cercopithecidae. In contrast, 14.7 and 42.2% of wild-born chimpanzees in Gabon and Congo were infected with HBV or had evidence of past HBV infection. At Centre International de Recherches Médicales (CIRMF) Primate Centre, 32.1% of chimpanzees and gorillas were HBV positive or had evidence of past infection. In the cases with past infection, 5.9% wild-born and 8.3% at CIRMF harboured HBV-DNA despite the presence of neutralizing HbsAb. Together with previous findings, we confirm the high HBV prevalence not only in humans but also in chimpanzees and gorillas in Gabon and Congo.     

Long-Term Habitat Use by Mountain Gorillas (Gorilla gorilla beringei). 1. Consistency, Variation, and Home Range Size and Stability

Mountain gorillas are highly folivorous. Food is abundant and perennially available in much of their habitat. Still, limited research has shown that single gorilla groups heavily used areas where food biomass and quality were relatively high and where they met daily nutritional needs with relatively low foraging effort. Also, ecological factors influenced solitary males less than groups with females. Long-term data on habitat use by multiple mountain gorilla social units and more extensive data on variation in food distribution, presented here, confirm that food distribution influences areal occupation densities across groups and over time. These data also confirm the group/solitary male distinction and show that food distribution became more important for one male once he acquired females. Groups used 25 km ² , and inter-annual home range and core area overlap was often low. Annual home range and core area size varied considerably within groups and across years. It bore no simple relationship to group size and estimated group biomass. Core areas were biased samples of total home ranges and were relatively good foraging areas. One group abruptly shifted its home range in response to male mating competition. Home ranges of two others expanded from 1981 to 1987, though at a decreasing rate. Data on one such group, which varied considerably in size, are consistent with arguments that costs of scramble competition are low except in unusually large groups. Low site fidelity, low scramble costs, and high home range overlap should decrease the ecological costs of female transfer.     

A preliminary study of selective visual attention in female mountain gorillas (Gorilla gorilla beringei)

Visually attending to conspecifics can give group-living primates important ecological information, help them to anticipate the behavior of others and to regulate interactions with them, and provide other valuable social information. Variation in the importance and quality of social relationships should influence the way individuals selectively attend to fellow group members. Preliminary data on visual monitoring of conspecifics by wild female mountain gorillas (Gorilla gorilla beringei) show that selective attention mirrors variation in social relationships. Social bonds between males and females are central to gorilla society; correspondingly, females are more likely to stop feeding, and focus their attention on males who walk into view than on females, especially when males give dislays. Females are more likely to focus on other females with whom they have antagonistic relationships than those (mostly close relatives) with whom they have affiliative, cooperative ones. Further research on the context and consequences of visual monitoring could help to address questions about the regulation of social relationships and about social cognition in gorillas.     

Gorilla ecology and behavior

Many factors influence the evolution of primate grouping patterns, including phylogeny, demographic and life-history variables, and ecological factors such as access to food, predation pressure, and avoidance of infanticide. The interaction between these factors determines social organization.¹ Because western lowland and mountain gorillas differ so dramatically in their habitats and foraging strategies, they provide a valuable opportunity to assess how changes in ecology influence this balance. Mountain gorillas live in high-altitude montane forests, are herbivorous, and live in stable and cohesive groups. Western lowland gorillas live in lowland rainforest and are much more frugivorous than mountain gorillas. It is not yet clear to what extent incorporating significant quantities of fruit in the diet influences western lowland gorilla sociality because they have been studied much less than have mountain gorillas. However, what is known about their behavior hints that there may also be considerable differences in their social organization, including changes in group size and cohesion and in the frequency and type of intergroup encounters. Gorillas thus provide a unique opportunity to reevaluate proposed models of ecological influences on social organization in African apes.© 1998 Wiley-Liss, Inc.     

Interactions between zoo‐housed great apes and local wildlife

Although there are published reports of wild chimpanzees, bonobos, and orangutans hunting and consuming vertebrate prey, data pertaining to captive apes remain sparse. In this survey‐based study, we evaluate the prevalence and nature of interactions between captive great apes and various indigenous wildlife species that range into their enclosures in North America. Our hypotheses were threefold: (a) facilities housing chimpanzees will report the most frequent and most aggressive interactions with local wildlife; (b) facilities housing orangutans and bonobos will report intermediate frequencies of these interactions with low levels of aggression and killing; and (c) facilities housing gorillas will report the lowest frequency of interactions and no reports of killing local wildlife. Chimpanzees and bonobos demonstrated the most aggressive behavior toward wildlife, which matched our predictions for chimpanzees, but not bonobos. This fits well with expectations for chimpanzees based on their natural history of hunting and consuming prey in wild settings, and also supports new field data on bonobos. Captive gorillas and orangutans were reported to be much less likely to chase, catch and kill wildlife than chimpanzees and bonobos. Gorillas were the least likely to engage in aggressive interactions with local wildlife, matching our predictions based on natural history. However unlike wild gorillas, captive gorillas were reported to kill (and in one case, eat) local wildlife. These results suggest that some behavioral patterns seen in captive groups of apes may be useful for modeling corresponding activities in the wild that may not be as easily observed and quantified. Furthermore, the data highlight the potential for disease transmission in some captive settings, and we outline the associated implications for ape health and safety. Am. J. Primatol. 71:458–465, 2009. © 2009 Wiley‐Liss, Inc.