The birth of a wild mountain gorilla (Gorilla gorilla beringei)

The live birth of a wild mountain gorilla is described. Details of the birth process are given along with the mother’s periparturitional behaviour and the responses of others to the birth. The mother, a multiparous female, gave no observable indication that birth was imminent, and delivered her infant quickly and easily. During parturition, a subadult male showed interest in the proceedings. Following delivery, several animals approached the mother, who then left the birth site. The mother cleaned her infant and by two hours postpartum had established ventro-ventral contact and, possibly, nursing. These data are related to relevant observations from the literature and unpublished data on other animals from this study.     

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.     

Observations on a small group of mountain gorillas (Gorilla gorilla beringei).

On the basis of a 5-month field study, data are presented on home range, patterns of daily movement and social behaviour in a small free-living gorilla group. Changes in group composition that occurred during the study are analysed with special reference to their effect on inter-individual distance. The relationships with other groups and lone individuals whose home ranges overlapped with that of the group studied are examined on the basis of intergroup contacts.     

Fracture toughness of mountain gorilla (Gorilla gorilla beringei) food plants

Mountain gorillas, the largest extant primates, subsist almost entirely on plant matter. Moreover, their diet includes a substantial amount of structural material, such as bark and stems, which other primates tend to avoid. Accordingly, the robust masticatory apparatus of gorillas may be adaptive to this presumably tough diet; however, quantitative information on this subject is lacking. In this study the fracture toughness of mountain gorilla foods was examined for the first time. Samples of 44 food plants from Bwindi-Impenetrable National Park (BINP) and Mgahinga Gorilla National Park (MGNP) were tested. These parks are inhabited by two gorilla populations that regarded by some as being distinct at the subspecific taxonomic level. Although food toughness did not differ between the two populations, both diets contained tough items. Tree barks were the toughest food items (varying from 0.23 to 8.2 kJ/m2), followed by shrub barks, pith, and stems. The toughness of leaves and fruit was negligible compared to that of bark. The toughness of bamboo was low in comparison to the toughest food items. Accordingly, the prominent toughness of bark, pith, and stems may be key factors in the evolution of orofacial robusticity in mountain gorillas.     

Activity rhythm and the ranging of a solitary male mountain gorilla (Gorilla gorilla beringei)

To clarify the advantages of solitary life in gorilla males, a lone silverback mountain gorilla (Gorilla gorilla beringei) was studied for nine months in the natural habitat of the Virunga volcanoes. While the time budget for each activity and daily activity cycle were similar to those of groups, his daily journey distance and ranging patterns differed from those of groups. His movements were little influenced by the distribution and abundance of foods, which strongly influence the movements of groups. He notably increased his day journey distances when he encountered neighboring groups. He persistently followed the groups for days and went out of his usual range area. These encounters shifted his monthly range from his natal group's range to that of other groups. When the silverbacks of the encountered groups noticed his presence, they usually gave hoots and chest-beats and sometimes fought violently with him, while females and immatures did not show positive responses towards him. Lone males could have more chance to contact females and to lure them away from their groups than silverbacks within groups. The lone male stage, accompanied by frequent contacts with different groups, probably provides maturing males with useful knowledge of neighboring groups and areas.     

Comparison of arthritis characteristics in lowland Gorilla gorilla and mountain Gorilla beringei

Gorilla gorilla and the less-studied G. beringei occupy very different, geographically separate habitats. We studied the occurrence of various forms of arthritis to examine possible nature/nurture causality. The macerated skeletons of 38 G. beringei and 99 G. gorilla individuals were examined macroscopically for the presence of articular and osseous pathologies. Contrasting with only isolated osteoarthritis and infectious arthritis was the frequent occurrence of a form of erosive arthritis associated with joint fusion. Twenty-one percent of the G. beringei and 20% of G. gorilla specimens were afflicted, which are statistically indistinguishable frequencies. While both had prominent axial disease, they differed in patterns of peripheral arthritis. Whereas G. beringei showed a pauciarticular pattern, the pattern in G. gorilla was more often polyarticular. Susceptibility to spondyloarthropathy was apparently genetically imprinted before Gorilla separated into G. gorilla and G. beringei. However, the different patterns of peripheral joint involvement suggest a causality resulting from lifestyle (e.g., the presence/absence or extent of knuckle walking) or a habitat-related infectious agent.     

Brain structure variation in great apes, with attention to the mountain gorilla (Gorilla beringei beringei)

This report presents data regarding the brain structure of mountain gorillas (Gorilla beringei beringei) in comparison with other great apes. Magnetic resonance (MR) images of three mountain gorilla brains were obtained with a 3T scanner, and the volume of major neuroanatomical structures (neocortical gray matter, hippocampus, thalamus, striatum, and cerebellum) was measured. These data were included with our existing database that includes 23 chimpanzees, three western lowland gorillas, and six orangutans. We defined a multidimensional space by calculating the principal components (PCs) from the correlation matrix of brain structure fractions in the well-represented sample of chimpanzees. We then plotted data from all of the taxa in this space to examine phyletic variation in neural organization. Most of the variance in mountain gorillas, as well as other great apes, was contained within the chimpanzee range along the first two PCs, which accounted for 61.73% of the total variance. Thus, the majority of interspecific variation in brain structure observed among these ape taxa was no greater than the within-species variation seen in chimpanzees. The loadings on PCs indicated that the brain structure of great apes differs among taxa mostly in the relative sizes of the striatum, cerebellum, and hippocampus. These findings suggest possible functional differences among taxa in terms of neural adaptations for ecological and locomotor capacities. Importantly, these results fill a critical gap in current knowledge regarding great ape neuroanatomical diversity.     

Geophagia by mountain gorillas (Gorilla gorilla beringei) in the Virunga Mountains,Rwanda

Mountain gorillas (Gorilla gorilla beringei) occasionally eat material from weathered regolith (subsoil) sediments in the Virunga Mountains of northwestern Rwanda. The possible nutritional significance of this behaviour has been investigated by analyzing the geochemistry, primary mineral composition, and clay content of several regolith and surface soil (paleosol) samples. Iron, Na, and Br content may be important in geophagy, and clay present in the soil may also have nutritional importance.     

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.     

The influence of male mating tactics on habitat use in mountain gorillas (Gorilla gorilla beringei)

Previous research has shown that both ecological and social factors influence mountain gorilla habitat use. New data on habitat use by a male gorilla and by a group confirm that male mating competition influences short- and long-term habitat use patterns, and show that its influence can supersede that of ecological factors on a long-term basis. When solitary, the male regularly approached and sometimes followed groups. His monthly home range size and equitability of home range use were directly proportional to the number of such interactions per month. His relationships with other groups became more conservative after he gained females, and, contrary to expectations based on metabolic needs, he used a much smaller area. The group considered here gradually expanded its home range and shifted its areas of intensive use throughout a three-year period. It then made a complete home range shift after three dramatic interactions, during which it was temporarily fragmented and two females emigrated. The group shared its home range with many other social units; overlap with most of these decreased after the shift. The degree of overlap and the lack of site fidelity by males and their groups support the argument that transfer is not ecologically costly to mountain gorilla females.