Physical maturation,life‐history classes and age estimates of free‐ranging western gorillas—insights from Mbeli Bai,Republic of Congo

Physical maturation and life‐history parameters are seen as evolutionary adaptations to different ecological and social conditions. Comparison of life‐history patterns of closely related species living in diverse environments helps to evaluate the validity of these assumptions but empirical data are lacking. The two gorilla species exhibit substantial differences in their environment, which allows investigation into the role of increased frugivory in shaping western gorilla life histories. We present behavioral and morphological data on western gorilla physical maturation and life‐history parameters from a 12.5‐year study at Mbeli Bai, a forest clearing in the Nouabalé‐Ndoki National Park in northern Congo. We assign photographs of known individuals to different life‐history classes and propose new age boundaries for life‐history classes in western gorillas, which can be used and tested at other western gorilla research sites. Our results show that western gorillas are weaned at a later age compared with mountain gorillas and indicate slower physical maturation of immatures. These findings support the risk‐aversion hypothesis for more frugivorous species. However, our methods need to be applied and tested with other gorilla populations. The slow life histories of western gorillas could have major consequences for social structure, mortality patterns and population growth rates that will affect recovery from population crashes of this critically endangered species. We emphasize that long‐term studies can provide crucial demographic and life‐history data that improve our understanding of life‐history evolution and adaptation and help to refine conservation strategies. Am. J. Primatol. 71:106–119, 2009. © 2008 Wiley‐Liss, Inc.     

Mitochondrial DNA phylogeography of western lowland gorillas (Gorilla gorilla gorilla)

The geographical distribution of genetic variation within western lowland gorillas (Gorilla gorilla gorilla) was examined to clarify the population genetic structure and recent evolutionary history of this group. DNA was amplified from shed hair collected from sites across the range of the three traditionally recognized gorilla subspecies: western lowland (G. g. gorilla), eastern lowland (G. g. graueri) and mountain (G. g. beringei) gorillas. Nucleotide sequence variation was examined in the first hypervariable domain of the mitochondrial control region and was much higher in western lowland gorillas than in either of the other two subspecies. In addition to recapitulating the major evolutionary split between eastern and western lowland gorillas, phylogenetic analysis indicates a phylogeographical division within western lowland gorillas, one haplogroup comprising gorilla populations from eastern Nigeria through to southeast Cameroon and a second comprising all other western lowland gorillas. Within this second haplogroup, haplotypes appear to be partitioned geographically into three subgroups: (i) Equatorial Guinea, (ii) Central African Republic, and (iii) Gabon and adjacent Congo. There is also evidence of limited haplotype admixture in northeastern Gabon and southeast Cameroon. The phylogeographical patterns are broadly consistent with those predicted by current Pleistocene refuge hypotheses for the region and suggest that historical events have played an important role in shaping the population structure of this subspecies.     

Proximity patterns of female western lowland gorillas (Gorilla gorilla gorilla) during the six months after parturition

Studies of wild mountain gorillas have demonstrated strong preferences among new mothers for the silverback. Protection against infanticide has been theorized to be the primary cause for this attraction. We examined social partner preferences in captive western lowland mothers during the 6 mo after parturition and found that juveniles and females were the primary members of the new mothers' social network. Mothers spent significantly more time in close proximity with both of these subgroups than with the silverback. Proximity patterns changed over time: new mothers spent more time near females in the month after parturition than in the month prior to parturition, and spatial proximity increased again in months 2-6 postparturition. These findings differ from those reported for wild mountain gorillas, which may reflect the lack of infanticide risk in captivity. Given current hypotheses that infanticide may be a limited in western lowland gorillas, the social partner preferences observed here may be indicative of patterns in wild populations.     

Species and sex identification of western lowland gorillas (Gorilla gorilla gorilla), eastern lowland gorillas (Gorilla beringei graueri) and humans

Methods for the identification of the sex and species of individuals from samples non-invasively taken from humans and gorillas were established. Amplification of a segment of amelogenin (AMG), which is an X–Y homologous gene, using two pairs of primers from human AMG, revealed both X- and Y-specific bands. The possibility of sex identification was examined by typing the AMG gene using hair and fecal samples from captive western lowland gorillas (Gorilla gorilla gorilla) in Japan and hair samples from wild eastern lowland gorillas (Gorilla beringei graueri) in the Kahuzi-Biega National Park, Democratic Republic of Congo, which were sexed by direct observation. Species-specific bands of AMG in gorillas and humans were identified by restriction fragment length polymorphisms analysis. These tests could be used for sexing unidentified individuals of wild western and eastern lowland gorillas, and screening contamination of human DNA from non-invasively acquired samples.     

Hand-clapping as a communicative gesture by wild female swamp gorillas

Hand-clapping is a form of gestural communication commonly observed in captive great apes yet only isolated instances of this behaviour have been documented in the wild. Nearly 20 years ago Fay recorded the first observations of hand-clapping in western lowland gorillas (Gorilla gorilla gorilla) in the Central African Republic. Here we present observations of Likouala swamp gorillas using hand-clapping as a form of gestural communication in previously undocumented contexts in the wild. We observed hand-clapping on four different occasions in four different groups. The hand-clap was always exhibited by an adult female and always consisted of two consecutive claps conducted in front of the body. We suggest the functional significance of the behaviour was to maintain and enforce group cohesiveness during instances of alarm. These observations suggest western lowland gorillas have a means of communicating that is thus far absent in their eastern counterparts (Gorilla beringei ssp.). This could be a gestural culture found only in western lowland gorillas which should be investigated further to shed light on the evolution of communication among hominoids.     

The cryptic genetic structure of the North American captive gorilla population

Western lowland gorillas (Gorilla gorilla gorilla) were imported from across their geographical range to North American zoos from the late 1800s through 1974. The majority of these gorillas were imported with little or no information regarding their original provenance and no information on their genetic relatedness. Here, we analyze 32 microsatellite loci in 144 individuals using a Bayesian clustering method to delineate clusters of individuals among a sample of founders of the captive North American zoo gorilla collection. We infer that the majority of North American zoo founders sampled are distributed into two distinct clusters, and that some individuals are of admixed ancestry. This new information regarding the existence of ancestral genetic population structure in the North American zoo population lays the groundwork for enhanced efforts to conserve the evolutionary units of the western lowland gorilla gene pool. Our data also show that the genetic diversity estimates in the founder population were comparable to those in wild gorilla populations (Mondika and Cross River), and that pairwise relatedness among the founders is no different from that expected for a random mating population. However, the relatively high level of relatedness (R = 0.54) we discovered in a pair of known breeding pairs reveals the need for incorporating genetic relatedness estimates in the captive management of western lowland gorillas.     

Brain organization of gorillas reflects species differences in ecology

Gorillas include separate eastern (Gorilla beringei) and western (Gorilla gorilla) African species that diverged from each other approximately 2 million years ago. Although anatomical, genetic, behavioral, and socioecological differences have been noted among gorilla populations, little is known about variation in their brain structure. This study examines neuroanatomical variation between gorilla species using structural neuroimaging. Postmortem magnetic resonance images were obtained of brains from 18 captive western lowland gorillas (Gorilla gorilla gorilla), 15 wild mountain gorillas (Gorilla beringei beringei), and 3 Grauer's gorillas (Gorilla beringei graueri) (both wild and captive). Stereologic methods were used to measure volumes of brain structures, including left and right frontal lobe gray and white matter, temporal lobe gray and white matter, parietal and occipital lobes gray and white matter, insular gray matter, hippocampus, striatum, thalamus, each hemisphere and the vermis of the cerebellum, and the external and extreme capsules together with the claustrum. Among the species differences, the volumes of the hippocampus and cerebellum were significantly larger in G. gorilla than G. beringei. These anatomical differences may relate to divergent ecological adaptations of the two species. Specifically, G. gorilla engages in more arboreal locomotion and thus may rely more on cerebellar circuits. In addition, they tend to eat more fruit and have larger home ranges and consequently might depend more on spatial mapping functions of the hippocampus. Am J Phys Anthropol 156:252–262, 2015. © 2014 Wiley Periodicals, Inc.     

Ontogenetic changes in limb bone structural proportions in mountain gorillas (Gorilla beringei beringei)

Behavioral studies indicate that adult mountain gorillas (Gorilla beringei) are the most terrestrial of all nonhuman hominoids, but that infant mountain gorillas are much more arboreal. Here we examine ontogenetic changes in diaphyseal strength and length of the femur, tibia, humerus, radius, and ulna in 30 Virunga mountain gorillas, including 18 immature specimens and 12 adults. Comparisons are also made with 14 adult western lowland gorillas (Gorilla gorilla gorilla), which are known to be more arboreal than adult mountain gorillas. Infant mountain gorillas have significantly stronger forelimbs relative to hind limbs than older juveniles and adults, but are nonsignificantly different from western lowland gorilla adults. The change in inter-limb strength proportions is abrupt at about two years of age, corresponding to the documented transition to committed terrestrial quadrupedalism in mountain gorillas. The one exception is the ulna, which shows a gradual increase in strength relative to the radius and other long bones during development, possibly corresponding to the gradual adoption of stereotypical fully pronated knuckle-walking in older juvenile gorillas. Inter-limb bone length proportions show a contrasting developmental pattern, with hind limb/forelimb length declining rapidly from birth to five months of age, and then showing no consistent change through adulthood. The very early change in length proportions, prior to significant independent locomotion, may be related to the need for relatively long forelimbs for climbing in a large-bodied hominoid. Virunga mountain gorilla older juveniles and adults have equal or longer forelimb relative to hind limb bones than western lowland adults. These findings indicate that both ontogenetically and among closely related species of Gorilla, long bone strength proportions better reflect actual locomotor behavior than bone length proportions.     

Comparative life history patterns of female gorillas

Objectives

Several theories have been proposed to explain the impact of ecological conditions on differences in life history variables within and between species. Here we compare female life history parameters of one western lowland gorilla population (Gorilla gorilla gorilla) and two mountain gorilla populations (Gorilla beringei beringei).

Materials and Methods

We compared the age of natal dispersal, age of first birth, interbirth interval, and birth rates using long-term demographic datasets from Mbeli Bai (western gorillas), Bwindi Impenetrable National Park and the Virunga Massif (mountain gorillas).

Results

The Mbeli western gorillas had the latest age at first birth, longest interbirth interval, and slowest surviving birth rate compared to the Virunga mountain gorillas. Bwindi mountain gorillas were intermediate in their life history patterns.

Discussion

These patterns are consistent with differences in feeding ecology across sites. However, it is not possible to determine the evolutionary mechanisms responsible for these differences, whether a consequence of genetic adaptation to fluctuating food supplies (“ecological risk aversion hypothesis”) or phenotypic plasticity in response to the abundance of food (“energy balance hypothesis”). Our results do not seem consistent with the extrinsic mortality risks at each site, but current conditions for mountain gorillas are unlikely to match their evolutionary history. Not all traits fell along the expected fast-slow continuum, which illustrates that they can vary independently from each other (“modularity model”). Thus, the life history traits of each gorilla population may reflect a complex interplay of multiple ecological influences that are operating through both genetic adaptations and phenotypic plasticity.

     

Recent divergences and size decreases of eastern gorilla populations

Compared with other African apes, eastern gorillas (Gorilla beringei) have been little studied genetically. We used analysis of autosomal DNA genotypes obtained from non-invasively collected faecal samples to estimate the evolutionary histories of the two extant mountain gorilla populations and the closely related eastern lowland gorillas. Our results suggest that eastern lowland gorillas and mountain gorillas split beginning some 10 000 years ago, followed 5000 years ago by the split of the two mountain gorilla populations of Bwindi Impenetrable National Park and the Virungas Massif. All three populations have decreased in effective population size, with particularly substantial 10-fold decreases for the mountain gorillas. These dynamics probably reflect responses to habitat changes resulting from climate fluctuations over the past 20 000 years as well as increasing human influence in this densely populated region in the last several thousand years.     

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.     

Western lowland gorillas (Gorilla gorilla gorilla) change their activity patterns in response to frugivory

The most important environmental factor explaining interspecies variation in ecology and sociality of the great apes is likely to be variation in resource availability. Relatively little is known about the activity patterns of western lowland gorillas (Gorilla gorilla gorilla), which inhabit a dramatically different environment from the well‐studied mountain gorillas (G. beringei beringei). This study aims to provide a detailed quantification of western lowland gorillas' activity budgets using direct observations on one habituated group in Bai Hokou, Central African Republic. We examined how activity patterns of both sexes are shaped by seasonal frugivory. Activity was recorded with 5‐min instantaneous sampling between December 2004 and December 2005. During the high‐frugivory period the gorillas spent less time feeding and more time traveling than during the low‐frugivory period. The silverback spent less time feeding but more time resting than both females and immatures, which likely results from a combination of social and physiological factors. When compared with mountain gorillas, western lowland gorillas spend more time feeding (67 vs. 55%) and traveling (12 vs. 6.5%), but less time resting (21 vs. 34%) and engaging in social/other activities (0.5 vs. 3.6%). This disparity in activity budgets of western lowland gorillas and mountain gorillas may be explained by the more frugivorous diet and the greater dispersion of food resources experienced by western lowland gorillas. Like other apes, western lowland gorillas change their activity patterns in response to changes in the diet. Am. J. Primatol. 71:91–100, 2009. © 2008 Wiley‐Liss, Inc.     

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.     

Paternity determination in captive lowland gorillas and orangutans and wild mountain gorillas by microsatellite analysis

Paternity exclusion studies provide useful information for testing certain theories of behavioral ecology and for the management and conservation of both wild and captive populations of endangered species. This study used eight human nuclear microsatellite loci, in the absence of species-specific PCR primers, to genetically identify the sires of 12 captive lowland gorillas (Gorilla gorilla gorilla) and 2 captive orangutans (Pongo pygmaeus pygmaeus andPongo p. abelii). Parentage assignments were confirmed by excluding all except a single potential sire for each offspring with the least two loci. Sire-offspring relationships were verified in 12 of the 14 cases, and reassigned in the case of two gorilla offspring. The orangutan paternity typing was supplemented by DNA fingerprinting. Additionally, five of the eight microsatellite loci, in conjunction with behavioral data, were used for a non-exhaustive set of paternity exclusions for five wild mountain gorillas (Gorilla g. beringei). The eight loci described in this study should be useful additions to the tools available for the study of genetics in the great apes.     

Genetic diversity of North American captive‐born gorillas (Gorilla gorilla gorilla)

Western lowland gorillas (Gorilla gorilla gorilla) are designated as critically endangered and wild populations are dramatically declining as a result of habitat destruction, fragmentation, diseases (e.g., Ebola) and the illegal bushmeat trade. As wild populations continue to decline, the genetic management of the North American captive western lowland gorilla population will be an important component of the long‐term conservation of the species. We genotyped 26 individuals from the North American captive gorilla collection at 11 autosomal microsatellite loci in order to compare levels of genetic diversity to wild populations, investigate genetic signatures of a population bottleneck and identify the genetic structure of the captive‐born population. Captive gorillas had significantly higher levels of allelic diversity (t₇ = 4.49, P = 0.002) and heterozygosity (t₇ = 4.15, P = 0.004) than comparative wild populations, yet the population has lost significant allelic diversity while in captivity when compared to founders (t₇ = 2.44, P = 0.04). Analyses suggested no genetic evidence for a population bottleneck of the captive population. Genetic structure results supported the management of North American captive gorillas as a single population. Our results highlight the utility of genetic management approaches for endangered nonhuman primate species.     

Infanticide and social flexibility in the genus <Emphasis Type="Italic">Gorilla</Emphasis>

Based on the cases of infanticide by male mountain gorillas reported from the Virunga volcanic region, the socioecological and life history features of gorillas satisfy the conditions for which infanticide may be expected. However, there are considerable variations in the occurrence of infanticide between habitats. We analyze the recent reports of infanticides that were directly observed or are suspected based on field evidence in two populations of eastern and western lowland gorillas (Kahuzi and Mbeli Bai, respectively) along with previous reports on mountain gorillas, and consider which social features are linked with and which factors influence the occurrence of infanticide in the gorilla populations. All victims were suckling infants and most of them were killed by males who seemed unrelated to them. Dependent infants are most vulnerable to infanticide when the protector male (its putative father in most cases) is absent, and so male protection ability seems to be important in determining female transfer decisions. Two cases observed in Kahuzi suggest that the infanticidal male may discriminate between infants to accept and those to kill according to his previous interactions with their mothers. Mating for a prolonged period prior to parturition is necessary for immigrant females to avoid infanticide by the new male of the group that they join. Infanticide was usually associated with female transfer, and the patterns of female association at transfer may shape variations in social structure between populations. Female mountain gorillas prefer large groups with multiple males and tend to transfer alone in order to seek more protection against infanticide in Virunga. By contrast, female eastern and western lowland gorillas tend to transfer with other females to small groups or solitary males, and maturing silverbacks take females to establish new groups through group fission in Kahuzi and Mbeli Bai. These differences may result in more multi-male and larger groups in the Virungas than in Kahuzi and Mbeli Bai. Rapid changes in density of gorilla social units and their relations following drastic environmental changes caused by recent human disturbances may also increase the probability of infanticide.     

Social structure and life-history patterns in western gorillas (Gorilla gorilla gorilla)

Life-history traits and ecological conditions have an important influence on primate social systems. Most of what we know about the life-history patterns and social structure of gorillas comes from studies of eastern gorillas (Gorilla beringei sp.), which live under dramatically different ecological conditions compared to western gorillas (Gorilla gorilla sp.). In this paper we present new data on western gorilla social structure and life histories from four study sites, and make comparisons with eastern gorilla populations. Data were obtained from two study sites with gorilla groups undergoing the habituation process (Lossi, Democratic Republic of Congo and Bai Hokou, Central African Republic) and two "bai" studies (Maya Nord and Mbeli Bai, Republic of Congo). The size and structure of these groups were similar to those seen in eastern gorillas. However, differences in the occurrence of various group transitions (group formations, changes between one-male and multimale composition, and group disintegrations) exist, and western gorillas notably exhibit much higher rates of male emigration and correspondingly fewer multimale groups compared to mountain gorillas. Certain phenomena have been observed only rarely, including predation by leopards. The preliminary data show no significant differences in birth rates between western gorillas and mountain gorillas. The ecological variability across gorilla habitats likely explains the flexibility in the social system of gorillas, but we need more information on the social relationships and ecology of western gorillas to elucidate the causes for the similarities and differences between western and eastern gorillas on the levels of individuals, social groups, and population dynamics.     

Nutritional Aspects of Western Lowland Gorilla (Gorilla gorilla gorilla) Diet During Seasons of Fruit Scarcity at Bai Hokou,Central African Republic

Traditionally, gorillas were classified as folivores, yet 15 years of data on western lowland gorillas (Gorilla gorilla gorilla) show their diet to contain large quantities of foliage and fruit, and to vary both seasonally and annually. The consumption of fruit by gorillas at Bai Hokou, Central African Republic, is correlated with rainfall and ripe fruit availability (Remis, 1997a). We investigated the nutritional and chemical content of gorilla foods consumed at Bai Hokou during two seasons of fruit scarcity as measured by phenological observations and compared our findings with the nutrient content of gorilla foods at other African sites. We conclude that during lean times, Bai Hokou gorillas consumed fruits with higher levels of fiber and secondary compounds than those of other populations of western lowland or mountain gorillas. Conversely, leaves consumed by Bai Hokou gorillas were relatively low in fiber and tannins. Bai Hokou gorillas appeared to meet their nutritional needs by eating a combination of fruit and foliage. They ate fruits comparatively high in secondary compounds and fiber when necessary. While gorillas are selective feeders, wherever and whenever preferred foods are scarce, their large body size and digestive anatomy enable them to consume and process a broader repertoire of foods than smaller bodied-apes.     

Impact of ecological and social factors on ranging in western gorillas

We examined the influence of ecological (diet, swamp use, and rainfall) and social (intergroup interaction rate) factors on ranging behavior in one group of western gorillas (Gorilla gorilla gorilla) during a 16-month study. Relative to mountain gorillas, western gorillas live in habitats with reduced herb densities, more readily available fruit (from seasonal and rare fruit trees), and, at some sites, localized large open clearings (swamps and "bais"). Ranging behavior reflects these ecological differences. The daily path length (DPL) of western gorillas was longer (mean=2,014 m) than that of mountain gorillas, and was largely related to fruit acquisition. Swamp use occurred frequently (27% of days) and incurred a 50% increase in DPL, and 77% of the variation in monthly frequency of swamp use was explained by ripe fruit availability within the swamp, and not by the absence of resources outside the swamp. The annual home-range size was 15.4 km2. The western gorilla group foraged in larger areas each month, and reused them more frequently and consistently through time compared to mountain gorillas. In contrast to mountain gorillas, intergroup encounters occurred at least four times more frequently, were usually calm rather than aggressive, and had no consistent effect on DPL or monthly range size for one group of western gorillas. High genetic relatedness among at least some neighboring males [Bradley et al., Current Biology, in press] may help to explain these results, and raises intriguing questions about western gorilla social relationships.     

Ecological Constraints on the Foraging Effort of Western Gorillas (Gorilla gorilla gorilla) at Bai Hoköu, Central African Republic

Recent studies demonstrate that western lowland gorillas incorporate much more fruit into their diet than Virunga mountain gorillas do. Very little is known, however, about how the frugivorous behavior of western gorillas influences their daily ranging behavior, which may ultimately affect social factors such as group size and structure. I examined the influence of diet and the spatiotemporal availability of plant foods on the foraging effort of nonhabituated western lowland gorilla groups during 17 months at Bai Hoköu in the Dzanga-Ndoki National Park, Central African Republic. I determined diet from indirect methods and gorilla plant food availability and spatial distribution from phenology and line transects. Daily path length gives an estimate of foraging effort and was the distance paced, following fresh gorilla trails, from morning to evening nest sites. The availability and distribution of fruit and its consumption by gorillas varied seasonally. When concentrating on fruits, gorillas traveled significantly farther (mean = 3.1 km/day) than when their diet consisted mostly of nonfruit vegetation, such as leaves and woody pith, stems, and bark (mean = 2.1 km/day). The amount of herbaceous vegetation in the diet did not vary seasonally and did not influence daily path length. The best environmental predictor of foraging effort was fruit density, or a measure combining both density and spatial pattern: coefficient of dispersion. In addition, when fruit patches were small, path length tended to increase but not significantly. Compared with results of other studies, gorillas at Bai Hoköu travel farther (mean = 2.6 km/day) than gorillas in Gabon (mean = 1.7 km/day) and five times farther than mountain gorillas in the Virungas (mean = 0.5 km/day). Increased foraging effort of gorillas in this region, especially during the fruiting season, may have profound effects on group size and structure.