Patterns of mandibular variation in Pan and Gorilla and implications for African ape taxonomy

Pan and Gorilla taxonomy is currently in a state of flux, with the number of existing species and subspecies of common chimpanzee and gorilla having been recently challenged. While Pan and Gorilla systematics have been evaluated on the basis of craniometric and odontometric data, only a handful of studies have evaluated multivariate craniometric variation within P. troglodytes, and none have evaluated in detail mandibular variation in either P. troglodytes or Gorilla gorilla. In this paper, we examine ontogenetic and adult mandibular variation in Pan and Gorilla. We test the hypothesis that patterns and degrees of mandibular variation in Pan and Gorilla closely correspond to those derived from previous analyses of craniometric variation. We then use these data to address some current issues surrounding Pan and Gorilla taxonomy. Specifically, we evaluate the purported distinctiveness of P.t. verus from the other two subspecies of Pan troglodytes, and the recent proposals to recognize Nigerian gorillas as a distinct subspecies, Gorilla gorilla diehli, and to acknowledge mountain and lowland gorillas as two separate species. Overall, patterns and degrees of multivariate mandibular differentiation parallel those obtained previously for the cranium and dentition. Thus, differences among the three conventionally recognized gorilla subspecies are somewhat greater than among subspecies of common chimpanzees, but differences between P. paniscus and P. troglodytes are greater than those observed between any gorilla subspecies. In this regard, the mandible does not appear to be more variable, or of less taxonomic value, than the face and other parts of the cranium. There are, however, some finer differences in the pattern and degree of morphological differentiation in Pan and Gorilla, both with respect to cranial and dental morphology, and in terms of the application and manner of size adjustment. Mandibular differentiation supports the conventional separation of bonobos from chimpanzees regardless of size adjustment, but size correction alters the relative alignment of taxa. Following size correction, intergroup distances are greatest between P.t. verus and all other groups, but there is considerable overlap amongst chimpanzee subspecies. Amongst gorillas, the greatest separation is between eastern and western gorillas, but adjustment relative to palatal vs. basicranial length results in a greater accuracy of group classification for G.g. gorilla and G.g. graueri, and more equivalent intergroup distances amongst all gorilla groups. We find no multivariate differentiation of the Nigerian gorillas based on mandibular morphology, suggesting that the primary difference between Nigerian and other western lowland gorillas lies in the nuchal region. Though intergroup distances are greatest between P.t. verus and other chimpanzee subspecies, the degree of overlap amongst all three groups does not indicate a markedly greater degree of distinction in mandibular, as opposed to other morphologies. Finally, mandibular differentiation corroborates previous craniodental studies indicating the greatest distinction amongst gorillas is between eastern and western groups. Thus, patterns and degrees of mandibular variation are in agreement with other kinds of data that have been used to diagnose eastern and western gorillas as separate species.     

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.     

A Genome-Wide Survey of Genetic Variation in Gorillas Using Reduced Representation Sequencing

All non-human great apes are endangered in the wild, and it is therefore important to gain an understanding of their demography and genetic diversity. Whole genome assembly projects have provided an invaluable foundation for understanding genetics in all four genera, but to date genetic studies of multiple individuals within great ape species have largely been confined to mitochondrial DNA and a small number of other loci. Here, we present a genome-wide survey of genetic variation in gorillas using a reduced representation sequencing approach, focusing on the two lowland subspecies. We identify 3,006,670 polymorphic sites in 14 individuals: 12 western lowland gorillas (Gorilla gorilla gorilla) and 2 eastern lowland gorillas (Gorilla beringei graueri). We find that the two species are genetically distinct, based on levels of heterozygosity and patterns of allele sharing. Focusing on the western lowland population, we observe evidence for population substructure, and a deficit of rare genetic variants suggesting a recent episode of population contraction. In western lowland gorillas, there is an elevation of variation towards telomeres and centromeres on the chromosomal scale. On a finer scale, we find substantial variation in genetic diversity, including a marked reduction close to the major histocompatibility locus, perhaps indicative of recent strong selection there. These findings suggest that despite their maintaining an overall level of genetic diversity equal to or greater than that of humans, population decline, perhaps associated with disease, has been a significant factor in recent and long-term pressures on wild gorilla populations.     

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.     

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.     

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.     

Gorillas (Gorilla gorilla gorilla) in the Likouala swamp forests of north central Congo: Preliminary data on populations and ecology

Until recently it was generally believed that gorillas avoid water. In two recent, independent faunal surveys in the Likouala swamp of north central Congo, we have found that gorillas occur at high densities, extending the known range of Gorilla gorilla gorillavery close to the Oubangui River. The gorillas in this area fed on the abundant ground vegetation of herbaceous monocotyledons in the swamp forest. In adjacent islands of terra firmathere was little evidence of gorilla nesting or feeding, indicating that they spend at least part of the year almost exclusively in the swamp forests. Recent reports from several sites in West Africa indicate that swamp forests may generally provide abundant food resources that are used regularly by gorillas.     

Nuclear integrations of mitochondrial DNA in gorillas

Great ape systematics, particularly at the species level and below, is currently under debate, due in part to the recent influx of molecular data. The phylogenies of previously published mitochondrial control region (or D-loop) DNA sequences in gorillas show deep splits within West African gorillas (Gorilla gorilla gorilla), and very high levels of nucleotide diversity in this subspecies. Here we demonstrate that several previously reported D-loop haplotypes from West African gorillas are in all likelihood nuclear integrations of mitochondrial DNA. Revised estimates of the amount and pattern of mitochondrial DNA diversity in gorillas are provided, revealing two reciprocally monophyletic and highly divergent groups of gorillas, concurrent with their geographic distribution.     

Newly Discovered Gorilla Population in the Ebo Forest,Littoral Province,Cameroon

We report on a new population of gorillas discovered in November 2002 in the Ebo Forest, Littoral Province, Cameroon. We observed A group of q7 gorillas directly for 83 min, and they were in auditory range for 155 min. Further evidence of gorilla presence included 8 nest groups totaling 38 nests, distinctive feeding signs accompanied by footprints, and a gorilla cranium collected from the nearby village of Iboti. This newly discovered gorilla population is geographically intermediate between the 2 extant populations of western gorillas: Gorilla gorilla gorilla, the most populous gorilla subspecies living in Gabon, Equatorial Guinea, Congo-Brazzaville, Central African Republic and Cameroon to the south of the Sanaga River, and G. g. diehli or the Cross River gorilla, a small population of ca. 250 individuals on the Cameroon-Nigeria border. It is not possible to assign the new gorilla population to either subspecies on the basis of measurements of the single male cranium. Genetic analyses of freshly shed hairs, collected from gorilla nests, may help to resolve the taxonomic status of the Ebo gorillas.     

Are the gorillas in Bwindi Impenetrable National Park “true” mountain gorillas?

The gorillas that inhabit Bwindi Impenetrable National Park in Uganda are the least known of the eastern gorillas. Because they are an allopatric population living a minimum of 25 km from the well‐studied population of mountain gorillas (Gorilla beringei beringei) in Rwanda and have certain morphological and ecological differences from these gorillas, their taxonomic status has been in question in recent years. This study presents new craniodental metrics from Bwindi individuals and compares them to Virunga individuals as well as to eastern lowland gorillas, G. gorilla graueri. Multivariate statistics, including MANCOVA, least‐squares, regression, and principal components analyses, were used to evaluate how closely the Bwindi crania resemble the Virunga crania and how both relate to G. g. graueri. Results indicate that the Bwindi gorillas have generally smaller crania than the Virunga gorillas, but when metrics are log‐transformed, the only variable that distinguishes the Bwindi individuals is a longer face. When both populations are compared to G. g. graueri, they cluster together separately from the eastern lowland gorillas, sharing such features as higher rami, wider bigonia, longer mandibles, and wider and shorter mandibular symphyses in relation to G. g. graueri. Functional morphological explanations for these differences are discussed, but lacking measurements of the physical properties of G. g. graueri, they cannot fully be explained. Results clearly indicate that at least pertaining to the cranium, upon which most gorilla taxonomy is based, the Bwindi gorillas are proper mountain gorillas (G. b. beringei). Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

The veterinary management of a laryngeal air sac infection in a free-ranging mountain gorilla.

A purulent laryngeal air sac infection was diagnosed in a free-ranging adult male mountain gorilla (Gorilla gorilla beringei) in Central Africa. Successful treatment included the administration of antibiotics via blow dart and surgical drainage in the field. This case occurred during an outbreak of respiratory disease of undetermined etiology in this and other gorillas in the population.     

Mitochondrial DNA sequence from an enigmatic gorilla population (Gorilla gorilla uellensis)

Although today gorillas are found in only two widely separate, discontinuous western and eastern African populations, rumors of the existence of an additional gorilla population in central Africa have inspired recent unsuccessful field expeditions in search of the "mystery ape" termed Gorilla gorilla uellensis. Such a gorilla population would have considerable conservation and scientific interest, and would presumably have descended from a population of gorillas that was thought to exist until the end of the 19th century on the Uele River in the current-day Democratic Republic of Congo. However, the sole evidence for the existence of these gorillas is three skulls and one mandible brought to the Royal Museum for Central Africa (Tervuren, Belgium) in 1898. We determined a mitochondrial DNA sequence from one of these specimens and compared it to sequences from other gorillas. Contrary to expectations, the sequence obtained did not exhibit the phylogenetic distinctiveness typical of a representative of a peripheral isolated population. Rather, the results suggest a scenario in which the museum specimens did not originally derive from the northern Congo, but were brought from the area of current distribution of western gorillas to that location; the subsequent discovery and collection of the specimens there gave rise to the false inference of a local gorilla population.     

Insectivory by Gorilla gorilla gorilla in Southeast Cameroon

Our study extends quantitative analyses of insect-eating by gorillas (Gorilla gorilla gorilla) to Cameroon. During a 2-mo period (May–June 2001), we recorded and analyzed feeding traces on plants and insects and in gorilla feces. We found 180 feeding traces, 17% of which involved insectivory. Seventy-eight percent of the fecal samples (n = 36) contained insects. Ants were found in 61% of the samples, termites in 39%, while 56% of the samples contained remains of other insects. We added 14 new species to the known insect diet of western lowland gorillas. Overall, social insects are predominant. The choice of prey by the Ntonga gorillas gives new clues for the existence of cross-cultural differences among gorilla populations. A comparison of the overall frequency of insectivory with those at other sites in Central Africa indicate a possible effect of forest disturbance on the insectivorous behavior of gorillas. The study suggests the existence of temporal variation in ant- and termite-eating by gorillas.     

Seasonality in the Ecology and Life Histories of Mountain Gorillas (Gorilla gorilla beringei)

The abundance of food, especially that of fruit and often that of young leaves, varies considerably over time for most primates. This variation can depend on or be independent of seasonality in rainfall. Mountain gorillas (Gorilla gorilla beringei) in the Virungas are exceptional: their habitat contains almost no edible fruit, and they mostly eat perennially available herbs and vines that are densely and evenly distributed in much of their habitat. Earlier studies documented little consistent temporal variation in mountain gorilla diets and habitat use, except for seasonal use of bamboo by some groups, and documented no birth seasonality. Long-term data ( 7 years) on 6 mountain gorilla groups confirm these results for habitat use, except that they show some seasonality in use of the upper altitudinal extremes of the gorillas' home ranges for unclear reasons. Relatively low and inconsistent variation in habitat quality over time should lower the costs of grouping for gorillas compared to other apes. Long-term data also confirm the absence of seasonality in births and conceptions. However, they show that mortality rates and risk of respiratory infections vary directly with rainfall. These relationships are probably causal and may be mediated through thermoregulatory stress.     

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.     

Movement patterns of a group of free-ranging mountain gorillas (Gorilla gorilla beringei) and their relation to food availability

Relationships between the movement patterns of free-ranging mountain gorillas (Gorilla gorilla beringei) and the abundance and distribution of foods in their home range were examined. During an 18-month field study, the ranging of one group of G. gorilla beringei was recorded on a 250 × 250 m grid system, from which measurements of frequency and duration of use, travel rate, and rate of revisitation of each quadrangle by the group were derived. Food items were sampled in selected quadrangles throughout the home range and various measures of food abundance, frequency and diversity were calculated. Analyses based on both spatial and temporal variation in food availability give supporting evidence for the prediction that mountain gorilla ranging patterns are influenced by the distribution and abundance of foods. Quality of food appears to be an important factor, as shown in analyses of yearly patterns and monthly changes in ranging. The gorillas studied spent more time in the higher quality areas of their home range, responded to a correlate of decreasing food abundance by increasing their rate of travel and area used, and revisited regions more frequently when the renewal rate of foods was clearly greater. Each of their foraging tactics can be explained as serving to increase the efficiency of harvesting foods.     

Feeding Ecology of Cross River Gorillas (Gorilla gorilla diehli) at Mawambi Hills: The Influence of Resource Seasonality

Determining the composition of primate diet and identifying factors that affect food choice are important in understanding habitat requirements of primates and designing conservation plans. We studied the diet of Cross River gorillas (Gorilla gorilla diehli) in relation to availability of food resources, in a semideciduous lowland forest site (Mawambi Hills) in Cameroon, from November 2009 to September 2011. Based on 109 d of feeding trail data, 203 fecal samples, and 22 mo of phenological monitoring, we determined that gorillas consumed a total of 242 food items, including 240 plant items from 186 species and 55 taxonomic families. Mawambi gorillas diversified fruit consumption when fruit availability increased, and consumed more fibrous foods (pith, leaf, bark) during times of fruit scarcity, consistent with results of other gorilla studies. However, fruit availability was not related to rainfall, and the period of fruit scarcity was more pronounced at Mawambi than at other gorilla study sites, due to a single long dry season and extreme rainfall at the end of the rainy season that delayed fruit production and ripening. We found no relationship between the daily path length of the gorillas and fruit consumption. We found feeding habits of Mawambi gorillas to be notably similar to those of a population of Cross River gorillas at Afi Mountain, Nigeria, although subtle differences existed, possibly due to site-specific differences in forest composition and altitude. At both sites the liana Landolphia spp. was the single most important food species: the leaves are a staple and the fruits are consumed during periods of fruit scarcity. Snails and maggots were consumed but we observed no further faunivory. We suggest that tree leaves and lianas are important fallback food sources in the gorilla diet in seasonally dry forests.     

Y-Chromosome variation in hominids: intraspecific variation is limited to the polygamous chimpanzee

Background

We have previously demonstrated that the Y-specific ampliconic fertility genes DAZ (deleted in azoospermia) and CDY (chromodomain protein Y) varied with respect to copy number and position among chimpanzees (Pan troglodytes). In comparison, seven Y-chromosomal lineages of the bonobo (Pan paniscus), the chimpanzee''s closest living relative, showed no variation. We extend our earlier comparative investigation to include an analysis of the intraspecific variation of these genes in gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus), and examine the resulting patterns in the light of the species'' markedly different social and mating behaviors.

Methodology/Principal Findings

Fluorescence in situ hybridization analysis (FISH) of DAZ and CDY in 12 Y-chromosomal lineages of western lowland gorilla (G. gorilla gorilla) and a single lineage of the eastern lowland gorilla (G. beringei graueri) showed no variation among lineages. Similar findings were noted for the 10 Y-chromosomal lineages examined in the Bornean orangutan (Pongo pygmaeus), and 11 Y-chromosomal lineages of the Sumatran orangutan (P. abelii). We validated the contrasting DAZ and CDY patterns using quantitative real-time polymerase chain reaction (qPCR) in chimpanzee and bonobo.

Conclusion/Significance

High intraspecific variation in copy number and position of the DAZ and CDY genes is seen only in the chimpanzee. We hypothesize that this is best explained by sperm competition that results in the variant DAZ and CDY haplotypes detected in this species. In contrast, bonobos, gorillas and orangutans—species that are not subject to sperm competition—showed no intraspecific variation in DAZ and CDY suggesting that monoandry in gorillas, and preferential female mate choice in bonobos and orangutans, probably permitted the fixation of a single Y variant in each taxon. These data support the notion that the evolutionary history of a primate Y chromosome is not simply encrypted in its DNA sequences, but is also shaped by the social and behavioral circumstances under which the specific species has evolved.     

Sexual Dimorphism and Facial Growth Beyond Dental Maturity in Great Apes and Gibbons

The great apes and gibbons are characterized by extensive variation in degree of body size and cranial dimorphism, but although some studies have investigated how sexual dimorphism in body mass is attained in these species, for the majority of taxa concerned, no corresponding work has explored the full extent of how sexual dimorphism is attained in the facial skeleton. In addition, most studies of sexual dimorphism combine dentally mature individuals into a single “adult” category, thereby assuming that no substantial changes in size or dimorphism take place after dental maturity. We investigated degree and pattern of male and female facial growth in Pan troglodytes troglodytes, Pan paniscus, Gorilla gorilla gorilla, Pongo pygmaeus, and Hylobates lar after dental maturity through cross-sectional analyses of linear measurements and geometric mean values of the facial skeleton and age-ranking of individuals based on molar occlusal wear. Results show that overall facial size continues to increase after dental maturity is reached in males and females of Gorilla gorilla gorilla and Pongo pygmaeus, as well as in the females of Hylobates lar. In male Pongo pygmaeus, adult growth patterns imply the presence of a secondary growth spurt in craniofacial dimensions. There is suggestive evidence of growth beyond dental maturity in the females of Pan troglodytes troglodytes and Pan paniscus, but not in the males of those species. The results show the presence of statistically significant facial size dimorphism in young adults of Pan paniscus and Hylobates lar, and of near statistical significance in Pan troglodytes troglodytes, but not in older adults of those species; adults of Gorilla gorilla gorilla and Pongo pygmaeus are sexually dimorphic at all ages after dental maturity. The presence of sex-specific growth patterns in these hominoid taxa indicates a complex relationship between socioecological selective pressures and growth of the facial skeleton.     

Gorillas of Bwindi-Impenetrable Forest and the Virunga Volcanoes: Taxonomic implications of morphological and ecological differences

Based on their geographic proximity to the Virunga Volcanoes (≈ 25 km), the Bwindi-Impenetrable Forest gorillas have been referred to the subspecies Gorilla gorilla beringei. Differences in anatomy, habitat, ecology, and behavior, however, suggest Bwindi gorillas are distinct from those in the Virungas. Relative to Virunga gorillas, Bwindi gorillas live at lower elevations, in warmer temperatures, are much more arboreal, have longer day ranges and larger home ranges, and eat much more fruit and pith, and less bamboo and leaves. Morphological differences reflect the differences in ecology, habitat, and behavior. Bwindi gorillas measured have smaller bodies, relatively longer limbs, hands, and feet, shorter trunks, thumbs, big toes, and tooth row lengths, and narrower trunks and orbital breadths than Virunga gorillas. These differences indicate Bwindi gorillas do not belong to G.g. beringei and should not be referred to as “mountain gorillas.” How unique the distinguishing features of Bwindi gorillas are, and whether or not they should be assigned to a new taxon, depends on the expression of these features in eastern lowland gorillas (G.g. graueri). © 1996 Wiley-Liss, Inc.