Categories: Gorilla Journal, Journal no. 44, Taxonomy
Present Gorilla Taxonomy
A species is a population (or group of populations), distinguished by the possession of one or more consistent heritable differences from other such populations. A subspecies is a geographic segment of a species, distinguished by the possession at high frequencies, but not as much as 100%, of one or more heritable differences from other such segments.
Most experts today accept two gorilla species, the eastern and the western gorilla, each with two subspecies. This taxonomy is also used in the IUCN Red List of Threatened Species.
Western gorilla, Gorilla gorilla
Western lowland gorilla, Gorilla gorilla gorilla (Cameroon, Gabon, Equatorial Guinea, Cabinda, Congo Republic, Democratic Republic of the Congo, Central African Republic)
Cross River gorilla, Gorilla gorilla diehli (Cross River area, on the border between Nigeria and Cameroon)
Eastern gorilla, Gorilla beringei
Grauer’s (or eastern lowland) gorilla, Gorilla beringei graueri (eastern D. R. Congo)
Mountain gorilla, Gorilla beringei beringei (Virunga volcanoes and Bwindi forest)
But often it is also important to have a look at differences in certain populations to see the complete diversity of a species. The following brief review summarizes what we know about gorillas.
By far the most usual way in which new species form is when populations become isolated from one another. The two gorilla species are separated by about 900 km of forest, and the reason for this gap is the changing climate in Africa in the past. During the Plio-Pleistocene, lower temperatures and greater aridity changed the area covered by forests periodically. About 2.8 million years ago, ice sheets in the temperate zones became large enough to influence climate at tropical latitudes; local climate in Africa went through cooler and more arid, and warmer and wetter periods, respectively. Species that were adapted to tropical climates and life in forests, like gorillas, survived in forest refugia during the arid phases of the Pleistocene. When the climate had stabilized again, migration and dispersal recommenced linking previous refugia. The gorillas dispersed into the regrowing forests, but rivers prevented the western and eastern gorillas from coming into contact again.
Isolation of populations is also the origin of subspecies. It usually develops slowly while some degree of gene flow may continue; this means that some individuals (in gorillas usually lone silverbacks) travel between populations until the distance between the populations becomes too large. Consequently, the differences between populations vary according to the length of time they have been isolated, and it is not easy to decide at what point they can be called different subspecies. The science to search for hints and to justify decisions about categories is called taxonomy.
Methods in Taxonomy
The traditional method that has been used to describe and differentiate taxa is to observe differences in colour and colour pattern, size and shape, and other bodily features, and take measurements. Bones, especially the skull, are the parts most frequently measured. But the methods have changed considerably since the scientific description of the first gorillas; on the one hand various statistical tests have been developed to compare measurements, and on the other hand, completely new methods of study have emerged – especially genetics.
Morphology: It is not easy to decide which differences are most important in taxonomy. In theory, any difference is useful, as long as it is heritable. Some evolutionary changes may have cascading effects, and lead to many more changes – for example, the bipedality of humans is the reason for many morphological differences between apes and humans, not just in locomotor anatomy. A more intractable problem is that it is often difficult to know for certain which differences actually are heritable. For example, it is known that gorillas, like many animals, grow more quickly and mature earlier in captivity than in the wild, and may end up larger – they may weigh more and females more often develop a sagittal crest.
Ecological conditions, for example food composition, may cause morphological adaptations that are not fixed genetically but develop during ontogeny. Certain kinds of food require a certain jaw form and a certain musculature; it is often difficult to discern what is genetically fixed and what was shaped by the food consumed over the years of development: for example, gorillas in mountainous regions often have strongly everted jaw angles, and it is suspected that this is simply a response to the pull of the masseter muscle because of a fibrous diet, but it is very difficult to prove one way or the other. It is therefore important to examine carefully how skull features vary, for example in association with terrestrial/arboreal differences; examination of growth allometries can show how adult differences are produced and can help us to understand how postnatal development can aid in our functional interpretation of morphologies.
In his early studies, Colin P. Groves found that Grauer’s gorillas are intermediate between Virunga gorillas and western gorillas in many respects. Regarding dental and craniomandibular features, however, Grauer’s gorillas do not appear to be morphologically intermediate between western and mountain gorillas, as might be predicted by dietary expectations given their feeding ecology. This shows that although differences between populations may sometimes be attributed to environmental influence, in many cases this cannot be the only explanation. In some cases morphological differences may reflect purely environmentally produced differences (“phenotypic plasticity”), although they may of course be due to genetic adaptations; even if there is little doubt that many of the gross morphological differences between gorilla taxa truly are heritable, differences in DNA sequences may be a more reliable indication for evolutionary developments.
Genetics: The present gorilla taxonomy was influenced considerably by studies examining genetic distance between gorilla populations. Many geneticists have asked how much genetic difference, or how much time since separation, is required to consider two populations different species, and there have been needless disagreements over this, but increasingly geneticists have argued that there is no reason to adopt different standards when looking at DNA from those that we use when looking at morphology; as we noted above, a species is distinguished by the possession of one or more consistent heritable differences from other species, and there is no difference in principle between fixed DNA base pair differences and consistent morphological ones.
In general, of course, conclusions from genetic studies have helped considerably to understand the evolution of gorillas. When two populations separate, the two sub-populations usually have different frequencies of certain gene variants or alleles that may be a result of genetic drift or of different selective forces. The distribution of these gene types (haplotypes) may be important for taxonomy.
Differences between Eastern and Western Gorillas
External Characters: Eastern and western gorillas are distinct in colour: black in the east, grayer and browner in the west. The saddle of silverback males extends to the thighs in western gorillas and is restricted to the back in eastern gorillas, except in old age. In western silverbacks part of the back often is almost completely bare. In western gorillas the hair is short and sparse on the brows, and the colour is often brightly reddish on the crown.
Another difference is the form of the nose. The nostril shape in Gorilla beringei is angular, in Gorilla gorilla it is rounded and padded at the sides, often called the “squashed tomato” nose shape. In eastern gorillas the nostril breadth is narrow, in western gorillas it is flared. A “lip” above the septum is present in western gorillas, while it is weak or absent in eastern gorillas. The nose in general spreads further down the upper lip in western gorillas.
The feet of western gorillas are quite distinct from those of eastern gorillas; the big toe has a smaller angle of abduction in eastern than in western gorillas, slightly more humanlike; western gorillas show an abduction angle more similar to chimpanzees.
Morphology: Eastern gorillas have shorter limbs compared to the length of the trunk, and a shorter and broader hand, though we need more measurements to be sure that they hold especially for some populations of Gorilla beringei graueri. There are some differences in the skull, such as the generally longer, narrower skull and, in particular, the long palate of eastern gorillas, and this is an especially marked difference between the two. More differences are: eastern gorillas have much larger cheekteeth, rather smaller incisors, and rather more sexual dimorphism in molar size than western gorillas, as well as more sexual dimorphism in the upper canines. In western gorillas the teeth are relatively small. These traits are associated with a more folivorous diet in eastern gorillas, although the meaning of the greater degree of sexual dimorphism in canine size is not clear.
The distinctive medial cuneiform anatomy that distinguishes eastern from western gorillas probably represents a longstanding adaptive divergence between the two lineages. It is explained functionally by increased terrestriality in eastern gorillas. If the more terrestrial adaptations present in the medial cuneiform of modern eastern gorillas are apomorphic (meaning “evolutionarily derived”) within the genus, then these features probably evolved after gorillas first expanded into more montane environments, which may have occurred well before the last glacial period (before 0.1 million years).
Genetics: When certain parts (loci) of mitochondrial DNA (mtDNA) are compared, the amount of nucleotide sequence divergence between eastern and western gorillas is very nearly the same as that between chimpanzees and bonobos. In nuclear DNA, there are fewer nucleotide substitutions than between the two Pan species. The reason for this difference between mtDNA and nuclear DNA may be that gorilla males travel much farther than Pan males, so that more difference can accumulate between different populations in mtDNA, which is purely maternally inherited, than in nDNA, which is derived half from the male line.
The initial population divergence of western and eastern gorillas might have occurred 0.9–1.75 million years ago, but some gene flow in both directions seems to have persisted until as recently as 78,000 years ago.
Eastern Gorillas: Differences of Subspecies/Populations
External Characters: Mountain gorillas are distinguished by the much stouter and more stocky build, much thicker pelage and shining black, long, shaggy hair. The hair of mountain gorillas is especially long and shaggy on the scalp (the supraorbital torus, the heavy bar of bone above the eyes, is also covered with shaggy hair). Virunga gorillas have a more developed beard than Grauer’s gorillas. Grauer’s gorillas may have brownish hair on the top of the head while mountain gorillas’ hairs are all black. Although in Grauer’s gorillas the hairs are generally shorter, especially on the scalp and around the face, they are very long on the arms in silverback males.
The nostril shape in Virunga gorillas is angular, in Grauer’s gorillas, and at least some of those in Bwindi, more rounded; the outline above the nostril in Virunga gorillas is clear, in some Bwindi and in Grauer’s gorilla it is not. The upper lip padding in mountain gorillas (including those in Bwindi) is weak, in Grauer’s gorillas the lip is strongly, diffusely padded, making it convex in lateral view when the mouth is closed, and this padding extends about two thirds down the lip.
In Virunga gorillas the lateral toes are often webbed in between the digits and the great toe is less divergent, more adducted to the sole. The big toe is alsoshorter in Grauer’s gorillas than in mountain gorillas, and the heel-to-big-toe-tip is only 84% of heel-to-second-tip. Several features of the medial cuneiform distinguish Grauer’s from mountain gorillas; this has been studied recently by the noted comparative anatomist Matt Tocheri. The distinguishing characteristics of these two taxa appear unrelated to differences in the ability to abduct the big toe and to the frequency of arboreality.
The Skeleton: Mountain gorillas have a very large facial skeleton, wider than in Grauer’s gorillas which have a noticeably narrow face which can be detected externally as well as on the skull; the ascending ramus (the ascending branch of the lower jaw) is higher in mountain gorillas, especially in females; the jaw angles are strongly flared in adults while they are not flared in Grauer’s gorillas, although this could be phenotypic plasticity, as we suggested above. The long palate, which distinguishes all eastern gorillas from western gorillas, is extreme in mountain gorillas, which also have extremely large molar and premolar teeth. In Grauer’s gorilla the whole skull is smaller, the humerus is longer and the clavicle shorter.
Morphological differences in the scapula, in limb proportions, hands and feet reflect the greater adaptation of G. b. beringei to terrestrial life, especially the Virunga gorillas.
Genetics: Mitochondrial DNA studies showed that 15 unique eastern gorilla haplotypes fall into two distinct clades: one includes all the analyzed Bwindi and Virungas gorillas, the other one the individuals from Tshiaberimu, Kahuzi-Biega (mountain and lowland sector) and the captive Grauer’s gorillas. No haplotypes were shared between Gorilla beringei graueri and G. b. beringei, nor were any haplotypes seen previously in western lowland gorillas found in any of the eastern gorillas. Within the subspecies all populations shared haplotypes with others, indicating recent gene flow.
Genetic data suggest that mountain and Grauer’s gorillas split 380,000 years ago. Population expansions (or bottlenecks) for both subspecies are estimated to have occurred around 25,100 and 22,100 years ago after the last glacial maximum.
So far it has not been possible to determine significant molecular and morphological differences among the Grauer’s gorilla populations. Grauer’s gorillas show substantial morphological signatures of hybridization, and this, combined with molecular evidence for migration of individuals from further west, strongly suggest that this region – or part of it – is a former zone of introgression.
The differences between Gorilla beringei beringei and Gorilla beringei graueri seem very marked, and as we have seen they even appear to be absolute as far as mtDNA is concerned. But caution is required before we start claiming that they differ 100%, therefore they must be distinct species: importantly, some graueri populations approach beringei morphologically, so the morphological differences at least seem to be found in a majority of individuals, but not all of them; while as far as mtDNA is concerned, not all populations of Grauer’s gorilla have been sampled, and until a complete sampling has been done, we cannot be certain that 100% of individuals can be distinguished. Unfortunately, some populations of graueri no longer exist, such as those on the mountains west of Lake Edward (with the exception of a very small population on Mt. Tshiaberimu itself), while others are very poorly known, such as those on the Itombwe mountains. Maybe DNA can be extracted from museum specimens, and so fill in the gap – then we will know more exactly whether mountain and Grauer’s gorillas are truly units, distinct species, or whether (as at present classified) they do overlap in heritable characters and so are subspecies within the same species.
Differences between Montane and Lowland Forest Gorillas
Even if it is not clear which of the differences are adaptations to the extreme heights in the Virungas, these comparisons are interesting. Altitude strongly influences the availability and distribution of food. In general temperature decreases and wind speed tends to increase with altitude, and moisture from fog is very high. This results in marked differences in plant structure and availability, with canopy height and species diversity being greatest at lower elevations. Plants that provide fruit and lianas occur at greater densities and diversities in lowland forests. When fruits are scarce, gorillas travel less and consume primarily low-quality terrestrial herbaceous foods.
In the Virungas, most gorilla foods are constantly available, and this results in short daily path lengths and relatively large, stable groups with low levels of within-group feeding competition. Their ability to adapt to this diet may have enabled gorillas, rather than chimpanzees, to live in highland forest, at least in this area (chimpanzees are found in highland forest elsewhere, for example on the Rwenzoris). Lowland gorillas are more frugivorous than mountain gorillas, but fruiting trees show large seasonal variation in fruit availability. This leads to both longer path lengths and more arboreal behaviour. When fruits are available in large quantities, lowland gorillas travel large distances to search for this preferred food.
Differences in resource availability combined with reduced folivory could also have direct effects on western gorilla development. Western lowland gorillas have slower life histories than mountain gorillas. In Bwindi, their development resembles more the lowland gorilla development than that of Virunga gorillas; although they live on mountains too, the altitude is lower and the gorillas consume more fruits than in the Virungas. Other factors that may lead to slower development are stronger seasonality in the habitat and lower herb density.
Although in eastern gorillas the clearest split is between mountain and Grauer’s gorillas, the Mt. Tshiaberimu population seems to have a special position within Gorilla beringei graueri. It is not clear whether their life on a high mountain is the reason for this. The Mt. Tshiaberimu gorillas approach the mountain gorillas in many respects, such as flaring jaw angles and a more adducted great toe. In some respects they are morphologically transitional between the gorilla populations of Virunga and Utu (the lowlands east of the Lualaba River, including the lowland sector of the Kahuzi-Biega National Park), while the Mt. Kahuzi gorillas show some features like Tshiaberimu and Virunga and some like Utu, and the Itombwe gorillas are more like the lower altitude groups.
Differences between Populations: Virunga and Bwindi Gorillas
Externally, Virunga and Bwindi gorillas are rather easy to tell apart – especially the nose of Bwindi gorillas looks more like a Grauer’s gorilla nose than that of a Virunga gorilla, except for the usually shorter, less padded upper lip. The Virunga gorillas tend to be slightly larger in overall size than the Bwindi gorillas. These body size differences correlate well with the observed differences in diet, and the different altitudes and temperatures in the two areas also correspond to body size differences and explain some of the differences between the two populations: forms with larger body size and shorter limbs generally are better able to conserve heat and endure the cold than forms with a smaller body size and longer limbs.
The body hair is short and blackish in Bwindi gorillas, with brownish tint in sunlight; Virunga gorillas have long, shaggy, jet-black hair, especially long on the arms. The facial hair in Bwindi gorillas is short and does not hide the ears, they have no beard, they have sparse hair on their brows and adult males have some white hair on their face, while Virunga gorillas have long facial hair that hides the ears and forms a beard or whiskers on the face. The brows are hairy in Virunga gorillas.
In Bwindi gorillas the bare skin below the eyes shows very fine wrinkling; in Virunga gorillas it is heavily wrinkled, which results in a characteristic “nose print”. The nose has no dorsal cleft in Bwindi gorillas; Virunga gorillas show a strong dorso-nasal cleft, the nostrils are relatively large.
The feet in the two populations are also different. In Bwindi gorillas the big toe cleft reaches the first metatarsal head (the metatarsals are the bones that support the toes; they are enclosed within the sole of the foot) and deeper than the level of the second metatarsal head. In Virunga gorillas the big toe cleft does not reach as far as the base of the toe itself, and is level with the base of the second toe. In some morphological indexes Bwindi gorillas are even more similar to western lowland gorillas: the foot breadth index in Gorilla gorilla gorilla averages 28, in Virunga gorillas 32 and in Bwindi gorillas 28.9. For the big toe cleft index the numbers are 63, 75, and 63.2, respectively.
Bwindi gorillas have longer faces, which may or may not have a biomechanical explanation. They also have lower and narrower ascending rami, shorter mandibles than the Virunga gorillas and some other differences in the mandible. There are minimal differences between the two mountain gorilla populations in skull measurements.
This variation cannot yet be fully explained by functional morphology related to diet, although some of the skull and tooth differences do probably correspond to dietary differences. As already mentioned, gorillas in Bwindi appear to develop more slowly than Virunga gorillas, which may also be associated with differences in their diet. Clearly, there is no simple picture relating to dietary toughness and masticatory adaptations that can be presented in gorillas at this time. One must also keep in mind that the morphological differences may not all have biomechanical explanations, but instead may be due to genetic drift, because both populations are rather small, and it is possible that some of them may be due to phenotypic plasticity (as we explained above) rather than genetic at all.
Bwindi and Virunga gorillas were in recent reproductive contact, which explains their genetic similarity despite differences in anatomy; perhaps the two populations have undergone rapid morphological divergence since their separation, or possibly there always was selection for differences between them because of the different altitudes at which they live.
Western Gorillas: Differences of Subspecies/Populations
External Characters: There are only a few photos of wild Cross River gorillas taken from a distance, and only one adult female of this subspecies lives in captivity – at least, only one who is known to be a Cross River gorilla with certainty. Not much is known about the ecology, group structure, behaviour and life history of Cross River gorillas, although it is clear that they are living at medium-high altitudes, well above the altitudes where most other western gorillas live. There are not sufficient data, and comparisons of the two western gorilla subspecies are therefore very difficult.
Rothschild noted in 1908 that the skin of a Cross River gorilla showed a beard as long and thick as that of the Virunga gorilla. Unfortunately, there is very little material, and there has been no study that shows whether a beard is a general characteristic trait of Cross River gorillas. Some zoo gorillas have beards, but as their exact origin is not clear, it is possible that some western lowland gorillas have beards too.
The captive Cross River gorilla female in Limbe has comparably light body hair, but there is not enough information for a general comparison of the two subspecies. Western gorilla females in captivity show a wide variety of body colours.
Cross River gorilla feet, as far as our insufficient information goes, are shorter than those of western lowland gorillas, which suggests a greater degree of terrestriality.
Morphology: A striking difference between most Cross River gorillas and other gorillas is the broad, low nuchal surface of the skull (the area where the postural muscles are attached at the back of the skull). The greatest skull length, cranial length and face height in Cross River gorillas are shorter than in western lowland gorillas, but relatively broader. In males the palate is shorter and narrower, and in many males the sagittal crest is poorly developed.
The cheek teeth tend to have a smaller surface area than do other western gorillas. This may suggest that their diet is less abrasive and requires less dental processing.
Genetics: The mitochondrial DNA of all Cross River gorillas is the same as that of the gorillas of Ebo forest and of many in the main forests of the Cameroon Plateau region. Some 17,800 years ago, western lowland and Cross River gorillas diverged but substantial gene flow (~ 4 individuals per generation) between the two western gorilla subspecies probably ceased only about 420 years ago. Although only mtDNA has been studied, the very fact that Cross River gorillas do not differ from some populations of G. g. gorilla indicates that the separation between them cannot be very deep, so despite the fact that G. g. diehli is derived from a subset of western lowland gorillas the habitat differences are clearly so intense that there has been strong selection in the Cross River population in the face of this strong gene flow.
As shown by genetic data, marked decline of Cross River gorilla population size began only about a hundred years subsequent to their separation from other western gorillas – that would be about 1700 A.D., probably following human population increase and agricultural expansion in the region. In contrast, the population size of western lowland gorillas increased after the divergence from the Cross River gorillas, probably due to the changing climate conditions since the late Pleistocene that led to repeated expansion and contraction of forest, as well as by more recent increased human impact on it.
Diversity within a Subspecies: Western Lowland Gorillas
Western lowland gorillas are by far the most numerous and the most diverse subspecies. Major river courses have played an important role in shaping boundaries of regional genetic groups, notably the Sangha River, the Ogooué River, and the Sanaga River. The Ivindo/Ayina River may have also influenced postglacial expansion of the remaining gorillas by directing the southern extension into northeastern Gabon. As far as their skulls are concerned, gorillas from the hinterland of Cameroon (the Cameroon Plateau) tend to be large with a broad skull; those from the coast of Cameroon and Gabon are smaller, with narrower skulls; those from the swampy Sangha River region are large like the Cameroon hinterland population but with shorter faces and smaller jaws. None of these differences will identify anything like a majority of individuals, and there is no question of recognising different subspecies within western lowland gorillas.
Study of mtDNA shows that there is some regional differentiation, but again it is only an average. One haplotype is confined to the Cameroon coast; a second haplotype characterises all gorillas from southern Gabon and neighbouring parts of the Congo Republic, and also occurs (more rarely) in northern Gabon; a third is found in the hinterland of Cameroon and northern Gabon; a fourth characterises all gorillas from east of the Sangha, and some from south-eastern Cameroon and northern Gabon; and a fifth is found all through Cameroon, and is the only one found in Ebo forest and among G. g. diehli.
Special Populations: Ebo and Bondo Gorillas
Very little is known about the small gorilla population in the Ebo forest that lies between the Cross River and the western lowland gorilla distribution areas. There are no photos and no individuals in captivity. The single skull that has become available does not resemble that of Cross River gorillas, but is most like skulls from the Cameroon Plateau; genetically, they share the same mitochondrial haplotype that is common in Cameroon Plateau gorillas (and is also the only one to be found in Cross River gorillas). Until we get more information, such as Y-chromosome or autosomal DNA, the affinities of this population will remain obscure.
The existence, until the early years of the 20th century, of gorillas in the district around Bondo and the Itimbiri River, in the northern D. R. Congo, is disputed by some people, but the evidence seems reasonable that they really did survive there until that period. In both cranial morphology and mtDNA they resemble western lowland gorillas. Whether they were isolated in that small region, or whether there had been, until shortly beforehand, intermediate populations stretching west of the Ubangi River joining them with other western lowland populations, is unclear. Evidently they will have been separated from the nearest populations of Gorilla beringei by unsuitable habitat, probably the vast Gilbertiodendron forests of the Ituri district, but genetic data indicate that there had been intermittent gene exchange between the two gorilla species even until the Late Pleistocene.
As we mentioned in Gorilla Journal 30 (2005), there was a proliferation of species and subspecies described in the late 19th and early 20th centuries, but study of more abundant cranial material, in particular, from the 1960s onward has put this into perspective and shown that most of the differences are on average only: two species, Gorilla gorilla and Gorilla beringei, are clearly very different, not on average but 100%, and the genetic data that are available so far fully support their classification as distinct species. Within G. beringei there are two very well-distinguished subspecies, but we need to know more about them, particularly about the variation from place to place within both of them, before we can confirm whether they are “merely” subspecies or fully distinct species in the sense of being Units of Biodiversity. Within G. gorilla, the Cross River population is clearly distinct morphologically, and rates as a distinct subspecies, even though it shares its mtDNA haplotype with neighbouring populations of western lowland gorillas. Morphology and genetics combine to show a complex pattern of interrelationships among populations within G. g. gorilla.
Angela Meder and Colin P. Groves
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