Physical Characteristics of Cross River Gorillas

Cross River Gorilla skulls are smaller than those of other gorillas. Male Cross River Gorilla skulls are particularly distinct. Cross River Gorillas have the smallest mean cheek tooth (molar and premolar) surface area, and the difference in this measure is greater between them and the Western Lowland Gorillas than between the two types of eastern gorilla. There is no corresponding difference in body size, so Cross River Gorilla skulls are smaller relative to their body size, as well as absolutely. (9)

Cross River GorillaNo distinguishing characteristics could be established from the scarce coat and skeleton specimens available for study by Sarmiento. Cross River Gorillas may have shorter hands and feet than other gorillas. The single male skeleton had a thumb at the short end of the range for western gorillas, and a large opposability index (indicating a short thumb relative to the index finger). The female skeleton was bigger than is usual for female Western Lowland Gorillas. (9)

The unique features of the Cross River Gorilla skull and dentition imply that these gorillas consume smaller food items (smaller gape in males) that require less dental processing and may be harder (smaller cheek tooth surface area, shorter palate in males, possibly greater biting force) than those eaten by Western Lowland Gorillas. Many of the qualitative features of the skull could also be related to stronger masticatory forces. Cross River Gorillas may spend less time chewing their food. The shape of the incisors (the front teeth, used for biting rather than chewing) and the fact that they are usually very worn, implies that they are more important to Cross River Gorillas than to other western gorillas, and  may be used for more of the feeding time, or include harder and more abrasive foods. (9)

The shorter (and shorter relative to body size) hand and foot length, and the higher opposability index are all suggestive of a more terrestrial life than is apparent from the skeletons of other western gorillas. The skull and tooth characteristics that distinguish the Cross River Gorilla have been associated in other anthropoid primates with shifts to more open habitats. These characters could also be related to lower fruit abundance. (9)


Evolution and the Pleistocene Forest Refuges

The Cross River Gorilla may have differentiated from other gorillas during the Pleistocene dry phase, when there was a decline in arboreal food sources and a greater emphasis on herbivory and terrestrial behaviours. At this time the Cross River region was probably a forest refuge, and the species within it evolved apart from those in other, separate refuges. The spread of these unique species must have been prevented in later times by various barriers, such as their dependence on the specialized local ecology, or geographical features such as the Sanaga River, leaving the Cross River region inhabited by many endemic species and subspecies. (9)

The current habitat may not represent the environment in which Cross River Gorillas differentiated and to which they are best suited. The Obudu Plateau and Bamenda Highlands once supported a unique montane forest ecosystem, since lost to grassland, and this could have been the environment in which they evolved. (9)

Cross River Gorillas are distinguished from Western Lowland Gorillas by their significantly lower cheek tooth surface area, smaller vault volume, narrower biglenoid diameter and narrower incisor row and palate width. Male Cross River Gorillas are further distinguished by their significantly lower average palate lengths and facial length than Western Lowland Gorilla males. A combination of qualitative skull characteristics is also diagnostic of Gorilla gorilla diehli, though not all Cross River Gorilla skulls will show all of these characters, and Gorilla gorilla gorilla may on occasion show them independently. (9)

Tooth morphology is less likely than bone morphology to reflect developmental and physiological plasticity. The distinctiveness of Cross River Gorilla teeth is therefore likely to be a result of genetic differences, at least in part. Since there are no significant dental differences between Mountain and Eastern Lowland Gorillas, there may be a greater distinction between the two subspecies of western gorilla than there is between the eastern subspecies. (9)