Gorilla are terrestrial knuckle-walkers, but also have arboreal behaviours. The least arboreal of these sub species is the mountain gorilla (Gorilla beringei beringei, an eastern gorilla). Due to their location in regions of high altitude fruit tress are less common so they do not need to leave the ground to search for food. in contrast the western gorilla (Gorilla gorilla gorilla) lives in forested areas at a lower altitude where they will exploit seasonal fruit as well as building nests in the trees. Finally in between these species is the grauer gorilla (Gorilla beringei graueri, another eastern gorilla) who inhabit both types of environment with some living in areas similar to the mountain gorilla and others living in areas more similar to the western gorillas.
Due to the difference in foraging behaviour it may be expected to see a difference in the morphological features which are associated with arboreal locomotion. This would provide evidence that morphological evolution is associated with ecological factors. These differences may be seen in the appendages of the gorillas and may vary in flexibility and ability to grasp.
A study by Tocheri et al. (2011) attempts to investigate this by looking at the foot of the three sub species of gorilla and using a CT scan to create 3D models. It was found that the cuneiform of western gorillas were distinctively different from the eastern gorilla as well as differing angles of the facets of the foot and a larger surface area of the fist metatarsal.
The cuneiform shape was also distinguishable between the two eastern gorilla species, but to a lesser degree and not in the way that the researchers predicted. This means that theses differences are not related to the ability or frequency of arboreal locomotion and are therefore not morphologically intermediate between western and mountain gorillas. However, the study suggests a hypothesis for the morphology difference that a coalition (bridge of tissue) of the navicular and intermediate cunieform may affect the shape of the cuneiform. It is undecided whether this is a result of a hybridization or other evolutionary process between western and eastern gorilla
Full citation of the article:
Tocheri, M. W., C. R. Solhan, et al. (2011). “Ecological divergence and medial cuneiform morphology in gorillas.” Journal of Human Evolution 60(2): 171-184.