The Hand Of Mountain Gorillas Is As Well Adapted To Vertical Climbing Behaviors As The Hand Of Chimpanzees

Grasping is a hallmark adaptation among primate hands. All primates need to climb in search of fruits in the upper forest canopy. Prehensile grasping is an important prerequisite for climbing because it provides stability in any position of the three-dimensional environment that primates inhabit.

Although there are some records about the frequency of vertical climbing behavior in wild great apes, the time spent on vertical climbing varies depending on the species and population studied. This is particularly apparent in mountain gorillas, which are considered to not climb very much as other African apes and thus, for which their climbing strategy has not yet been studied in detail. Chimpanzees, by contrast, engage in arboreal locomotion more frequently but how they use their hand and forelimb during vertical climbing has not yet been examined beyond preliminary studies in captivity.

Male mountain gorilla (Wikipedia)

Previous attempts to correlate behavioral and environmental differences with variation in skeletal morphology are severely limited because little is known about the actions that the hands and forelimbs are performing on various natural supports of vertically climbing apes, particularly in the wild. Moreover, observations on arboreal hand use were mainly conducted in captive settings and are scarce, particularly in regards to the role of the thumb, whose functional importance in grasping arboreal substrates has traditionally been downplayed. The gorilla’s hand proportion of a long thumb relative to fingers is very much like ours and different to chimpanzees, which has been related functionally to enhanced manipulation. To better understand how species differences in hand morphology, as well as differences in body size, may relate to or be adaptive for vertical climbing, we need more naturalistic studies on how the digits grasp various substrates in mountain gorillas and chimpanzees.

Studying Moutain Gorilla Hand Grips In Uganda

In a recent study, researchers from the University of Kent and the Max-Planck-Institute of Evolutionary Anthropology provided first insights into the hand use and forelimb posture of 15 wild, habituated mountain gorillas during vertical climbing on natural substrates in comparison to eight semi-free-ranging chimpanzees.

Contrary to the Virunga Mountains, tree use by mountain gorillas is relatively common at the Bwindi Impenetrable National Park, Uganda. Bwindi has a higher mean annual temperature, greater plant diversity and a greater availability of arboreal fruits compared to the ecological extreme of mountain gorilla range at the Karisoke Research Centre, the best studied part of the Virunga Mountains.

The dense forest with fruiting trees makes Bwindi ideal for collecting much-needed data on hand use during vertical climbing in mountain gorillas. Fieldwork in the rainforest, however, can be very challenging as conditions of observations may be very difficult (e.g., low light, bad visibility due to foliage or height), or the frequency of observations may be too low (e.g., infrequent contact with particular individuals).  All you need is close attention to detail, passion and a positive attitude towards hard and tiring work that enable you to collect these data under extreme physical and mental demands. Although a sanctuary is not the wild, this location allows you to get within a much closer observation range within a natural forest environment than is often possible with wild, habituated populations. I strongly believe that functional morphologists should study their animals in the natural habitat and thus, I feel, the contribution to preserve their environments is a moral obligation of every scientist studying their behavior.

The study revealed that mountain gorillas and chimpanzees are capable of using particular hand grips and forelimb postures to accommodate variation in substrate size during vertical climbing. Both apes preferred the same grips and similar forelimb postures on similarly sized locomotor supports. Furthermore, the demands of grasping substrates during vertical climbing elicited three thumb postures in both apes. As predicted, differences in morphology between both apes appear to elicit slightly different grasping strategies. Unlike chimpanzees, gorillas show a complete loss of ulnocarpal articulation that allows for a greater mobility (i.e., ulnar deviation) of their wrist and hand. The greater ulnar deviation at the wrist could be observed when large-bodied mountain gorillas descended lianas. This extremely ulnarly-deviated wrist posture also enabled mountain gorillas to use the opposed thumb as an additional point of contact if needed, demonstrating for the first time the supportive role of the gorilla’s thumb during vertical climbing.

Together, this comparative study provides much needed data on how the hand and forelimb are used during vertical climbing in a natural habitat that can, in turn, help us to interpret the differences in skeletal features. Likewise, more work on how living apes use their hands for arboreal locomotor behaviors is needed to gain greater insight into the potential range of behaviors that might be capable with a given bony morphology. Ultimately, without our understanding of the diversity in behavior, both within and between species, there would be no starting point for reconstructing early human behaviors.

This study, Comparison of hand use and forelimb posture during vertical climbing in mountain gorillas (Gorilla beringei beringei) and chimpanzees (Pan troglodytes), was published by Dr. Johanna Neufuss from the University of Kent in the American Journal of Physical Anthropology.

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