Chimpanzee Skeletons at Kibale

Category: Paleopathology
Posted on: March 23, 2009 10:26 PM, by afarensis, FCD

Posted on: March 23, 2009 10:26 PM, by afarensis, FCD

Paleopathology, for all practical purposes, is the study of the diseases and traumas that affect humans in the past. Necessarily, it is restricted to the study of the skeleton which severely limits the scope of what diseases can be studied. Even with that restriction a wide variety of questions can be addressed. We can, for example, ask how the change in lifestyle from hunter-gatherer to agriculturalist impacted human health. Or we can look at disease patterning in a given lifestyle. We can also look at whether disease and trauma differentially affect a given group such as young versus old or male versus female.

Since chimpanzees are our closest living relatives, understanding the diseases and traumas that impact the chimp skeleton might shed some light on human evolution. We can ask, for example, what selective factors impact chimpanzees It goes without saying that it would also be helpful to conservation biologists as well. There is a growing body of literature on the subject.

One such paper concerns skeletal trauma in chimpanzees at Kibale National Park. The paper analyzed the remains of twenty chimpanzees collected at Kibale National Park during 1997-2000. Of these twenty, twelve included post cranial material, while eight were represented by craniums or skulls alone. The sample included six females, eleven males, and three of undetermined sex. Age composition was as follows: four juveniles (age 3-10 years), five young adults (age 10-20 years), six prime adults (age 20-35 years), and six old adults (age 35+ years) - the paper does not explain where the extra individual came from.

Four different categories were looked at: arthropathy, trauma, bone formation and loss, and developmental abnormalities.

Arthropathy

All but five skeletons showed signs of moderate to severe degenerative joint disease (DJD). In thirteen of these the DJD was related to trauma, while in two (both female) the DJD was related to old age. In humans, one of the primary locations for DJD is in the lumbar spine, not so in chimpanzees (however, see below). In the chimpanzees, the primary location is in the extremities (and is more prevalent in the forelimbs than the hindlimbs). Most of the crania showed signs of severe degenerative arthitis at the temporo-mandibular joint (see below).

Trauma

Thirteen individuals showed signs of healed trauma, but this figure may have been biased downward due to the fact that postcranial material was lacking in eight of the individuals. Eleven of the individuals postcranial material showed signs of healed trauma. Four of these had long bone fractures as well as healed rib or hand fractures. In total, seven individuals had fractures in their hands (which seem to be in the metacarpals and phalanges). Eight crania had fractures and seven (five females and 2 males) and depressed fractures indicative of bites. One female had five depressed fractures attributes to bites. One of the males had a tooth fragment, from an unidentified species, embedded in his ulna.

Bone Formation and Loss

Five specimens displayed periostitis related to trauma. Three specimens displayed evidence of ossified ligaments or tendons. There was also a few crania and mandibles showing signs of infection.

Developmental Abnormalities

One individual showed bilateral accessory naviculars in the feet, while another displayed a rare hip dysplasia called coxa valga.

What Does It Mean?

So how can these patterns of trauma, and so forth, be explained? There are actually several causes at work. The first involves falls from the canopy which accounts for a large majority of the trauma observed. There is some data which indicates that safety during climbing has had an impact on chimpanzee morphology. It can also be seen in the fact that chimpanzees prefer the energetically more expensive terrestrial locomotion to arboreal locomotion.

It has been proposed, based on other studies, that cranial trauma shows sex specific patterns. Some of this was seen in the Kibale sample. Specifically, in females the crania were more likely to show fractures, whereas in males the face was more likely to show fractures. Such was not the case with bitemarks.

When compared to the chimpanzees at Gombe, some interesting differences are seen. In the Gombe chimps there is no evidence of TMJ problems. Fractures in the hands and feet are much more prevalent at Kibale and, this I find really interesting, DJD is found in the lumbar area much more frequently at Gombe. I would have expected a repeat of the Kibale pattern (i.e. more prevalent in the extremities than in the spine) and have to wonder what the chimpanzees at Gombe were doing differently.

There is one form of trauma I did not mention above. That involves trauma due to contact with humans. At Gombe there are a few skeletons which show evidence of poliomyeltitis - contracted via human contact. At Kibale, however, there is evidence of death by gunshot and by machete. There is also evidence for amputation of hands and feet, and mutilation of fingers and toes all due to snares set for game animals.

Fig. 2. Examples of healed traumatic injury in the Kibale chimpanzees. A: Depressed lesion (probable bite wound), indicated with white circle, on cranial vault of KFB 3. B: Upper limbs long bones of KFB156 showing the smaller left arm, which resulted from a left hand injury. C: The mandible of KVC1, showing the fractured and extensively modified right ramus and condyle. D: Fused metatarsals and distal tarsal row (all cuneiforms and cuboid) of KFB 152. E: Extensively remodeled right pelvis of KFB107, the probable result of a fall from height.

Literature Cited

Carter, Pontzer, Wrangham, and Peterhans (2008) Skeletal Pathology in Pan troglodytes schweinfurthii in Kibale National Park, Uganda. AJPA 135(4):389-403

Source and photographs