Analysis of Skeletal Lesions

Abstract

This chapter comprises a quantitative analysis of skeletal lesions in the Gombe chimpanzees. I begin by comparing the skeletal sample with the living population of chimpanzees at Gombe as well as the population of chimpanzees from the Kasekela community who have died from 1960 to 2006. While the skeletal sample diverges from the living sample, there are several similarities between the skeletal sample and the death sample. Differences include that the skeletal sample includes more chimpanzees with a known cause of death. The chapter continues with a short analysis of arthropathy in a sample of 20 adult chimpanzees. The number of joints affected by degenerative disease increases with age. Next, I describe an analysis of dental lesions and related pathologies (n = 23 adult chimpanzees). I report how many chimpanzees are affected by dental lesions and results of tests for sex differences. Males have a statistically higher rate of dental caries than females, while no other lesion type demonstrated a detectable difference. The linear relationship between antemortem tooth loss and age was not significant, nor did the ages of chimpanzees with and without osteoarthritis of the temporomandibular joint (TMJ) differ statistically. The larger-scale analyses of skeletal lesions testing for sex differences, trends related to age, and differences between dominance rank categories examine a sample of 36 complete chimpanzee skeletons. Most animals are affected by skeletal trauma, pathology, or both. I report which body regions were most commonly affected by skeletal lesions, as well as the proportion of observed elements affected by lesions, and note several sex differences in lesion frequency. Age and traumatic lesions are related, but not in a simple, linear way. This is because traumatic lesion rate can depend on cause of death. Pathologic lesions, on the other hand, are strongly related to increased age. While there was some evidence that high-ranking males experience higher rates of skeletal trauma and pathology, the effect size of this relationship was smaller than the effect size for age. Chimpanzees accumulate skeletal lesions as they age, though the rate at which this happens is influenced by individual selective pressures. The chapter ends with a comparison of skeletal lesions in this sample from Gombe versus a chimpanzee skeletal sample from Kibale, Uganda. While there are broad similarities between the two samples, there are several significant differences likely driven by the limited samples. Because chimpanzees are a long-lived species, it is only possible to begin detecting patterns of skeletal lesions and the ways they are influenced by demographic categories after decades of study.