It is well established that aging of the human skeleton includes the loss of bone mass or density, but little comparative information on age-related osteopenia in other primates is available. In order to determine whether bone loss occurs in normally locomoting nonhuman primates, radiographs of 139 rhesus monkey skeletons from the Cayo Santiago collection were examined, and measures of percent cortical bone (PCT) at the midshaft of second metacarpals, humeri, and femora were calculated. An age-related osteopenia was observed in the metacarpal of adult female macaques, although much individual variation was present. This variability could not be explained by reproductive history (number of offspring), matrilineal affiliation, or degree of osteoarthrosis. However, in a subsample of 15 animals, females who had given birth in the three months prior to death showed lower PCTs than those who had not conceived in the previous mating season. In adult males, PCT did not vary significantly with age, but males that weighed less and those with no/little osteoarthrosis showed lower PCTs than heavier or more arthritic animals, respectively. The multifactorial nature of the variation in PCT observed here suggests that similar processes of bone behavior are involved in the skeletal aging of both humans and rhesus monkeys.

In order to discuss the rate and onset of adult aging in rhesus monkeys, 55 adult animals from the Wisconsin Regional Primate Research Center and the University of Wisconsin Psychology Primate Laboratory were examined. Degenerative joint disease (DJD) at the hip and spine was scored, and loss of passive joint mobility at the hip was measured. Development of DJD at both the hip and spine was significantly and positively correlated with age. Spinal changes, especially at the thoraco/lumbar intervertebral symphyses, appeared to develop somewhat more rapidly than hip degeneration. In some individuals, DJD was observed soon after the completion of growth, but pronounced changes seldom occurred before the middle of the second decade of life. Similarly, age-dependent losses of passive joint mobility appeared to begin early in ontogeny and to become increasingly pronounced in the aging adult. Although interspecific comparisons are difficult due to intraspecies and intraindividual variation, the timing of musculoskeletal aging in the rhesus spine and hip differs from that observed in humans in a way that parallels previously documented species differences in patterns of musculoskeletal growth. These observations and data on age-related change in other systems, suggest that rates and durations of many ontogenetic processes in rhesus monkeys are approximately three times as fast and one-third as long as those of the corresponding human processes. Importantly, differences in the timing of reproduction do not appear to follow the same scaling factor observed in other systems. Although reproduction may, therefore, be under separate control, the consistent pattern observed in other aspects of somatic growth and aging supports the hypothesis (Cutler, 1976; Sacher, 1978) that evolutionary changes in ontogeny have resulted from selection acting upon a few genes with widespread regulatory effects.

In order to describe ontogenetic change in the musculoskeletal system of rhesus monkeys, 126 Macaca mulatta from Cayo Santiago, ranging in age from 7 months to 21 years, were examined under anesthesia. Passive joint excursions were measured at the wrist, elbow, shoulder, hip, and knee. Mean ranges of excursion at these joints differed significantly between age groups and by sex. The potential for most movements appeared to decrease approximately 25 degrees over the first two decades of the macaque life span, and males generally showed less potential for movement than females in all age groups. These results are similar to those obtained for humans and are consistent with patterns of positional behavior, trauma, and osteoarthritis observed in this rhesus monkey population. Thus, to fully describe the locomotor strategy of rhesus monkeys, age- and sex-related variation in locomotor anatomy and functional capacity must be considered.

Ten vertebral elements from the AL-288 partial hominid skeleton and 11 elements from the AL-333 collection are described. The AL-288 column presents a marked kyphosis at the level of thoracic vertebrae 6 through 10, with pronounced new bone formation on the ventral surfaces of these vertebrae. These features, associated with narrowed disc space and minor osteophytosis, resemble Scheuermann disease in the human. Even though this diagnosis is consistent with a basically human, bipedal locomotor repertoire, the presence of Scheuermann disease suggests that lifting, climbing, or acrobatic activities may have been important in early hominids.

Asymmetry was investigated in the forelimbs of 150 rhesus monkey (Macaca mulatta) skeletons using measurements of right and left humerii, radii, ulnae, second metacarpals, and femora. Seven of the ten forelimb dimensions were larger on the right than on the left side. Paired t-tests revealed that the mean of the right side was significantly larger than that for the left for two measurements of the ulna and two of the humerus. No measurement was significantly larger on the left than on the right side. These results indicate a small but significant asymmetry in the forelimb bones of rhesus monkeys and, as is the case for humans, the direction of asymmetry favors the right side. Our findings are consistent with an interpretation of hypertrophy of certain muscles and opens the question of whether rhesus monkeys preferentially use their right forelimbs for manipulative tasks that require manual dexterity, as is the case for humans. These forelimb skeletal asymmetries are discussed in light of the recent literature on cortical asymmetry and handedness in nonhuman primates.