Aging in the musculoskeletal system of rhesus monkeys: II. Degenerative joint disease.

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.