The Nonhuman Primate NHP Model of Aging

The use of nonhuman primates as a model for the mechanisms of biological aging is well known (Corr, 2000). Crews (2003) correctly notes that, because of the shared evolutionary history of human and nonhuman primates, greater insight into the complex processes of aging and senescence can be gained through comparisons of phylogenetically similar organisms that have the most parallels to aging humans. Both human and nonhuman primates experience similar morphological and physiological decline with increasing age, including chronic degenerative conditions and an increased risk of disease, making nonhuman primates invaluable as a model of human aging (Crews and Garruto, 1994). Investigations of the associations of age with social behavior in nonhuman primates, however, have not proven as informative.

Primatologists have tested most of the proposed gerontological theories of human social or behavioral aging across various species of nonhuman primates. Primate gerontology, however, is a relatively new field that is based on very few studies, many of which present conflicting evidence (Corr, 2002). In tests of the social disengagement theory, for instance, Pavelka (1991), using Japanese macaques (Macaca fuscata), found no support for social disengagement in older monkeys, but Naka-michi (1984), also working with Japanese macaques, found the opposite and reported evidence that sociality did decrease with increasing age. Additionally, Corr (2000), in a comparison between rhesus macaque (Macaca mulatta) age groups, reported finding decreased sociality of aged rhesus macaque females, but increased levels of sociality in aged rhesus males in the same population. Moreover, two studies focusing on social behavior in aging lemurs (Picq, 1992; Taylor, 1998) also reported conflicting levels of social withdrawal. In apes, Bloomsmith et al. (2000) reported that their observations of Yerkes chimpanzees and gorillas showed significant behavioral differences in some categories between younger and older animals, but no significant differences between age groups in rates of social behavior.

Clearly, the search for aged behavioral universals in primates has not provided consistent insights into the social behavior of aged individuals. One reason may be that the relationship between social behavior and physical aging is greatly impacted by a number of factors, both cultural and ecological. All primates, human and nonhuman, become aged within a complex network of varying environments and social structures. If we are to clarify and broaden our understanding of becoming old, these variations must be fully considered. Such a broadened horizon, for instance, could explain the differences found in aged sociality between the described two, free-ranging populations of Japanese macaques living in different environments: one in mountainous Japan and one in the desert of Texas. Also, considerations of species-specific sex-based roles and life histories could explain why males and females of the same rhesus population would differ in their social behavior with increasing age as each sex lives very different lives. Primate aging occurs within a multiplicity of factors: basic biology, species-level behavioral repertoires, varying adaptations to environmental pressures, widely varying social structures, and so on. One of the hallmarks of the Order Primates is plasticity of behavior and diet, which allows primates to adjust and adapt to varying environmental pressures. It would seem, therefore, that consideration of individual and population-level differences in aging must be investigated within that context.

Although nonhuman primates make an excellent biological model for human aging, there are several important impediments to their use in aging research that should be noted. First, nonhuman primates do not have a comparable life span to humans, preventing direct comparison. Second, due to predation pressures and lack of long-term field studies that follow identified individuals over the life span, known aged primates are rare in wild populations. Third, though established colonies offer higher numbers of aged primates, they are semi-free ranging, provisioned populations, which could confound results. Finally, the use of aged nonhuman primates under laboratory conditions has become increasingly difficult due to the lack of elders that have known histories and that are not compromised from participation in past projects. Moreover, funding limitations and ethical considerations place additional limitations on the scope of aging research. Still, the use of nonhuman primates as a human aging model is predicted to increase, particularly in rhesus monkeys (Roth et al., 2004).

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