Several macaque species have been routinely used for studies on aging, including rhesus (M. mulatta), cyno-molgus (M. fasicularis), pigtail (M. nemestina), stump-tail (M. arctoides), and bonnet (M. radiata). Obtaining sufficient numbers of aging animals is difficult, and estimating age in wild-caught individuals is a challenge. Some investigators have estimated age by tooth wear, but this is often inaccurate. Age of young animals may be estimated based on maturity, stature, and pubertal status. However, birth records are really the only reliable data.
In the rhesus macaque, there are Chinese- and Indian-derived monkeys, with the latter being most represented in the literature. These monkeys may be considered subspecies in that they differ somewhat in their physical characteristics. There have been some studies that have attempted to identify characteristics that differ between these subspecies to determine if they can both be used in a single study. An excellent study was conducted to compare the subspecies and to evaluate their growth characteristics (for review, see Clarke and O'Neill, 1999). Chinese-origin adult males were larger than their Indian-origin counterparts, whereas the Chinese-derived females were generally larger as young animals. Interestingly, the Indian-derived females caught up and surpassed the Chinese-origin females as adults. To further complicate this issue, there is evidence that individuals born in the laboratory were larger as adults compared to those monkeys caught in the wild. In addition, there are some data that suggest differences in immune response between these subspecies (Messaoudi and Nicolich-Zugich, personal communication). Clearly, the distribution of individuals of these subspecies must be considered when developing an experimental design in which there are limitations in the number of animals and availability of these subspecies.
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