If the focus of gerontology is on the human species, then it makes sense for us to select our closest relatives as models of aging, provided these species indeed age differently than humans (Figure 2.2).
As mentioned above, our closest living relative is the chimpanzee, in which aging appears to occur earlier than in humans. Similarly, all the great apes show signs of aging at younger ages than humans, though it is unknown whether they age differently from each other (Erwin et al., 2002). As we move further away from humans and great apes, species tend to be smaller, less intelligent, and shorter-lived. This is apparent among Old World monkeys (family: Cercopithecidae), which are generally shorter-lived than apes. Two species of Old World
monkeys have already been studied in the context of aging: rhesus monkeys and baboons. Both appear to age considerably faster than humans and great apes, making them potentially useful models for comparative studies of aging. Interestingly, baboons have an MRDT of roughly 4 years (Bronikowsky et al., 2002) while rhesus monkeys appear to have an MRDT not smaller than that of humans (Finch, 1990). Nonetheless, physiological studies suggest that rhesus monkeys age about twice as fast as humans (Finch, 1990; also see Chapter 38 by Roth and colleagues in this book and references in AnAge). Baboons and rhesus monkeys demonstrate how comparing aging rates among different species can be difficult and how the MRDT is not always an accurate estimate of rates of aging.
Among New World monkeys, also termed Platyrrhini, we find species much shorter lived than apes and humans. Marmosets are a good example, such as the common marmoset Callithrix jacchus. The record longevity for these animals is little over 16 years, and numerous age-related changes have been reported in their second decade of life (see AnAge for references). They also reach sexual maturity at about one year of age—which is much sooner than apes—suggesting shorter generation cycles, shorter lifespans, and hence in accordance with a faster aging process. Therefore, if our choice of species is aimed at discovering what determines rate of aging among primates, with humans as our ultimate goal, then these shorter lived primates are certainly a good choice. In contrast, some New World monkeys are longer lived, attaining sexual maturity at older ages. Examples include members of the genera Alouatta, Ateles, Cacajao, and Cebus. The white-faced capuchin (Cebus capucinus) is a good example with a record longevity of almost 55 years and attaining sexual maturity with at least 5 years of age. Therefore, New World monkeys offer a variety of aging phenotypes suitable for comparative studies of aging. The large variation in rates of aging among such closely related species argues, once again (Miller, 1999), that genetic factors determine rate of aging in primates and makes New World monkeys a valuable source of models of aging.
Moving further away from humans, tarsiers (family: Tarsiidae; genus: Tarsius) also appear to be short-lived with short generation cycles (Austad, 1997a). In contrast, a greater diversity is found among Strepsirrhini, one of the two primate suborders (Figure 2.2). Lemurs of the Lemuridae genus are relatively long-lived when compared to their closest relatives. The brown lemur can live 37 years, which is impressive considering it reaches sexual maturity at about age 2. It was also reported that a hybrid between a brown and a black lemur lived for 39 years (Jones, 1982). In contrast, dwarf and mouse lemurs (family: Cheirogaleidae) do not live more than 20 years, and age-related changes have been described in their second decade of life (see AnAge). For instance, the fat-tailed dwarf lemur (Cheirogaleus medius) has been argued as an example of a fast-aging primate (Austad, 1997a). Lorisiforms such as the slender loris (Loris tardigradus) also appear to be short-lived with a fast development (Austad, 1997a), although the slow loris (Nycticebus coucang) has been reported to live over 26 years. As in New World monkeys, the Strepsirrhini suborder appears to feature a variety of aging rates.
While there are reasons to focus only on primates in comparative studies of aging (Austad, 1997a), rodents may also serve as a potential models. First of all, mice and rats (Rattus norvegicus) are well-established models in biomedical and aging research. Secondly, rodents and primates diverged roughly 58 million years ago (mya), not long before the two primate suborders—Strepsirrhine and Haplorrhini—diverged 49 mya (Springer et al., 2003). Lastly, the short life cycles and fast aging processes of mice and rats have not been observed in any primate. By incorporating rodents and primates, we thus obtain a range of aging rates close to that of the entire Mammalia class. In Table 2.1 I recap all of the species mentioned above.
If we aim to investigate the factors regulating aging in primates having the human species as our priority, then our choice of species need not go further than primates and rodents. As argued before (Austad, 2005), more is learned by the study of closely-related species that differ considerably in the trait of interest. Of course, diversity is always welcomed and other species can be incorporated into the comparative biology of aging. Nonetheless, solely using primates and rodents in the comparative biology of aging may be adequate to determine which genetic factors regulate the human aging rate. Certainly, there are major difficulties in studying primates, but that depends on which factor is being studied. In the modern age of genomics it may be necessary, not to keep animals in captivity, but rather to have their genome sequenced. It is with this prospect that I suggest these animals as choices for aging research. Hopefully, some of these animals, like short-lived primates, may also be incorporated as experimental models, as suggested before (Austad, 1997a).
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When over eighty years of age, the poet Bryant said that he had added more than ten years to his life by taking a simple exercise while dressing in the morning. Those who knew Bryant and the facts of his life never doubted the truth of this statement.