Weaning hand-reared infant primates of the usual laboratory species from a bottle to solid food is not particularly difficult. For infants fed semipurified milk replacers, the same ingredients can be formulated into solid diets with gelling agents, such as agar. Sugars, such as lactose or glucose, can be used in place of the starch, dextrins, and dextromaltose that are so commonly used in semipurified diets for adults. Provision of a solid diet simultaneously with the liquid diet allows infants to become accustomed to the novelty of a new food, providing opportunities to smell, touch, taste, and carry it around long before appreciable quantities are consumed. By the age of 2-4 months, infant monkeys still consuming liquid diets with lactose or glucose as a carbohydrate can be converted to solid diets containing ‘‘adult’’ carbohydrates or any of a variety of natural-ingredient-based products. Older monkeys generally prefer to handle and chew their food rather than drink it.
Humans share many age-related phenomena with great apes and Old World monkeys. If biologic aging of nonhuman primates is studied longitudinally, data representing a substantial portion of the human life span can be obtained within relatively few years (Short et al., 1987). Because of their close genomic relationship, the most relevant models of human aging may be chimpanzees (Pan troglodytes) or bonobos (P. paniscus), but the costs of acquisition and care of these species, combined with longevities of more than 5 decades in captivity, renders their use prohibitive.
Perhaps rhesus (Macaca mulatta), pigtail (Macaca nemestrina), and celebes (M. nigra) macaques are more practical; all have been used as models for studies of aging (Hansen et al., 1981; Howard, 1983; Kemnitz et al., 1993; Lane et al., 1996). They share aging maladies with humans, including atherosclerotic vascular disease, altered plasma lipid metabolism, signs of Alzheimer’s disease, menopause, diabetes mellitus, rheumatoid arthritis, obesity, and osteoporosis (Brown et al., 1974; Howard, 1983; Kaplan et al., 1985; DeRousseau, 1985a,b; Willcox et al., 1986; Sumner et al., 1989; Hansen, 1992; vom Saal et al., 1994; DeRousseau, 1994; Austad, 1997; Cefalu et al., 1997; Colman and Kemnitz, 1998). Early reviews described age-related changes in old primates (Bowden, 1979; Davis and Leathers, 1985; Short et al., 1987). Many current studies in primate gerontology are focused on age-related disorders that are influenced by nutrition.
Undoubtedly, many factors accelerate aging, but alterations in diet composition and limitations in the amount of food consumed have proved to be effective modulators of this process. Diet restriction, in the absence of essential-nutrient deficiencies, plays a positive and fundamental role in increasing survival and in delaying the onset and slowing the development of degenerative aging conditions (McCay et al., 1935; Tannenbaum, 1940; Merry and Holehan, 1979; Bodkin et al., 1995; Lane et al., 1996; Cefalu et al., 1997; Verdery et al., 1997). It is the only intervention consistently shown to extend both median and maximal life span in mammals (Weindruch and Walford, 1988; Weindruch et al., 1995; Roth et al., 1995). Many types of diets work. Both highly purified diets and commercial diets increase maximal life span when fed in reduced amounts, provided that all essential nutrients are present and moderate reductions in caloric intake are achieved (Weindruch, 1995).
It is probable that experimentally increasing the maximal age of research animals at death will yield important insights into the systematic processes of aging (Hayflick, 1985). Likewise, increasing the life span of research subjects will assist in the definition of biomarkers of aging, attributes that generally change with age and could help in predicting health and length of life (Ingram et al., 1993).
A preliminary aging study with 30 male rhesus macaques (M. mulatta) 0.6-25 years old and 30 male squirrel monkeys (Saimiri sciurius) 1 to more than 10 years old, representing Old World and New World species, respectively, was begun in 1987 for the National Institute on Aging (NIA) at the National Institutes of Health (NIH) Primate Unit of the National Center for Research Resources in Poolesville, MD (Ingram et al., 1990; Lane et al., 1992; Moon and Taylor, 1994; Roth et al., 1995). These species have natural life spans of about 40 and 20 years, respectively. The study was later expanded to include 120 female and male rhesus macaques and 30 male squirrel monkeys of various ages. All except the oldest animals, were caged as pairs (Weindruch et al., 1995).
Separate pelleted natural-ingredient diets were fed to each species. The proximate proportions of components of the diets (by weight) for rhesus and squirrel monkeys, respectively, were as follows: 15.4% and 20.3% crude protein; 5.0% and 8.0% crude fat; and 5% and 5% crude fiber. Gross energy (GE) concentrations in the rhesus and squirrel monkey diets were 3.77 kcal·g-1 and 4.03 kcal·g-1, respectively. Each diet was supplemented with vitamins and minerals at concentrations 40% above recommended allowances (Ingram et al., 1990; Weindruch et al., 1995). Diet-restricted primates were fed about 30% less than normally fed controls of each species and were adapted to the lower intakes over a 3-month period. Daily diet allotments during the preliminary study were based on National Research Council (National Research Council, 1978) estimates of GE requirements in kilocalories per kilogram of body weight (BW). Daily GE intakes by juvenile, adult, and old rhesus macaques fed ad libitum were