Although, in general, primates eat to meet their energy requirements, some captive primates seem to consistently eat in excess of immediate energy needs and become obese. Thus, it might be necessary to limit intake of diets that are energy-dense and very palatable. Long-term studies (Ingram et al., 1990) exploring the effects of restricted energy intake on life span have demonstrated that primates can adjust to moderate energy restriction as long as nutrient intakes are sufficient to maintain basic body functions. They do it either by decreasing accretion of body tissue, particularly fat, or by decreasing physical activity to match energy consumption.
High protein intakes can have satiating effects beyond the calories provided. When Jen et al. (1985) administered a liquid intragastric infusate containing casein as 36% of ME calories, satiation of rhesus macaques receiving the infusate and consuming a nutritionally adequate solid diet occurred more quickly than when the same percentage of ME in the infusate was provided by either carbohydrate or fat. The putative effects of high-protein diets in suppressing appetite were concluded to have potential for weight control. When adult male rhesus macaques received 50% of their ME intake as protein (oral plus intragastric infusion) compared with 14%, a doubling in plasma branched-chain amino acid (valine, isoleucine, and leucine) concentration and a consistently reduced caloric intake (by 24.7%) were noted (Hannah et al., 1990). Gibbs and Smith (1977) found that gastric preloads of L-phenylalanine, but not of D-phenylalanine, produced large reductions in meal size among rhesus monkeys, as did intravenous infusions of cholecystokinin, a gut hormone released in response to L-phenylalanine and regarded as an endogenous “satiety signal.” Young adult male baboons (Papio cynocephalus) responded with a 44% decrease in meal size when cholecystokinin octapeptide at 25 ng·kg-1 of BW was given intravenously before a 30-min meal (Figlewicz et al., 1995).
Plasma concentrations of glucose and insulin modulate feeding behavior, and blood concentrations of these compounds can be influenced by diet composition. When solutions of maltose, sucrose, or glucose (molar concentrations not specified) were provided to rhesus macaques at the beginning of a 24-h feeding period, the intake of a commercially prepared complete diet was significantly reduced, and total energy intake matched need. However, when fructose solutions were offered, reduction in food intake was only 37% of that induced by the other sugars. The difference in food intake was evident 3 h after presentation of the sugar solutions; this suggested an association with absorptive or immediate postabsorptive events and was presumably due to the failure of fructose to increase plasma glucose concentrations, as do the other sugars, or to elicit an insulin response (Kemnitz and Neu, 1986).
Variations in the concentrations of essential vitamins and minerals and the presence of aversive compounds, can also influence food intake and animal performance substantially (Newberne, 1975).
Most feeding programs for nonhuman primates in captivity use dry extrusions as the chief source of nutrients. In some management systems, food is offered ad libitum; in others, a fixed amount of food is presented one or more times per day. Some animal caretakers feed the same number or volume of extrusions. However, the densities and sizes of extrusions vary, not only between products made by different manufacturers but between batches of the same product. Thus, feeding by number or volume can lead to unintended changes in energy and nutrient intake. Weight is the recommended measure upon which the amount of food offered should be based.
The nutritional implications of feeding pellets ad libitum or in amounts limited to what can be consumed in 1 h, twice a day, to baboons (Papio cynocephalus) have been explored by Phillips and Clemens (1981). Food consumption and digestibility were not significantly different, nor were there differences in total transit times of fluid and particulate digesta markers. However, ad libitum-fed baboons passed 2-mm and 10-mm particulate markers more quickly and had a shorter 85% marker-recovery interval than did limit-fed baboons.
Extrusions can make the entire diet or be supplemented with other foods, such as nutritionally complete treats, vegetables, fruits, and insects. Such supplements often are more palatable than the extrusions, and supplement intake must be controlled lest overall intake become nutritionally unbalanced (Shimwell et al., 1979).
With the exception of browse for such primates as colobus monkeys (Colobus spp.), langurs (Presbytis spp.), and howlers (Alouatta spp.), which have a well-developed digestive capacity for fermenting fiber, supplemental foods are commonly fed for environmental enrichment rather than for nutritional reasons. When used, such foods should be nutritionally complete or result in minimal nutritional distortion of the diet. In some cases, nutritionally complete “treats” are available from commercial manufacturers, but care should be taken to assure that the supplement is nutritionally complete before incorporating it into a feeding program. Aside from nutritionally complete supplements or treats, appropriate environmental enrichment food choices would be those high in moisture and low in calories, such as vegetables and some fruits, rather than