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Experiments such as those described above are a means to examine the nutrient regulatory responses of animals to simulated variation in highly simplified experimental environments, but they do not tell us whether and how these responses operate in the realistic setting of natural food environments. In recent years, several studies have examined this issue through recording the dietary intakes of individual animals in unmanipulated or minimally manipulated natural environments. Much of this work has concerned primates because they readily habituate to the presence of human observers, enabling detailed observation over entire days or even multiple consecutive days. The focus on primates is beneficial from our perspective because it helps to place the human nutritional research discussed below into a broader biological context.

It is now clear that animals in the wild, as in the laboratory, employ nutrient‐specific appetites to compose diets with specific amounts and ratios of macronutrients. Felton et al. [28] found that spider monkeys in Bolivia have a strong preference for Ficus boliviana figs, and when these are not available, eat other food combinations to form a diet with a protein‐energy ratio very similar to that of the figs. Mountain gorillas in Bwindi and Virunga compose nutritionally similar diets from very different food combinations [29,30]. Johnson et al. [31] found that a baboon studied for 30 consecutive days composed daily diets with similar percentage energy from protein, even though she ate very different food combinations on different days. Other studies have demonstrated that wild primates change the selected diet to track specific changes in nutrient requirements. Guo et al. [32] showed that the intake of fat and carbohydrate by golden snub‐nosed monkeys living in a highly seasonal environment increased during the cold winters by an amount that closely matched the increased energy requirements for maintaining body temperature in the cold, whereas protein intake did not change. Cui et al. [33] found that rhesus macaques increased their energy intake by ~30% when lactating, but in this case, there was no difference in the ratio of macronutrients selected.

Several studies have examined the regulatory responses of wild primates when natural variation in food availability constrains them from achieving their macronutrient target. A variety of responses has been recorded for different species. Spider monkeys [34] (Fig. 6.3), chimpanzees [35], and blue monkeys [36] show protein prioritization, where protein intake is maintained at or close to the target level while fat and carbohydrate intake vary with the percentage of energy contributed by protein in the diet (Fig. 6.1c). Mountain gorillas show the opposite response, in which they over‐eat protein to maintain a constant intake of non‐protein energy [37]. Rhesus macaques show an intermediate strategy, in which the deficit of one nutrient matches the surplus ingested of the other, and consequently, total energy intake is maintained constant with variation in dietary macronutrient ratios [33].

Figure 6.3 Regulatory responses by free‐ranging Peruvian spider monkeys (Ateles chamek) to ecologically imposed variation in dietary macronutrient ratios. Each point represents the daily protein and non‐protein energy intake of an individual. The overall pattern of intakes suggests protein prioritization, in which the target intake of protein is maintained, and fat and carbohydrate intake vary with daily variation in dietary macronutrient balance (blue pattern in Fig. 6.1).

Source: Modified from Felton et al. [34].