1993b). A comparative description of growth in humans and chimpanzees has been published by Smith et al. (1975) and in chimpanzees and gorillas by Leigh and Shea (1996). However, energy requirements for growth were not reported.

Because dietary energy requirements differ so widely among various animal species, generalizations about daily requirements for growth must be viewed cautiously. The composition of tissues deposited during growth and measured as weight gain significantly influences required dietary energy inputs. Each gram of protein deposited represents about 5.4 kcal of net energy; each gram of fat, about 9.1 kcal of net energy (Scott, 1986; Robbins, 1993b). However, this is far from the full story. Energy losses are associated with digestion of the gross energy in food and with metabolism of the absorbed energy as growing tissues are synthesized. In low-birth-weight human infants, it has been calculated that 10.8 kcal of dietary ME is invested for each gram of fat gain and 13.4 kcal for each gram of protein gain; this is similar to calculations for other animal species with simple stomachs (Roberts and Young, 1988). The average amount of dietary ME used for maintenance and activity in low-birth-weight infants fed different diets was 34.7 kcal·BWkg-1·day-1. Total EE data collected with DLW from low-birth-weight infants suggests that these infants have a total EE and, therefore, an energy requirement about 20% greater than that of normal-birth-weight infants (Davies, 1998). Reviews of total EE measurements of normal-weight babies indicate that energy intakes during the first year of life are considerably below current international recommendations. Those recommendations—95 and 84 kcal ME·BWkg-1·day-1 for infants from birth to 6 months and from 6 to 12 months, respectively—were based on intakes by healthy infants in developed countries (FAO/ WHO/UNU, 1985). They are based on energy expenditure plus energy storage as determined with deuterium. The estimates can be used to calculate dietary ME requirements if the ME values of foods consumed are estimated correctly.

Energy stored in new tissue of growing human infants can be estimated by monitoring changes in BW over time, assuming that each gram of BW gained or lost represents 5.6 kcal (FAO/WHO/UNU, 1985; Davies, 1998). If an infant gains 40 g in a week, 224 kcal of energy would be stored as new tissue per week, assuming that that new tissue has a consistent energy density of 5.6 kcal·g-1. Dietary energy (as ME) expended each day for growth has been estimated to be 1.9 kcal·BWkg-1 at 10-15 years, 0.96 kcal·BWkg-1 at 15 years, and 0.48 kcal·BWkg-1 at 16-18 years (FAO/WHO/UNU, 1985).

A similar assumption has been made for growth in other animals, in that 5.6 kcal per gram of expected BW gain is intermediate between a theoretical maximum of about 9 kcal·g-1 for fat deposition and a low of 1.5-3.5 kcal per gram of BW gain reported for white-tailed deer, field mice, and voles, animals that accumulate relatively little fat during neonatal growth (Robbins, 1993b). However, diversity in body size among nonhuman primate species creates difficulty in computation of energy needs and efficiencies. In comparing energy allocations with growth and homeothermy, McClure and Randolph (1980) found that the smaller cotton rat (Sigmodon hispidus) has a shorter gestation period, is weaned sooner, has larger litters, and reaches sexual maturation faster than the larger eastern wood rat (Neotoma floridana). Thus, it appears that young of the smaller species can allocate their energy preferentially to rapid development of physiologic functions rather than to growth. Conversely, young of the larger species can emphasize efficient growth and experience a long period of dependence on maternal investment in that growth. The authors advanced the hypothesis that, in general, large species defer onset of active thermoregulation until body masses of the young are greater (when mass-specific metabolic rates are lower) to permit more-efficient early growth. Small species sacrifice growth efficiency in favor of rapid attainment of early independence and consequently pay high energy costs to do so. If valid, that hypothesis might also apply to nonhuman primates, with species that range in adult weight from less than 100 g to more than 200 kg.

When daily intakes of semipurified diets by male and female squirrel monkeys (S. sciureus) weighing 846-1,552 g were measured for a 26-week period, the estimated ME requirement (mean ± SD) for maintenance was 179 ± 19 kcal·BWkg-1·day-1. Sex, caloric density of the diet, and dietary fat content did not affect the maintenance requirement. After 24 weight gain or loss periods were measured, the cost of weight gain or loss was determined to be about 7.7 kcal·g-1 (only slightly higher than the previously discussed factor of 5.6 kcal·g-1). Body-composition changes were not determined (Ausman et al., 1981).

It has been proposed that energy requirements for infant New World monkeys are 300-500 kcal GE·BWkg-1·day-1 compared with 200-300 kcal GE·BWkg-1·day-1 for infants of the larger Old World species (NRC, 1978; Nicolosi and Hunt, 1979). However, the New World data were derived only from studies of very small species, and such a generalization seems unwise. Both Old World and New World monkeys were reported to have an adult energy requirement that was lower by 30-50% on a kcal·BWkg-1·day-1 basis than requirements for growth (Nicolosi and Hunt, 1979). That is similar to the finding in humans that, although total daily EE increases between the age of 10 years and maturity (on the basis of BMR and activity estimates), daily EE per kilogram of BW decreases by about 34% and 30% for males and females, respectively (FAO/ WHO/UNU, 1985).

Smaller primate species exhibit higher mass-specific energy requirements for growth than larger primate spe-

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