Bioenergetics, the study of the use and transfer of energy, can provide important insights into the ecology and evolution of early hominids. Energy dynamics represent a central interface between an organism and its environment; how energy is extracted from limited environmental resources and allocated to various somatic functions has consequences in terms of survival and reproduction (McNab, 2002; Leonard and Ulijaszek, 2002; Leonard et al., 2007). Thus, energy provides a useful currency for measuring fitness. Energy dynamics also shape aspects of an organism’s life history, given that energy used for functions related to maintenance (e.g., resting metabolic rate [RMR], physical activity, and thermoregulation) cannot be used for production, such as the metabolic costs associated with growth and reproduction.
Energetic studies offer a window into hominid brain evolution, as an increase in the size of this metabolically expensive organ requires a shift in energy allocation—either an absolute increase in energy intake or a reduction in the portion of energy allotted to other components of energy expenditure. Consequently, encephalization may affect an organism’s life history pattern and shape variables such as the timing of weaning, age at maturity, and reproductive scheduling (Bogin, 1999, 2002). Non-human primates, including hominids, are distinct from most other mammals in having relatively large brains for their body size, a pattern noted by numerous authors (e.g., Martin, 1990). Modern humans have extended this trend and, with brains averaging approximately 1300 g, are outside the range of other living primates (Jerison, 1973; Leonard and Robertson, 1992).
The metabolic cost of brain tissue is approximately 240 kcal/kg/day and as such is considerably higher than certain tissues such as skeletal muscle (13 kcal/kg/day, at rest), similar to other organs such as the liver (200 kcal/kg/day), and lower than others such as the heart (440 kcal/kg/day) (Holliday, 1986; Elia, 1992). Given that humans and other primates (including great apes) have RMRs similar to other mammals (Leonard and Robertson, 1992,
1994; Aiello and Wheeler, 1995; Snodgrass et al., 2007), despite their relatively large brains, a comparatively large proportion of the resting energy budget is expended on brain metabolism in living humans (20-25%) and non-human primates (8-10%) compared to other mammals (3-5%) (Leonard and Robertson, 1994; Aiello and Wheeler, 1995).
While many studies of primate brain evolution have concentrated on identifying the causal selective factors associated with encephalization in non-human primates and hominids, other studies have taken a different approach and considered the factors associated with the ability to grow and maintain large brains in these taxa (e.g., Leonard and Robertson, 1994; Aiello and Wheeler, 1995; Leonard et al., 2003). These latter studies have concentrated on elucidating the ways that non-human primates and hominids, in particular, overcame the energetic constraints on encephalization. Following a similar approach, in the present chapter we use comparative data on living mammals (including humans and other primates) coupled with information on fossil hominids to consider the energetics of brain evolution as related to diet, body composition, and body size. We use these comparative data to test several hypotheses. First, we hypothesize that among non-human primates dietary quality (i.e., the energy and nutrient density of the diet) will be inversely related to body mass, as predicted by the Jarman-Bell relationship (Bell, 1971; Jarman, 1974). Second, we predict among non-human primates relative brain size and relative diet quality will be positively associated (i.e., species with relatively large brains will consume relatively high quality diets. Third, we hypothesize that non-human primates will have smaller gut sizes than non-primate mammals, as predicted by the Expensive Tissue Hypothesis (Aiello and Wheeler, 1995). Finally, we predict that non-human primates will have less total skeletal muscle mass compared to non-primate mammals.
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