Male and female energetic constraints

Male and female energetic constraints (the costs of metabolism, activity and reproduction) result from two factors: (1) the degree of sexual dimorphism in body weight that exists between the sexes, and (2) the reproductive state of the individual.

Male primates are often larger than female primates. Large differences in body size have significant energetic consequences. While metabolic rate increases with body size as around body weight0'75, commonly the rate of food intake may increase less rapidly with body weight (Clutton-Brock, 1994). Furthermore, heavier bodied animls are further constrained by their greater weight and size and are thus less able to exploit certain areas of food patches such as the outer branches of fruiting trees (Doran, 1993a, 1993b). Conversely, weaker, smaller animals are less able to exploit resources that require greater strength for their processing such as the stripping of bark. Smaller animals are also more likely to be displaced at food sources by larger ones (Wheatley, 1982).

The degree of sexual dimorphism in body mass will therefore affect the amount of sexual segregation within a habitat. A large degree of sexual dimorphism produces males as large-bodied foragers and females as smaller bodied foragers. Males and females are thus under differing selective pressures due to their body size and effectively occupy different niches within the same habitat. This has the advantage of lowering feeding competition between the sexes. Low sexual dimorphism results in greater feeding competition between the sexes because males and females are similar in body size and strength and will therefore overlap in foraging strategy. If feeding competition becomes too high, the sexes may have to segregate by altering foraging behaviour relative to each other in order to lower feeding competiton by exploiting different resources within the same habitat.

Although males are larger than females (so we would expect them to have greater daily energetic costs), female primates suffer the extra energy costs of internal fertilisation, pregnancy, extended lactation and prolonged dependency of young (Sadleir, 1969; Portman, 1970; Pond, 1977). These extra costs often mean that a female must feed for longer than an even larger male (relative to just how much larger that male is). For example, lactating female gelada baboons are found to spend up to 30% more time feeding per day than non-lactating females (Dunbar, 1992). Alternatively, females may select a different high-quality diet that requires extra searching and processing times (Pollock, 1977; Post, Hausfater and McCuskey, 1980; Harrison, 1983; Clutton-Brock, Albon and Guinness, 1984).

Therefore, when the degree of sexual dimorphism that occurs between the sexes is low (such as in bonobos and chimpanzees), female daily energetic costs may overlap with male costs. This is because any sex differences in daily energetic costs cannot be attributed solely to body weight disparity alone, but include the costs of reproductive demands made upon females. The social structure as well as environmental variation between populations may also influence these emergent differences.

Sex differences in foraging, as well as being energetically sensible for the individual, expand the feeding options for dispersed individuals and small groups whose subsistence activities are mostly self-regulated (Galdikas and Teleki, 1981). The exploitation of differing ecological niches between the sexes promotes a reduction in intersexual competition for food and is partly related to an existing energetic separation of the sexes that is the result of a greater degree of sexual dimorphism in body mass. With a difference in the subsistence activities of males and females, the sharing of resources within a group can become adaptive. 'Complementary activities' allow females to benefit from the eating of foods that they normally would not have the time, energy or 'capacity for risk' to attain. Food sharing allows males to 'buy' a female's affection and perhaps secure a mating preference. Among chimpanzees, the selection by a male of the individuals with whom he shares his kill is significant, and 80% of sharing in chimpanzees involves adults of both sexes getting meat from males. Female chimpanzees that are cycling are more successful in getting meat than non-cycling females (Teleki, 1973).

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