Primatologists have long sought explanations for the diversity of social systems found among and within primate species. Apes have received much attention given their close relationship to Homo, and the fact that within this group there are as many social systems as there are genera. Differences among genera, species, subspecies, and in some cases populations within a subspecies are presumed to be due to adaptations to varied environments.
An important way in which the environment varies is in the spatiotemporal availability of different food sources. Folivorous items, such as mature leaves and terrestrial herbaceous vegetation (THV), are often difficult to digest, low in energy and high in protein. In addition, they are generally available year-round, abundant and evenly distributed in the environment. On the other hand, fruits (and in most cases, young leaves) are often easily digested, high in energy, perhaps low in protein, only seasonally available, and are rarely evenly distributed in the environment (see Remis, this volume for nutritional information).
The inconsistent and unpredictable availability of fruit presents a number of obstacles for primates. For one, it requires them to travel farther each day to fulfill their nutritional requirements than folivores (Milton and May, 1976; Clutton-Brock and Harvey, 1977). Furthermore, the distance traveled is intrinsically related to the size of the social group (Wrangham et al., 1993; Janson and Goldsmith, 1995).