Studies of positional behavior have helped our understanding of postcranial adaptation in primates and this in turn has contributed to discussions concerning how and why particular directions in primate evolution occurred. At first, anatomists observed positional behavior, whether in captivity or in the wild, in order to describe what primates actually do. These observations led to simple categorization of primate species into, for example, brachiators or arboreal quadrupeds, as well as evolutionary scenarios reconstructing adaptive pathways in primate and human locomotor evolution (e.g. Keith, 1923; Clark, 1959; Napier and Walker, 1967). With the proliferation of field studies, many of the early categories proved less than useful and the association between particular anatomical features and specific behaviors came under closer scrutiny (e.g. Stern and Oxnard, 1973; Mittermeier and Fleagle, 1976; Morbeck et al., 1979). These works, which used more sophisticated methods, specifically the quantification of primate positional behavior, helped to direct studies of positional behavior towards ecology as well as morphology. Despite these efforts made in the 1970s, surprisingly few species have been adequately sampled quantitatively in the wild, and, perhaps more importantly, very few studies have focused upon a particular research problem (for example, changes in body size and its effect on arboreal locomotion; Napier, 1967; Cartmill, 1974; Fleagle and Mittermeier, 1980; Fleagle, 1985; Jungers, 1985).
The concept of carrying capacity is fundamental in determining a species' density and biomass, which enter into the equations used in ecological theory to predict species diversity and community structure (MacArthur, 1972). Therefore, you might think that a great deal was known about how to determine the carrying capacity of species in nature; however, this is not the case. In the vast majority of species, if the carrying capacity is measured at all, it is estimated as the population density of individuals associated with a zero growth rate (Dennis & Taper, 1994). The more fundamental question of what resources determine that population level is often left unanswered.
Debate has raged over the past two decades as to the extent or even existence of resource limitation in primate populations. Early observations (Struhsaker, 1969) suggested that many fruit-eating primates faced an overabundance of resources in the trees that they used. Later study quantitatively confirmed that some trees produced more fruit than a group of monkeys could consume in one sitting (Janson, 1988b), but also showed that many trees did not produce enough fruit to satiate even a single animal. In one of the few attempts to relate primate population densities to resources directly, Coehlo et al. (1976) compared the biomass and consumption rates of howler and spider monkeys in Guatemala to the availability of the major food resources that they used. They found that over an annual cycle total food production vastly exceeded the requirements of the population density in that site. However, in Peru, Janson (1984) found that the productivity per unit area of food species used by capuchin monkeys varied enough so that in the dry season insufficient fruit was produced to sustain the population. At that time, these fruit-eating monkeys were seen to switch to alternate plant products.
Africa is an immense continent covering approximately 30 million km2 and encompassing 49 countries. For decades Africa has been considered a continent of great mystery, partly stemming from the fact that when Europeans first traveled to Africa, they found large expanses of seemingly impenetrable forests. Further, the first explorers often pressed inland by following rivers and thus often encountered only long stretches of riverine forest. In reality, the majority of the rainforest in Africa is situated in a belt that extends less than 10° north and south of the equator, and it is frequently broken by savanna or dry forest (e.g., the 300 km wide Dahomey Gap in West Africa). Thus, unlike the initial impressions of continuous homogenous forest, Africa actually contains a myriad of habitats from multistrata tropical forest, to dry deciduous forest, woodland, savanna, and desert. Along with the variety of habitats found within the continent, Africa harbors a great diversity of primate communities: at least 64 species of primates are found in Africa (15 prosimians, 46 monkeys, and 3 apes, Oates, 1996a; Fig. 1.1).
The objective of this chapter is to provide a template with which to begin to understand the diversity of primate communities in Africa's tropical forests. To do this, we first review the nature of the forested habitats in which primates occur, describing general habitat characteristics and, when possible, providing detailed contrasts of rainfall and forest structure.
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