INTRODUCTION 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.