Bioenergetic Constraints on Primate Abundance

Abstract

Explaining variation in primate population densities is central to understanding primate ecology, evolution, and conservation. Yet no researchers to date have successfully explained variation in primate population density across dietary class and phylogeny. Most previous work has focused on measures of food availability, as access to food energy likely constrains the number of individuals supported in a given area. However, energy output may provide a measure of energy constraints on population density that does not require detailed data on food availability for a given taxon. Across mammals, many studies have shown that population densities generally scale with body mass−0.75. Because individual energy expenditures scale with body mass0.75, population energy use (the product of population density and individual energy use) does not change with body mass, which suggests the existence of energy constraints on population density across body sizes, i.e., taxa are limited to a given amount of energy use, constraining larger taxa to lower densities. We examined population energy use and individual energy expenditure in primates and tested this energy equivalence across body mass. We also used a residual analysis to remove the effects of body mass on primate population densities and energy expenditures using basal metabolic rates (BMR; kcal/d) as a proxy for total daily energy expenditure. After taking into account phylogeny, population energy use did not significantly correlate with body mass. Larger primates, which use more energy per day, live at lower population densities than smaller primates. In addition, we found a significant negative correlation between residuals of BMR from body mass and residuals of population density from body mass after taking phylogeny into account. Thus, energy costs constrain population density across a diverse sample of primates at a given body mass, and primate species that have relatively low BMRs exist at relatively high densities. A better understanding of the determinants of primate energy costs across geography and phylogeny will ultimately help us explain and predict primate population densities.