Investigating how ecology and demography influence folivorous primate biomass in the western Amazon

The aim of this study is to examine the relationships between ecology, demography and behavior in order to address the question of which factors are influencing density in two populations of red howler monkeys ( Alouatta seniculus) occurring at different densities and in different forest types in Yasuni National Park, Eastern Ecuador. Attempting to narrow down and distinguish between the many closely related biological factors thought to affect primate density is important not only because it may further our understanding of the process of behavioral adaptations, but also because it is a critical component in aiding conservation endeavors. This project employs a multivariate approach which tests eight ecological factors, particularly those suggested by previous research to be important influences on folivore density, that are distinguished by how they would affect the behavior of folivorous primates. Specifically, each factor is accompanied by a set of mutually exclusive a priori predictions regarding how each should, all other aspects remaining equal, affect the behavior of folivorous primates in areas of high versus low population density. This enables a comparison to determine which ecological variable(s) best predict the observed patterns of Alouatta behavior at the high density site, and thus identify those aspects of forest ecology which are influencing population density. This study has three main goals. The first goal is to evaluate whether the predicted differences in the ecological variables occur at the site with high howler monkey population density. The second is to identify whether the predicted trends in demography with increasing population density proposed for other primates are present at the high density site, and the third is to test which ecological variables best predicted the observed patterns of Alouatta behavior in order to determine which variables are playing the largest roles influencing Alouatta densities.;Out of eight variables examined, six aspects of forest ecology are supported to possibly contribute to higher howler monkey population density: (1) hunting, (2) higher soil fertility, (3) higher phenological productivity, (4) higher resource quality in the form of leaves with higher protein levels, (5) higher density of leaves with higher protein levels and (6) higher resource density of leaves lower in total phenolic content. There is a pronounced difference in the number of groups present at each site (ten at the higher density site versus three at the low density site); however no significant differences are found in the demographic patterns at either the population or the group level between sites, and birth rates appears to be similar. These results suggest that density per se may not be affecting population or group composition because both populations appear to be at equilibrium and thus equally constrained reproductively. Lastly, when predicted behavioral patterns for each of the supported ecological variables are compared to the observed patterns of Alouatta behavior, it is found that both higher plant quality (higher protein and lower phenolic levels) and higher density of these resources are influencing differences in Alouatta density. While low levels of hunting is likely affecting population density at the low density site, it is suggested that resource quality in addition to abundance are the primary factors contributing to the differences in densities, supporting previous findings in folivorous Old World primates.