Nutrient drivers and movement ecology of wild Bornean orangutan (Pongo pygmaeus wurmbii) foraging choices
DiGiorgio, Andrea L.
MetadataShow full item record
Understanding why animals make the foraging choices they do has been an interdisciplinary research goal for decades. This question is especially salient in biological anthropology, as we seek to understand how the human diet evolved by looking to non-human primate models. Optimal Foraging Theory (OFT), and more recently the Geometric Framework for Nutrition (GF) and Movement Ecology paradigms provide models and heuristics that aid in our understanding of what drives the foraging decisions of animals. Yet until recently, most research examining the foraging decisions of frugivorous herbivores has focused on the OFT based strategy of maximizing and obtaining fruit foods. My research examines alternative nutrient priorities in a frugivorous primate, the Bornean orangutan (Pongo pygmaeus wurmbii), to determine whether fruit-seeking and energy maximization form a sufficient explanation of foraging behavior. Using behavioral and geospatial data from full-day focal animal follows collected in 2015-2016 in Gunung Palung National Park, I first demonstrate that orangutans leave available fruit resources to eat non-fruit foods, suggesting that orangutans are intentionally seeking out non-fruit foods, and clearing the way for a foraging model beyond strict energy maximization via fruit seeking. Further, I find that orangutans do consume non-fruit food when fruit is in visual proximity. I next test whether the Marginal Value Theorem (MVT) of OFT can account for these fruit patch departures by testing one critical assumption of MVT – that feeding rates in a patch decrease over time. Feeding rates did not decrease over patch residence, thus MVT does not explain orangutan fruit patch departures. Instead, I find that orangutans maintain an average 10:1 ratio of non-protein energy to protein, and prioritize protein intake. These findings could explain fruit departure. Geospatial data suggest that fruit is not the only goal of orangutan foraging and that these apes navigate to other food types, in particular, leaves. Taken together, these findings suggest that GF provides a good explanation of orangutan foraging in tandem with OFT energy maximization. I discuss the similarities in nutrient goals between orangutans, modern humans, and extinct hominins, and the conservation implications of my research.