UC Santa Barbara

Energy is a fundamental resource that drives biological processes, and how animals acquire and mobilize energy from their environments is essential for their survival and reproductive success. Competition for these limited resources is a near-universal force shaping species interactions and ecological communities, but its intensity and outcomes are difficult to quantify and have been historically under-studied in primates. My research aims to investigate these dynamics by examining how fluctuations in competitive pressure influence food access, foraging behavior, physical condition, and reproduction in red-tailed monkeys (Cercopithecus ascanius). Red-tails offer a unique opportunity to study these interactions; they are small-bodied, arboreal and fruit-eating primates, whose physiology and existence among several large and dominant competitors make them particularly vulnerable to the costs of competition. To determine the energetic costs of competition, I studied red-tailed monkeys across three sites in and around Kibale National Park in Uganda with varying degrees of competition from chimpanzees (Pan troglodytes), grey-cheeked mangabeys (Lophocebus albigena), and blue monkeys (C. mitis), their main competitors. Using a combination of observational data, metabolic biomarkers, and remote-sensing tools, I evaluated how these competitors affect red-tail physical condition and reproduction.

In Chapter 2, I examined how fluctuations in fruit availability and the presence of chimpanzees, who are dominant frugivorous competitors and predators of red-tails, influence red-tailed monkeys’ foraging and energy balance. I evaluated urinary C-peptide, a non-invasive biomarker of energy balance, from six social groups at Ngogo Research Station in Kibale National Park, against fruit availability, foraging behavior, and fluctuations in competitive pressure from chimpanzees. I found that red-tail energy balance decreases dramatically when chimpanzees are drawn to their home range and limit the red-tails’ ability to profit from local resources. Polyspecific association with mangabeys also had a similar effect, but blue monkeys did not, underscoring the complexity of interactions within primate feeding guilds. These findings challenge the assumption that more food automatically leads to better nutritional outcomes for all primates.

Since human-measured fruit availability does not represent what is accessible for these subordinate primates, in Chapter 3, I tested an alternative measure of habitat productivity. I used remote-sensed Enhanced Vegetation Index (EVI) data to predict fruit availability and energy balance in frugivorous primates in tropical rainforests. I performed a lag analysis to test whether EVI minima, which correspond with leaf flushing that occurs approximately 2.5 months before fruiting, predicts red-tail C-peptide. EVI minima correlated with high C-peptide values 96 days (~3 months) later and with increased fruit availability 32 days later (~1 month), establishing a link between remote-sensed measures of habitat productivity and frugivorous primate foraging behavior in biodiverse environments.

Lastly, in Chapter 4, I used a comprehensive metabolic biomarker panel for an in-depth investigation of energy mobilization across a gradient of competitive pressure. Using energy balance (urinary C-peptide), relative muscle mass (creatinine), metabolic rate (triiodothyronine), ketosis (ketones), and protein catabolism (stable nitrogen isotopes), I identified four distinct metabolic profiles spanning different physical conditions. Groups facing higher competition exhibit prolonged energy deficits compared to those with few or no competitors. Competition had no effect on reproduction, indicating that red-tails prioritize reproduction over long-term physical condition, a strategy consistent with fast-paced life histories.

By integrating physiological, ecological, and technological approaches, this dissertation advances our understanding of how small-bodied primates adapt to competitive pressures in dynamic ecosystems. The findings highlight the critical role of competition in shaping energy allocation strategies and primate life histories, while drawing connections between species-level interactions and intrinsic energy mobilization. This underexplored aspect of primate ecology provides new insights into the complex challenges small primates face in under fluctuating competitive landscapes, with broader implications for conservation efforts in increasingly fragmented habitats.