Why do primates eat the leaves of one tree and ignore those on an adjacent tree of the same species?
At a broad level, these are the kinds of questions that inspire my research.
My students and I conduct several interlinked projects to understand the role of nutrition in primate species abundance and coexistence. Most of my fieldwork is currently in Kibale National Park, an evergreen forest in western Uganda that hosts one of the highest densities of primates in the world. I also study mountain gorillas in Bwindi Impenetrable National Park, where half the world's population of these endangered apes live.
Nutrition and Primate Biomass
Many researchers have demonstrated that the protein-to-fiber ratio of the mature leaves in primate habitats is a good predictor of folivore abundance at regional and local scales across Africa and Asia. Although this model has excellent predictive power, from a nutritional viewpoint this relationship is unexpected. One puzzling aspect is that folivorous primates at many of these sites consume crude protein that is well in excess of their estimated protein requirement, suggesting that protein is not a limiting nutrient. We are examining a number of nutritional factors that could be driving this relationship, including protein quality and the importance of energy.
Global Change and Primate Nutrition
Using long-term datasets that are uniquely available from Kibale National Park through the work of Tom Struhsaker and Colin Chapman, we are investigating some of the cascading effects of anthropogenic alterations on the nutritional composition of foods such as climate change, logging, human use and edge effects. For example, through experimental studies it has been demonstrated that the elevated temperature, rainfall, and CO2 associated with climate change reduces leaf protein, increases fiber, and increases production of plant defensive chemicals, all of which could have a severe impact on nutrition. We are also investigating evolutionary responses to changes by comparing feeding strategies exhibited today to those quantified 30+ years ago.
Nutrition and Juvenile Growth
Since nutrition early in life has been suggested as an important predictor of lifetime fitness, we are investigating the role of juvenile nutrition on growth of two species of colobine monkeys (red colobus and black-and white colobus). We quantify the nutritional gains of juvenile primates during their first year of life, and monitor the growth of individual monkeys remotely using parallel lasers attached to a digital camera. This research will provide insights into the nutritional factors influencing population growth and improve our understanding of the role of nutrition in contributing to juvenile recruitment into primate populations, and the social systems of these two sympatric colobines.
Nutritional Balancing Acts
Using innovative frameworks of nutritional ecology, including the geometric framework and the right angle mixture triangle, we are investigating the nutritional priorities of folivorous and frugivorous monkeys and apes. Since evolutionarily animals will ingest the balance, amount and combinations of nutrients that enhance fitness, we are investigating the ways in which sympatric species make food choices that will prioritize the intake and/or balance of specific nutrients and toxins. This research has implications for understanding how individual food items contribute to meeting nutritional goals, and is important for understanding the evolution of human diet and primate sociality. This research is conducted in collaboration with David Raubenheimer (University of Sydney).
Intraspecific Variability in Primate Resources
Our previous studies have demonstrated that there can be dramatic withinspecies variability in the nutritional content of primate foods. We are further examining this, and are conducting a fine scale analysis of the within-species variation in nutritional composition of primate foods to assess the heterogeneity resource distribution. One example of this exploration is our study on the vertical allocation of nutrients within a tree. Since leaves in the upper crown receive more sunlight and therefore photosynthesize at a faster rate, we expect that they may differ nutritionally from leaves in the bottom of a crown. If so, preferential feeding on high-quality leaves may affect the placement of monkeys within a tree and have important implications for primate ecology and sociality. To deal with this variation, we are currently exploring the use of near-infrared reflectance spectroscopy as a means to rapidly analyze large numbers of samples for a suite of nutrients.
Mountain Gorilla Nutrition
My Ph.D. research at Cornell University focused on understanding how endangered mountain gorillas meet their nutritional requirements. I conducted my fieldwork in Bwindi Impenetrable National Park, a montane rainforest in southwestern Uganda located on the border with the Democratic Republic of Congo. Gorilla habitat in Bwindi is ecologically different from the well-studied mountain gorillas in the Virunga Region of Rwanda just 35 km away. In Bwindi, there are more fruiting trees and a greater diversity of plant species. In collaboration with Andy Plumptre (Albertine Rift Program, Wildlife Conservation Society), I compared the nutritional strategies of the Virunga gorillas to those in Bwindi. Despite the fact that the Bwindi and Virunga gorillas had a very different resource base and ate different food parts and species, we found that the nutritional quality of their diets was remarkably similar, indicating that diets containing fruit are not necessarily nutritionally different to those containing leaves.
The individuals in the study group exhibited variable nutritional strategies. Silverback males ate more food to meet the needs of their large body size, but spent less time feeding than adult females and juveniles by increasing their intake rate. Adult females and juveniles consumed more protein and energy per kg of metabolic body mass than silverbacks to meet the increased nutritional needs of lactation (adult females) and growth (juveniles). Females and juveniles met these increased needs by eating more food per kg of metabolic body mass than silverbacks, and did not select higher protein and higher energy food items. Although available energy may be seasonally insufficient for other apes, both protein and energy did not appear to be limiting for the Bwindi gorillas, rather an important mineral, sodium, appeared to be deficient, and over 95% of the sodium in gorilla diets was contributed by an unusual resource - decaying wood. In collaboration with Julie Frey, Esther Angert, Mike Cranfield and John Bosco Nizeyi we characterized the bacteria in gorilla feces and found that gorillas harbored bacteria that had strong similarity to those that degrade fiber and secondary compounds.
This nutrition work was done in collaboration with my PhD advisors and colleagues, Alice Pell, Ellen Dierenfeld, Skip Hintz, Julie Frey, Esther Angert and Peter Van Soest
I continue to conduct research in Bwindi on the mountain gorilla diet. In collaboration with David Raubenheimer and Colin Chapman, I used the geometric framework to demonstrate that gorillas prioritize the intake of energy and eat large amounts of protein in order to meet their energy needs. This research is counter to the widespread view that nitrogen is limiting in animal ecosystems and that gorillas select particular foods for their protein content. We also demonstrated that during the fruiting seasons, gorillas eat a balance of protein and energy that is similar to the American Heart Association's recommendation for healthy humans. When fruits are scarce, they eat a very high protein diet, similar to the concentrations of protein in human high-protein weight loss diets (but gorilla diets are very low in fat!).
In collaboration with Dwight Bowman, I also considered the importance of intestinal parasites to gorilla nutritional strategy. The gorillas harbored at least 5 intestinal parasites. Unlike chimpanzees, Bwindi gorillas did not appear to intentionally consume items for their medicinal purposes; however, properties of their staple diet, particularly variation in tannin consumption, affected intestinal parasites.
In collaboration with NYCEP graduate student Claudia Astorino, we found that the gorillas apparently consume many "functional foods" that have antioxidant properties, which may provide important health benefits.
Nutritional Ecology of Primates Across the Globe- Collaborative Studies
I have many collaborations with researchers at other universities on a wide variety of primate taxa and topics, including: