Kevin B. Potts

Diet of chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda, 1. Diet composition and diversity.

Chimpanzees (Pan troglodytes) are ecologically flexible omnivores with broad diets comprising many plant and animal foods, although they mostly eat fruit (including figs). Like other ecologically flexible nonhuman primates (e.g., baboons, Papio spp.) with broad diets, their diets vary across habitats. Much data on diets come from short studies that may not capture the range of variation, however, and data are scant on variation within habitats and populations. We present data on diet composition and diversity for chimpanzees at Ngogo, in Kibale National Park, Uganda, collected over a 15‐year period, with a focus on the plant components of the diet. We compare Ngogo data to those on chimpanzees at the nearby Kibale site of Kanyawara, on other chimpanzee populations, and on some other frugivorous–omnivorous primates. Results support the argument that chimpanzees are ripe fruit specialists: Ngogo chimpanzees ate a broad, mostly fruit‐based diet, feeding time devoted to fruit varied positively with fruit availability, and diet diversity varied inversely with fruit availability. Comparison of Ngogo and Kanyawara shows much similarity, but also pronounced within‐population dietary variation. Chimpanzees fed much more on leaves, and much less on pith and stems, at Ngogo. Figs accounted for somewhat less feeding time at Ngogo, but those of Ficus mucuso were quantitatively the most important food. This species is essentially absent at Kanayawara; its abundance and high productivity at Ngogo, along with much higher abundance of several other important food species, help explain why chimpanzee community size and population density are over three times higher at Ngogo. High inter‐annual variation at Ngogo highlights the value of long‐term data for documenting the extent of ecological variation among chimpanzee populations and understanding how such variation might affect population biology and social dynamics. Am. J. Primatol. 73:1–16, 2011. 

Diet of chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda, 2. temporal variation and fallback foods

Highly frugivorous primates like chimpanzees (Pan trogolodytes) must contend with temporal variation in food abundance and quality by tracking fruit crops and relying more on alternative foods, some of them fallbacks, when fruit is scarce. We used behavioral data from 122 months between 1995 and 2009 plus 12 years of phenology records to investigate temporal dietary variation and use of fallback foods by chimpanzees at Ngogo, Kibale National Park, Uganda. Fruit, including figs, comprised most of the diet. Fruit and fig availability varied seasonally, but the exact timing of fruit production and the amount of fruit produced varied extensively from year to year, both overall and within and among species. Feeding time devoted to all major fruit and fig species was positively associated with availability, reinforcing the argument that chimpanzees are ripe fruit specialists. Feeding time devoted to figs—particularly Ficus mucuso (the top food)—varied inversely with the abundance of nonfig fruits and with foraging effort devoted to such fruit. However, figs contributed much of the diet for most of the year and are best seen as staples available most of the time and eaten in proportion to availability. Leaves also contributed much of the diet and served as fallbacks when nonfig fruits were scarce. In contrast to the nearby Kanywara study site in Kibale, pith and stems contributed little of the diet and were not fallbacks. Fruit seasons (periods of at least 2 months when nonfig fruits account for at least 40% of feeding time; Gilby & Wrangham., Behavioral Ecology and Sociobiology 61:1771–1779, 2007) were more common at Ngogo than Kanyawara, consistent with an earlier report that fruit availability varies less at Ngogo [Chapman et al., African Journal of Ecology 35:287–302, 1997]. F. mucuso is absent at Kanyawara; its high density at Ngogo, combined with lower variation in fruit availability, probably helps to explain why chimpanzee population density is much higher at Ngogo.

Suitable Habitats for Endangered Frugivorous Mammals: Small-Scale Comparison, Regeneration Forest and Chimpanzee Density in Kibale National Park, Uganda

Landscape patterns and chimpanzee (Pan troglodytes schweinfurthii) densities in Kibale National Park show important variation among communities that are geographically close to one another (from 1.5 to 5.1 chimpanzees/km2). Anthropogenic activities inside the park (past logging activities, current encroachment) and outside its limits (food and cash crops) may impact the amount and distribution of food resources for chimpanzees (frugivorous species) and their spatial distribution within the park. Spatial and temporal patterns of fruit availability were recorded over 18 months at Sebitoli (a site of intermediate chimpanzee density and higher anthropic pressure) with the aim of understanding the factors explaining chimpanzee density there, in comparison to results from two other sites, also in Kibale: Kanyawara (low chimpanzee density) and Ngogo (high density, and furthest from Sebitoli). Because of the post-logging regenerating status of the forest in Sebitoli and Kanyawara, smaller basal area (BA) of fruiting trees most widely consumed by the chimpanzees in Kanyawara and Sebitoli was expected compared to Ngogo (not logged commercially). Due to the distance between sites, spatial and temporal fruit abundance in Sebitoli was expected to be more similar to Kanyawara than to Ngogo. While species functional classes consumed by Sebitoli chimpanzees (foods eaten during periods of high or low fruit abundance) differ from the two other sites, Sebitoli is very similar to Kanyawara in terms of land-cover and consumed species. Among feeding trees, Ficus species are particularly important resources for chimpanzees at Sebitoli, where their basal area is higher than at Kanywara or Ngogo. Ficus species provided a relatively consistent supply of food for chimpanzees throughout the year, and we suggest that this could help to explain the unusually high density of chimpanzees in such a disturbed site.

Nutritional Ecology and Reproductive Output in Female Chimpanzees (Pan troglodytes): Variation Among and Within Populations

The efficiency with which animals procure calories and nutrients affects their growth, survival, and reproductive success (Bercovitch 1987; Altmann 1991, 1998; Koenig et al. 1997; Borries et al. 2001). In female primates, foraging efficiency may be a primary regulator of ovarian function, thus influencing the probability of conception and the duration of lactational amenorrhea (Knott 1998; Bentley 1999; Di Bitetti and Janson 2000; Brockman and van Schaik 2005). In particular, theoretical and empirical work has demonstrated that the quality and abundance of resources available and/or utilized during periods of relative food scarcity can have profound impacts on socioecology and population dynamics. Recently, attempts have been made to provide operational definitions of such resources (“fallback foods”; Lambert 2007; Marshall and Wrangham 2007; Marshall et al. 2009) and to generate testable hypotheses regarding their influence on primate ecology and evolution (see Potts et al. 2009; Harrison and Marshall 2011).

Variability in Population Density Is Paralleled by Large Differences in Foraging Efficiency in Chimpanzees (Pan troglodytes)

The assumption that population density restricts the foraging efficiency of individuals in the population via increased competition for resources underpins socioecological models of female social relationships in primates. We examined this assumption by comparing quantitative measures of foraging efficiency in two communities of chimpanzees (Pan troglodytes schweinfurthii) that inhabit the same contiguous forest in Kibale National Park, Uganda, but differ substantially in size and density. To calculate net caloric gain rates (our measure of foraging efficiency) we obtained data directly from focal follows of individual chimpanzees on 1) residency time in a feeding patch, 2) feeding rate in the patch, 3) the nutrient content of the items fed upon, and 4) the distance walked between feeding patches. We collected foraging efficiency data over 1059 h at Ngogo and 961 h at Kanyawara. We found that individuals in the high-density community (Ngogo) had higher mean foraging efficiency values than those in the low-density community (Kanyawara), and that foraging efficiency varied less over time at Ngogo when assessed over the time scales of individual feeding/traveling bouts and of daily net caloric gain rates. Fluctuation in net caloric gain rates on a monthly time scale was greater at Ngogo than at Kanyawara, but this was likely due to the nutritional effects of a mast fruiting event by one of the most important species at the site. These findings suggest that high population density at Ngogo has not precluded high foraging efficiency. The classic view of increased population density inevitably increasing feeding competition and reducing foraging efficiency likely underemphasizes the ability of primates, especially those in rich habitats, to maximize caloric intake.

Ecological factors influencing habitat use by chimpanzees at Ngogo, Kibale National Park, Uganda.

Although numerous ecological and social factors influence range use in vertebrates, the general assumption is that ranging patterns typically accord with principles of optimal foraging theory. However, given temporal variability in resource abundance, animals can more easily meet nutritional needs at some times than at others. For species in which sociality is particularly important for fitness, such as chimpanzees (Pan troglodytes) and other group‐living primates, the influences of social factors can be particularly strong, and likely interact closely with ecological factors. We investigated home range use by a community of chimpanzees at Ngogo, Kibale National Park, Uganda, to determine whether range use corresponded to energy‐based optimality principles. Chimpanzees were particularly attracted to areas of the home range where individuals of Ficus mucuso (a large but low‐density resource) were found, but only if those areas also offered other preferred or important resource classes. The aggregation of large foraging parties at F. mucuso crowns (frequently seen year‐round) facilitates a number of socially beneficial activities for both males and females. Because chimpanzees apparently seek out F. mucuso in areas where other high‐quality feeding opportunities exist, these social benefits likely do not come at the expense of fitness benefits accrued from feeding on high‐quality resources. Am. J. Primatol. 78:432–440, 2016.

Remembering Jerry Lwanga: A Perspective from His Colleagues

Tony L. Goldberg · Samuel Angedakin · Gilbert M. Isabirye Basuta · Michelle Brown · Thomas M. Butynski · Colin A. Chapman · Lauren Chapman · Sholly Gunter · Innocent Kato · Jean-Michel Krief · Sabrina Krief · Joanna E. Lambert · Kevin E. Langergraber · John C. Mitani · Martin N. Muller · Sherry V. Nelson · Patrick Omeja · Emily Otali · Kevin B. Potts · Elizabeth A. Ross · Jessica M. Rothman · Carolyn Rowney · Eric Sande · Thomas T. Struhsaker · Dennis Twinomugisha · David P. Watts · Geoffrey Weny · Richard W. Wrangham