Nutritional ecology is of central importance to understanding primate foraging and social behavior (Lambert and Rothman, 2015; Bryer and Uwimbabazi, 2024). There is considerable interest in nutritional aspects of primate feeding ecology (Glander, 1977; Milton, 1981; Rothman et al., 2008a; Ganas et al., 2009; Heesen et al., 2013; Irwin et al., 2014) and a growing literature on the quantitative assessment of the nutritional content of the diets of wild chimpanzees (Pan troglodytes) (Conklin-Brittain et al., 1998; Reynolds et al., 1998; Wrangham et al., 1998; Matsumoto-Oda and Hayashi, 1999; Takemoto, 2003; Deblauwe and Janssens, 2008; Hohmann and Fruth, 2008; O'Malley and Power, 2012; McLennan and Ganzhorn, 2017; Uwimbabazi et al., 2019, 2021; Lindshield et al., 2021a). The vast majority of chimpanzee studies are based on populations in forest and forest-mosaic landscapes (Lindshield et al., 2021b; for definitions of savannas, forests, and forest-mosaics, see van Leeuwen et al., 2020), and there has not yet been a nutritional assessment of the diets of chimpanzees in savanna landscapes (hereafter, ‘savanna chimpanzees’; reviewed in Lindshield et al., 2021b), where forest vegetation comprises a tiny fraction of land cover. In general, savanna environments have a lower density of food resources than forests, with fewer large fruiting trees, as well as few herbaceous food options during the dry season (Bromage and Schrenk, 1995; Lindshield et al., 2021b). Furthermore, given that protein and sugar concentrations in some plant foods are sensitive to sun exposure (Ganzhorn, 1995; Houle et al., 2014), it is possible that savanna flora differ in nutritional composition relative to flora in forests and forest-mosaics. Chimpanzees occupying these habitats constitute a minority of wild populations but are of high interest because of the window they offer into the evolution of early hominins during periods of relative aridity (Potts, 1998; Bonnefille, 2010; Barboni, 2014; Uno et al., 2016; Nelson and Hamilton, 2017; Negash et al., 2024).

Extant species are invaluable to understanding and interpreting the fossil record (e.g., Negash et al., 2024), and primates, specifically, serve as models for paleoanthropological reconstructions by way of homology or analogy (Moore, 1996; Elton, 2006; Pilbeam and Lieberman, 2017). Oftentimes, chimpanzees and bonobos (genus Pan) are the standard-bearers in this regard (Gruber and Clay, 2016) as they are the closest living relatives of Homo sapiens and are anatomically similar to many early hominins (Pilbeam and Lieberman, 2017). African great apes are not living replicas either, as they retain adaptations lost in early hominins (Pilbeam and Lieberman, 2017), and therein offer clues about the paleoenvironmental pressures contributing to the Homo-Pan divergence. This comparative approach situates hominins within a phylogenetic context to identify homologous and derived traits as well as homoplasies (Pilbeam and Lieberman, 2017). Moreover, primate models enable us to apply patterns of the present to reconstruct past behaviors (Ungar and Sponheimer, 2011). There is a compelling case to be made for chimpanzees as ideal representatives of the last common ancestor with humans based on the evidence for evolutionary conservatism in the Pan lineage (e.g., Muller, 2017; but see Marks, 2002; Sayers and Lovejoy, 2008 for contrasting viewpoints). Savanna chimpanzees are useful models, given that paleoenvironmental reconstructions reveal a geographically widespread distribution of savanna landscapes, including mosaics of woodlands, bushlands/shrublands, and wooded grasslands, which were contemporaneous with several early hominin sites (e.g., Ardipithecus ramidus at Aramis, Ethiopia: Negash et al., 2024). Moreover, dietary stable isotope comparisons among early hominins and African great apes indicate that savanna chimpanzee diets closely resemble those of Ar. ramidus in terms of plant food ingestion (δ13C: Nelson and Hamilton, 2017).

Foods are packages of nutrients that vary in quality among parts (e.g., fruits, flowers, leaves, and insects), species, phenophases, and individuals and across time periods and study areas (Conklin-Brittain et al., 1998; Chapman et al., 2003; Houle et al., 2014; Lambert and Rothman, 2015; McLennan and Ganzhorn, 2017). Foragers presumably select foods from their complex environments based on several intrinsic (e.g., digestion physiology: Lambert, 1998; Felton et al., 2009; Lambert and Rothman, 2015) and extrinsic (e.g., local resource availability: Heller, 1980; Wrangham et al., 1991; Watts et al., 2012; Uwimbabazi et al., 2019; intragroup feeding competition: Goodall, 1986; Emery Thompson et al., 2007; Gilby et al., 2017) factors that are consequential to fitness (Emlen, 1966; MacArthur and Pianka, 1966; Stephens and Krebs, 1986; Stephens et al., 2007; Raubenheimer et al., 2009, 2015; Watts et al., 2012; Lambert and Rothman, 2015; Uwimbabazi et al., 2021). Compared to sympatric frugivorous monkeys, chimpanzees target high-energy foods more often, such as ripe fleshy fruits (e.g., berries, drupes, and figs: Balcomb et al., 2000) even during periods of low fruit availability (Wrangham et al., 1998; Yamagiwa and Basabose, 2006; Head et al., 2011). Fruits are generally high in sugar and moderate in fiber levels, and when they are ripe, they are even higher in sugar and lower in fiber and secondary metabolites (Houle et al., 2014; Lambert and Rothman, 2015). The importance of ripe fruit to chimpanzees is evidence of a preference for sugary foods (Remis, 2002, 2006) and, when combined with fast ingestion rates, results in high energy intake (Uwimbabazi et al., 2019; Lindshield et al., 2021a).

While ripe fruit is central to their diet (reviewed in Watts et al., 2012), chimpanzees frequently ingest complementary foods for nutrient balancing (Uwimbabazi et al., 2021), such as leafy material and animal prey. They eat substantial amounts of leaves, which are generally lower in sugar and higher in fiber and protein than fruit (Lambert and Rothman, 2015). Ingesting leafy material may help individuals meet their protein intake targets (Uwimbabazi et al., 2021) or serve as important energy sources during periods of fruit scarcity (Isabirye-Basuta, 1989). Animal prey (e.g., mammals and insects) contribute protein, fat, and essential vitamins to their diet (Tennie et al., 2009; O'Malley and Power, 2014). Chimpanzees may depend on relatively common foods with adequate macronutrient levels to meet daily requirements (i.e., ‘staples’), such as Ficus fruits (Conklin and Wrangham, 1994; Newton-Fisher, 1999; Yamagiwa and Basabose, 2009; Potts et al., 2011; Uwimbabazi et al., 2019), and incorporate seasonal foods with high caloric values (N'guessan et al., 2009) or ephemeral foods for nutrient supplementation (e.g., vitamin B12: Tennie et al., 2009) based on local resource availability. In response to ripe fruit scarcity, the dietary breadth expands (Watts et al., 2012) and chimpanzees increase consumption of lower-quality foods (i.e., ‘fallback’ foods; Marshall et al., 2009; Lambert and Rothman, 2015), such as piths, stems, and bark cambium (Tutin et al., 1991; Wrangham et al., 1991; Bogart and Pruetz, 2011; Lapuente et al., 2020).

While these dietary trends apply to many chimpanzee communities (e.g., ripe fruit is important to all chimpanzee groups), a common theme across studies is that food quality (Hohmann et al., 2010) and food availability (Newton-Fisher, 1999; Potts et al., 2011) vary among sites and across time periods to a degree that makes it difficult to generalize about diets at the species level (Watts et al., 2012). For instance, one chimpanzee community may allocate as much as 24% of annual feeding time to insects, while another excludes insects from the diet (Bogart and Pruetz, 2011). For this reason, it is important to study the nutritional ecology of chimpanzees across a range of environments.

The diets of savanna chimpanzees mostly correspond to species-wide trends, including omnivory, frugivory, and sensitivity to local resource availability (Hunt and McGrew, 2002; Bogart and Pruetz, 2011; Webster et al., 2014; Piel et al., 2017). Savanna biomes, with their overall lower tree biomass than forests and forest-mosaics (van Leeuwen et al., 2020), influence food availability and thus impact diet as well (Lindshield et al., 2021b). Some distinguishing features of savanna chimpanzee diets include a greater reliance on plant foods other than fleshy ripe fruits, such as dry-adapted pods and seeds (Suzuki, 1969; McGrew et al., 1988; Schoeninger et al., 1999; van Casteren et al., 2018; Lindshield et al., 2021a, 2021b), unripe fruits (Suzuki, 1969; Pruetz, 2006; Lindshield et al., 2021a), flowers (Pruetz, 2006; Watts et al., 2012), and underground storage organs (Hernandez-Aguilar et al., 2007). The exocarps of many fruits appear to be tougher and stiffer in savannas, potentially as an adaptation to the drier environment (Suzuki, 1969; McGrew et al., 1988; van Casteren et al., 2018). Chimpanzees in savannas may exhibit slightly lower rates of meat ingestion, not necessarily because of lower hunting rates but because they are hunting smaller prey more often, such as bushbabies and rodents (Moore et al., 2017). Insects feature prominently in their diets, but there is high variation in consumption among sites (Webster et al., 2014). For termites, specifically, ingestion may occur throughout the year at relatively high rates (e.g., Macrotermes at Fongoli: Bogart and Pruetz, 2011) or seasonally pulse (e.g., Macrotermes at Issa, Tanzania: Stewart and Piel, 2013; Phillips et al., 2023).

The most intensively studied savanna chimpanzee group inhabits a semiarid savanna woodland at Fongoli, Senegal, where temperatures in excess of 40 °C are common (Pruetz, 2025). This population has been of special interest to anthropologists because of their behavioral adjustments to living in a hot, dry environment, including using caves to take refuge from the heat, soaking in water, hand-digging wells, and exhibiting frequent nocturnality (Pruetz, 2018, 2025; Wessling et al., 2018a, 2018b; Lindshield et al., 2021b). A particularly intriguing behavior among Fongoli chimpanzees is their regular use of tools to hunt vertebrate prey, a behavior exhibited more frequently by females than males (Pruetz et al., 2015). This behavior has inspired discussion on the potential for tool-assisted hunting to have developed as an adaptation to the environmental challenges of living in a savanna, with particular relevance to how early hominins may have done the same (Pruetz et al., 2015). These environmental challenges may include a patchier distribution of resources (Pruetz et al., 2015) and a lower availability and smaller body size of vertebrate prey than in forests, potentially requiring adaptive strategies to boost protein intake (Bogart and Pruetz, 2011; Pickering and Domínguez-Rodrigo, 2012). In line with this idea, Fongoli chimpanzees spend more time in termite fishing than other populations studied to date, and they do so year-round (Bogart and Pruetz, 2011).

This study is the first to document the nutrient concentrations of chimpanzee foods from a semiarid, savanna environment. First, we aim to identify how chimpanzees in the savanna select their foods. More specifically, we assessed whether food intake was associated with nutrient content, food availability, and social factors. Uwimbabazi et al. (2021) found that protein intake was regulated within a narrow range across seasons in female chimpanzees in Kibale. It is possible that chimpanzees maximize energy intake while ensuring they have a steady intake of protein within their optimal intake range, as suggested by Uwimbabazi et al. (2021). Therefore, we predict that chimpanzees at Fongoli consume foods according to their energetic yield. We also expect that chimpanzees ingest clumped or abundant foods more than those that are scattered or scarce as a way to minimize search effort, on the basis that savanna chimpanzees range in areas with less forest and tree cover (van Leeuwen et al., 2020; Wessling et al., 2020). Food availability varies among months, and Fongoli chimpanzees allocate more feeding time to fruit when it is abundant (Pruetz, 2006; Bogart and Pruetz, 2011; Wessling et al., 2018a; Lindshield et al., 2021a). Baobab (Adansonia digitata) fruit is an especially important food at Fongoli when it is in season as it is relatively rich in sugars and energy (Lindshield et al., 2021a). As food is a limited resource, access to key foods or feeding sites can be influenced by an individual's social status while foraging in a group. For instance, dominant individuals may steal food from subordinates (Teleki, 1973; Goodall, 1986; Gilby et al., 2017) or supplant them from feeding locations (Goodall, 1986). We therefore predict that chimpanzees feed more on high-yield foods when they are in season and when their social rank allows them priority access.

Our second aim is to compare and contrast foods between savanna and forest sites. Savannas have more dry-adapted flora, and fruits consumed by chimpanzees in savannas in East Africa have tougher and stiffer exocarps than those in the forest (van Casteren et al., 2018). Savanna fruits, however, may be higher in sugars as there is a positive relationship between sugar concentration and sunlight exposure in wild fruits (Houle et al., 2014). Plants receive more sunlight in open-canopy vegetation (woodlands, grasslands: Lindshield et al., 2017); most foods at Fongoli are found in open vegetation types, and grasslands and woodlands dominate this landscape (Pruetz and Bertolani, 2009). There is also a particular importance of nonfleshy fruits (e.g., capsules, follicles, pods) and unripe fruits in chimpanzee diets at Fongoli (Pruetz, 2006; Lindshield et al., 2021a). We therefore compare the nutritional compositions of these fruit types to ripe fleshy fruits (berries, drupes, and figs), predicting that nonfleshy fruits are lower in water and higher in sugar than fleshy fruits. In comparison to fruits consumed at forest sites, we predict that fruits consumed by chimpanzees at Fongoli will be higher in sugars. Our study contributes to a better understanding of food selection in savanna chimpanzees, enables more detailed dietary comparisons among savanna and forest groups, and provides novel observations of dry-adapted plant foods and their nutrients to enhance diet reconstructions for early hominins.