Why do chimpanzees hunt? Considering the benefits and costs of acquiring and consuming vertebrate versus invertebrate prey

Sodium content in plant and insect food resources consumed by chimpanzees (Pan troglodytes schweinfurthii) in Gombe National Park, Tanzania

OBJECTIVES

Many nonhuman primate diets are dominated by plant foods, yet plant tissues are often poor sources of sodium-a necessary mineral for metabolism and health. Among primates, chimpanzees (Pan troglodytes), which are ripe fruit specialists, consume diverse animal, and plant resources. Insects have been proposed as a source of dietary sodium for chimpanzees, yet published data on sodium values for specific foods are limited. We assayed plants and insects commonly eaten by chimpanzees to assess their relative value as sodium sources.

MATERIALS AND METHODS

We used atomic absorption spectroscopy to determine sodium content of key plant foods and insects consumed by chimpanzees of Gombe National Park, Tanzania. Dietary contributions of plant and insect foods were calculated using feeding observational data.

RESULTS

On a dry matter basis, mean sodium value of plant foods (n = 83 samples; mean = 86 ppm, SD = 92 ppm) was significantly lower than insects (n = 12; mean = 1549 ppm, SD = 807 ppm) (Wilcoxon rank sum test: W = 975, p < 0.001). All plant values were below the suggested sodium requirement (2000 ppm) for captive primates. While values of assayed insects were variable, sodium content of two commonly consumed insect prey for Gombe chimpanzees (Macrotermes soldiers and Dorylus ants) were four to five times greater than the highest plant values and likely meet requirements.

DISCUSSION

We conclude that plant foods available to Gombe chimpanzees are generally poor sources of sodium while insects are important, perhaps critical, sources of sodium for this population.

Free hand hitting of stone-like objects in wild gorillas

The earliest stone tool types, sharp flakes knapped from stone cores, are assumed to have played a crucial role in human cognitive evolution. Flaked stone tools have been observed to be accidentally produced when wild monkeys use handheld stones as tools. Holding a stone core in hand and hitting it with another in the absence of flaking, free hand hitting, has been considered a requirement for producing sharp stone flakes by hitting stone on stone, free hand percussion. We report on five observations of free hand hitting behavior in two wild western gorillas, using stone-like objects (pieces of termite mound). Gorillas are therefore the second non-human lineage primate showing free-hand hitting behavior in the wild, and ours is the first report for free hand hitting behavior in wild apes. This study helps to shed light on the morphofunctional and cognitive requirements for the emergence of stone tool production as it shows that a prerequisite for free hand percussion (namely, free hand hitting) is part of the spontaneous behavioral repertoire of one of humans' closest relatives (gorillas). However, the ability to combine free hand hitting with the force, precision, and accuracy needed to facilitate conchoidal fracture in free hand percussion may still have been a critical watershed for hominin evolution.

Pathways to paternal care in primates

Natural selection will favor male care when males have limited alternative mating opportunities, can invest in their own offspring, and when care enhances males' fitness. These conditions are easiest to fulfill in pair-bonded species, but neither male care nor stable "breeding bonds" that facilitate it are limited to pair-bonded species. We review evidence of paternal care and extended breeding bonds in owl monkeys, baboons, Assamese macaques, mountain gorillas, and chimpanzees. The data, which span social/mating systems and ecologies, suggest that there are multiple pathways by which conditions conducive to male care can arise. This diversity highlights the difficulty of making inferences about the emergence of male care in early hominins based on single traits visible in the fossil record. We discuss what types of data are most needed and the questions yet to be answered about the evolution of male care and extended breeding bonds in the primate order.

Cost-Benefit Trade-Offs of Aquatic Resource Exploitation in the Context of Hominin Evolution

While the exploitation of aquatic fauna and flora has been documented in several primate species to date, the evolutionary contexts and mechanisms behind the emergence of this behavior in both human and non-human primates remain largely overlooked. Yet, this issue is particularly important for our understanding of human evolution, as hominins represent not only the primate group with the highest degree of adaptedness to aquatic environments, but also the only group in which true coastal and maritime adaptations have evolved. As such, in the present study we review the available literature on primate foraging strategies related to the exploitation of aquatic resources and their putative associated cognitive operations. We propose that aquatic resource consumption in extant primates can be interpreted as a highly site-specific behavioral expression of a generic adaptive foraging decision-making process, emerging in sites at which the local cost-benefit trade-offs contextually favor aquatic over terrestrial foods. Within this framework, we discuss the potential impacts that the unique intensification of this behavior in hominins may have had on the evolution of the human brain and spatial ecology.

Balancing costs and benefits in primates: ecological and palaeoanthropological views

Maintaining the balance between costs and benefits is challenging for species living in complex and dynamic socio-ecological environments, such as primates, but also crucial for shaping life history, reproductive and feeding strategies. Indeed, individuals must decide to invest time and energy to obtain food, services and partners, with little direct feedback on the success of their investments. Whereas decision-making relies heavily upon cognition in humans, the extent to which it also involves cognition in other species, based on their environmental constraints, has remained a challenging question. Building mental representations relating behaviours and their long-term outcome could be critical for other primates, but there are actually very little data relating cognition to real socio-ecological challenges in extant and extinct primates. Here, we review available data illustrating how specific cognitive processes enable(d) modern primates and extinct hominins to manage multiple resources (e.g. food, partners) and to organize their behaviour in space and time, both at the individual and at the group level. We particularly focus on how they overcome fluctuating and competing demands, and select courses of action corresponding to the best possible packages of potential costs and benefits in reproductive and foraging contexts. This article is part of the theme issue 'Existence and prevalence of economic behaviours among non-human primates'.

Meat eating by nonhuman primates: A review and synthesis

Most nonhuman primates prey on vertebrates. Meat-eating, defined as ingestion of vertebrate tissue, occurs in 12 families, ≥39 genera, and ≥89 species. It is most common in capuchins (Cebus and Sapajus spp.), baboons (Papio spp.), bonobos (Pan paniscus), and chimpanzees (Pan troglodytes) and modestly common in blue monkeys (Cercopithecus mitis), callitrichids (Callithrix spp. and Saguinus spp.), and squirrel monkeys (Saimiri spp.). It is uncommon in other cercopithecines, rare in other haplorhines and in lemurs, and virtually absent in colobines. Birds are the prey class eaten by the most species (≥53), followed by reptiles (≥48), amphibians (≥38), mammals (≥35), and fish (≥7). Major hypotheses for the importance of meat eating are that it is (1) mainly an energy source, especially (1a) when plant-source foods (PSFs) with high energy return rates are scarce (energy shortfall hypothesis); (2) mainly a protein source; and (3) mainly a source of micronutrients scarce in PSFs. Meat eating bouts sometimes provide substantial energy and protein, and some chimpanzees gain substantial protein from meat monthly or annually. However, meat typically accounts for only small proportions of feeding time and of total energy and protein intake, and quantitative data are inconsistent with the energy shortfall hypothesis. PSFs and/or invertebrates are presumably the main protein sources, even for chimpanzees. Support is strongest for the micronutrient hypothesis. Most chimpanzees eat far less meat than recorded for hunter-gatherers, but the highest chimpanzee estimates approach the lowest for African hunter-gatherers. In fundamental contrast to the human predatory pattern, other primates only eat vertebrates much smaller than they are, tool-assisted predation is rare except in some capuchins and chimpanzees, and tool use in carcass processing is virtually absent. However, harvesting of small prey deserves more attention with reference to the archaeological and ethnographic record.

Foraging strategies underlying bird egg predation by macaques: A study using artificial nests

Bird egg predation is widespread in non-human primates. Although nest predation is often described as opportunistic, little is known about foraging strategies and nest detection in primates. Since it is the prevalent cause of nest failure in the tropics, birds select nest sites within specific microhabitats and use different nest types to increase nesting success. Identifying the nests targeted by the northern pigtailed macaques (Macaca leonina), an omnivorous cercopithecine species, and known nest predator, will shine light on nest foraging strategies in primates. The aim of this research was to reveal if nest predation is a selective or opportunistic feeding behavior. We studied, using artificial nests and camera traps, the influence of nest type (open-cup vs. cavity), microhabitat (i.e., understory density, canopy cover, canopy height, ground cover, and presence vs. absence of thorns and lianas), and nest height, on nest predation by a troop of northern pigtailed macaques in the Sakaerat Biosphere Reserve (Thailand), a degraded environment. In our study, macaque predation on artificial nests was high; out of the 200 nests that were set up, 112 were plundered by macaques. Although predation rates decreased with nest height, nest type, and microhabitat had no significant effect on predation by macaques. Nest detectability and accessibility did not affect predation rates. Macaques actively searched for nests in different microhabitats, suggesting that nest predation by this primate might be considered a selective feeding behavior in this degraded habitat. Consequently, nest predation by this primate might have important conservation implications on the population dynamics of forest-dwelling bird species. Behavior observation methods, such as instantaneous scan sampling, may underestimate nest predation by primates, a furtive and cryptic behavior.

Chimpanzee vertebrate consumption: Savanna and forest chimpanzees compared

There is broad consensus among paleoanthropologists that meat-eating played a key role in the evolution of Homo, but the details of where, when, and why are hotly debated. It has been argued that increased faunivory was causally connected with hominin adaptation to open, savanna habitats. If savanna-dwelling chimpanzees eat meat more frequently than do forest chimpanzees, it would support the notion that open, dry, seasonal habitats promote hunting or scavenging by hominoids. Here we present observational and fecal analysis data on vertebrate consumption from several localities within the dry, open Ugalla region of Tanzania. Combining these with published fecal analyses, we summarize chimpanzee vertebrate consumption rates, showing quantitatively that savanna chimpanzee populations do not differ significantly from forest populations. Compared with forest populations, savanna chimpanzees consume smaller vertebrates that are less likely to be shared, and they do so more seasonally. Analyses of chimpanzee hunting that focus exclusively on capture of forest monkeys are thus difficult to apply to chimpanzee faunivory in open-country habitats and may be misleading when used to model early hominin behavior. These findings bear on discussions of why chimpanzees hunt and suggest that increases in hominin faunivory were related to differences between hominins and chimpanzees and/or differences between modern and Pliocene savanna woodland environments.

Reproductive state and rank influence patterns of meat consumption in wild female chimpanzees (Pan troglodytes schweinfurthii)

An increase in faunivory is a consistent component of human evolutionary models. Animal matter is energy- and nutrient-dense and can provide macronutrients, minerals, and vitamins that are limited or absent in plant foods. For female humans and other omnivorous primates, faunivory may be of particular importance during the costly periods of pregnancy and early lactation. Yet, because animal prey is often monopolizable, access to fauna among group-living primates may be mediated by social factors such as rank. Wild chimpanzees (Pan troglodytes) across Africa habitually consume insects and/or vertebrates. However, no published studies have examined patterns of female chimpanzee faunivory during pregnancy and early lactation relative to non-reproductive periods, or by females of different rank. In this study, we assessed the influence of reproductive state and dominance rank on the consumption of fauna (meat and insects) by female chimpanzees of Gombe National Park, Tanzania. Using observational data collected over 38 years, we tested (a) whether faunivory varied by reproductive state, and (b) if high-ranking females spent more time consuming fauna than lower-ranking females. In single-factor models, pregnant females consumed more meat than lactating and baseline (meaning not pregnant and not in early lactation) females, and high-ranking females consumed more meat than lower-ranking females. A two-factor analysis of a subset of well-sampled females identified an interaction between rank and reproductive state: lower-ranking females consumed more meat during pregnancy than lower-ranking lactating and baseline females did. High-ranking females did not significantly differ in meat consumption between reproductive states. We found no relationships between rank or reproductive state with insectivory. We conclude that, unlike insectivory, meat consumption by female chimpanzees is mediated by both reproductive state and social rank. We outline possible mechanisms for these patterns, relate our findings to meat-eating patterns in women from well-studied hunter-gatherer societies, and discuss potential avenues for future research.

'Impact hunters' catalyse cooperative hunting in two wild chimpanzee communities

Even when hunting in groups is mutually beneficial, it is unclear how communal hunts are initiated. If it is costly to be the only hunter, individuals should be reluctant to hunt unless others already are. We used 70 years of data from three communities to examine how male chimpanzees 'solve' this apparent collective action problem. The 'impact hunter' hypothesis proposes that group hunts are sometimes catalysed by certain individuals that hunt more readily than others. In two communities (Kasekela and Kanyawara), we identified a total of five males that exhibited high hunt participation rates for their age, and whose presence at an encounter with red colobus monkeys increased group hunting probability. Critically, these impact hunters were observed to hunt first more often than expected by chance. We argue that by hunting first, these males dilute prey defences and create opportunities for previously reluctant participants. This by-product mutualism can explain variation in group hunting rates within and between social groups. Hunting rates declined after the death of impact hunter FG in Kasekela and after impact hunter MS stopped hunting frequently in Kanyawara. There were no impact hunters in the third, smaller community (Mitumba), where, unlike the others, hunting probability increased with the number of females present at an encounter with prey.

New evidence on the tool-assisted hunting exhibited by chimpanzees (Pan troglodytes verus) in a savannah habitat at Fongoli, Sénégal

For anthropologists, meat eating by primates like chimpanzees (Pan troglodytes) warrants examination given the emphasis on hunting in human evolutionary history. As referential models, apes provide insight into the evolution of hominin hunting, given their phylogenetic relatedness and challenges reconstructing extinct hominin behaviour from palaeoanthropological evidence. Among chimpanzees, adult males are usually the main hunters, capturing vertebrate prey by hand. Savannah chimpanzees (P. t. verus) at Fongoli, Sénégal are the only known non-human population that systematically hunts vertebrate prey with tools, making them an important source for hypotheses of early hominin behaviour based on analogy. Here, we test the hypothesis that sex and age patterns in tool-assisted hunting (n=308 cases) at Fongoli occur and differ from chimpanzees elsewhere, and we compare tool-assisted hunting to the overall hunting pattern. Males accounted for 70% of all captures but hunted with tools less than expected based on their representation on hunting days. Females accounted for most tool-assisted hunting. We propose that social tolerance at Fongoli, along with the tool-assisted hunting method, permits individuals other than adult males to capture and retain control of prey, which is uncommon for chimpanzees. We assert that tool-assisted hunting could have similarly been important for early hominins.

New actualistic data on the ecology and energetics of hominin scavenging opportunities

For decades, the 'hunting-scavenging debate' has been an important research focus in Plio-Pleistocene hominin behavioral ecology. Here I present new data on potential scavenging opportunities from fresh carnivore kills on a conservancy in central Kenya. This ecosystem is dominated by felids (mainly lions) and has a different carnivore guild than in many earlier studies of scavenging opportunities that took place in areas such as Ngorongoro and Serengeti in Tanzania and Maasai Mara in Kenya, where lions face high levels of inter-specific competition from bone-crunching hyenas. I found that while scavenging opportunities vary among carcasses, most carcasses retained some scavengeable resources. Excluding within-bone resources, even the scavengeable meat on 'defleshed' larger sized prey carcasses is usually substantial enough to meet the total daily caloric requirements of at least one adult male Homo erectus individual. I argue, as others have before me, that scavenging opportunities in a particular ecosystem will vary in part due to carnivore taxon, density and guild composition; prey size, biomass and community structure; and habitat (e.g., vegetation, physiography). We should expect variability in scavenging opportunities in different locales and should focus our research efforts on identifying which variables condition these differences in order to make our findings applicable to the diversity of ecological settings characterizing the past.

The energetic and nutritional yields from insectivory for Kasekela chimpanzees

Insectivory is hypothesized to be an important source of macronutrients, minerals, and vitamins for chimpanzees (Pan troglodytes), yet nutritional data based on actual intake are lacking. Drawing on observations from 2008 to 2010 and recently published nutritional assays, we determined the energy, macronutrient and mineral yields for termite-fishing (Macrotermes), ant-dipping (Dorylus), and ant-fishing (Camponotus) by the Kasekela chimpanzees of Gombe National Park, Tanzania. We also estimated the yields from consumption of weaver ants (Oecophylla) and termite alates (Macrotermes and Pseudacanthotermes). On days when chimpanzees were observed to prey on insects, the time spent in insectivorous behavior ranged from <1 min to over 4 h. After excluding partial bouts and those of <1 min duration, ant-dipping bouts were of significantly shorter duration than the other two forms of tool-assisted insectivory but provided the highest mass intake rate. Termite-fishing bouts were of significantly longer duration than ant-dipping and had a lower mass intake rate, but provided higher mean and maximum mass yields. Ant-fishing bouts were comparable to termite-fishing bouts in duration but had significantly lower mass intake rates. Mean and maximum all-day yields from termite-fishing and ant-dipping contributed to or met estimated recommended intake (ERI) values for a broad array of minerals. The mean and maximum all-day yields of other insects consistently contributed to the ERI only for manganese. All forms of insectivory provided small but probably non-trivial amounts of fat and protein. We conclude that different forms of insectivory have the potential to address different nutritional needs for Kasekela chimpanzees. Other than honeybees, insects have received little attention as potential foods for hominins. Our results suggest that ants and (on a seasonal basis) termites would have been viable sources of fat, high-quality protein and minerals for extinct hominins employing Pan-like subsistence technology in East African woodlands.