Insectivory of savanna chimpanzees (Pan troglodytes verus) at Fongoli, Senegal

Little is known about the behavior of chimpanzees living in savanna-woodlands, although they are of particular interest to anthropologists for the insight they can provide regarding the ecological pressures affecting early hominins living in similar habitats. Fongoli, Senegal, is the first site where savanna chimpanzees have been habituated for observational data collection and is the hottest and driest site where such observation of chimpanzees occurs today. Previously, indirect evidence suggested these chimpanzees consumed termites throughout the year, an unusual occurrence for western and eastern chimpanzees. Although meat eating by chimpanzees continues to receive much attention, their use of invertebrate prey has received less emphasis in scenarios of hominin evolution. Here, we further examine the invertebrate diet of Fongoli chimpanzees using direct observational methods and accounting for potential environmental influences. Termite feeding positively correlated with high temperatures. Fongoli chimpanzees spend more time obtaining termites than any other chimpanzee population studied, and this extensive insectivory contributes to the list of distinctive behaviors they display relative to chimpanzees living in more forested habitats. We suggest that savanna chimpanzees at Fongoli differ significantly from chimpanzees elsewhere as a result of the selective pressures characterizing their harsh environment, and this contrast provides an example of a viable referential model for better understanding human evolution. Specifically, our results support the hypotheses that invertebrate prey may have figured more prominently into the diet of early hominins in similar habitats, especially given that invertebrates are an important source of protein and other essential nutrients in a highly seasonal environment.     

Features of meat digestion by captive chimpanzees (Pan troglodytes)

The pronounced carnivory of many human populations contrasts sharply with feeding habits of other Hominoidea. Of extant great apes, only chimpanzees (Pan spp.) actively seek out vertebrate prey, but meat is only a minor portion of their diet. Some accounts suggest that wild chimpanzees digest prey inefficiently. To investigate the capacity of chimpanzees to digest meat, feeding trials were carried out on three captive chimpanzees (Pan troglodytes) using a fixed amount of nonpurified diet with and without a predetermined amount of boned cooked chicken. The results showed no significant differences in the rate of passage of digesta and digestion of diets with and without chicken. Meat ingestion did not change the nitrogen (N) concentration of feces or the total amount of N defecated. Visual inspection of fecal matter showed no evidence of undigested meat. Taken together, the results indicate that chimpanzees are able to digest meat of the type and quantity consumed during these trials.     

Gastrointestinal Parasites of Savanna Chimpanzees (Pan troglodytes schweinfurthii) in Ugalla,Tanzania

Understanding variability in patterns of parasite infections requires studies of multiple populations inhabiting a variety of habitats. Gastrointestinal parasites of chimpanzees (Pan troglodytes) have been studied extensively at several forested sites, but the parasite fauna of chimpanzees living in dry, open habitats is less well known. We studied the parasites of savanna chimpanzees (Pan troglodytes schweinfurthii) living in the Issa Valley, Ugalla (Tanzania). We examined 119 fresh fecal samples using standard coproscopical methods. We detected protozoans including Blastocystis sp., Entamoeba coli, E. histolytica/dispar, Iodamoeba buetschlii, Troglodytella abrassarti, and Troglocorys cava, but only two types of spirurid nematodes among the helminths. The parasites of the Ugalla chimpanzees differ from those of forest chimpanzees in the absence of Strongyloides sp. and strongylid nematodes and a high prevalence of spirurids. Strongylids and Strongyloides sp. have thin-shelled eggs and larvae, which develop in the external environment; thus they may not be able to survive for prolonged periods in the extreme environment of Ugalla. The Ugalla chimpanzees also live at a lower population density and exhibit a larger home range than forest chimpanzees, factors that may lead to lower exposure to infective nematode larvae. Spirurid eggs, however, have thick shells and a life cycle dependent on intermediary hosts, making their survival and transmission in such extreme conditions more feasible. These differences between parasite fauna of closed and open forest chimpanzees contribute to our understanding of the ecology of infectious disease, and have the potential to contribute to conservation policies and practices.     

Hand Preferences for a Haptic Task in Chimpanzees (Pan troglodytes)

We tested the hand preferences of 20 chimpanzees (Pan troglodytes) for a haptic task requiring individuals to search for grapes in an opaque bucket filled with water. We compared these data to the hand preferences displayed by the same chimpanzees during reaching and bimanual feeding tasks. The chimpanzees displayed no significant hand preference for the reaching or bimanual feeding tasks, but exhibited a right-hand preference while performing the haptic task. In contrast, New and Old World monkeys display left-hand preferences for similar tasks. We discuss the relevance of these findings for the evolution of handedness in primates.     

Forest fragments become farmland: Dietary Response of wild chimpanzees (Pan troglodytes) to fast-changing anthropogenic landscapes

Behavioral flexibility, including an ability to modify feeding behavior, is a key trait enabling primates to survive in forest fragments. In human-dominated landscapes, unprotected forest fragments can become progressively degraded, and may be cleared entirely, challenging the capacity of primates to adjust to the changes. We examined responses of wild chimpanzees (Pan troglodytes schweinfurthii) to major habitat change: that is, clearance of forest fragments for agriculture. Over 7 years, fragments in Bulindi, Uganda, were reduced in size by 80%. We compared the chimpanzees’ diet at the start and end of this period of rapid deforestation, using data derived mainly from fecal analysis. Similar to other long-term study populations, chimpanzees in Bulindi have a diverse diet comprising over 169 plant foods. However, extensive deforestation seemed to impact their feeding ecology. Dietary changes after fragment clearance included reduced overall frugivory, reduced intake of figs (Ficus spp.; formerly a dietary “staple” for these chimpanzees), and reduced variety of fruits in fecal samples. Nevertheless, the magnitude of most changes was remarkably minor given the extent of forest loss. Agricultural fruits increased in dietary importance, with crops accounting for a greater proportion of fruits in fecal samples after deforestation. In particular, cultivated jackfruit (Artocarpus heterophyllus) became a “staple” food for the chimpanzees but was scarcely eaten before fragment clearance. Crops offer some nutritional benefits for primates, being high in carbohydrate energy and low in hard-to-digest fiber. Thus, crop feeding may have offset foraging costs associated with loss of wild foods and reduced overall frugivory for the chimpanzees. The adaptability of many primates offers hope for their conservation in fragmented, rural landscapes. However, long-term data are needed to establish whether potential benefits (i.e. energetic, reproductive) of foraging in agricultural matrix habitats outweigh fitness costs from anthropogenic mortality risk for chimpanzees and other adaptable primates.     

Uncertain availability of a preferred food affects choice in a captive group of chimpanzees (Pan troglodytes)

Food-choice was investigated in a social group of 16 chimpanzees (Pan troglodytes) maintained in a large outdoor compound. Three feeding stations located along the periphery of the compound were considered analogous to food patches. Color-coded aluminum panels temporarily covered each feeding apparatus, with one color corresponding to nonpreferred food (commercial biscuits) available at two locations and other colors corresponding to the certain or uncertain availability of preferred food (oranges) available at one location. Only nine chimpanzees met the criterion for learning the color/food associations and thus only those animals were included in the analysis. There was a significant decrease in choosing the station associated with oranges when the probability of availability of oranges was reduced from 0.5 to 0.1 but not from 1.0 to 0.5. In addition, there was a significant increase in the frequency with which the subjects made no choice when the probability of availability of oranges was reduced from 1.0 to 0.1. The data indicate that the uncertain availability of preferred food in a choice situation affects choice behavior in a social group of chimpanzees studied under controlled conditions.     

Dexterity and technique in termite fishing by chimpanzees (Pan troglodytes troglodytes) in the Goualougo Triangle,Republic of Congo

Although the phenomenon of termite fishing by chimpanzees (Pan troglodytes) has historical and theoretical importance for primatology, we still have a limited understanding of how chimpanzees accomplish this activity, and in particular, about details of skilled actions and the nature of individual variation in fishing techniques. We examined movements, hand positions, grips, and other details from remote video footage of seven adult and subadult female chimpanzees using plant probes to extract Macrotermes muelleri termites from epigeal nests. Six chimpanzees used exclusively one hand (left or right) to grip the probe during termite fishing. All chimpanzees used the same repertoire of actions to insert, adjust, and withdraw the probe but differed in the frequency of use of particular actions. Chimpanzees have been described as eating termites in two ways—directly from the probe or by sweeping them from the probe with one hand. We describe a third technique: sliding the probe between the digits of one stationary hand as the probe is extracted from the nest. The sliding technique requires complementary bimanual coordination (extracting with one hand and grasping lightly with the other, at the same time). We highlight the importance of actions with two hands—one gripping, one assisting—in termite fishing and discuss how probing techniques are correlated with performance. Additional research on digital function and on environmental, organismic, and task constraints will further reveal manual dexterity in termite fishing.     

Predation on mammals by the chimpanzee (Pan troglodytes)

With respect to prey selectivity and predation frequency, chimpanzees (Pan troglodytes) show local differences as well as diachronic variability within the same population. When data on predation from three long-term studies at Mahale, Gombe, and Tai are compared, some differences and similarities emerge; Mahale is more like Gombe than Tai in regard of prey selection but features of hunting at Tai with respect to predation frequency are not conspicuous. The most responsible factor for diversity in prey selectivity is a distinct “prey image” maintained by chimpanzees of different populations, although it is necessary to clarify in future studies why and how such tradition develops. Relative body size of chimpanzees to prey species and/or the degree of cooperation among members of a hunting party may explain the variability in prey size selected at each site, the latter influencing the frequency of successful hunts at the same time. Although various degrees of habituation and different sampling methods including artificial feeding might have obscured the real differences, recent data from the three populations do not seem to be biased greatly by such factors. Nevertheless, it is still difficult to make strict comparisons due to the lack of sufficient standardized data across the three populations on the frequency of hunting and predation. It is suggested that the size or demographic trend of a chimpanzee unit-group, especially the number of adult males included, necessarily influences its hunting frequency as well as its prey profile. It is also suggested that factors which bring these males together into a party (e.g. fruit abundance, swollen females, conflict between unit-groups etc.) strongly affect theactual hunting and kill rates. Other possible factors responsible for the local differences are forest structure (e.g. tree height), skilful “hero” chimpanzees, and competition with sympatric carnivorous animals. A total of at least 32 species have been recorded as prey mammals of chimpanzees from 12 study sites and the most common prey mammals are primates (18 species), of which 13 species are forest monkeys. Forest monkeys, colobine species in particular, are often the most common victims of the predation by chimpanzees at each site. We may point out a tendency toward selective hunting for the forest monkeys in terms of the selectivity of prey fauna among all three subspecies of chimpanzees, including populations living in drier environment. The mode of chimpanzee hunting seems to correspond to the highest available biomass of gregarious, arboreal monkeys in the forest, colobine species in particular. In contrast, bonobos (P. paniscus) are less carnivorous than chimpanzees, only rarely preying on a few species of small mammals. The sharp contrast of the two allied species in their predatory tendencies appears to have something to do with the differences in the structure of primary production between their habitats.     

Seed Reingestion in Savannah Chimpanzees (<Emphasis Type="Italic">Pan troglodytes verus</Emphasis>) at Fongoli,Senegal

Coprophagy is common in captive primates but has also been reported in the wild. For example, wild apes extract and reingest items from faeces. We term this behavior seed reingestion because the dung matrix is not consumed. We assessed the importance of seed reingestion in a population of savannah chimpanzees at Fongoli, southeastern Senegal, one of the hottest and driest areas of the species’ range, where chimpanzees have a relatively narrow dietary repertoire. We observed habituated chimpanzees on 122 d during 8 mo in 2005 and 2006, employing both focal subject and scan sampling of identified individuals for 1278 h of data collection. Chimpanzees reingested seeds of 2 species: Parkia biglobosa and Adansonia digitata. Both seed species have a hard protective shell, and the embryos are rich in proteins and lipids. Chimpanzees initially ate the fruit matrix pulp and swallowed intact seeds before reingesting and chewing/destroying seeds. Seed reingestion accounted for almost 2% of feeding time. We suggest that at Fongoli this behavior may be an adaptive strategy to maximize food intake, by softening the seed’s shell and making the seed’s content more accessible.     

Army ant prey availability and consumption by chimpanzees (Pan troglodytes vellerosus) at Gashaka (Nigeria)

Army ant predation by chimpanzees has been studied as an intriguing example of tool use and a possible case of cultural variation. However, the importance of army ant prey in chimpanzee diet and feeding ecology is still only poorly understood. We studied the availability and consumption of army ants in a population of the chimpanzee subspecies Pan troglodytes vellerosus in Nigeria. Army ants were collected from nests and trails (workers) and near artificial light sources (males). Three potential prey species were found: Dorylus rufescens, Dorylus gerstaeckeri and Dorylus kohli . Dorylus rufescens was by far more abundant than the other two species. Only remains from D. rufescens were present in chimpanzee faeces. This is the first report of consumption of this ant species by chimpanzees. However, because of the low availability of the other two species, it is unclear whether this pattern reflects a preference for D. rufescens . Although D. rufescens ' availability varied with weather conditions, the occurrence as well as the absolute and relative numbers of Dorylus fragments in faeces did not. This finding, together with the considerable difficulties encountered by human observers in their efforts to locate nests by following trails, suggests that the chimpanzees in this population do not harvest army ants from trails and do not use trails to locate nests. The overall occurrence of army ant fragments in 42.3% of all faecal samples is the highest ever recorded in any chimpanzee population. This indicates that in this chimpanzee population, army ant prey is not a fallback during periods of sparse availability of plant food, but quantitatively important throughout the year. Future studies will be needed to clarify which cues and strategies chimpanzees use to locate army ant nests and to assess the role of myrmecophagy with respect to macro- or micronutrient demands.     

Assessing the effects of cognitive experiments on the welfare of captive chimpanzees (Pan troglodytes) by direct comparison of activity budget between wild and captive chimpanzees

We investigated the effects of cognitive experiments by direct comparison of activity budgets between wild and captive chimpanzees. One goal of captive management is to ensure that the activity budgets of captive animals are as similar as possible to those of their wild counterparts. However, such similarity has rarely been achieved. We compared the activity budget among three groups of chimpanzees: wild chimpanzees in Bossou (Guinea, n = 10), and captive chimpanzees who participated in cognitive experiments (experimental chimpanzees, n = 6) or did not participate in the experiments (nonexperimental chimpanzees, n = 6) at the Primate Research Institute (Japan). The experimental chimpanzees voluntarily participated in computer‐controlled cognitive tasks and small pieces of fruits were provided as rewards. The data from captivity were obtained on the experimental days (weekdays) and nonexperimental days (weekends). In both study sites, we followed each chimpanzee from about 7 a.m. until the time when chimpanzees started to rest in the evening. The behaviors were recorded every 1 min. The results showed that on weekdays, feeding time and resting time of the experimental chimpanzees were almost the same as those of wild chimpanzees. However, for the nonexperimental chimpanzees, feeding time was significantly shorter and resting time was longer than those of the wild chimpanzees. In contrast, no difference was found in feeding time or resting time of the two groups of captive chimpanzees on weekends. The results suggested that the cognitive experiments worked as an efficient method for food‐based enrichment. Am. J. Primatol. 73:1231–1238, 2011. © 2011 Wiley Periodicals, Inc.     

Feeding Consequences of Hand and Foot Disability in Wild Adult Chimpanzees (<Emphasis Type="Italic">Pan troglodytes schweinfurthii</Emphasis>)

Many wild primates experience long-term limb disability, and their ability to cope with disability has implications for survival and fitness. We quantified the arboreal feeding and postural behaviors of adult chimpanzees to study the consequences of physical limb disabilities. We collected behavioral data for a total of 8 mo on chimpanzees at Sebitoli, Kibale National Park, Uganda, focusing on the time spent feeding, common feeding tree species, body postures, and substrate use. Of the 51 chimpanzees we observed, 16 (31%) exhibited limb anomalies, which varied in form and severity. Disabled chimpanzees climbed as high as chimpanzees without disabilities and did not differ from nondisabled chimpanzees in the amount they used feeding tree species. Adult chimpanzees with severe hand disability spent significantly more time feeding than nondisabled individuals. In addition, manually disabled adults did not suspend themselves from branches during feeding as frequently as nondisabled adults and used larger substrates for gripping and sitting than nondisabled adults. These results indicate that disabled individuals compensate to carry out feeding activities in trees.     

The Foraging Costs of Mating Effort in Male Chimpanzees (Pan troglodytes schweinfurthii)

Costs of mating effort can affect the reproductive strategies and lifetime fitness of male primates, but interspecific and interindividual variation in the magnitude and distribution of costs is poorly understood. Male costs have primarily been recognized in seasonally breeding species that experience concentrated periods of mating competition. Here, we examine foraging costs associated with male mating effort in chimpanzees (Pan troglodytes schweinfurthii), a polygynandrous species, in which mating opportunities occur intermittently throughout the year. To quantify male feeding, aggression, and mating, we conducted focal follows on 12 males in a wild community (Kanyawara, Kibale National Park, Uganda) for 11 mo. Males fed less on days when high-value mating opportunities (estrous parous females) were available than on days without any mating opportunities. Reductions in feeding time were related to increased rates of aggression and copulation, indicating that the proximate cause of changes in male foraging was mating effort. Surprisingly, however, there was no relationship between dominance rank and the extent to which feeding time was reduced. High costs of mating effort may reduce the degree of reproductive skew and limit the use of possessive tactics in chimpanzees. We suggest that male bonding in chimpanzees may be favored not only for its benefits but because intragroup competition is so costly. Our results complement the available data on mammals, and primates in particular, by showing that mating effort can have measurable foraging costs even in species, in which breeding is aseasonal and only moderately skewed.     

The macronutrient composition of wild and cultivated plant foods of West African chimpanzees (Pan troglodytes verus) inhabiting an anthropogenic landscape

Agricultural expansion encroaches on tropical forests and primates in such landscapes frequently incorporate crops into their diet. Understanding the nutritional drivers behind crop-foraging can help inform conservation efforts to improve human-primate coexistence. This study builds on existing knowledge of primate diets in anthropogenic landscapes by estimating the macronutrient content of 24 wild and 11 cultivated foods (90.5% of food intake) consumed by chimpanzees (Pan troglodytes verus) at Bossou, Guinea, West Africa. We also compared the macronutrient composition of Bossou crops to published macronutrient measures of crops from Bulindi, Uganda, East Africa. The composition of wild fruits, leaves, and pith were consistent with previous reports for primate diets. Cultivated fruits were higher in carbohydrates and lower in insoluble fiber than wild fruits, while wild fruits were higher in protein. Macronutrient content of cultivated pith fell within the ranges of consumed wild pith. Oil palm food parts were relatively rich in carbohydrates, protein, lipids, and/or fermentable fiber, adding support for the nutritional importance of the oil palm for West African chimpanzees. We found no differences in the composition of cultivated fruits between Bossou and Bulindi, suggesting that macronutrient content alone does not explain differences in crop selection. Our results build on the current understanding of chimpanzee feeding ecology within forest-agricultural mosaics and provide additional support for the assumption that crops offer primates energetic benefits over wild foods.     

Longitudinal analysis of length growth in the chimpanzee (Pan troglodytes)

The adolescent growth spurt in linear dimension in humans is considered to be unique among mammals, but few comparative studies have been done, even on chimpanzees. Growth of the summed length of crown to rump, thigh, and leg was studied longitudinally in 12 chimpanzees. We took body weight growth and reproductive maturation into consideration. Reproductive maturation was monitored by the swelling of sexual skin and menarche in females, and by testicular development in males. We applied two relationships found in humans between body length growth and the environment to the chimpanzees. The first relationship was the robustness of the growth spurt, meaning that the spurt is absent only in individuals under the most severe environmental pressure. Subjects maturing in a favorable or even mediocre environment are anticipated to show the growth spurt. The second relationship was catch-up growth, where, when the environment is ameliorated, growth may be accelerated to attain the target size. Catch-up growth at the end of the juvenile period may mimic the adolescent growth spurt. Results showed that subjects living under favorable conditions did not exhibit a growth spurt, and that it was only the subjects who had delayed growth in the juvenile period that showed a spurt in adolescence, the period when reproductive maturation occurred. Although we have concluded that chimpanzees do not have an adolescent growth spurt, except in cases of catch-up growth, this does not mean that they have a different growth pattern from that of humans. The absence of a growth spurt may be associated with adaptations to chimpanzee patrilineal society, where adolescent males are incorporated into the adult hierarchy at a low rank.     

Chimpanzees (Pan troglodytes) tolerate some degree of inequity while cooperating but refuse to donate effort for nothing

In cooperative hunting, a carcass cannot be divided equally, and hunts may be unsuccessful. We studied how chimpanzees respond to these two variables, working for unequal rewards and no rewards, which have been rarely included in experimental cooperative tasks. We presented chimpanzees with a task requiring three chimpanzees to work together and varied the reward structure in two separate experiments. In Experiment 1, two individuals received more rewards than the third, making the outcome unequal. We wanted to know if cooperation would continue or break down, and what mechanisms might maintain performance. Experiment 2 used equal rewards, but this time one or more locations were left unbaited on a proportion of trials. Thus, there was a chance of individuals working to receive nothing. In Experiment 1, the chimpanzees worked at a high rate, tolerating the unequal outcomes, with rank appearing to determine who got access to the higher-value locations. However, equal outcomes (used as a control) enhanced cooperative performance, most likely through motivational processes rather than the absence of inequity aversion. In Experiment 2, performance dropped off dramatically when the chimpanzees were not rewarded on every trial. Their strategy was irrational as donating effort would have led to more rewards in the long run for each individual. Our results lead to a hierarchy of performances by condition with equity > inequity > donating effort. Chimpanzees therefore tolerate mild inequity, but cannot tolerate receiving nothing when others are rewarded.     

Imaginary Play in chimpanzees (Pan troglodytes)

Imaginary Play was studied in a group of five signing chimpanzees and it was found that chimpanzees engage in imaginary play similar to that found in human children. Fifteen hours of remote videotapes were analyzed for instances of imaginary play. Behaviors were defined as imaginary play by meeting a predetermined criteria which allowed them to be classified into one of six different categories of imaginary play. Six instances of imaginary play were found and these were classified into the two categories of Animation and Substitution. Observations of imaginary play in other research with chimpanzees were discussed.     

Food Acceptance and Social Learning Opportunities in Semi‐Free Eastern Chimpanzees (Pan troglodytes schweinfurthii)

When confronted with novel foods, chimpanzees’ responses combine a mixture of curiosity and cautiousness. Once the item is in the mouth, the initial cautiousness is followed by an aversion to bitter taste that is mediated mainly by the TAS2R gene family. For instance, variations on the TAS2R38 locus which has been studied extensively in humans have been associated with different acceptance of bitter substances. Surprisingly, while cautiousness and bitter taste aversion were selected to prevent any risk of poisoning, very few studies on novel food acceptance have included the vegetative parts of plants. Moreover, the studies were usually carried out with captive apes faced to a very restricted variety of non‐toxic plants, hardly making the results representative. This study aims to replicate previous findings obtained in zoos while controlling for these limitations. We provided nine subgroups of eastern chimpanzees (Pan troglodytes schweinfurthii) living in the Ugandan sanctuary of Ngamba Island with novel plants known to be consumed by wild chimpanzees of the same subspecies, as well as domestic plants, wild sapota fruit and grey clay used by human local communities. We also genotyped their TAS2R38 gene. Our results confirm the very low genetic heterogeneity for TAS2R38 in this chimpanzee subspecies. Chimpanzees were particularly cautious towards the vegetative parts of novel plants, likely reflecting their behavioural strategy for avoiding toxic compounds. We also confirmed their higher propensity towards testing sapota and clay, reflecting their ability to expand their diet. In contrast with the results found in zoos, familiar and novel less‐palatable vegetative parts of plants did not elicit many interindividual observations. This may be explained by the items presented, which could have been so novel to be considered as enrichment in captive conditions, where the apes are rarely exposed to plants.     

Innovation in wild chimpanzees (Pan troglodytes)

Innovations of behavior have major implications for the concept of culture in animals. Innovation has rarefy been documented in wild animal populations. The chimpanzees of the Taï National Park, Côte d’Ivoire, spontaneously included new patterns of behavior in their repertoire during our study. Some innovations were incorporated into a minority of the group members’ repertoire, whereas others, such as building day nests in trees and on the ground more frequently, became general behaviors. Similarly, new contexts for use of leaf- clipping behavior and novel ways of feeding on some leaves appeared and rapidly became part of the behavior of most group members. The environmental parameters were too stable to explain these new forms of behavior, as most of them took place within 1 month during the same dry season. In a similar way, leaf- grooming acquired a new function in Gombe chimpanzees. A process similar to social conventions could explain the emergence of these new functions for an existing behavior in a way rather similar to human “fashion. ” Three major characteristics of human culture — the absence of individual variations in the performance of the behavior, the general use of the behavior by group members, and the ability to modify the function of a behavior — characterize leaf-clipping in Taï chimpanzees and leaf- grooming in Gombe chimpanzees.