Flexible feeding on cultivated underground storage organs by rainforest-dwelling chimpanzees at Bossou, West Africa

It has been proposed that exploitation of underground storage organs (USOs) played an important role in the evolution of the genus Homo, these items serving as ‘fallback foods’ during periods of low food availability. The use of USOs as food by wild chimpanzees is infrequent and seen mostly in populations inhabiting relatively arid environments, such as the savanna. Here, we specifically test the hypothesis that chimpanzees (Pan troglodytes verus) inhabiting tropical wet forest at Bossou (Republic of Guinea, West Africa) exploit USOs as a fallback food during periods of fruit scarcity. Chimpanzees were never observed feeding on wild USOs, that is, those that were never cultivated, and rarely on other underground plant parts. However, direct observations revealed regular consumption of the USOs of cultivated cassava (Manihot esculenta), a spatially abundant and continuously available plant, although the chimpanzees did not use tools when acquiring and feeding on cassava. In agreement with the fallback foods hypothesis, our results show that chimpanzees exploited cassava USOs more frequently when both wild and cultivated fruits were scarce, and consumption patterns of cassava paralleled those of wild fallback foods. These seasonal extractive USO foraging strategies by chimpanzees can strengthen attempts to construct a clearer picture of the importance of USO feeding in hominoid evolution.     

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.     

Fruit sharing between wild adult chimpanzees (Pan troglodytes schweinfurthii): a socially significant event?

Although food sharing is a habitual aspect of chimpanzee (Pan troglodytes) life, sharing of plant foods between unrelated adults is rare. Observations of such behavior have typically been interpreted as the outcome of a process by which individuals that are otherwise unable to gain access to the food manage to obtain a nutritional benefit. Here we present behavioral details and an acoustic analysis regarding an observation of food sharing between unrelated adult wild chimpanzees that we suggest cannot be explained using traditional nutrition-based models. Instead we propose that the exchange is only understandable as a socially important event, and we cite two further observations in the same population that support this suggestion.     

A preliminary list of chimpanzees' alimentation at Bossou,Guinea

A preliminary list of all the plant and animal foods for wild chimpanzees at Bossou, south-east of the Republic of Guinea, is presented for comparison with that of other study areas and as the basic data for the further studies. Altogether 205 food items (species × part eaten) from 156 species in 106 genera of plant food are recorded (Table 1). On the other hand little of animal food has been recorded (Table 2). Thus only one species of mammal (tree pangolin) and bird (wood owl) are represented in the diet along with only a few types of other animals. As calculated from number of food items fruits and seeds (nuts) occupy more than half of the diet; and leaves, shoots, and twigs (stems) constitute about one third.     

Nutritional significance of the selective ingestion of Albizia zygia gum exudate by wild chimpanzees in Bossou, Guinea

The selective ingestion of plant gum exudates by chimpanzees has been frequently observed at various study sites. At Bossou, Guinea, chimpanzees also frequently ingest Albizia zygia gum exudate. A functional explanation for this behavior is lacking, so we evaluated its possible contribution of energy in the form of short-chain fatty acids (SCFA) as well as minerals. An in vitro fermentation study of A. zygia gum using the fecal bacteria of a Bossou chimpanzee showed that carboxylic acids were produced with a 6-hr lag phase up to 44 mmol/l by 18 hr of incubation. Acetate was the most abundant acid produced, followed by lactate and propionate. The energy supplied from the fermentation of a piece of gum exudate (20-30 g) was negligible in comparison with the estimated daily energy requirements of chimpanzees in the wild. However, A. zygia gum exudate (20-30 g) can supply sufficient amounts of calcium, manganese, magnesium, and potassium to fulfill the daily requirements for these minerals in chimpanzees.     

Dietary responses to fruit scarcity of wild chimpanzees at Bossou,Guinea: Possible implications for ecological importance of tool use

A 13-month ecological study was conducted at Bossou, Guinea, West Africa, to elucidate how a community of wild chimpanzees (Pan troglodytes verus) deals with the scarcity of main foods. During the study period, fruit availability fluctuated radically. The chimpanzees were confirmed to depend heavily on three “keystone resources” which were available when their main foods (fruit pulp) were scarce. These were fruits of Musanga cecropioides, oil-palm (Elaeis guineensis) nuts, and oil-palm pith. These are abundant in the chimpanzees' home range and their nutritional contents compensate for a decrease in nutritional intake from fruit pulp. The presence of these excellent backup foods may explain the high reproductive performance of Bossou chimpanzees. Here, chimpanzees consumed two of the three keystone foods using two types of tool behavior: nut-cracking for oil-palm nuts and pestle-pounding for oil-palm pith. These tool-using behaviors accounted for 31.9% of the total feeding time spent in June (the month in which the highest frequency occurred) and 10.4% in total for the year. It is suggested that the Bossou chimpanzees depend strongly on tools for their subsistence. This implies a possible function for tool technology in the evolution of our human ancestors. Am J Phys Anthropol 106:283–295, 1998. © 1998 Wiley-Liss, Inc.     

Nutritional Characteristics of Wild and Cultivated Foods for Chimpanzees (<Emphasis Type="Italic">Pan troglodytes</Emphasis>) in Agricultural Landscapes

Primate habitats are being transformed by human activities such as agriculture. Many wild primates include cultivated foods (crops) in their diets, calling for an improved understanding of the costs and benefits of crop feeding. We measured the macronutrient and antifeedant content of 44 wild and 21 crop foods eaten by chimpanzees (Pan troglodytes schweinfurthii) in a mosaic habitat at Bulindi, Uganda, to evaluate the common assertion that crops offer high nutritional returns compared to wild forage for primates. In addition, we analyzed 13 crops not eaten at Bulindi but that are consumed by chimpanzees elsewhere to assess whether nutritional aspects explain why chimpanzees in Bulindi ignored them. Our analysis of their wild plant diet (fruit, leaves, and pith) corresponds with previous chemical analyses of primate plant foods. Compared to wild food equivalents, crops eaten by the chimpanzees contained higher levels of digestible carbohydrates (mainly sugars) coupled with lower amounts of insoluble fiber and antifeedants. Cultivated fruits were relatively nutritious throughout the ripening process. Our data support the assumption that eating cultivated foods confers energetic advantages for primates, although crops in our sample were low in protein and lipids compared to some wild foods. We found little evidence that crops ignored by the chimpanzees were less nutritious than those that they did eat. Nonnutritional factors, e.g., similarity to wild foods, probably also influence crop selection. Whether cultivated habitats can support threatened but flexible primates such as chimpanzees in the long term hinges on local people’s willingness to share their landscape and resources with them.     

Use of leaves as cushions to sit on wet ground by wild chimpanzees

A new type of tool use, leaf cushion, by wild chimpanzees (Pan troglodytes verus) at Bossou, Guinea, was found. We report two cases: one is indirect evidence; the other is direct observation of a chimpanzee who used the tool. Both cases indicate that chimpanzees used a set of leaves as a cushion while sitting on wet ground. Chimpanzees at Bossou show various kinds of tool use, some of which are unique to the community. Most of these behavioral patterns are subsistence tool use for obtaining food, as at other study sites. The use of leaves as a cushion adds to the few instances of nonsubsistence, elementary technology seen used by wild chimpanzees. Am. J. Primatol. 44:215–220, 1998. © 1998 Wiley-Liss, Inc.     

New data on the use of stone tools by chimpanzees in Guinea and Liberia

Two types of use of “hammers” for cracking nuts by wild-living chimpanzees have been distinguished: (1) Relatively small stones are used by the chimpanzee community at Bossou in Guinea to crack the nuts of oil palms growing on abandoned farmland, while no nuts of wild tree species are cracked. (2) Larger hammer stones (and, at some sites, wooden clubs) are used in a more sophisticated manner to crack the nuts of wild trees, but not of oil palms, in an area ranging from south-east Sierra Leone through Liberia to the south-west of the Ivory Coast. The first author (1986) has proposed that Type I has been copied by the chimpanzees, under pressure of food shortage, from the local human population. New data now indicate that, at Bossou, while habitat deterioration has continued, the number of hammer and anvil stones per utilized oil palm tree has approximately tripled in the last six years. The quantity of food obtained from oil palm nut kernels, however, amounts to only a few percent of the total diet. For the rest these apes depend to a large extent on many other agricultural products cultivated at Bossou which they are allowed freely to consume, including even cassava (manioc) roots and sweet potatoes dug by them from the ground. Some factors determining the chosen size of hammers were analyzed. Two abnormal hammers were found whose wear suggested a tentative, human-like manner of use. No evidence has been found to indicate the use of stone tools by chimpanzees in the adjoining chimpanzee-inhabited areas around the range of the Bossou community. Type II stone tool use was found, however, in a primary forest on a mountain≈13 km west of Bossou. This is especially intriguing because the site is separated by a wide belt of drier rain forest from the belt of very humid rain forest in the south where all the other known Type II sites are located. More research on the geographical distribution of the use of stone tools by chimpanzees and on the underlying ecological factors is recommended.     

Pestle-pounding behavior of wild chimpanzees at Bossou,Guinea: A newly observed tool-using behavior

A new type of tool-using behavior was observed in a group of wild chimpanzees (Pan troglodytes verus) at Bossou, Guinea. The chimpanzees used the leaf-petiole of oil-palm trees (Elaeis guineensis) as a pounding tool to deepen a hole in the oil-palm crown which appeared after the chimpanzees had pulled out the central young shoots. Finally, the chimpanzees extracted and ate the apical meristem or apical bud of the oil-palm tree which is edible but inaccessible without such tool use. The motor pattern which the chimpanzees employed is similar to that used for termite-nest digging but it is more exaggerated and requires great force. The behavior is reminiscent of pestlepounding. The chimpanzees exploit substantial amounts of food with this tool-using skill, compensating for insufficient fruit foods in the primary forest. This tool-using behavior was first observed in 1990 and, to date, almost half of the group members have been confirmed to use the pestle tool. It appears that this tool-using behavior was invented recently and has since spread widely throughout the group as a habitual one.     

Demographic parameters and life history of chimpanzees at Bossou, Guinea

Demographic parameters of wild chimpanzees at Bossou, Guinea, are presented and compared with those of other populations. The population size of Bossou chimpanzees has been stable over the last 26 years, except during two incidents of partial deforestation. The annual birth rate for a female (mean = 0.194, but 0.165 when the infant survived more than 4 years) and interbirth interval are not much different from those of other study sites. The primiparous age of Bossou chimpanzees, however, is far younger (mean = 10.9 years) than for all other known wild chimpanzee populations. The infant and juvenile survival rate is also the highest (female = 0.64, male = 0.52 for the first 8 years). As a result, the lifetime reproductive success of Bossou chimpanzees is estimated to be highest among long-term study sites. The rate of disappearance from Bossou dramatically increases during the adolescent stage, and most young chimpanzees disappear before or around maturation. Probably because the environmental capacity for chimpanzees at Bossou is at its limit, many young independent males, as well as females, have to disperse, though others may die. For chimpanzee alpha males of other populations, mature males may be needed as collaborators to defend resources. In the case of Bossou, however, a lack of adjacent groups, conspecific competitors, predators, and perhaps medium-sized mammals as prey for group hunting may eliminate this need of the alpha male for other males. The reasons why all males of other chimpanzee populations persist in being philopatric for life and maintain kin-related male bonds differing from most mammal species, including humans, are discussed.     

Mortality rates among wild chimpanzees

In order to compare evolved human and chimpanzees' life histories we present a synthetic life table for free-living chimpanzees, derived from data collected in five study populations (Gombe, Ta?, Kibale, Mahale, Bossou). The combined data from all populations represent 3711 chimpanzee years at risk and 278 deaths. Males show higher mortality than females and data suggest some inter-site variation in mortality. Despite this variation, however, wild chimpanzees generally have a life expectancy at birth of less than 15 years and mean adult lifespan (after sexual maturity) is only about 15 years. This is considerably lower survival than that reported for chimpanzees in zoos or captive breeding colonies, or that measured among modern human hunter-gatherers. The low mortality rate of human foragers relative to chimpanzees in the early adult years may partially explain why humans have evolved to senesce later than chimpanzees, and have a longer juvenile period.     

Social structure and dynamics of wild chimpanzees at Bossou,Guinea

A small population of wild chimpanzees was studied at Bossou, Guinea, for six months from November 1976 to May 1977. All the chimpanzees except dependent infants were identified without artificial feeding and were observed from within 20 m almost every day. The population size of 21 chimpanzees was little changed from 1967. Although the sex ratio (male/female) of immatures (infants, juveniles, and adolescents) was 0.833 in January 1977, the adult sex ratio was 0.429. More than half of the males must therefore have disappeared. No desertion of males from the Bossou group was confirmed during the study period but two emigrated males from other groups did come to join the Bossou group for a short period. Four out of six mothers had two or three children. From the age discrepancy of brothers/sisters, the mean birth interval from a mother was estimated to be about four years. An elder sister of two infants who had an age-gap of only three years, or perhaps less, disappeared without receiving sufficient care from her mother. Mothers who had infants aged about 1 year or more showed swelling of their sexual skin and were confirmed to mate with males.     

Paternity discrimination and inter-group relationships of chimpanzees at Bossou

A small group of wild chimpanzees at Bossou, southeastern Guinea, is semi-isolated, occupying a home-range which is several kilometers from those of other groups. The group has had only one adult male since 1985, raising the probability of inbreeding. Direct observation suggests that this male was the father of all infants born in the group since 1985. In 1991 individually identified samples of food wadges, hair, and feces were collected from most of the group members. These samples were analyzed using GT dinucleotide repeat polymorphisms to determine familial relationships. The genetic analysis revealed that the resident adult male was the probable father of only three out of the four infants analyzed. It is suspected that an adult male from a neighboring group was the father of the fourth infant, born in late 1986 or early 1987.     

Patterns of food sharing among mother and infant chimpanzees at Gombe National Park, Tanzania

The patterns of food sharing among mother and infant chimpanzees at the Gombe National Park, Tanzania, are primarily influenced by the infants' ability to procure and process foods independently. Foods infants could find and process on their own were not shared by mothers as frequently as foods infants could not obtain independently. Temporal changes were observed in the patterns of interactions as infants matured. It is suggested that food sharing may be considered a behavioral strategy which facilitates the infant's transition from dependent suckling to independent foraging.     

Deactivation of snares by wild chimpanzees

Snare injuries to chimpanzees (Pan troglodytes) have been reported at many study sites across Africa, and in some cases cause the death of the ensnared animal. However, very few snare injuries have been reported concerning the chimpanzees of Bossou, Guinea. The rarity of snare injuries in this study group warrants further consideration, given the exceptionally close proximity of the Bossou chimpanzees to human settlements and the widespread practice of snare hunting in the area. Herein we report a total of six observations of chimpanzees attempting to break and deactivate snares, successfully doing so on two of these occasions. We observed the behavior in 5 males, ranging in age from juveniles to adults. We argue that such active responses to snares must be contributing to the rarity of injuries in this group. Based on our observations, we suggest that the behavior has transmitted down the group. Our research team at Bossou continues to remove snares from the forest, but the threat of ensnarement still remains. We discuss potential ways to achieve a good balance between human subsistence activities and the conservation of chimpanzees at Bossou, which will increasingly be an area of great concern in the future.     

Observations on the population dynamics and behavior of wild chimpanzees at Bossou,Guinea, in 1979–1980

A small population of wild chimpanzees was studied at Bossou, Guinea, for 80 days from December 1979 to March 1980 in its natural habitat. Since the beginning of the previous study in November 1976, the population decreased from 21 to 19 animals due to the disappearance of five members and three births. Three out of four adult and adolescent males disappeared, but no adult and adolescent female changed membership during the 39 months. The high social cohesion of this group and related behavior patterns changed little between the studies in spite of the disappearance of dominant and other males. In the moving range of the group, 127 mature oil-palm trees were examined; the base of 39 were used for palm-nut cracking by chimpanzees. The selection of cracking sites is discussed. A possible sign of leaf-sponging for drinking water from the hollow of the tree was found. The leaf-clipping display in which frustration or mild aggression is manifested by breaking stiff leaves with the mouth was observed throughout both study periods. A juvenile male chimpanzee was seen to catch a tree-pangolin which was later eaten by an adult female but other group members took little interest in the meat. It is clear that hunting and meateating are not habitual but accidental among the chimpanzees of Bossou.     

Tool-use for catching ants by chimpanzees at Bossou and Monts Nimba,West Africa

The use of tools by wild chimpanzees to catch ants was studied at Bossou and Monts Nimba, south-eastern Guinée. Insect-eating by chimpanzees at Bossou appears to be opportunistic and ant-catching was seen only in certain years. The most common prey species wasDorylus (Dorylus) molestus Gerstaecker. Sixty wands for catching ants were found at Bossou. Half of them were made from Zingiberaceae or Marantaceae, the stems and long petioles of which are usually straight, and have few branches. The length of the wand was variable. The mean length of ant-catching wands (46.7 cm) was between those found at Gombe and Tai. Ants' nests were usually dug up by hand, but on one occasion a digging stick was used. Four chimpanzees who used only one hand to manipulate the wand had also been observed using a stone hammer for nut-cracking. All of them used the same hand in wand-manipulation as in nut-cracking. As reported for chimpanzees at Gombe, Mahale, and Tai, more females than males tended to catch ants with wands though the sex difference was not demonstrated statistically. Ants which climbed up the wand were directly squeezed off using the lower-lip and eaten. This is the same method as seen at Tai, but different from most episodes observed at Gombe. Juveniles and infants also caught ants using wands, however, they seemed to do this more out of curiosity than as a means of feeding. At Goera, over 15 km from Bossou and separated from it by Monts Nimba, the same characteristics of ant-catching techniques were evident from traces.     

Food Preferences Among Captive Western Gorillas (Gorilla gorilla gorilla) and Chimpanzees (Pan troglodytes)

I used a zoological park setting to address food preferences among gorillas (Gorilla gorilla gorill) and chimpanzees (Pan troglodytes). Gorillas and chimpanzees are different sizes, and consequently, have been traditionally viewed as ecologically distinct. Sympatric western gorillas and chimpanzees have proved difficult to study in the wild. Limited field data have provided conflicting information about whether gorillas are fundamentally different from chimpanzees in diet and behavior. Fruit eating shapes the behavior of most apes, but it is unclear whether the large-bodied gorillas are an exception to this rule, specifically whether they are less selective and more opportunistic fruit eaters than chimpanzees are. My research provides experimental observational data to complement field data and to better characterize the diets and food preferences of the African apes. During laboratory research at the San Francisco Zoological Gardens, I examined individual and specific differences in food preferences of captive gorillas and chimpanzees via experimental paired-choice food trials with foods that varied in nutritional content. During the study, I offered 2500 paired-food choices to 6 individual gorillas and 2000 additional pairs to them as a group. I also proffered 600 food pairs to 4 individual chimpanzees. Despite expectations of the implications of body size differences for diet, gorillas and chimpanzees exhibited similar food preferences. Both species preferred foods high in non-starch sugars and sugar-to-fiber ratios, and low in total dietary fiber. Neither species avoided foods containing tannins. These data support other suggestions of African apes sharing a frugivorous adaptation.     

Responses to novel foods in captive chimpanzees

Hesitancy to eat novel foods hampers the immediate enlargement of the diet but serves to limit the risk of ingesting toxic foods. Neophobia has been systematically investigated in only a few primate species, in which it appears to be affected by social influences. Surprisingly, little is known about neophobia in chimpanzees. We studied the response of eight adult captive chimpanzees to 16 foods (foods commonly eaten by humans and never tasted before by chimpanzees). Each novel food was presented twice to the chimpanzee by a familiar or an unfamiliar human. Between the two trials the human ate the food face to face with the chimpanzee (demonstration). Results showed that some foods were almost unanimously accepted, whereas others were not. Moreover, there were marked interindividual differences in food acceptance and consumption; chimpanzees ranged from being almost completely neophobic to accepting almost all foods. Familiarity with the human and the human's demonstration did not affect responses to the foods. The humans' predictions concerning the chimpanzees' acceptance of the different foods were rather good; furthermore, in seven cases out of eight the humans' preferences did not correlate with their predictions on the chimpanzees' preferences. The finding that most captive chimpanzees are initially cautious toward novel foods supports the little information there is regarding this subject in wild chimpanzees. However, the lack of influence of the humans' familiarity and demonstration on the response to food by the chimpanzees calls for more naturalistic studies, in which social influences are provided by group members. Since novel stimuli provide sensory stimulation and elicit exploration and social interest, occasional presentation of novel foods could be a promising and cheap device for feeding enrichment. Zoo Biol 21:539–548, 2002. © 2002 Wiley‐Liss, Inc.