Wide-ranging chimpanzees at Mt. Assirik,Senegal

A 4-year study of the ecology and ethology of wild chimpanzees (Pan troglodytes verus)was carried out in far western Africa. Contacts with chimpanzees and the locations of their nests were noted to determine which types of habitat were most used and to estimate the density of the population and the size of its home range. The results show that this community has one of the lowest densities and largest home ranges of all populations of chimpanzees studied so far. As such, it may provide a useful model for the reconstruction of hominid evolution in the Plio-Pleistocene.     

Feeding, foraging, and food sharing behavior of immature chimpanzees.

Observations of chimpanzees under naturalistic conditions show that nutritional autonomy increases as young individuals mature. Substantial proportions of feeding time are spent eating food obtained through solicitation. Food sharing at SOPF occurs among mother and infant pairs most frequently; less frequently between adult males and infants or juveniles, and least frequently between adult females and immature individuals. Mothers tend to share what their offspring cannot obtain independently while others primarily share what is easiest to replace. These patterns of solicitation and distribution of food are consistent with predictions based upon benefits to individuals through kin selection and/or energetic efficiency.     

Baboons, chimpanzees, and tools

Wild chimpanzees make and use tools with far greater frequency and variety than wild baboons. Sampling differences; differences in the sensory, motor, or cognitive capacities for skilled tool use; or environmental differences are not responsible. Fortuitous discovery of novel forms of tool behavior probably occurs as rarely among chimpanzees as among baboons. However, research on wild and captive chimpanzees reveals that the discovery is transmitted quickly among chimpanzee groups by observation learning, and thus becomes well-established in the group's behavioral repertoire. In contrast, study of captive baboons reveals that they acquire little information about tool behavior by observation. With little observation learning the behavior is not likely to be acquired by other group members since repeated independent discovery is improbable. Thus chimpanzees' more proficient tool behavior is due mainly to their greater capacity for observation learning. This suggests that the advent of hominid tool traditions was accompanied by a capacity for facile observation learning and thus, perhaps, by language ability.     

Weight growth of colony-reared chimpanzees.

Body weight data, collected routinely from the US Air Force chimpanzee colony at Holloman AFB, was used for a longitudinal analysis of growth. Data were fitted to a model based on human growth patterns, which is described and compared to other treatments applied to growth data. Results are compared to other observations of chimpanzee and of human growth.     

Resocialization of a group of ex-laboratory chimpanzees, Pan troglodytes.

In order to determine if socially deprived chimpanzees could establish an integrated social unit and reproduce, a group of wild-born individuals from a single-cage laboratory environment were released onto a small island in a South Florida tourist park in 1975. The present study was undertaken during June and July of 1988 to evaluate the long term results of the experiment and to document the reproductive history of the chimpanzees since their release on the island. The evidence presented in this paper suggests that it is possible for laboratory chimpanzees to be resocialized and to reproduce successfully.     

Establishment,maintenance, and behavior of free-ranging chimpanzees on Ossabaw Island,Georgia, U.S.A.

A small number of adult chimpanzees were released on Ossabaw Island, Georgia, in order to evaluate the feasibility of establishing a free-ranging reproductive colony of chimpanzees in a semitropical North American climate. First, three females and one male were released in June 1972. Following the unexpected deaths of two of these females, four more females were added. Except for one newborn that was removed from the colony, the chimpanzee colony remained intact, and was still in existence as of February 1975. Preparation for and maintenance of the chimpanzees is described. Behavioral adaptations to the environment, and some aspects of chimpanzee social behavior are reported. The potential importance of colonies such as this, for reducing over-exploitation of naturally occurring primate populations is discussed. The financial support of Yerkes Regional Primate Research Center, Emory University, Atlanta, Georgia, and The Pennsylvania State University, Department of Poultry Science, University Park, Pennsylvania, permitted us to make these observations. Funding for planning and construction of the island facilities, and relocation and maintenance of the chimpanzees was provided by a National Institutes of Health grant toGeoffery Bourne, Director of the Center.     

Social organization of savanna-dwelling chimpanzees,Pan troglodytes verus,at Mt. Assirik,Senegal

A community of chimpanzees at Mt. Assirik in south-eastern Senegal subsists in a hot, dry and open environment. This wide-ranging, savanna-living group provides an opportunity for comparisons of social organization with other population elsewhere in Africa living in forest and woodland. The group numbered about 28 over the four-year study, and its composition by age and sex was typical. The average size of parties (i.e., temporary sub-groups) did not differ from other populations. However, a high proportion of the group tended to remain together in such parties. The composition of parties resembled that found elsewhere, but some differences emerged between the wet and dry seasons. Larger, mixed parties containing adult males were much more common in open, non-forested habitats than were solitary individuals or parties without adult males. Large parties tended to form for travelling, especially for rapid movement over long distances. Such aspects of social organization seem unlikely to be related to the availability of food in any simple way. Instead, they seem to be adaptations to threat from predators and patchily distributed food, water and nesting sites. This results in occasional bivouacs and mass-migrations from one part of the home-range to another, especially in the dry season.     

Reproductive function in aged female chimpanzees.

Reproductive function was evaluated in ten female chimpanzees (Pan troglodytes) aged 35-48 years. Forty-eight years is the longevity record for the chimpanzee. Data on cycle frequency and duration was available for seven animals. Most were cycling regularly until death, and all had experienced at least one menstrual cycle within one year of death. After exclusion of periods when the animals were pregnant or in postpartum amenorrhea, the mean cycle frequency (+/- standard error)/year was 9.54 +/- 0.20 in seven animals aged 15-25 (432 cycles analyzed) compared to 8.6 +/- 0.76 in the same animals at age 35 + years (405 cycles analyzed); this effect approached significance (p = 0.072, Mann-Whitney U-test). Cycle length of 16 cycles in each of seven animals aged 15-25 was 32.23 +/- 0.38 days. The same animals when aged over 35 had mean cycle lengths of 35.59 +/- 0.73. This difference was not significant, although cycle lengths clearly increased with age in some individual animals. In five aged animals for which mating data was available, appropriate exposure to a male occurred in 52 cycles, but only two pregnancies occurred; one pregnancy resulted in a live birth at age 38, the other in a stillbirth at age 40. This conception rate was 3.85% compared with 20% in the same animals aged 15-25. These data suggest greatly reduced fertility after age 35, although menstrual cycle frequency remained high. The persistence of menstrual cyclicity until death, which occurred due to natural causes at latest in the fifth decade, is in striking contrast to the human female in which menopause occurs in the fifth decade and death is often postponed for several more decades.     

Language: Origins, definitions and chimpanzees

Language and its evolutionary origins are examined in the light of empirical data from studies on artificial language acquisition, human language acquisition in the chimpanzee and the definitional problems surrounding language. Rather than providing solutions, questions are raised in regard to the various approaches to the problems of language origin and definitions of language.     

Children, but not chimpanzees, prefer to collaborate

Human societies are built on collaborative activities. Already from early childhood, human children are skillful and proficient collaborators. They recognize when they need help in solving a problem and actively recruit collaborators [1, 2]. The societies of other primates are also to some degree cooperative. Chimpanzees, for example, engage in a variety of cooperative activities such as border patrols, group hunting, and intra- and intergroup coalitionary behavior [3-5]. Recent studies have shown that chimpanzees possess many of the cognitive prerequisites necessary for human-like collaboration. Chimpanzees have been shown to recognize when they need help in solving a problem and to actively recruit good over bad collaborators [6, 7]. However, cognitive abilities might not be all that differs between chimpanzees and humans when it comes to cooperation. Another factor might be the motivation to engage in a cooperative activity. Here, we hypothesized that a key difference between human and chimpanzee collaboration-and so potentially a key mechanism in the evolution of human cooperation-is a simple preference for collaborating (versus acting alone) to obtain food. Our results supported this hypothesis, finding that whereas children strongly prefer to work together with another to obtain food, chimpanzees show no such preference.