Frances J. White

Feeding Competition and Patch Size in the Chimpanzee Species Pan Paniscus and Pan Troglodytes

The relative importance of feeding competition in Pan paniscus and Pan troglodytes schweinfurthii is examined in an attempt to understand the major differences in social organization of the two species. P. paniscus at Lomako is characterized by a stronger tendency for association among females than among female P. troglodytes at Gombe. Party size in P. paniscus is dependent on patch size. Feeding competition was more important in small patches than in large patches. The total amount of feeding time by a party in a patch (chimp-minutes) was a measure of patch size that was available for both chimpanzee species. P. paniscus was found to have larger party sizes and to use larger food patches than P. troglodytes. The importance of dispersed ground foods for each species of chimpanzee was compared and, although the results are not conclusive, they indicate that this type of food was equally important in the diets of both populations. Two hypotheses of the ecological basis for differences in social structure are compared in light of this evidence.

Party composition and dynamics inPan paniscus

The pygmy chimpanzee, or bonobo, Pan paniscus,diplays a fission-fusion social organization in which individuals associate in parties that vary in size and composition. Data from a 2-year field study of nonprovisioned P. paniscusshow that party composition varies with party size. Although females, on average, outnumber males, the proportion of males in the party increases in larger parties. This effect was not due to the greater number of known females. Both females and males will join and leave a party in the company of others, but only males appear frequently to join or leave as lone individuals. All-male parties were not observed, but all-female (nonnursery) parties were relatively common. These trends reflect greater cohesion among females than observed in P. troglodytes schweinfurthii.Cohesion between males and female P. paniscusmay increase with party size.

Seasonality and Socioecology: The Importance of Variation in Fruit Abundance to Bonobo Sociality

The assumption that nonseasonal, evergreen, rain forests contain more continuously available food resources than seasonal rain forests is fundamental to comparisons made between the socioecology of the male-bonded Pan troglodytes and the female-based social system of the Pan paniscus. Chimpanzee females may be less social due to the high costs of feeding competition, whereas in the more food-rich central African rain forests such as the Lomako forest, female bonobos can associate and socially bond. The Lomako Forest experiences two wet and two dry seasons a year. Data on fruit abundance and sociality show that despite monthly variation in fruit availability, there was no consistent seasonal variation in fruit abundance or dietary breadth. Bonobo use of nonfig fruits, figs, THV, and leaves did not follow seasonal patterns. Leaves and THV may act as complementary sources of plant protein and their use was inversely correlated. Monthly variation in fruit abundance was associated with a significant decrease in the number of males in a party but not in the number of females. Focal males were frequently solitary during 1 of the 3 months with the smallest party sizes. In contrast, females remained social with each other throughout the year. Therefore, seaonality at Lomako appeared to be less marked than at comparable chimpanzee sites, such that the variation in fruit abundance did not fall below a level that prohibits female sociality.

Distribution of Ruffed Lemurs (Varecia variegata) in Ranomafana National Park, Madagascar

The distribution of Varecia variegata at 3 sites in Ranomafana National Park is consistent with the hypothesis that this specialized frugivore is the most susceptible of 12 sympatric lemur species to habitat disruption brought about by selective logging. Varecia is most abundant at the least disturbed site and absent from the most intensively logged site. The logged area showed lower values for overall tree basal area, crown diameters and tree height and an increase in the number of trees, presumably because of growing saplings. Varecia select large food trees of species that are preferentially logged. Although tree species diversity may be higher after logging, many Varecia food trees were absent from disturbed forests.

Changes in Behavior in Free-Ranging Lemur catta Following Release in a Natural Habitat

The adjustment of captive‐reared and developmentally deprived ringtailed lemurs (Lemur catta) to supported release on St. Catherines Island, Georgia, was studied over 7 years to examine if these animals developed behavior comparable to wild populations. Initial changes after release included decreased obesity and increased agility as well as foraging for appropriate novel plants. Ranging, daily behavior cycles, and vocalizations developed more slowly over 1–3 years, but eventually the behavior resembled that of wild groups. Group composition and social structure changed through conflict to resemble wild and captive troops in social organization, including the emergence of matrilineal dominance and male emigration. Since behavior eventually resembled that seen in the wild, some resilience of species‐typical wild behavior in captivity is supported. Am. J. Primatol. 47:15–28, 1999.

Pan paniscus 1973 to 1996: Twenty-three years of field research

Although the pygmy chimpanzee is not smaller than a chimpanzee, it has long intrigued anthropologists with its more gracile build and greater arboreality. The social organization of the pygmy chimpanzee is unique among primates in showing female‐bonding among nonrelatives, little male bonding despite male residence, and relative inability of males to outrank females. The crucial ecological basis for this difference from the male‐bonded system of chimpanzees is that pygmy chimpanzees do not go through an extended season of low food availability during which party sizes fall to low levels.

Contrasting chimpanzees and bonobos: nearest neighbor distances and choices

In an effort to understand factors underlying differences in the social organization of Pan troglodytes and P. paniscus, we measured the nearest neighbor distances and choices for chimpanzees in Kibale National Park, Uganda, and for bonobos in Lomako Forest, Zaire. We assume that the spatial organization of a set of individuals should reflect the underlying relationships between them. Bonobos were found to have smaller nearest neighbor distances than chimpanzees. The distribution and variability of the distances suggested that chimpanzees have a more restricted range of nearest neighbor distances than bonobos, avoiding situations of very close proximity. Selection of the sex of the nearest neighbor by a focal animal differed between the species. For example, male bonobos rarely had another male as their nearest neighbor, while male chimpanzees frequently did. Similarly, male bonobos tended to move apart when observed together, while it was female chimpanzees who tended to move apart when together. These observations are considered with respect to the nature of the social interactions in these species.

Male Transfer in Captive Ruffed Lemurs, Varecia Variegata Variegata

Black and white ruffed lemur (Varecia variegata variegata) groups have been studied under both semi-free-ranging conditions, at the Duke University Primate Center (DUPC), since 1985 and in the wild, within Ranomafana National Park in southeastern Madagascar, since 1988. In 1990, fences separating two established enclosure groups of V. variegata at the DUPC were removed. The lowest ranking males from each group first visited and then transferred from their natal group into the neighboring group. One male transferred into a single-male, single female group and rapidly attained dominance over the older resident male. The male transferring into the larger group (6–8 individuals) rose gradually in rank and mated with both resident adult females (mother and daughter). In V. variegata, female mating with multiple males may be an adaptation to reduce the risk of infanticide, but single male matings appear to be necessary for paternal care (guarding). In the wild, males have also been observed visiting between study groups.

Sex differences in tool use acquisition in bonobos (Pan paniscus)

All the great ape species are known tool users in both the wild and captivity, although there is great variation in ability and behavioral repertoire. Differences in tool use acquisition between chimpanzees and gorillas have been attributed to differing levels of social tolerance as a result of differences in social structure. Chimpanzees also show sex differences in acquisition and both chimpanzees and bonobos demonstrate a female bias in tool use behaviors. Studies of acquisition are limited in the wild and between species comparisons are complicated in captivity by contexts that often do not reflect natural conditions. Here we investigated tool use acquisition in a captive group of naïve bonobos by simulating naturalistic conditions. We constructed an artificial termite mound fashioned after those that occur in the wild and tested individuals within a social group context. We found sex differences in latencies to attempt and to succeed where females attempted to fish, were successful more quickly, and fished more frequently than males. We compared our results to those reported for chimpanzees and gorillas. Males across all three species did not differ in latency to attempt or to succeed. In contrast, bonobo and chimpanzee females succeeded more quickly than did female gorillas. Female bonobos and female chimpanzees did not differ in either latency to attempt or to succeed. We tested the social tolerance hypothesis by investigating the relationship between tool behaviors and number of neighbors present. We also compared these results to those reported for chimpanzees and gorillas and found that bonobos had the fewest numbers of neighbors present. The results of this study do not support the association between number of neighbors and tool behavior reported for chimpanzees. However, bonobos demonstrated a similar sex difference in tool use acquisition, supporting the hypothesis of a female bias in tool use in Pan. Am. J. Primatol. 75:917–926, 2013.

Variation in the Social Systems of Extant Hominoids: Comparative Insight into the Social Behavior of Early Hominins

The observed social systems of extant apes and humans suggest that the common ancestral state for Miocene hominoids was living in multimale–multifemale groups that exhibited a tendency to fission and fusion in response to ecological and/or social variables. The Hominoidea share a set of social commonalities, notably a social niche that extends beyond kin and beyond the immediate social group, as well as extensive intraspecific flexibility in social organization. We propose that an essential feature of hominoid evolution is the shift from limited plasticity in a generalized social ape to expanded behavioral plasticity as an adaptive niche. Whereas in most nonhominoid primates variability and flexibility take the shape of specific patterns of demographic flux and interindividual relationships, we can consider behavioral flexibility and plasticity as a means to an end in hominoid socioecological landscapes. In addition, the potential for innovation, spread, and inheritance of behavioral patterns and social traditions is much higher in the hominoids, especially the great apes, than in other anthropoid primates. We further suggest that this pattern forms a basis for the substantial expansion of social complexity and adaptive behavioral plasticity in the hominins, especially the genus Homo. Our objectives in this article are threefold: 1) summarize the variation in the social systems of extant hominoid taxa; 2) consider the evolutionary processes underlying these variations; and 3) expand upon the traditional socioecological model, especially with respect to reconstructions of early hominin social behavior. We emphasize a central role for both ecological and social niche construction, as well as behavioral plasticity, as basal hominoid characteristics. Over evolutionary time these characteristics influence the patterns of selection pressures and the resulting social structures. We propose that a mosaic of ecological and social inheritance patterns should be considered in the reconstruction of early hominin social systems.