L. van Elsacker

Dominance and its behavioral measures in a captive group of bonobos (**Pan paniscus**)

We investigated the existence of a social dominance hierarchy in the captive group of six adult bonobos at the Planckendael Zoo. We quantified the pattern of dyadic exchange of a number of behaviors to examine to what extent each behavior fits a linear rank order model. Following de Waal (1989), we distinguish three types of dominance: agonistic dominance, competitive ability and formal dominance. Fleeing upon aggression is a good measure of agonistic dominance. The agonistic dominance hierarchy in the study group shows significant and strong linearity. The rank order was: 1. female (22 yr), 2. female (15 yr)., 3. male (23 yr.), 4. female (15 yr.), 5. male (9 yr.), 6. male (10 yr.). As in the wild, the females occupy high ranks. There is prominent but nonexclusive female agonistic dominance. Teeth-baring does not fulfil the criteria of a formal submission signal. Peering is a request for tolerance of proximity. Since its direction within dyads is consistent with that of fleeing interactions, it is a useful additional measure to determine agonistic ranks in bonobos. In competitive situations, the females acquire more food than other group members do. The rank obtained from access to food resources differs from the agonistic rank due to female intrasexual social tolerance, expressed in food sharing. We typify the dominance styles in the group as female intrasexual tolerance and male challenging of rank differences. The agonistic rank order correlates significantly with age and has a strong predictive value for other social behaviors.

Effect of posture on hand preferences during experimental food reaching in bonobos (Pan paniscus)

The effect of posture on hand preferences was examined in an experiment with 5 bonobos (Pan paniscus). To obtain a food reward, the animals had to adopt 1 out of 7 different postures. These postures represented an increasing problem to the maintenance of body equilibrium. It was expected that an increasing demand for equilibrium maintenance would elicit individual preferences and a population-level bias. All animals showed an increasing trend toward left-handedness while shifting to a bipedal posture from a seated posture by way of a quadrupedal posture. The importance of bipedalism in the evolution of left- and right-handedness is discussed briefly.

High chimpanzee and gorilla densities in a non-protected area on the northern periphery of the Dja Faunal Reserve, Cameroon

Information on the densities of threatened species in non-protected areas is crucial for assessing the degree of isolation of adjacent protected areas and consequently their potential for preserving species from extinction. Relatively few studies, however, provide such information. We present the results of a survey of the densities of two great ape species, the gorilla Gorilla gorilla gorilla and chimpanzee Pan troglodytes troglodytes , in a non-protected area on the northern periphery of Dja Faunal Reserve, Cameroon. Densities of chimpanzees and gorillas were estimated to be 1.1 and 3.8 weaned individuals per km 2 , respectively. The results confirm that gorillas prefer building nests in vegetation types with limited visibility, and that within preferred vegetation types for nesting, gorillas select patches that are the most difficult to penetrate, resulting in less conspicuous nests. Although the opposite tendencies were exhibited by chimpanzees, no firm conclusions could be drawn from our data. Despite its non-protected status and past and ongoing logging activities in the area, the densities of gorillas and chimpanzees on the northern periphery of Dja Faunal Reserve are comparable to those found within the reserve itself, indicating the need for developing alternative conservation action to protect these important populations. The creation of a Communal Wildlife Zone in this area is legislatively possible, and could be an effective conservation tool because it has to originate from the local people.

A review of terminology on aggregation patterns in bonobos ( Pan paniscus )

We review terms that describe the levels of group structure in Pan paniscus.We discuss points of confusion that result from unclear definition of terms, which could lead to inaccurate intra- and interspecific comparisons. We recommend terms that are applicable to captive populations.

Current status of the bonobo (Pan paniscus) in the proposed Lomako Reserve (Democratic Republic of Congo).

Abstract The creation of a 3800 km2 forest reserve along the Lomako river (Equateur Province, Democratic Republic of Congo) is considered one of the main actions necessary for the conservation of the bonobo (Pan paniscus). We conducted a one-month qualitative survey in parts of the Lomako forest, in order to determine the presence of bonobos and the nature and degree of human disturbance. Results are based on information obtained by questionnaires at human settlements, direct evidence of human activities and forest visits in search of bonobos. There is still a rather large bonobo population in the south-central part of the Lomako forest, relatively free of hunting. However, the northern population seems to be decimated, except in the most impenetrable forest parts. The threats to the proposed reserve are twofold. First, the indigenous inhabitants of the Lomako forest have started intensifying commercial bushmeat hunting as an alternative to the loss of their agricultural economy. Secondly, an increasing number of commercial hunters are entering the area. In addition to the creation of the Lomako Forest Bonobo Reserve, we consider that support of local agriculture and the presence of researchers are the most important factors for the continued preservation of the local bonobo population by the indigenous inhabitants of the forest.

Food calling by captive bonobos (**Pan paniscus**) : an experiment

We examined (i) whether bonobos display a specific food-calling behavior when discovering a hidden food resource, (ii) whether the presence of competitors affects this behavior, and (iii) whether food quantity or gender influences its appearance. We carried out experiments (n = 108) within a captive group of eight bonobos at the Animal Park Planckendael (Mechelen,Belgium). We hid highly preferred food items (n = 7 or 25) in their enclosure and recorded vocal behavior and interactions between discoverer and group members. As a control, we gave the same number of items to the individuals when isolated from the group, a situation without potential food competition (n = 38). The only vocalization frequently uttered by the discoverer was the food peep. They uttered food peeps significantly more often when no food competition was possible. The amount of food had no significant influence on whether food peeps were uttered. The same applies to the individuals’ identity or gender. Although the costs of food calling behavior seemed much higher for males, both sexes uttered food calls to the same extent. We hypothesize thai males signal food presence in order to attract potential mates and are willing to give up the discovered food resource in return for sex: sex for food exchange. In contrast, females may vocalize to attract coalition partners. Through these coalitions, they can monopolize food resources vis-à-vis males. It is also possible that females have less reason to suppress food calk, since they are dominant to males. This study suggests that bonobos are able to give shaded signals about their environment and have the potential to communicate this information in order to promote their sexual strategy.

Insectivory by Gorilla gorilla gorilla in Southeast Cameroon

Our study extends quantitative analyses of insect-eating by gorillas (Gorilla gorilla gorilla) to Cameroon. During a 2-mo period (May–June 2001), we recorded and analyzed feeding traces on plants and insects and in gorilla feces. We found 180 feeding traces, 17% of which involved insectivory. Seventy-eight percent of the fecal samples (n = 36) contained insects. Ants were found in 61% of the samples, termites in 39%, while 56% of the samples contained remains of other insects. We added ≥14 new species to the known insect diet of western lowland gorillas. Overall, social insects are predominant. The choice of prey by the Ntonga gorillas gives new clues for the existence of cross-cultural differences among gorilla populations. A comparison of the overall frequency of insectivory with those at other sites in Central Africa indicate a possible effect of forest disturbance on the insectivorous behavior of gorillas. The study suggests the existence of temporal variation in ant- and termite-eating by gorillas.

Signaling of Reproductive Status in Captive Female Golden-Headed Lion Tamarins (Leontopithecus chrysomelas)

In species in which paternal care has an important impact on the offsprings fitness, concealment of reproductive status has been proposed as a strategy employed by females to prevent males from practicing desertion and polygamy, which would then lead to monogamous or polyandrous mating systems or both. We investigated whether the females reproductive status is being concealed in golden-headed lion tamarins, which exhibit extensive paternal care and a mainly monogamous/polyandrous mating system. We used a combination of behavioral observations and endocrine data to determine female reproductive status and to examine changes in sociosexual behaviors over the ovarian cycle and between conceptive and nonconceptive cycles. Females clearly signaled their reproductive status by way of proceptive sexual presenting. Males showed increased frequencies of anogenital sniffing and mounting during the fertile period, indicating that they detected changes in olfactory and behavioral cues emitted by females, and they adjusted their mounting behavior accordingly. Males and females also remained in closer proximity before and during the fertile period, which suggests the existence of mate guarding. We discuss a possible function of behavioral advertisement of reproductive status in shaping the mating system in Leontopithecus chrysomelas.