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The majority of information from wild bonobos is obtained by observations of focal individuals. These records are combined with physiological and molecular data to explore causes and consequences of social behavior. Using material such as urine, feces, and hair that can be collected non-invasively from wild apes, we are able to measure hormones, stable isotopes and other physiological markers. The work on wild bonobos at LuiKotale is complemented by data from a large number of captive bonobos. This approach takes advantage of having control of social and environmental parameters and the fact that both sexes can be monitored beyond the age when females transfer to other groups.

Nutrition and energy status

by Gottfried Hohmann & Barbara Fruth

Diet composition is often regarded to constitute a species specific trait with certain food types being dominant over others. In the context of human evolution, the change from a plant based diet (herbivory) to a mixed diet combining several types of food (omnivory) is regarded a prime mover paving the way for other developments such as cooperation, food sharing, and technologies of food processing. Evolutionary models associate the emergence of these traits with relatively dry habitats, pronounced seasonality, and shortage of fruit bearing trees which implies that these traits would not have evolved in high productivity forest habitats. Some traits of bonobo feeding behavior, namely the absence of tool use, the relatively low investment in food processing, and the relatively low frequency of meat eating seem to support such models. Our work combines data on the individual nutritional status with measures of energy supply and food abundance, using stable isotopes and other nutritional markers that can be extracted from non-invasive samples such as hair, urine and feces.

This work is conducted in collaboration with Sylvia Ortmann, Leibnitz Institute for Zoo and Wildlife Research, Berlin and with Vicky Oelze, Anthropology Department, UC Santa Cruz

Resource competition among females

by Niina Nurmi

Socio-ecology provides models that link characteristics of food resources to different modes of competition within and between groups. In addition the model predicts differential net energy gain to characteristics of dominance hierarchies and social relationships. In short, food resources that are defendable and worth defending lead to direct (or contest) competition that will take place within and/or between groups depending on patch size. On the other hand, indirect (or scramble) competition occurs over resources that are limited but indefensible. Different modes of competition lead to specific combinations of negative or positive individual net energy gain when associated with group size and dominance rank. This will select for behavioral kin biases, cooperation, and matrilineal hierarchies in some cases and egalitarian societies or social tolerance in others. The aim of this project is to evaluate socio-ecological models that link characteristics of the main food resources that females compete over with differential net energy gain.

The project is conducted jointly with Oliver Schülke from the Courant-Research Center Evolution of Social Behavior, Georg August Universität Göttingen

Female mate choice and mating strategies in wild bonobos

by Pamela Heidi Douglas

The main objective of this study is to elucidate the role of females in the mating system of wild bonobos. Long-standing questions pertaining to female sexuality are being addressed in the free-ranging community of bonobos at Luikotale, using an integrated approach combining information on behaviour, sexual morphology, and reproductive physiology. Female mate choice is an important evolutionary process that imposes sexual selection on males, yet for nearly a century after it was proposed in Darwin’s seminal work (1871); females were considered to be less sexually strategic than males. Only in recent decades have female mate choice and mating strategies received considerable theoretical and empirical attention, and evidence that females actively pursue their own mating and reproductive strategies is continually increasing.

To address this question, I am conducting focal observations and collecting non-invasive hormone samples of female bonobos to obtain data on mating behaviour and reproductive endocrinology. More specifically, I will relate female mating behaviour to reproductive state and conception likelihood by considering visual signs of oestrus and timing of ovulation as assessed by urinary concentrations of sex steroids.

The data collected in this study are expected to answer the following questions:

  1. What mating strategies do female bonobos employ?
  2. How does female reproductive state influence female mating strategies and choice of mating partners?
  3. Is there evidence of female mate choice in bonobos, and if so, what is the function of female choice within the mating system of wild bonobos?

Cooperation without genetic ties: a behavioral endocrine model in bonobos

by Liza Moscovice

Humans are distinguished from other species by their high levels of sociality and cooperation with a diverse range of social partners, including relatives and also unrelated individuals. In non-human primates as well, individuals form selective social bonds that confer a range of fitness benefits. Most research on primate social bonds has focused on relationships among the philopatric sex, involving kin or same-sex peers who have grown up together. As a result, we currently know very little about the extent to which nonhuman primates have adaptations to maintain flexible social bonds outside of kinship, and we do not understand the function of differentiated social relationships for primates.

Bonobos deviate from predictions of socio-ecological models as females, the dispersing sex, engage in close bonds while the weakest associations occur between the philopatric males. This research project explores the causes and consequences of individual variation in affiliation and cooperation among unrelated female bonobos within and between communities.

This work is conducted in collaboration with Adrian Jaeggi, Dept. of Anthropology, Emory University

Male dominance relations, mating strategies and reproductive success

By Gottfried Hohmann & Barbara Fruth

Male bonobos comprise a suit of behaviors that is exceptional when compared with other non-human primates:
1. While physically superior to females, males and females are co-dominant.
2. While females engage in differentiated relations and sometimes support each other in conflicts, social relations among males are neutral and there is no evidence for coalitions with other males.
3. The strongest and most consistent social bond of males is to its mother who may intervene on behalf of her son in conflicts of males.
4. As other primate species male community members form a linear dominance hierarchy and high ranking males are more aggressive than low ranking males. However, common forms of male aggression such as infanticide and aggression against females are absent or rare. This raises a number of questions regarding the extent of mate competition among males and its efficiency in terms of mating and reproductive success.

Another feature of bonobo society is the lack of territorial defense and relaxed inter-community relations. In chimpanzees one important role of male community members is territorial defense (resource defense polygyny). In addition of protecting the home range from invasion by neighbors, male chimpanzees launch cooperative attacks and injure or kill members of neighboring groups. In contrast, in bonobos intercommunity encounters are relatively relaxed and the intense aggression between males from different groups has never been observed. This lack of xenophobia in male aggression is considered as being part of a behavioral syndrome that is characterized by low level aggression and/or low intensity of aggression. However, the selective advantage deriving from the pacifistic nature of male behavior remains ambiguous. Long-term data from males of one community are combined with corresponding information from another community and are updated with current information.

This work is done in collaboration with Martin Surbeck and Linda Vigilant

Life history patterns of bonobos and chimpanzees and their relation to modern humans

by Verena Behringer

Hominoid primates and humans share a suit of life history traits such as slow maturation, a delay in reproductive activity, and long life expectancy. Maturation until adulthood may account for one third of total life span and there is often a considerable gap between the emergence of reproductive competence and the onset of reproduction. While many components of the life history of modern humans reflect cultural and economic achievements of postindustrial societies, basic trends of developmental changes display phylogenetic heritage. The project aims to collect information on the physiological; development of bonobos and chimpanzees, two hominoid species that are equally closely related to humans. In this context we will explore physiological markers that have not been the subject of primate studies but that are known to be indicative of critical stages in the life history of humans. In order to establish the bench mark for comparative analyses, the project compiles a data set from modern humans that allows direct comparisons with the data from hominoid primates. The proximate goal of the project is a cross-sectional study that uses samples from a relatively large number of bonobos, chimpanzees, and modern humans to identify temporal patterns of fluctuation of physiological markers. The ultimate goal is to initiate a long-term project to monitor ontogenetic changes in physiology, physical development and behavior of individual bonobos and chimpanzees from birth to adulthood.

This project is conducted jointly with Jeroen Stevens, University of Antwerp.

Development and maturation

By Sean Lee

Differences between bonobos and chimpanzees are most obvious in adult individuals. To investigate when and how species specific traits emerge, we collect data from 25 immature individuals in two habituated bonobo communities at the field site of Lui Kotale. The project (PhD by Sean Lee, Georg Washington University) was launched in 2013 and is ongoing. Data collection includes morphometric measures and dental development using photogrammetry, and development of social behavior. Urine samples from the same subjects facilitate investigation of age-related changes in hormones that are indicative of specific developmental stages such as adrenarche, puberty, and onset of reproduction. This work is embedded into another long-term project that involves a large number of captive bonobos (N=170) and chimpanzees (N=195).

This work is conducted in collaboration with Shannon McFarlin and Carson Murray from the Department of Anthropology, Georg Washington University