Female Activity Budgets and Offspring Development in Wild Bonobos and Chimpanzees
Open AccessSocioecological models posit that feeding ecology is a key determinant of behavior, social organization, and life history (Brown, 1964; Emlen & Oring, 1977; Sterck, Watts, & van Schaik, 1997). Across mammals, females are particularly impacted by access to food resources because they must cope with the energetic demands of both rearing offspring and ensuring their own survival. Therefore, feeding ecology can have both direct impacts on offspring development by shaping life history as well as indirect impacts by shaping maternal strategies. In this thesis, I take a comparative perspective to considering how differences in feeding ecology relate to female activity budgets and offspring development in the genus Pan.Given their close phylogenetic relationship, the two great apes that comprise the genus Pan, bonobos (Pan paniscus) and chimpanzees (P. troglodytes), exhibit numerous similarities in morphology and behavior, including general locomotor adaptations and promiscuous, multi-female, multi-male fission-fusion social organizations (reviewed in Gruber & Clay, 2016). Despite these similarities, the two species exhibit stark differences in female feeding ecology. Female bonobos experience less intense feeding competition than female chimpanzees, which relates to salient differences in female sociality: female bonobos are more gregarious and frequently engage in cooperative behavior, while female chimpanzees are generally less social. One might expect that these differences in feeding ecology and social relationships manifest in species-specific patterns of maternal investment and offspring development.However, despite several decades of field studies on both Pan species, surprisingly little is known about how differences in female feeding ecology and gregariousness influence maternal strategies and offspring development in wild populations. I utilize novel data on wild bonobos paired with existing data on wild chimpanzees to evaluate differences between the two species in female activity budgets and offspring development. In Chapter 1, I compared activity budgets between lactating females in the two species and found that they did not differ in time allocated to foraging effort (feeding and travel budgets) or overall social effort. This suggests that, despite differences in feeding ecology between the species, females allocate comparable amounts of time to feeding and social interactions. This may be a consequence of differences in fission-fusion dynamics between females of the two species. However, I found that the distribution of social partners differed between females of the two species: while female bonobos biased social interactions towards their dependent offspring, female chimpanzees biased social interactions towards other community members. The prioritization of community members instead of their dependent offspring is surprising given that female chimpanzees spend substantial time ranging alone with their dependent offspring. I suggest that female chimpanzees may be prioritizing interactions with community members when in groups to maintain their social bonds in spite of extended periods of solitary ranging.In Chapter 2, I compared behavioral development in infant and juvenile females of the two species, an age-sex class of particular interest given stark differences in adult female sociality between the two species. I again found broad similarities between the two species, in metrics related to the attainment of independence from mothers and the development of social skills. In Chapter 3, I compared the affiliative social preferences of infant and juvenile females of the two species, hypothesizing that they differ due to the different social integration strategies adopted by the two species upon immigration. Given high costs associated with dispersal, selection should act on individuals prior to dispersal in a manner that promotes their exposure to the types of social relationships that are most relevant for when they attempt to integrate into new communities. I found that infant and juvenile female bonobos engaged in affiliative social interactions with nonmaternal kin adult social partners proportionately more than did infant and juvenile female chimpanzees. I also found that, of these nonmaternal kin adult social partners, infant and juvenile female bonobos interacted with males more than expected by chance.My results demonstrate a remarkable similarity in activity budgets between females of the species, both during adulthood and immaturity. Aspects of behavior that differed between species were predominantly associated with the distribution of social partners. These results highlight the need for additional study on the development of less frequent social behaviors, including aggression and coalition formation, as well as the need for data collection on adolescents, in order to better understand the developmental correlates of differences in adult behavior between females of the two Pan species.
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