‘Adoption’ by Maternal Siblings in Wild Chimpanzees

PLOS ONE, Dec 2019

The adoption of unrelated orphaned infants is something chimpanzees and humans have in common. Providing parental care has fitness implications for both the adopter and orphan, and cases of adoption have thus been cited as evidence for a shared origin of an altruistic behaviour. We provide new data on adoptions in the free-living Sonso chimpanzee community in Uganda, together with an analysis of published data from other long-term field sites. As a default pattern, we find that orphan chimpanzees do not become adopted by adult group members but wherever possible associate with each other, usually as maternal sibling pairs. This occurs even if both partners are still immature, with older individuals effectively becoming ‘child household heads’. Adoption of orphans by unrelated individuals does occur but usually only if no maternal siblings or other relatives are present and only after significant delays. In conclusion, following the loss of their mother, orphaned chimpanzees preferentially associate along pre-existing social bonds, which are typically strongest amongst maternal siblings.

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‘Adoption’ by Maternal Siblings in Wild Chimpanzees

Citation: Hobaiter C, Schel AM, Langergraber K, Zuberb uhler K ( 'Adoption' by Maternal Siblings in Wild Chimpanzees Catherine Hobaiter 0 Anne Marijke Schel 0 Kevin Langergraber 0 Klaus Zuberbu hler 0 Odile Petit, CNRS (National Center for Scientific Research), France 0 1 Centre for Social Learning and Cognitive Evolution and Scottish Primate Research Group, School of Psychology and Neuroscience, University of St Andrews , St Andrews, Scotland , 2 Department of Psychology, University of York, York, United Kingdom, 3 Institute Jean Nicod, Ecole Normale Supe rieure, Pavillon Jardin, Paris, France, 4 Department of Anthropology, Boston University , Boston , Massachusetts, United States of America, 5 Department of Primatology, Max Planck Institute for Evolutionary Anthropology , Leipzig, Germany , 6 Department of Comparative Cognition, Institute of Biology, University of Neuchatel , Neuchatel, Switzerland, 7 Budongo Conservation Field Station, Masindi , Uganda The adoption of unrelated orphaned infants is something chimpanzees and humans have in common. Providing parental care has fitness implications for both the adopter and orphan, and cases of adoption have thus been cited as evidence for a shared origin of an altruistic behaviour. We provide new data on adoptions in the free-living Sonso chimpanzee community in Uganda, together with an analysis of published data from other long-term field sites. As a default pattern, we find that orphan chimpanzees do not become adopted by adult group members but wherever possible associate with each other, usually as maternal sibling pairs. This occurs even if both partners are still immature, with older individuals effectively becoming 'child household heads'. Adoption of orphans by unrelated individuals does occur but usually only if no maternal siblings or other relatives are present and only after significant delays. In conclusion, following the loss of their mother, orphaned chimpanzees preferentially associate along pre-existing social bonds, which are typically strongest amongst maternal siblings. - Competing Interests: The authors have declared that no competing interests exist. Adoptions of orphaned infant and juvenile chimpanzees have been recorded at all long-term research sites [15]. In East African chimpanzee communities, adoption has been documented for older maternal siblings, nulliparous and infertile females [24] and by a maternal grandmother [5]. In contrast, in the West African communities of Ta Forest, Ivory Coast, adoptions by apparently unrelated group members are common, including adult males (one father) and parous females, particularly allies of the deceased mother with no known kin-relationship [1]. As in humans, adoption in chimpanzees involves the regular provision of allomaternal care, such as carrying, sharing food, defending, and grooming [1,2], by an adult individual in ways that do not differ from what is normally provided by the biological mother [1]. These observations led Boesch et al. [1] to suggest that adoption by wild chimpanzees should be interpreted as a potential example of altruistic behaviour in the animal kingdom, mainly because of the significant costs to the adopter [1,5]. The understanding of prosocial behaviour in non-human animals, and in particular altruism, has been hampered by a failure to establish and implement clear behavioural definitions [6 8]. We take prosocial behaviour to be a behaviour that increases the direct fitness of another individual [9]. Although recent research has provided within-species comparisons of prosocial behaviour, the emerging picture still remains unclear [see 10]. For instance, in chimpanzees there is evidence both for [11,12] and against [13] prosocial behaviour. One explanation may be that the expression of prosocial behaviour is task and situation specific. For example, in contrast to wild chimpanzees, captive individuals may not actively share food, but do help others to complete a food reward task [14]. Altruism is one possible motivation for prosocial behaviour, although there are other possibilities. An accepted evolutionary way to define behaviour, such as adoption, as altruistic is in terms of its lifetime fitness consequences [7,8]. For adoption to be an altruistic behaviour there must be an average cost to the lifetime fitness of the adopter, and an average benefit to the lifetime fitness of the orphan [8]. Boesch et al. [1] employ a definition for adoption that is based on immediate costs (to the adopter) and benefits (to the orphan) during the care period. However, as they note, adoption may also result in long-term benefits for the adopter, for example by gaining a future social ally [1]. If the initial cost to the adopter during the care period is met or exceeded by later benefit, adoption may be better described as mutualism [7]. Whether or not chimpanzees express prosocial, altruistically motivated behaviour has considerable implications for theories of human evoluti (...truncated)


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Catherine Hobaiter, Anne Marijke Schel, Kevin Langergraber, Klaus Zuberbühler. ‘Adoption’ by Maternal Siblings in Wild Chimpanzees, PLOS ONE, 2014, Volume 9, Issue 8, DOI: 10.1371/journal.pone.0103777