‘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.
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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)