Abstract
Tolerant food sharing among human foragers can largely be explained by reciprocity. In contrast, food sharing among chimpanzees and bonobos may not always reflect reciprocity, which could be explained by different dominance styles: in egalitarian societies reciprocity is expressed freely, while in more despotic groups dominants may hinder reciprocity.
We tested the degree of reciprocity and the influence of
dominance on food sharing among chimpanzees and
bonobos in two captive groups. First, we found that
chimpanzees shared more frequently, more tolerantly,
and more actively than bonobos. Second, among chimpanzees,
food received was the best predictor of food
shared, indicating reciprocal exchange, whereas among
bonobos transfers were mostly unidirectional. Third,
chimpanzees had a shallower and less linear dominance
hierarchy, indicating that they were less despotic than
bonobos. This suggests that the tolerant and reciprocal
sharing found in chimpanzees, but not bonobos, was
made possible by the absence of despotism. To investigate
this further, we tested the relationship between
despotism and reciprocity in grooming using data from
an additional five groups and five different study periods
on the main groups. The results showed that i) all
chimpanzee groups were less despotic and groomed
more reciprocally than bonobo groups, and ii) there was
a general negative correlation between despotism and
grooming reciprocity across species. This indicates that
an egalitarian hierarchy may be more common in chimpanzees,
at least in captivity, thus fostering reciprocal
exchange. We conclude that a shallow dominance
hierarchy was a necessary precondition for the evolution
of human-like reciprocal food sharing.
Jaeggi, A V