In this study, we assessed whether retaliatory aggression is used by mandrill (Mandrillus sphinx) victims as a way to enforce a reduction in further received aggression. Four possible types of retaliatory aggression were explored: direct or indirect and immediate or time-decoupled retaliation. Both immediate direct retaliation (counteraggression) and immediate indirect retaliation (kin-oriented redirected aggression, that is, redirected aggression aimed at the original aggressor's kin) were observed, but only the latter was effective in reducing the risk of renewed aggression by the original aggressor. Depending on the exact definition of functional punishment one is adopting, these results may be interpreted as suggesting kin-oriented redirected aggression represents a previously overlooked form of punishment based on the infliction of indirect costs.
