Tentative evidence for inequity aversion to unequal work-effort but not to unequal reward distribution in Goffin's cockatoos

One hallmark in the evolution of cooperation is the ability to evaluate one's own payoff for a task against that of another person. To trace its evolutionary history, there has recently been a surge in comparative studies across different species. In non-human animals, evidence of inequity aversion has so far been identified in several primate species, dogs, and rats. Research in birds revealed mixed findings so far: among corvids, crows and ravens did react sensitively to unequal payoffs and work-effort, while New Caledonian crows did not. Among psittacids, kea were studied so far: Yet, despite the fact that they live in large, hierarchically organized social groups that show complex interactions, they did not show a significant reaction to inequitable payoffs. Here we tested for the first time a Cacatua, the Goffin's cockatoo, using a standardized token exchange paradigm in which first the partner and then the subject could exchange a token for a food reward. Our results show that subjects did not react to unequal reward distributions. However, in comparison to the Equity Condition, the likelihood to exchange was lower in the condition in which the partner received the same reward as a gift (without having to work for it) whereas the subject had to perform a task involving substantial work-effort, suggesting that the Goffin's cockatoos do react aversively to work-effort inequity. In a follow-up experiment, subjects never received a reward but observed a conspecific receive a high-quality reward depending on condition. We found again no evidence for an aversion for the unequal reward distribution, but only that, independent of condition, subjects quickly lost their motivation to participate due to not receiving a reward. In summary, Goffins showed some sensitivity to increased unequal work-effort, but did not react to unequal reward distribution.