Abstract
Inequity aversion, the negative reaction to unequal treatment, is considered a mechanism for stabilizing cooperative interactions between non-kin group members. However, this might only be adaptive for species that switch cooperative partners. Utilizing a comparative approach, inequity aversion has been assessed in many mammalian species and recently also in corvids and one parrot species, kea, revealing mixed results. To broaden our knowledge about the phylogenetic distribution of inequity aversion, we tested four parrot species in the token exchange paradigm. We varied the quality of rewards delivered to dyads of birds, as well as the effort required to obtain a reward. Blue-headed macaws and African grey parrots showed no reaction to being rewarded unequally. The bigger macaws were less willing to exchange tokens in the “unequal” condition compared to the “equal high” condition in which both birds obtained high quality rewards, but a closer examination of the results and the findings from the control conditions reveal that inequity aversion does not account for it. None of the species responded to inequity in terms of effort. Parrots may not exhibit inequity aversion due to interdependence on their life-long partner and the high costs associated with finding a new partner.
Highlights
Inequity aversion, the negative reaction to unequal treatment, is considered a mechanism for stabilizing cooperative interactions between non-kin group members
We found that the blue-headed macaws and the African grey parrots exhibited no reaction to unequal treatment and continued to exchange tokens independent of the reward distribution
Inequity aversion did not seem to account for the response of either species
Summary
The negative reaction to unequal treatment, is considered a mechanism for stabilizing cooperative interactions between non-kin group members. Receiving equal outcomes for equal cooperative effort is a basic expectation in our everyday lives If this premise is violated by rewarding individuals unequally for the equal amount of work, a negative behavioural reaction, termed inequity aversion, arises[1]. It has been proposed that species that rely on cooperative interactions with varying non-kin group members may benefit from evaluating the equality of their cooperative payoffs in order to assess whether to stick with a certain partner or to look for another one with whom to gain better outcomes[14] This hypothesis has gained further support by studies on non-primate species; dogs[15], wolves, Canis lupus[16] and rats, Rattus norvegicus[17], all gregarious species that regularly cooperate with non-kin group members, have shown inequity aversion in paradigms adapted to their species-specific abilities. On the other end of the social spectrum, less gregarious species do not require such a mechanism since assessments of cooperative payoffs are not relevant in their everyday life[8]
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