Abstract
Sensitivity to inequity is considered to be a crucial cognitive tool in the evolution of human cooperation. The ability has recently been shown also in primates and dogs, raising the question of an evolutionary basis of inequity aversion. We present first evidence that two bird species are sensitive to other individuals' efforts and payoffs. In a token exchange task we tested both behavioral responses to inequity in the quality of reward (preferred versus non-preferred food) and to the absence of reward in the presence of a rewarded partner, in 5 pairs of corvids (6 crows, 4 ravens). Birds decreased their exchange performance when the experimental partner received the reward as a gift, which indicates that they are sensitive to other individuals' working effort. They also decreased their exchange performance in the inequity compared with the equity condition. Notably, corvids refused to take the reward after a successful exchange more often in the inequity compared with the other conditions. Our findings indicate that awareness to other individuals' efforts and payoffs may evolve independently of phylogeny in systems with a given degree of social complexity.
Highlights
The sensitivity regarding ones benefits of actions as well as efforts and payoffs for both the acting individual and its cooperation partner is proposed to be a key mechanism in the evolution of cooperation [1,2,3,4]
Concerning sensitivity to effort, our prediction held: birds tended to exchange less in the effort control compared to the equity condition (T = 2.62, p = 0.039, n.s. after Bonferroni). This is the first evidence of behavioral responses to inequity in birds
A) and to presence/absence of the reward; they were highly sensitive to inequity in working effort in both tasks
Summary
The sensitivity regarding ones benefits of actions as well as efforts and payoffs for both the acting individual and its cooperation partner is proposed to be a key mechanism in the evolution of cooperation [1,2,3,4]. Humans have been shown to have a strong preference regarding equity of reward distribution, referred to as fairness [5]. They experience dissatisfaction when perceiving themselves under- as well as over-rewarded in experimental bargaining games [6] and under more naturalistic conditions, e.g. job satisfaction [7,8,9]. Dogs do not respond to inequity in the quality of reward, raising the possibility that mammals other than primates possess a qualitatively different form of IA [18]. Elements of IA may evolve independently of phylogeny in systems with a given cognitive complexity and, notably, specific demands of social life, i.e. iterated cooperative interactions with different individuals [17]
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