The Influence of the Relationship and Motivation on Inequity Aversion in Dogs

One crucial element for the evolution of cooperation may be the sensitivity to others' efforts and pay-offs in comparison with one's own costs and gains. If an individual responds to a disadvantageous reward distribution, then it would likely increase its relative fitness compared with those who do not (Brosnan & de Waal, Nature 428:140, 2004). Recent experimental research indicates that sensitivity toward unequal reward distribution is not a uniquely human trait; non-human primates react to inequity if they witness a conspecific that obtains a more attractive reward for the same effort. However, primates are not the only species that cooperate and thus would benefit from inequity aversion. Canids also frequently cooperate with group members (e.g., wolves, wild dogs, etc.) or humans (domestic dogs), allowing for the possibility that they also are sensitive to the reward distribution. We report the findings of two studies of dogs' responses to inequity and the social factors that mediate such responses. In the first study, we investigated whether domestic dogs showed a response to the inequity of rewards received for the same action in pairs of familiar dogs. We found that dogs showed significant behavioral differences when tested without food rewards when in the presence of a rewarded partner compared to the baseline and asocial control situations. This indicated that the presence of a rewarded partner matters (Range, Horn, Viranyi, & Huber, Proc Natl Acad Sci USA 106(1):340-345, 2009). In contrast to primates, dogs did not react to differences in the quality of food or effort. In the second study, we analyzed whether individual (motivational or personality) characteristics determined the response of each subject to unequal reward distributions or whether the subject's responses depended on the specific relationship they had with its partner. We found that individual motivation and the quality of the relationship influenced the dogs' performance in test conditions, but that these factors varied across the control and reward inequity conditions. Overall, our results suggest that inequity aversion in dogs is conditional on their and their partner's rewards. Whether inequity aversion is based on the same mechanisms in both humans and non-human primates is still unclear.