New Caledonian Crows Behave Optimistically after Using Tools.

Are complex, species-specific behaviors in animals reinforced by material reward alone or do they also induce positive emotions? Many adaptive human behaviors are intrinsically motivated: they not only improve our material outcomes, but improve our affect as well [1-8]. Work to date on animal optimism, as an indicator of positive affect, has generally focused on how animals react to change in their circumstances, such as when their environment is enriched [9-14] or they are manipulated by humans [15-23], rather than whether complex actions improve emotional state. Here, we show that wild New Caledonian crows are optimistic after tool use, a complex, species-specific behavior. We further demonstrate that this finding cannot be explained by the crows needing to put more effort into gaining food. Our findings therefore raise the possibility that intrinsic motivation (enjoyment) may be a fundamental proximate cause in the evolution of tool use and other complex behaviors. VIDEO ABSTRACT.