Experiments with various species have shown that monkeys often express resentful behavior when a partner receives a greater reward for performing an identical task. Monkeys have been shown to stage strikes, refusing to participate and ignoring what they perceive as inferior compensation. Dogs are capable of similar, though less sensitive, discrimination, report Friederike Range and colleagues.
The researchers conducted experiments with pairs of domestic dogs accompanied by their owners. While the partner and subject dogs sat next to each other with their owner standing behind them, each dog was prompted to put its paw in the experimenter's hand, and upon complying, given a piece of sausage or bread.
Compared to a variety of control situations, the dogs reacted differently to unfair reward distribution, as measured by their reaction when the partner was given food for the task, but the subject was not. This resentment was quantified in the number of times the experimenters had to prompt the animals, or the number of times the dog would perform the task before refusing. The dogs did not appear to care exactly what reward they were given, or whether the partner did or did not have to perform the task before receiving food.
Dog envy may be an evolutionary precursor to more sophisticated primate emotion, the researchers say.
