In this sequence, a dog is told to give its paw, and does, without getting a reward. Then it watches as its buddy gets a reward for giving its paw. In the third frame, the first dog not only refuses to give its paw after seeing the unequal treatment, it won't even look at the researcher. (Friederike Range)

Envy has joined the long list of human emotions attributed to dogs.

Austrian researchers reached this conclusion by testing pairs of dogs to see how unequal treatment affected their tendency to co-operate.

Sure enough, giving one dog a treat when its friend doesn't get one leads the friend to reject the researcher's next request, a form of resentful behaviour.

"It was not the presence of the second dog but the fact that the partner received the food that was responsible for the change in the subject's behaviour," the team, led by Friederike Range of the University of Vienna, reported in a paper published Monday in the Proceedings of the National Academy of Sciences.

Recent studies of human behaviour suggested that unequal treatment may affect how people look at co-operating. Experiments with monkeys showed that they often went on strike by refusing to participate or ignoring lower-valued rewards — a kind of envious behaviour — when they were treated unequally.

Range, two colleagues from the university and one from Austria's Konrad Lorenz Institute for Evolution and Cognition Research, set out to try the experiment with dogs.

They took pairs of domestic dogs, and asked them to give their paws. They varied the circumstances and rewards to see how the changes affected the outcome. But in the key experiment, one of the pair was rewarded with a piece of sausage or black bread for presenting its paw, and the other wasn't.

The dogs that weren't rewarded reacted by making the experimenter ask repeatedly for the paw, and eventually refusing.

But unlike monkeys, the quality of the reward — sausage, a superior treat, and black bread, less so — did not affect the dogs, the report said.

The paper, "The absence of reward induces inequity aversion in dogs," was written by Range, Lisa Horn, Zsofia Viranyi and Ludwig Huber. All the researchers are from the University of Vienna except Viranyi, who is with the Lorenz institute.