Dogs can sniff out unfair situations and show a simple emotion similar to envy or jealousy, Austrian researchers reported Monday.
Dogs sulked and refused to "shake" paws if other dogs got treats for tricks and they did not, said Friederike Range, an animal psychologist at the University of Vienna, who led the study into canine emotions.
"It is a more complex feeling or emotion than what we would normally attribute to animals," said Range.
The study, which was published in the Proceedings of the National Academy of Sciences, also showed dogs licked and scratched themselves and acted stressed when they were denied rewards given to other dogs.
Other studies have shown monkeys often express resentful behavior when a partner receives a greater reward for performing an identical task, staging strikes or ignoring what they view as inferior compensation.
It turns out dogs are able to show a similar, if less sensitive, response, said Range in a telephone interview.
In a series of experiments using different breeds of dogs, the researchers looked at how two animals sitting next to each other reacted to unequal rewards after handing a paw to a researcher.
Dogs not given a treat licked their mouths, yawned, scratched and showed other signs of stress and stopped performing the task, Range said.
To show this was not just because the animals were not getting food, the researchers then tested the dogs alone and found that in this situation the envious canines cooperated longer before stopping.
"It is really about the unequal distribution of the reward," Range said. "If it was only about frustration they would stop at the same time."