Dogs, like people and monkeys, seem to have a sense of fairness.
"Animals react to inequity," said Friederike Range of the University of Vienna, Austria, who led a team of researchers testing animals at the school's Clever Dog Lab. "To avoid stress, we should try to avoid treating them differently."
Similar responses have been seen in monkeys.
Range said she wasn't surprised at the dogs reaction, since wolves are known to cooperate with one another and appear to be sensitive to each other. Modern dogs are descended from wolves.
Next, she said, will be experiments to test how dogs and wolves work together. "Among other questions, we will investigate how differences in emotions influence cooperative abilities," she said via e-mail.
In the reward experiments reported in Tuesday's edition of Proceedings of the National Academy of Sciences, Range and colleagues experimented with dogs that understood the command "paw," to place their paw in the hand of a researcher. It's the same game as teaching a dog to "shake hands."
Those that refused at the start — and one border collie that insisted on trying to herd other dogs — were removed. That left 29 dogs to be tested in varying pairs.
The dogs sat side-by-side with an experimenter in front of them. In front of the experimenter was a divided food bowl with pieces of sausage on one side and brown bread on the other.
The dogs were asked to shake hands and each could see what reward the other received.
When one dog got a reward and the other didn't, the unrewarded animal stopped playing.
When both got a reward all was well.
One thing that did surprise the researchers was that — unlike primates — the dogs didn't seem to care whether the reward was sausage or bread.
Possibly, they suggested, the presence of a reward was so important it obscured any preference. Other possibilities, they said, are that daily training with their owners overrides a preference, or that the social condition of working next to a partner increased their motivation regardless of which reward they got.
And the dogs never rejected the food, something that primates had done when they thought the reward was unfair.
The dogs, the researchers said, "were not willing to pay a cost by rejecting unfair offers."
Clive Wynne, an associate professor in the psychology department of the University of Florida, isn't so sure the experiment measures the animals reaction to fairness.
"What it means is individuals are responding negatively to being treated less well," he said in a telephone interview.
But the researchers didn't do a control test that had been done in monkey studies, Wynne said, in which a preferred reward was visible but not given to anyone. In that case the monkeys went on strike because they could see the better reward but got something lesser.
Range responded, however, that her team did indeed do that control test as well as others in which food was moved or held in the hand but not given to the dog being tested.
In dogs, Wynne noted, the quality of reward didn't seem to matter, so the test only worked when they got no reward at all.
However, Wynne added, there is "no doubt in my mind that dogs are very, very sensitive to what people are doing and are very smart."