Dec. 9 -- MONDAY, Dec. 8 (HealthDay News) -- It's a dog-envy-dog world, and a jealous dog isn't likely to take it lying down.
According to new research in this week's issue of the Proceedings of the National Academy of Sciences, dogs who sense that another canine is getting better treatment will go on strike, er, make that a paw strike. They refused to "shake hands" if they weren't getting equal "pay."
For this experiment, dogs and their owners were divided into pairs. The paired dogs sat next to each other, with owners behind them, and were asked to put their paw into the researcher's hand. Once the "shake" was successfully completed, each dog was rewarded with either a piece of sausage or a piece of bread.
But if the dog who "shook" was denied its treat and the partner was not, resentment seemed to build up, with the denied dog eventually refusing to lift its paw.
Dogs stopped cooperating earlier if another dog was present than if they were on their own in the same scenario. They also appeared more stressed if a partner was getting a reward and they weren't than if they were denied rewards in a solo setting.
The quality of the reward itself didn't matter to the canine subjects.
The immediate good news for dog owners is that you don't have to feel guilty if one dog is getting liver and the other a regular dog biscuit (as long as both pooches are getting something).
Of course, it's not truly clear if "envy" is the right word to describe these patterns. "Envy implies a certain emotional state and a thought process, at least by the classic definition," said Bonnie V. Beaver, a professor in the department of small animal clinical sciences, College of Veterinary Medicine & Biomedical Sciences at Texas A&M University in College Station. "We don't know what they're thinking. We know that at least without appropriate rewards, they're not willing to continue working. This gives us a scientific procedure that allows us to give better evaluation."
But precise definitions don't really matter, as the potential significance of the findings goes beyond that.
This "sensitivity to inequity" may be crucial in the evolution of cooperation.
According to the study authors, from the University of Vienna in Austria, it had once been believed that this trait belonged only to humans.
But previous studies have suggested capuchin monkeys and chimpanzees also express resentful behavior when there's not equal pay for equal work. However, in these species, the quality of the reward did matter.
Other animals, including wolves, African wild dogs and mongooses, also appear to cooperate with each other in certain activities, such as raising pups.
Dogs show cooperation with humans if not as much with each other. This connection with humans seems to foreshadow canines' reactions when humans give preferential treatment.
"This shows a behavioral connection [among animals]," Beaver said. "Previous studies have shown that primates will do this kind of behavior, much more like people, and now we've taken it back a little bit further to a less human form to see this in dogs. Is this in cats? In birds? . . . Is this a relatedness or is it something that evolved separately in different species? This gives us more information."
The Humane Society of the United States has more on dog behavior.
SOURCES: Bonnie V. Beaver, D.V.M., professor, Department of Small Animal Clinical Sciences, College of Veterinary Medicine & Biomedical Sciences, Texas A&M University, College Station; Dec. 8-12, 2008, Proceedings of the National Academy of Sciences, online