There was an interesting study at the University of Vienna, Austria's Clever Dog Lab that determined that dog's react negatively when they view other dogs getting better treatment. They asked dogs that sat side by side to "give paw" and rewarded one dog with food and did not reward the other. The unrewarded dog stopped working.

Friederike Range, who led the experiments, 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.

Read the entire story in the Chicago Tribune.

Jeff's Note: one question I have is whether the dog's stop working because they are not rewarded, or because the other dog is getting a reward. Any behavior is not rock-solid until there is the correct amount of rewards and the correct ratio of rewards. There is something called "ratio strain" that occurs if an animal is pushed too far in between rewards. The dogs in this experiment  might have just stopped working because of ratio strain.