The indignant dogs' refusal to participate further was accompanied by scratching, yawning, mouth licking, and avoiding the gaze of the partner dog as well as the experimenter.
The dogs didn't seem to worry if their partner got sausage—a premium treat—and they only got bread, or if the other dog didn't perform the trick but was rewarded anyway.
"It gets more complicated if it's about both effort and the reward, and maybe dogs can't do that yet," Range suggested.
Dogs' more basic form of envy—and the insistence on some degree of equality—is probably critical to survival in cooperative activities. Wolves and wild dogs are known to hunt and raise their pups in groups, where individuals who don't insist on compensation would likely be taken advantage of.
(Related: "Do Dogs "Read" Humans to Find Food?" [November 21, 2002].)
Double or Nothing
Primatologist Frans de Waal at Emory University in Atlanta, Georgia, said he would expect this kind of envy—really an aversion to unequal reward—in all animals that regularly cooperate.
"The dogs showed a stronger reaction when they received nothing for the task in the presence of a rewarded companion than with no companion at all," he noted.
"They were OK with no reward if no one else got one, which shows that it is a social reaction."
Scott Creel, a behavior ecologist at Montana State University, said the research suggests many social species may have mental processes scientists once believed were unique to humans, or at least primates.
"It seems logical that many of the same selection pressures that have shaped our cognition and emotions also operate in other social species," said Creel, who has studied the behavior of African wild dogs but was not involved in this study.
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