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
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