It's not just humans who can go green with envy

Dogs feel envy just like humans, but their jealousy is not complicated and underhand, nor does it revolve around the size of a new car or cost of a foreign holiday. Simply, it comes down to whether or not they are given a sausage.

Scientists have proved what many dog owners suspected: man's best friend is capable of feeling the human emotions of envy and resentment, especially when he feels the victim of unfair dealings in the handing out of meaty rewards.

An experiment with domestically trained dogs has shown that canines are capable of withdrawing their co-operation and friendship if they see another dog get tasty sausage morsels that they feel they deserve.

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An animal psychologist, Friederike Range and her University of Vienna colleagues tested canine envy by sitting two dogs next to one another. The first was told to offer its paw to shake hands; it did. But instead of rewarding the paw-offering dog, the researchers gave the piece of sausage to the second dog.

After doing this a few times, the first dog withdrew co-operation and turned its head away in disgust, in order to avoid eye contact with the human experimenter, the scientists explained.

"Furthermore, we showed that it was not the presence of the second dog but the fact that the partner received the food that was responsible for the change in the subjects' behaviour," they say in their study published in the journal Proceedings of the National Academy of Sciences.

The research is part of a wider investigation into the nature of "inequity aversion" in the animal kingdom, when highly social creatures attempt to do something to stop the perceived inequalities within their social group. Inequity aversion is believed to be central to the evolution of co-operative behaviour.

"Inequity aversion is defined as partners resisting inequitable outcomes. In humans, it seems to be based on the simultaneous evaluation of their costs and gains compared with those of their partner," the scientists said. "Until recently, it has been thought that sensitivity toward unequal reward or effort distribution is a uniquely human asset. However, several experiments carried out with capuchin monkeys and chimpanzees suggest otherwise.

"Our results suggest that species other than primates show at least a primitive version of inequity aversion, which may be a precursor of a more sophisticated sensitivity to efforts and pay-offs or joint interaction."

The dogs in the study were offered two kinds of reward, a bite of sausage or a piece of dark bread. Although the sausage is higher value in terms of a reward, the scientists saw no difference in the dogs' behaviour whichever was offered.

"Interestingly, our results differed from results of the primate studies in that we found no indication for sensitivity toward the quality of the food reward and the effort involved," the scientists said. "Primates react to the quality of food, not just the presence [or] absence, and show more negative reactions than the dogs in this study."

It's only natural: Animals and emotions

*Envy: All primates show fairly sophisticated levels of envy and resentment. Tests have shown that the level of envy displayed by monkeys can depend on the quality of reward another monkey receives. A high-reward food item, such as a banana, given to a rival monkey led to more resentful behaviour compared to when the rival was given something obviously less tasty.

*Grief: Elephants have been known to show outbursts of apparent grief over the bones of deceased relatives. Much of this observation is anecdotal and has been difficult to validate. However, there appears to be good supporting observations about how elephant mothers grieve for dead offspring.

*Pride: Horses are known to show off in front of other newcomers in a nearby field. Some scientists suggest this is a form of pride stemming from the hierarchical nature of groups of wild horses.