Dogs become unhappy when their partners receive food

In the same way as humans and chimpanzees, they appear to resent missing out on rewards given to others.

Scientists believe envy may be a hallmark of social species, and could play a role in the evolution of co-operation.

Like primates, dogs are social animals that engage in co-operative behaviour, such as hunting in packs.

Researchers in Austria conducted a series of tests with pairs of domestic dogs and their owners. The dogs were encouraged to put out their paws in return for a food reward - either a piece of bread, or a more tasty slice of sausage. How many times they were willing to extend their paws before being rewarded depended on their mood, the experimenters found.

Dogs became unhappy when they saw their partners getting food not being offered to them. They showed signs of distress, such as scratching and yawning, and stopped extending their paws while avoiding eye contact with the scientists.

The same kind of reaction did not occur when a reward was withheld but the partner dog was absent.

"The partner receiving a reward thus seemed a crucial factor responsible for refusing to give the paw in the reward inequity test," said researchers led by Dr Friederike Range, from the University of Vienna.

The researchers concluded: "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 of joint interactions." (Independent)