The Vienna-based researchers showed that dogs will stop doing a simple task when not rewarded if another dog, which continues to be rewarded, is present. The experiment consisted of taking pairs of dogs and getting them to present a paw for a reward.On giving this "handshake" the dogs received a piece of food.One of the dogs was then asked to shake hands, but received no food.The other dog continued to get the food when it was asked to perform the task. The dog without the reward quickly stopped doing the task, and showed signs of anger or stress when its partner was rewarded. To make sure that the experiment was really showing the interaction between the dogs rather than just the frustration of not being rewarded, a similar experiment was conducted where the dogs performed the task without the partner.Here they continued to present the paw for much longer. Dr Frederike Range from the University of Vienna says this shows that it was the presence of the rewarded partner that was the greater influence on their behaviour. "The only difference is one gets food and the other doesn't, they are responding to being unequally rewarded." she said. The researchers say this kind of behaviour, where one animal gets frustrated with what is happening with another, has only been observed in primates  before. Studies with various types of monkeys and chimpanzees show they react not only to seeing their partners receiving rewards when they are not, but also to the type of reward. The dog study also looked at whether the type of reward made a difference.Dogs were given either bread or sausage, but seemed to react equally to either.Dr Range says this may be because they have been trained. The dogs refused to give the paw when they  _  .
found another dog was given nothing felt they were not treated equally were aware they received less food were given too much reward