To the dog lover, the ability of man's best friend to bark at the postman and fetch a stick is proof of his intelligence. But they may be even brighter than that. Scientists have welcomed dogs into a select club of species capable of using abstract concepts. The research showed that dogs are able to mentally sort objects into categories, a talent for abstract thought that has only been shown in birds and primates before.

The researchers trained four dogs with a touch-screen test which involved choosing between two images which appeared simultaneously. One was an image of a dog, the other a landscape. The animals indicated their preference with a prod of their noses. When they chose the dog they were rewarded with food. If they chose the landscape they had to wait a few seconds before the same two images appeared again.

Next the team tested the dogs - two border collies, an Australian shepherd and a mongrel - with an unfamiliar set of dog pictures and landscapes. To pass they would need to realise that "dog" is a category of object that unfamiliar objects also fall into. Their success dropped slightly from about 80% to 72%. "It shows us that dogs are able to use more or less abstract concepts," said Dr Friederike Range, who carried out the work with colleagues at the University of Vienna.

Lastly, the experimenters used familiar landscapes as background with new dog images superimposed. Again the animals were able to choose the ones with the dogs. The research appears in the journal Animal Cognition and is reported today in New Scientist magazine.