ENGLAND (REUTERS) - Most dog owners already know it to be true, but Hope the labrador is taking part in an experiment that proves man's best friend really can recognise human emotions.

Dogs are shown either a happy or angry face of both humans and other dogs.

At the same time they'll hear a sound of the corresponding species that could be happy, angry or neutral.

The dogs are untrained and the voice is in an unfamiliar language. Here, Portuguese is presented to British dogs.

Scientists then analysed where the dog was looking and for how long.

"We wanted to see if the dogs could assess the emotional content of the human voices and whether they would actually discriminate the emotional information within them... What we found is that when dogs were hearing positive sounds they would look longer to positive faces, both human and dog. And when they were listening to negative sounds they would look longer to negative, angry faces," said Ms Natalia De Souza, a PhD student in experimental psychology from the University of Sao Paulo.

By combining two different sources of sensory information dogs form mental representations of positive and negative emotional states.

Dogs, the researchers say, must have an internal database of emotional states - something only previously studied in primates.

"This is the first empirical experiment that will show dogs can integrate visual and oratory inputs to understand or differentiate human emotion as dog emotion." said Dr Kun Guo, a reader in psychology at the University of Lincoln.

Dogs are a highly social species.

Scientists say this mood-detecting ability might have helped in their domestication over the generations, giving them a leg-up on their way to becoming man's faithful companion.