Pet dogs know exactly how to use facial expressions to communicate with their human companions.

Most mammals use facial expressions to let each other know how they're feeling and what they want, from baring their teeth to show aggression, to raising their eyebrows to make their eyes large and sad-looking.

Dogs use more facial expressions when they know a human is watching them, according to a study published in the journal Scientific Reports. This shows that the dogs are aware of when they have a human audience and adapt their behaviour appropriately to communicate as well as possible.

The finding shows dogs are more socially attuned than we have given them credit for. Dogs' facial expressions aren't an automatic reflex, but a trait used for communication and tailored to a particular audience.

"The findings appear to support evidence dogs are sensitive to humans' attention and that expressions are potentially active attempts to communicate, not simple emotional displays," said study author Juliane Kaminski, a researcher at the University of Portsmouth's Dog Cognition Centre.

Kaminski and her colleagues looked at 24 dogs of different breeds, with individuals aged between 1 and 12 years old. When their owners were looking towards them, the dogs used many more expressions than when their owner was turned away from them. Whether the owner had food or not had no impact on the dogs' use of facial expressions.

"This study moves forward what we understand about dog cognition. We now know dogs make more facial expressions when the human is paying attention," Kaminsky said.

It's not clear whether the dogs understand what they're doing, whether the behaviour has been bred into them and is 'hardwired', or whether it is a learned response from viewing other individuals' expressions.

In previous research, Kaminski found that dogs are very aware of how much attention a human is paying them. When people close their eyes or turn away, dogs are much more likely to steal food. In another study, Kaminski showed that dogs follow the gaze of humans if they make eye contact first.

"Domestic dogs have a unique history – they have lived alongside humans for 30,000 years and during that time selection pressures seem to have acted on dogs' ability to communicate with us."