The study found dogs are more likely to use puppy-dog eyes when humans are watching them.

Dogs make puppy-dog eyes for the benefit of humans and rarely use the imploring facial expression when on their own, a new study has shown.

It has long been assumed that animal facial expressions are involuntary and dependent on emotional state rather than a way to communicate.

But scientists at the Dog Cognition Centre at Portsmouth University have found that dogs mostly use facial expressions when humans are present, as a direct response to attention.

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Puppy-dog eyes, in which the brow is raised to make the eyes appear wider and sadder, was found to be the most commonly used expression in the study.

Researchers do not know whether the dogs are aware they look sadder, or have just learned that widening their eyes elicits sympathy and affection in humans.

Dog cognition expert Dr Juliane Kaminski said: "We can now be confident that the production of facial expressions made by dogs are dependent on the attention state of their audience and are not just a result of dogs being excited.

"In our study they produced far more expressions when someone was watching, but seeing food treats did not have the same effect.

"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."

The researchers studied 24 dogs of various breeds, aged 1 to 12. All were family pets. Each dog was tied by a lead a metre away from a person, and the dogs' faces were filmed throughout a range of exchanges, from the person being oriented towards the dog, to being distracted and with her body turned away from the dog.

They found that when they were not being watched by a human, the animal dropped facial expressions.

Kaminski said it is possible dogs' expressions have evolved as they were domesticated

"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," she said.

"We knew domestic dogs paid attention to how attentive a human is - in a previous study we found, for example, that dogs stole food more often when the human's eyes were closed or they had their back turned.

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

Previous research has shown some apes can also modify their facial expressions depending on their audience, but until now it was not known whether dogs also had the capability.