Dogs are much more facially expressive when they're around their human friends than when they're by themselves or with other dogs, and move their faces as a communication tool rather than to express emotion.

According to a new study, our canine companions don't use facial expressions to respond to food treats, suggesting that their excited facial expressions may not be as much about excitement or even being sorry as we think.

The information shouldn't be a surprise. There is extensive research in the field of domestic animal communication with humans.

We already have an inkling, for instance, that domestic cats meow a lot more than their feral counterparts, indicating that they honed the vocalisation as a tool for communicating with humans.

Apparently, dogs have done the same with facial expressions.

"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," said lead researcher Juliane Kaminski of the University of Portsmouth.

"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 study involved 24 pet dogs, aged between 1 and 12 years. These were leashed a metre away from a human, and their facial expressions analysed during a series of interactions, using scientific dog facial analysis tool DogFACS.

There were two states of human interaction, facing towards the dog, and turning her back to the dog. For each of these, the team tested the dog's facial expressions with food present and without food present.

The dogs showed far more facial expression when the human was facing them, whether food was present or not.

In addition, Kaminski's previous work has shown that dogs can tell when a human is paying attention (and the sneaky buggers are more likely to pilfer food when humans are distracted), so the finding indicates that the attention and the facial expressions are linked.

We already know that dogs can sense and respond to human emotion. We also know that the puppy-dog-eyes "guilty look" dogs affect when scolded has little to do with the dog's emotion; rather, it's a learned behaviour, acting submissive in response to anger.

Kaminski's team's research is consistent with this. The raised-eyebrow puppy-dog eyes look was the most common facial expression her team recorded - one that tends to elicit sympathy in humans. This, Kaminski believes, could be a sign that dogs have developed facial expressions due to domestication.

"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.

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

It is not known how well dogs understand their facial expressions - whether they can understand the emotional state of another being, or are just using learned behaviours for a given situation. So the jury is still out on whether dogs have cognitive empathy.

The research has been published in the journal Scientific Reports.