By Victoria Gill
Science and nature reporter, BBC News
The study was the first to measure how much dogs "prefer" their owners
Scientists have shown just how much dogs rely on seeing their owners faces in order to recognise them.
The researchers also measured how much dogs prefer to gaze at and follow their owners, rather than a stranger.
In the journal Animal Behaviour, the team described how dogs had difficultly recognising their human "best friend" when the person had their face covered.
The study sheds more light on how thousands of years of domestication has affected the behaviour of canines.
Paolo Mongillo from the University of Padua in Italy led the study. He explained that, although many researchers have studied how dogs interact with humans, no one had yet investigated how the animals focused on one person in preference to another - or just how much companion dogs "prefer" their owners.
Dr Mongillo's team at the university's department of experimental veterinary sciences invented an experiment to measure this.
"We had the dog in an empty room and we instructed the owner and another person - someone unfamiliar to the dog - to walk across the room several times," the scientist explained.
"The people walked in opposite directions, so they crossed many times in front of the dog and we measured how long the dog looked at one person versus another."
The research team then instructed the two people to leave the room via two different doors and allowed the dog to approach one of the doors.
"Most of the dogs gazed at their owners for most of the time and then chose to wait by the owner's door," said Dr Mongillo.
He described this as an "expected" result but something that no one has measured before.
"If you imagine a dog in a real setting in a city or anywhere in the middle of a crowd or a crowded space, you can see how the animal must have adapted to give preferential attention to its owner," said Dr Mongillo.
In the second part of the study, the scientists asked the people to cover their faces; the human volunteers then walked across the room with bags over their heads.
During this phase of the experiment, the dogs were much less attentive to their owners. This revealed just how much the animals relied on human faces for recognition.
Wild dogs rely on body signals and on cues from other animals in their social groups, but studies including this one suggest that domestic dogs are so attuned to human social groups that they are even able to recognise some human facial expressions.
When the people covered their faces the dogs paid less attention to their owners
"This is very likely to be a by-product of thousands of years of domestication," said Dr Mongillo.
Studies of the genetic differences between dogs and their wolf ancestors suggests that canines were first domesticated between 15,000 and 40,000 years ago.
Dogs and dementia
In the same study, the team investigated the effects of ageing on the dogs' attention.
They found that "aged" dogs - seven years and older - were less able to focus on their owner and also were less likely to choose the owner's door.
"There have been studies to show that dog ageing is similar to human ageing in terms of cognitive impairment," said Dr Mongillo.
So studying ageing in dogs could help our knowledge of human as well as animal age-related diseases.