You know that face your dog makes, the one that is a little bit quizzical, maybe a bit sad, a bit anticipatory, with the eyebrows slanted?
Sometimes you think it says: "Don't be sad. I can help." Other times it quite clearly asks: "No salami for me?"
Scientists have not yet been able to translate the look, but they have given it a very serious label: "AU101: inner eyebrow raise". And a team of evolutionary psychologists and anatomists reported in Proceedings Of The National Academy Of Sciences that dogs make this face more often and way more intensely than wolves. In fact dogs, but not wolves, have a specific muscle that helps raise those brows.
An example of the look can be found in a talking-dog video that nearly 200 million people have watched online. The dog in the video can twitch its inner eyebrow because it has a muscle called the levator anguli oculi medialis. It can talk because it is a YouTube video. But it hardly needs the voice-over to make its emotional point.
"I think the study is compelling," said Dr Clive Wynne, a psychologist at Arizona State University and head of the Canine Science Collaboratory there. It is, he said, "another piece of the puzzle of what connects dogs to people".
Dr Wynne, who was not associated with the study, said a greater number and variety of the two species would need to be studied to learn more about the general differences between dogs and wolves.
How humans and other animals communicate by looking at each other is a matter of great interest to scientists. Dr Anne Burrows, an anatomist at Duquesne University in Pittsburgh, studied chimpanzee faces. She and other researchers, including Dr Juliane Kaminski, an evolutionary psychologist at the University of Portsmouth in England, teamed up a few years ago to look at domestic animal facial expressions and musculature.
They started with horses and cats, and, she said, horses have facial movements similar to dogs, but cats do not. "It turned out they just don't really move their faces at all." The researchers did not explain how cats are still able to express highly sophisticated states of mind such as scepticism, disdain, deep self-satisfaction and world-weary ennui. That research may have to be left to the cats themselves.
Dogs were an obvious subject. As many dog owners have said: "Just look at that face!" There were studies on how dogs look to their owners when they cannot solve a problem, and evidence that dogs who indulged more in "AU101: inner eyebrow raise" were more likely to get adopted from shelters.
So the team tested dog and wolf behaviour by videotaping their reactions, and, as expected, dogs did raise their eyebrows more often and more intensely than wolves. Even though wolves do not have that muscle, they have a lot of other muscles, so they can do a bit of the look.
Researchers dissected the heads of four wolves and six dogs, acquired after deaths in which they had no part. As might be expected from animals so closely related, all the musculature was exactly alike except for the levator muscle, which none of the wolves had. One other muscle, which varied in the wolves and dogs, was also related to eye movement.
The scientists hypothesise that humans have unconsciously favoured eyebrow-raising dogs during fairly recent selective breeding. Dr Burrows said one tantalising hint that could lead to future study was that one of the dogs, a siberian husky, was more like the wolves and did not have the levator anguli oculi medialis. Huskies are more closely related to wolves than some breeds, and it could also be that talent in sled-pulling was more important than a soulful face in the breed's development.
"The next step is to look at more breeds," Dr Kaminski said, to see if the behaviour and the musculature vary. And, she said, she would want to know whether the upbringing of a dog has an effect on this behaviour.
And although animals might close one eye, and despite the vast amount of evidence on the Internet, there is no scientific evidence that dogs, or any other animals, wink.