You come home after work and the family pooch greets you, grinning ear to ear.
That's a true expression of emotion, right? Your dog is really showing she is happy to see you? Most likely, according to new research that for the first time documents and catalogues changes in the facial expression of laboratory mice in response to a particular emotion, pain.
And although a dog is different from a mouse, all pets probably express emotions in much the same way that humans do, according to Jeffrey Mogil, a psychologist and neuroscientist at McGill University in Montreal, senior author of a study published online in the journal Nature Methods.
The finding may not be all that surprising, because we all know our pets love us and express that emotion unconditionally, but it has been largely a matter of faith ever since Charles Darwin published his book, "The Expression of Emotion in Men and Animals," way back in 1872.
Oddly enough, there is still much debate over whether animals really feel human-like emotions, but Mogil is confident that his research shows that for at least one emotion -- pain -- mice feel it, and express it, in a way that is surprisingly similar to the way humans show they are in pain, which Darwin also predicted.
Research Confirms Humans Use Same Muscles as Other Animals to Express Emotion
"It's always good when you can confirm a prediction of Darwin," Mogil said in a telephone interview.
The great naturalist, who did more than anyone else to shape our understanding of life on earth, loved his dog and knew whereof he spoke.
"But man himself cannot express love and humility by external signs so plainly as does a dog, when with drooping ears, hanging lips, flexuous body, and wagging tail, he meets his beloved master," Darwin said in his book on animal emotions, an international bestseller as soon as it rolled off the presses. "Nor can these movements in the dog be explained by acts of volition or necessary instincts, any more than the beaming eyes and smiling cheeks of a man when he meets an old friend."
Since all species share a common ancestor if you go back far enough, Darwin reasoned that humans would use the same muscles as other animals to express emotions, and the new research confirms that.
Mice Express Pain Through Facial Contortions, Research Shows
"With mankind some expressions, such as the bristling of the hair under the influence of extreme terror, or the uncovering of the teeth under that of furious rage, can hardly be understood, except on the belief that man once existed in a much lower and animal-like condition," Darwin concluded.
The new research shows that mice really do express pain through contortions of the face, just as humans do.
"If it works in people and it works in mice, there's no reason I can think of why it wouldn't work in cats and dogs and horses and cows and any mammal. That's sort of the point," Mogil said, "this is evolutionarily based."
Darwin was wrong on some details, Mogil added, but he was right in arguing that humans and other animals even use the same muscles to express emotion.
Mice Have Mouse-Specific Expressions, Researcher Says
"A mouse's pain face is exactly the same as the human pain face, at least with respect to those features that they have in common (tightening of the eyes, bulging of the nose and bulging of the cheeks)," and humans and mice use the same muscles to create those signals, Mogil said. Mice also have two expressions that are "mouse-specific" (movement of the whiskers and ears,) he added.
Mogil teamed up with a fellow pain expert, Kenneth Craig of the University of British Columbia in Vancouver, to come up with a "mouse grimace scale" that veterinarians and researchers can use to measure pain in lab mice. That should be useful in the search for pain medications, and it should help ensure humane treatment of laboratory animals.
The pain induced in the mice used in this study "ranges from dipping the tail in hot water to the inflammation of body parts," Mogil said. "If you want to study pain you have to impose some pain, and this is very mild pain."
It was enough, however, for Craig's team to isolate photos of Mogil's mice that showed various levels of pain, and thus create a scale that requires only a little practice to master. It only took an hour of training for participants in the study to accurately determine the level of pain (from none to severe) 75 percent of the time, using the mouse grimace scale. More training, and better videos, boosted that success rate to 97 percent.
Study Should Have Positive Implications for Pain in Lab Mice, Researcher Says
Mogil expects attacks from animal-rights activists for inflicting even modest pain on lab mice, but he argues that the study should have "positive implications for pain in laboratory mice," in that researchers will be able to tell just how much pain the mouse is experiencing and treat it accordingly.
What is perhaps most surprising in all of this is that it took so long to come up with a way to document pain just through facial expressions.
There have been some studies of various animals and how they express emotions, mostly fear, because that's easier to create than a feeling of happiness. Mogil described the lack of research as "shocking."
So how did he manage to pull it off, at least for mice and pain?
"I have an advantage over most pain researchers in that I teach at an undergraduate institution, not a medical school, and because of that I have access to very high quality and very free labor" in the form of undergraduates, he said. "This method is quite labor intensive. You have to take video and then you have to isolate still photographs and that takes a long time."
Now, if they can just figure out how to make a mouse smile.