Mice can feel cagemate's pain

Canadian researchers observe empathy in mice.

by Lydia Fong -- July 12, 2006

If the world's smallest violin is tiny enough, perhaps a couple of mice could play the world's saddest song on it.

A Canadian research team in the Pain Genetics Lab at McGill University discovered that a mouse's response to pain is intensified in the presence of another mouse that is also in pain. In addition, according to a study published in the June 30th issue of Science, the mice appear to synchronize their pain responses.

"Both of those things, ultimately, are suggestive of empathy," said Jeffrey Mogil, a psychology professor and one of the study's lead authors.

Previously, empathy was a behavior thought to exist only in higher primates and humans.

To induce pain in their mouse subjects, the researchers used what is known as the "acetic acid writhing test," which simulates a stomachache.

In response, the mice displayed a subclass of empathy known as "emotional contagion," where one mouse recognizes and adopts the emotional state of another. Surprisingly, this only occurred if the mice knew each other-that is, if they had been cagemates for at least two weeks. This is the amount of time required for a mouse to familiarize itself with the pheromones of another mouse, according to Mogil.

Since pheromones play a big role in the interactions of mice, researchers guessed the mice would communicate their pain via scent. However, after using a zinc sulfate treatment to block the animals' sense of smell, researchers found they could still sense one another's pain. After disrupting other senses, the researchers determined that communication was only interrupted when an opaque Plexiglas divider was placed between the mice.

"The message 'I'm in pain' appears to be transmitted visually," Mogil said. "Humans are very, very good at telling when another human is in pain by looking at the face, and we think that probably mice can do it, too."

Frans de Waal, a psychology professor at Emory University who studies primate behavior, speculates that empathy first evolved in mammals because mothers needed to be able to respond immediately to the distresses of their offspring. It then expanded beyond the mother-offspring relationship.

"Animals need to be in tune with each other, with alarm and fear," said de Waal. "It's highly adaptive to be in tune with others."

According to Mogil, scientists may be able to use the mouse model to more closely study the mechanism of empathy, particularly the genes, neurochemicals, and brain areas involved. The finding could be useful for studying human conditions such as autism, which is associated with a reduced ability to empathize.

"Empathy is a very hot topic with humans, but the problem with humans is that we can't really do any experiments on them," Mogil said. "You can stick them under an imager and see what parts of the brain lights up, and that's about it."

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