Mice show evidence of empathy

Mice who watch their peers in pain are more sensitive to it themselves, Canadian researchers report this week in Science -- the first evidence of empathy between adult, non-primate mammals.

There is an "increasingly popular" view that this kind of basic, pre-cognitive response to social cues may be present in all mammals, said Frans de Waal at Emory University and the Yerkes National Primate Research Center, who did not participate in the study. "This "highly significant [paper]...confirms that empathy is an ancient capacity," he told The Scientist in an Email.

The response mice showed to their peers in pain is an example of emotional contagion, according to senior author Jeffrey Mogil. (The best known example found in both humans and chimpanzees is the contagious yawn.) In higher primates, emotional contagion can progress to the more complex behaviors commonly associated with the term empathy, such as when a human identifies with a friend's pain and is driven to help.

Mogil and his team at McGill University became interested in looking at empathy in mice after they stumbled onto an interesting pattern in a large data set suggesting that a mouse's sensitivity to a pain test depends on its exposure to others that have been through the test. The pattern suggested that mice "might be talking to each other" about their pain in ways that changed their response to it, he said.

In this study, the scientists injected acetic acid into one or both of each pair of same-sex adult mice they were studying, causing them to writhe in pain, and allowed them to observe each other. An injected mouse writhed more if its partner was also writhing, but only if the mouse had previously shared a cage with its partner for more than 14 days.

Observing another mouse also reduced a mouse's response to pain. When the researchers injected the paws of familiar mice pairs with varying doses of inflammation-causing formalin, mice whose partners experienced less pain tended to show less pain sensitivity (indicated by how long a mouse licked its paw).

The researchers also found that a writhing mouse became more sensitive to the acetic acid while watching its cagemate deal with a different painful stimulus -- heat. These findings suggest that mice experience a general increased sensitivity to pain, and don't simply imitate what they see.

To figure out what the mice were using to communicate pain to their similarly distressed peers, the researchers systematically blocked each of their senses, using physical barriers or rendering the animals deaf or unable to smell. They found that mice appeared to depend primarily on visual cues to generate an empathic response -- a surprise, since mice are known to be reliant on smell, along with ultrasonic vocalizations to care for their offspring. "Given that rodents don't use visual senses much...that was our last guess," said Mogil. However, it's "almost impossible" to knock out pheromone circuits, which mice use to identify their acquaintances in the first place, so pheromones may also be a significant mediator of empathy, he said.

There's a practical lesson here for mouse researchers, according to Mogil -- mice who observe each other during experiments may be "contaminating" the data. He added that he and his colleagues now routinely put up an opaque barrier between mice being tested simultaneously.

Empathy is "an evolutionary mechanism to maintain social cohesion. If you're evolving and you're in a group, you're more sensitive to the pain of other members in a group," explained James Harris at Johns Hopkins University, who did not participate in this study.

Greater empathy between individuals who are familiar goes back to the early evolution of maternal care in mammals, according to de Waal. "This may have driven initial evolution of being in tune with the emotions of others, after which all the fancy stuff that we associate with empathy came into play."

However, these findings in mice hinge on how one defines empathy, which is still under debate, de Waal noted. "Lots of psychologists think top-down, hence equate empathy with complex cognition... which requires introspection," he said. "In this view, mice shouldn't have empathy."

In 2004, British researchers used brain imaging to pinpoint the empathy center in humans. The next step here will be to find the mechanisms behind the phenomenon in mice, according to Mogil and Harris. Researchers "may have avoided looking at altruism [in rodents] because it seemed too ridiculous," Mogil said, but these findings have "opened our eyes [about the] abilities of rodents in terms of social interactions."

"If it turns out that the 'empathetic' effect in mice is mediated by the same brain mechanisms as human empathy," Jaak Panksepp at Washington State University, not a co-author, told The Scientist in an Email, "then the evidence would be truly compelling that their model actually reflects evolutionary continuity in a pro-social mechanism among many different mammalian species."

Ishani Ganguli
iganguli@the-scientist.com

Links within this article

D.L. Langford, et al, "Social modulation of pain as evidence for empathy in mice," Science, June 30, 2006.
www.sciencemag.org

E. Russo, "New views on mind-body connection: Studies into placebo effect and empathy suggest how the brain encodes subjective experience," The Scientist, August 2, 2004.
http://www.the-scientist.com/article/display/14865/

S.D. Preston and F.B. de Waal, "Empathy: Its ultimate and proximate bases," Behav Brain Sci, February 2002.
PM_ID: 12625087

Frans de Waal
http://en.wikipedia.org/wiki/Frans_de_Waal
http://www.emory.edu/LIVING_LINKS/OurInnerApe/meet_frans.html

Jeffrey Mogil
http://paingeneticslab.ca/4105/02_01_jeffrey_mogil.asp

James Harris
http://faculty.jhsph.edu/?F=James&L=Harris

T. Singer et al, "Empathy for pain involves the affective but not sensory components of pain," Science, February 20, 2004
PM_ID: 14976305

Panksepp, J. "Brief social isolation, pain responsivity, and morphine analgesia in young rats," Psychopharmacology, 1980.
PM_ID: 6781002

Jaak Panksepp
http://www.vetmed.wsu.edu/depts-vcapp/Panksepp-endowed.asp