In a highly controversial experiment, Canadian researchers showed that adult mice become more sensitive to pain after watching other mice in pain.
McGill University professor of psychology Dr. Jeffrey Mogil and colleagues in the Pain Genetics Lab in Canada discovered that mice who were co-housed (so familiar to each other) and able to see one another in pain were more sensitive to pain than those tested alone.
Mogil and his team 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 distressing study, which has received much criticism on ethical grounds, 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 cage-mate 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.
The response mice showed to their peers in pain is an example of emotional contagion, according to 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.
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 US Yerkes National Primate Research Center, who did not participate in the study. “This “highly significant [paper]…confirms that empathy is an ancient capacity.” he said.
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. According to Mogil and Harris the next step here will be to find the mechanisms behind the phenomenon in mice. 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, said “then the evidence would be truly compelling that their model actually reflects evolutionary continuity in a pro-social mechanism among many different mammalian species.”
The findings are not only unprecedented in what they tell us about animals, they may ultimately be relevant to understanding pain in humans. “Since we know that social interaction plays an important role in chronic pain behaviour in humans,” Dr. Mogil said, “then the mechanism underlying such effects can now be elucidated; why are we so affected by those around us?”
According to Mogil there’s a practical lesson here for researchers – 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.
In other experiments on mice, scientists from the US University of Wisconsin-Madison and Oregon Health and Science University (OHSU) showed that the ability to empathize with others is partially determined by genes .
In the experiments, one mouse observed as another mouse was placed in a test chamber and trained to associate a 30-second tone with a mild foot shock. Upon experiencing the shock, the test mouse emitted a short distress call or squeak.
Though having no direct knowledge of the foot shock, observers from a very social mouse strain learned from the distress calls to associate the test chamber and tone with something negative. When later placed in the test chamber and presented with the tone, they exhibited clear physiological signs of aversion, such as freezing in place, even though no shock was delivered.
A genetically different mouse strain with fewer social tendencies did not learn any connection between the cues and the other mouse’s distress, showing that the ability to identify and act on another’s emotions may have a genetic basis.
The experiment was designed to gain a better understanding of empathy in humans.
OneKind is firmly opposed to any experiments using animals that causes them pain or suffering. Where such experiments increase our understanding of animal sentience, we will report them in the long term interest of all animals. The more scientific evidence we have of the capacities of other animals as sentient creatures the stronger our case for re-evaluating our relationships with them to benefit us all.
- Langford D. J, Crager S. E, Shehzad Z, Smith S. B, Sotocinal S. G, Levenstadt J, Chanda M. L, Levitin D. J and Mogil J.S 2006 Social Modulation of Pain as Evidence for Empathy in Mice. Science, Vol. 312 no. 5782 , 1860-1861
- Chen Q, Panksepp J.B and Lahvis G.P 2009 Empathy Is Moderated by Genetic Background in Mice. PLoS ONE 4(2): e4387. doi:10.1371/journal.pone.0004387