PRIMATES aren鈥檛 unique in their ability to register a companion鈥檚 pain after all. Mice apparently feel pain more keenly if another mouse is going through the same experience, but only if the co-sufferer is a cage-mate.
鈥淲e found that if they know each other and can see each other, both mice are in more pain, and the pain behaviours seem to be synchronised,鈥� says Jeffrey Mogil, who led the research at McGill University in Montreal, Canada.
Mogil鈥檚 team fed mice with vinegar, which caused mild stomach ache for 30 minutes or so and made the mice wriggle in discomfort. When two mice that were strangers were given the vinegar, they did not wriggle very much, perhaps to avoid revealing vulnerability to a potential rival. However, cage-mates that had previously spent time together wriggled more, and seemed to synchronise their wriggles. The same happened in another test in which a chemical was used to cause mild inflammation in a paw (Science, vol 312, p 1967).
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The researchers say that the mice had to see each other suffer for the effect to kick in, but that pheromones that enable recognition play a role too.
He stresses that the reactions shouldn鈥檛 be mistaken for sympathy, which requires consciousness of another鈥檚 suffering. 鈥淚 don鈥檛 think consciousness is required at all for what we鈥檙e seeing,鈥� says Mogil. Instead, the 鈥渆mpathy鈥� is akin to collective yawning in humans. 鈥淚t suggests that empathy goes further down the physiological tree than we thought.鈥�
