The discovery was made during experiments in which well-acquainted mice were caused to suffer pain together. The findings have been published in the latest issue of New Scientist magazine.

In one experiment two mice were fed vinegar, which caused mild stomach-ache for around 30 minutes making the creatures wriggle² in discomfort³.

Researchers at the McGill University observed that cage-mates that had previously⁴ spent time together wriggled⁵ more, and seemed to synchronize⁶ their wriggling⁷.

But in tests where the mice were strangers to each other, they displayed few signs of pain.

It is thought that this masking of pain is done to avoid revealing vulnerability to a potential rival.

The same happened in another test in which a chemical was used to cause mild inflammation of a paw, the researchers said. They found that the mice had to see each other's sufferings for the effect to kick in. The pheromones that enable recognition also played a role, the scientists added.

They said the reactions should not be mistaken for sympathy, which requires consciousness of another's suffering.

The reactions are a kind of "empathy", which is akin¹ to collective yawning in humans and perhaps goes further down the physiological⁸ tree than had been previously thought, the researchers said.

中文参考：

　　加拿大专家说，在一个笼子中生活过的两只老鼠，能相互“传染”它们所感受到的痛苦，这种现象与某人打哈欠可能会“传染”旁人类似。
　　
　　据最新一期英国《新科学家》杂志报道，加拿大麦吉尔大学的研究人员报告说，他们进行了一项给老鼠喂醋的实验，醋可使老鼠胃部感觉轻微不适达３０分钟左右，使它们因不舒服而开始扭动。

　　研究人员发现，如果两只老鼠互不相识并同时被喂醋，它们并不会扭动得很厉害，这可能是为了避免在潜在对手面前暴露自身的弱点。但如果两只老鼠曾生活在一个笼子里，彼此熟识，那么在同时喂醋后，它们会扭动得较为剧烈，看上去它们的扭动像是在无意识的彼此影响下同步进行。

　　研究人员强调说，老鼠的这种反应不能被理解为它们有同情心，而应视为“移情作用”的体现，这类似于某些人看到别人打哈欠时，自己也会不自觉地打哈欠一样。这一实验说明，能被“移情作用”影响的物种可能远比研究人员想象的要多。