If the same is true for people, it may help explain conditions such as fibromyalgia, where people feel pain in the absence of an obvious medical cause.
"We've shown for the first time that you don't need an injury or inflammation to develop a pain state," says team leader Andrey Ryabinin from Oregon Health and Science University in Portland, Oregon. "Pain can develop simply because of social cues."
Ryabinin and his colleagues discovered the phenomenon in mice isolated from direct physical contact with other mice made to feel pain. The team found that the "bystander" mice became as hypersensitive to pain as the mice that were actually subjected to it. All the mice were in the same room, but the cages were at least 1 to 2 metres apart, and the animals couldn't see each other.
The team subjected the experimental mice to alcohol or heroin withdrawal, or injected a chemical irritant into their paws. Later, both they and the bystander mice were checked for hypersensitivity to pain in three different ways. One test involved tickling their feet with increasingly fine hairs, another dipped the end of their tail in warm water to see how quickly they withdrew it, and in the last a paw was injected with an irritant to see how frequently they licked it.
"When the pain was caused by alcohol or heroin withdrawal, the scores for the bystanders completely matched those of the experimental animals," says Ryabinin. By contrast, control mice in a neighbouring room were unaffected. When the experimental mice felt pain from an irritant, the hypersensitivity scores in the bystanders was about halved, but still higher than the controls.
Next, Ryabinin worked out that odours secreted by the pained mice contain the cues for hypersensitivity by showing that the bystander mice reacted in the same way when exposed only to bedding from the mice in pain.
Ryabinin speculates that the phenomenon may have evolved to warn companion animals of impending hazards, diseases or threats.
The results may begin to explain why some people experience discomfort with no obvious cause, he says. But working out how cues are transmitted in people may be far more difficult, as they're much less attuned to odour signals than mice.
"We don't know if smell is involved in humans, but there have been experiments clearly showing that humans are sensitive to, and change behaviour, in response to smells they don't consciously perceive," says Ryabinin. "But obviously, vision is incredibly important in humans, so we see a chronic pain patient grimace and this immediately triggers a response in us."
The study has potential implications for people who live with chronic pain patients, as they could be at risk themselves of developing chronic pain conditions, he says.
"It's all related to the functional significance of being sensitive to the states of others," says Frans de Waal at Emory University in Atlanta, who studies aspects of empathy. Being in tune with the situation of others is highly valuable for adapting and reacting to new situations, he adds.