A mouse's nose has a cluster of specialized cells that respond to the chemical signals sent out by fellow mice that are in distress, researchers report, meaning that mice can literally smell fear. A lump of nerve cells in the nose tip called the Grueneberg ganglion responds to the "fear pheromones" of imperiled creatures, sending a signal straight to the brain. As Grueneberg ganglia are known to exist in rodents, cats, apes, and humans, researchers say it's likely that the cells perform the same function in all mammals. In a new study, researchers dosed water dishes with mouse alarm pheromones, and put the dishes in cages with both normal mice and mice whose ganglia had been removed. The contrast was very striking, [lead researcher Marie-Christine] Broillet said. "The normal mouse immediately gets scared and goes to the corner of the box and freezes," she said. But mice without the ganglia carried on as before, seemingly unaware of the danger signals. Both groups were able to sniff out cookies hidden in their cages, however, suggesting the altered group's sense of smell was otherwise unaffected
The findings, reported in the journal Science [subscription required], solve an old puzzle about the function of the Grueneberg ganglion; when it was discovered in 1973 scientists couldn't determine its purpose, and it was then forgotten for over 30 years. Researchers rediscovered the structure
a few years ago when mice were genetically engineered to produce a green fluorescent protein in their neurons, Broillet says. Scientists were surprised to see the clusters of green neurons sitting all alone at the tip of the mice’s noses [Science News].
Warning systems that help animals detect a threat to their own species provide a clear boost in survival odds, so researchers say it makes sense that the ganglia evolved early and are present throughout the mammalian family tree.
Even certain plants release alarm pheromones to produce bitter and astringent tannins, so they can become less appetizing to hungry animals. In modern life, our own response to alarm pheromones might be hard to notice, but it is entirely possible that we still inadvertently react to their presence [Ars Technica].