ROBERT SIEGEL, host:
Fear has its own smell. That smell comes from what scientists call an alarm pheromone. Some animals produce it when they're scared or attacked. But the question of how animals make and detect these mysterious chemicals has stumped scientists for years.
Well, now, a team in Switzerland has taken a big step, or a big sniff. NPR's Christopher Joyce reports on the discovery of the organ in mice that actually smells fear.
CHRISTOPHER JOYCE: The organ in question is a tiny bundle of cells near the tip of a mouse's nose. Scientists at the University of Lausanne first thought it might detect the smell of a mother's nipples. It didn't. Then they got to thinking about pheromones, those strange biochemicals that animals, insects and even plants use as a form of chemical communication, and in particular, alarm pheromones.
Ms. MARIE-CHRISTINE BROILLET (Biologist, University of Lausanne): Basically, they are produced by stressed animals, like for example when an animal is injured or scared.
JOYCE: Marie-Christine Broillet is a biologist at the University. She wondered if that bundle of cells might detect alarm pheromones. So, her team collected air samples from cages, where older laboratory mice were being euthanized. When they exposed a live mouse to that air, the neurons in that bundle of cells might detect alarm pheromones. So her team collected air samples from cages where older laboratory mice were being euthanized. When they exposed a live mouse to that air, the neurons in that bundle of cells started to fire and the mouse's behavior changed.
Ms. BROILLET: It would just go to the opposite end of the cage and freeze.
JOYCE: Freezing is a common fear response in many animals. They try not to be noticed. In another experiment, Broillet's team surgically removed the detection cells from mice. Then, the pheromone didn't scare them at all and they were still able to find a buried Oreo cookie. So, their overall sense of smell was not affected.
One other type of animal that definitely uses this kind of alarm pheromone is fish. It's secreted from their skin when a fish is attacked by a bigger fish. Scientists haven't identified that chemical either, but they can get salmon to drop to the bottom of a tank and lie motionless just by putting some ground up salmon skin in the water.
Nathaniel Scholz is a zoologist with the National Oceanic and Atmospheric Administration, where he's done that very experiment. He worries that humans may be preventing fish from smelling danger.
Mr. NATHANIEL SCHOLZ (Zoologist, National Oceanic and Atmospheric Administration): We're focused on this area because there's a lot of different pollutants that run off terrestrial landscapes in storm water and get into fish habitats. And because the nose is directly exposed to the surrounding environment, water pollution can effectively interfere with the normal functioning of the nose.
JOYCE: Scholz has found that one pollutant in urban runoff, copper, prevents fish from smelling the alarm pheromone. Young salmon exposed to copper in a tank did not react and an adult cutthroat trout, our predator, quickly picked them off. Scholz says alarm pheromones may be helpful to a species. One animal may be eaten, but others, perhaps the animal's relatives, in fact, get away.
It's clear so far that fish, insects, plants and now, mammals use alarm pheromones to help them avoid danger. Marie-Christine Broillet says there's no reason to think humans would be different.
Ms. BROILLET: You get scared or you panic or you feel the fear from other people without seeing anything, so it might be that exactly enable to sense these alarm pheromones. Maybe we could use alarm pheromones too.
JOYCE: The research appears in this week's issue of the Journal of Science. Christopher Joyce, NPR News.
SIEGEL: And you can watch videos of a mouse responding to alarm pheromones at our Web site, npr.org.