Mammals Have "Alarm Detectors" in Noses

James Owen
for National Geographic News
August 21, 2008
Mammals, including humans, have a built-in alarm detector in the tip of the nose for sniffing out danger, new research suggests.

The tiny sensor, discovered in mice, is used to pick up chemical warning signals sent by fellow animals in distress, scientists say.

Many plants and animals emit airborne molecules called alarm pheromones, which alert members of their species to dangers such as predators.

But how mammals detected these pheromones has been a mystery.

Now a team from the University of Lausanne in Switzerland says the answer lies in a microscopic ball of cells in the nose called the Grueneberg ganglion.

(Read: "Plant Networks Can Send Warnings, Spread Viruses" [October 1, 2007].)

Danger Communication

The Grueneberg ganglion was first identified in 1973 in various types of mammals, including rodents, cats, apes, and humans.

These mammals probably all have a nose for danger, the study team said, although the discovery has only been made so far in mice.

The new discovery—to be reported tomorrow in the journal Science—was made during a study of the Grueneberg ganglion in mice using physiological techniques.

"The ganglion is the only [smell] sub-system that's completely functional at birth, so we were thinking it was important for nipple finding for the baby mouse," said study co-author Marie-Christine Broillet.

But after numerous tests for nipple finding and other possible functions, the team found that the ganglion played a role in danger communication.

The researchers then compared how mice with and without their Grueneberg ganglia responded to alarm pheromones.

The contrast was very striking, 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.

(Related: "Elephants Distinguish Human Friends From Foes by Smell" [October 18, 2007].)

The chemical sensor's nose-tip location is ideal for early detection of pheromones, Broillet added.

Scientific Advances

Stuart Firestein, professor of biology at Columbia University, New York, said that the new finding represents two important scientific advances.

"First of all, it extends a growing appreciation that the olfactory system is not a singular system but is really made up of several subsystems," he said.

Second, the study advances the understanding of how body cells and molecules function together within the nervous system.

The newfound mouse alarm detection system introduces "a new and likely powerful model" for such investigations, Firestein said.

The chemical makeup of alarm pheromones and where they are produced in the body remains unknown.

But the discovery of the pheromone sensor should finally enable researchers to identify the molecules involved, study co-author Broillet said.

Alarm pheromones, if they can be artificially produced, could have various uses, such as repelling pests or dispersing crowds, she added.

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