BioEd Online

Researchers in India have discovered that a single sniff is enough for a rat to locate the source of an enticing aroma.

Their work shows that rats can effectively smell in ‘stereo’: their two nostrils work independently in much the same way as our ears, with contrasting signals to the brain creating a spatial understanding of sensory information.

The team at the National Centre for Biological Sciences in Bangalore tested the ability of rats to discriminate between smells coming from their left or their right. They trained thirsty rats to drink from a water spout on the corresponding side in response to the odour.

Such is the rodent’s skill that, once trained, they required just 50 milliseconds to decide where the smell was coming from, report Upinder Bhalla and his colleagues in this week’s issue of Science¹. The rats selected the correct side with at least 80% accuracy, regardless of the odour presented; the researchers used banana, eucalyptus and rose water in the tests.

When one nostril was covered over, however, the rats lost their ability, showing that they need both nostrils to locate smells, the researchers add. This suggests that the two different nasal passages send contrasting signals to the brain, despite the fact that a rat’s nostrils are a mere 3 millimetres apart.

The effect probably works in a similar way to our own hearing, Bhalla explains. Our brain weighs up differences in the timing and intensity with which a sound reaches our two ears. If the sound is stronger and arrives earlier in our right ear, for example, the sound probably came from that direction.

Friends and foes

It is highly likely that other members of the animal kingdom also share this ability, Bhalla adds. “We don’t know what other animals have this effect. But given the excellent odour-localization ability of dogs, they seem likely to have some such ability,” Bhalla says.

Previous research has hinted that even humans might possess stereo noses, Bhalla adds, although our relatively pathetic sense of smell and overall reliance on vision has made this probably tiny effect very difficult to pin down.

Meanwhile, Bhalla’s team plans to investigate further the secret of the rat’s remarkable olfactory talents. “It was quite interesting to see how fast the whole process works,” Bhalla says. “And it is a good challenge now to figure out the neural mechanisms.”

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