You wouldn't even try to smell underwater ...

Cause your smell receptors are inside your nose; and trying this would mean to inhale liquid water into your lungs. And this is not just the case of the humans, but of mammals in general. "When mammals adapt to living in water, their sense of smell usually degenerates," explained Kenneth Catania, a biologist at Vanderbilt University. "The primary example is the cetaceans, whales and dolphins, which have lost their sense of smell."

So, you can imagine the surprise of the biologists when they discovered that a couple of tiny semi-aquatic mammals solved the problem. "This came as a total surprise because the common wisdom is that mammals can't smell underwater," said Catania.

When searching for food, land mammals are guided to their meal by sniffing air odors, especially tiny ones, which can not have a large visual spectrum. But Catania noticed that a small mammal - the star-nosed mole - blew a lot of bubbles while swimming. So he decided to test the animal: he stuck various objects (pieces of earthworm, small fish, and blobs of wax) to the bottom of a glass tank and used a high-speed video camera to monitor the moles' behavior. When the moles directed to the object, they rapidly exhaled bubbles through their noses, which came into contact with the object, and were then quickly sucked back in.

The rhythm was of 5-10 times per second, about the same rate that small terrestrial mammals (like mice and rats) sniff the air. "Because the olfactory nerves in the nose are covered with mucous, odorant molecules are all water soluble," Catania said. "So when these bubbles come into contact with an object, it is almost inevitable that odorant molecules will mix with the air and be drawn into the nose when the bubble is inhaled."

But the star-nosed mole's nose (photo) is very peculiar: it is surrounded by super-sensitive appendages that it can use to identify objects by touch, so Catania had to rule out this factor in order to prove the moles were actually smelling the objects. So he made underwater scent trails leading to food and put a steel grid between the moles and the scent trail that prevented the nose appendages from squeezing through, but allowed the bubbles to pass through.

Even without using their nose appendages, the moles had a 75 to 100 % success rate. When Catania used a finer grid that didn't allow bubbles to pass through, the moles were as likely to find the food as when not using smelling. Furthermore, Catania tested water shrews, which presented the same bubble-blowing behavior and tracked the food by scent. "Now the question is, 'What other semi-aquatic mammals do this?'," Catania said.

"Do animals like otters and seals do anything similar, or is there a size limit and it only works for smaller mammals?"

Photo credit: Keneth Catania