The mammal that can smell underwater
Moles blow bubbles to capture underwater scents.
Star-nosed moles have already snuffled their way into the Guinness Book of World Records as the world's fastest eaters. Now the tiny animals balls of grey-black fur with claws and a flowery pink nose have set another record: the first mammals to be caught sniffing out their prey underwater.
It has long been known that some aquatic animals have a sense of smell. Lobsters, for example, wave their antennae around, allowing specialized hairs to sweep through the surrounding water and pick up passing scent molecules. What wasn't clear, however, was whether mammals that spend part of their lives on dry land would be able to use a normal nose to sniff around below the surface.
"If you go through the literature on the historical ideas of olfaction, there's a lot of statements to the effect that 'Obviously, it's impossible for a mammal to smell underwater'." says Kenneth Catania, a neurobiologist at Vanderbilt University in Nashville, Tennessee and the author of the study.
Whales, for example, are missing some of the olfactory features that land mammals have. It is assumed that they lost these capabilities when they made the transition to a life aquatic. Semi-aquatic mammals were thought to use their olfactory system only on dry land.
Catania had been studying how star-nosed moles (Condylura cristata) feed on land when he decided to take a look at how they hunt underwater. So he set up a high-speed video camera under a glass-bottomed aquarium, tossed in an earthworm and a star-nosed mole, and started filming.
Right away, he noticed that the moles repeatedly blew bubbles out of their nostrils, only to quickly inhale them back in. The frequency (about 10 times a second) and volume of the bubbles was strikingly similar to the sniffing behavior that rats and mice use to track prey on land.
Catania wondered whether the bubbles were allowing odour molecules in the water to mix with air that was then drawn back into the mole's flamboyant nose and smelled.
To check, he trained the moles to follow an earthworm scent trail laid randomly on one of two paths in their underwater tank. Five moles tested used bubble-blowing and picked the correct path 75100% of the time, he reports in Nature1
But if a fine-meshed grid was placed between the scent trail and the mole, stopping bubbles from reaching the smelly molecules, the mole's ability to find the right path dropped to 50%, as would be expected by pure chance. This is about the same level of accuracy with which the practically moles could find unscented objects.
Catania has observed the same behaviour in the American water shrew (Sorex palustris), a mouse-like animal that is both "incredibly cute" and "an aggressive predator", he says. But whether bubble sniffing is universal among semi-aquatic mammals remains to be seen.
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- Catania C. C., et al. Nature, 444 . 1024 - 1025 (2006).
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