Mole sniffs the world in stereo

Nostrils of unlovely mammal recognize slight differences in odors

SMELLS GOOD  The common mole is blind and has a poor sense of touch, but it can smell in stereo. Each nostril receives slightly different scent cues that the animal uses to navigate toward its food, a new study finds.

K. Catania

The common mole may be homely but its nose is a wonder to behold.

THE NOSE KNOWS A common mole sniffs out a piece of food during a laboratory experiment. K. Catania

The eastern American mole, also known as the common mole, tracks down an earthworm treat by recognizing the slightly different odor cues entering each nostril, neurobiologist Kenneth Catania of Vanderbilt University in Nashville reports online February 5 in Nature Communications.

The finding suggests that even though mole nostrils are separated by a fraction of a centimeter, each gets its own scent information that can guide an animal’s actions. “It’s an elegant demonstration of what many people suspected,” says Peter Brunjes, a neuroscientist at the University of Virginia. Previous experiments with people and rats had reached contradictory conclusions regarding whether smell, like sight and hearing, is a bilateral sense.

Catania never expected the common mole, Scalopus aquaticus, to have uncommon abilities. “I’ve described it as the unlucky, stupid cousin of the star-nosed mole,” he says. Star-nosed moles, Condylura cristata, have an incredible sense of touch in their tentacled schnozzes and are among the world’s fastest foragers. But compared with other mole species, the eastern American mole has a poor sense of touch. The animals also can’t see. Catania turned to common moles because he thought they would have a hard time finding food and could be tested against star-nosed moles in future experiments.

But when he placed a common mole in a semicircular arena with a chopped up bit of earthworm as bait, he says, “it would wiggle its nose around and go in a beeline toward the food.”

Since the moles’ other senses are so bad, Catania wondered whether the animals locate their food by smell. He first plugged one of a mole’s nostrils with a short piece of plastic tubing. That caused the mole to veer off course in the direction of the open nostril. Next, Catania stuck small tubes into both nostrils and crossed the tubes so that the right nostril sniffed odors from the left side of the mole’s face and vice versa. The moles behaved as if they had gotten reversed directions to the food, searching to one side before finding the food or missing the earthworm treat entirely.

Catania’s discovery that crossing the nostrils’ inputs confuses the animals is strong evidence that moles, and probably other mammals, engage in bi-nostril smelling, Brunjes says. If moles simply sniffed their way toward an ever-stronger scent, then crossing the tubes wouldn’t make a difference.  “It’s kind of the perfect proof,” he says.

Moles may have evolved to precisely follow a scent trail because they have to dig for their food. “Being off by half an inch could be hugely energetically expensive,” Catania says.

He is interested in studying whether star-nosed moles traded their sense of smell for better touch. He also wants to know how scent information is wired into the brains of common moles.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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