Most mammals, including humans, see in stereo and hear in stereo. But whether they can also smell in stereo is the subject of a long-standing scientific controversy. Now, a new study shows definitively that the common mole (Scalopus aquaticus) – the same critter that disrupts the lawns and gardens of homeowners throughout the eastern United States, Canada and Mexico – relies on stereo sniffing to locate its prey. The paper that describes this research, "Stereo and Serial Sniffing Guide Navigation to an Odor Source in a Mammals," was published on Feb. 5 in the journal Nature Communications.
"I came at this as a skeptic. I thought the moles' nostrils were too close together to effectively detect odor gradients," said Kenneth Catania, the Stevenson Professor of Biological Sciences at Vanderbilt University, who conducted the research.
What he found turned his assumptions upside down and opened new areas for potential future research. "The fact that moles use stereo odor cues to locate food suggests other mammals that rely heavily on their sense of smell, like dogs and pigs might also have this ability," Catania said.
Catania's interest in the common mole's sense of smell dates back ten years when he was studying the remarkable sense of touch of the common moles' cousin, the star-nosed mole, which uses a set of fleshy tentacles surrounding its nose to detect edible objects as it burrows. He decided to test the common moles' capability to find prey for comparison purposes. "I expected the common mole, which is virtually blind and doesn't have a very good sense of touch, to be a lot worse than the star-nosed mole. So I was quite surprised when they turned out to be very good at locating prey. At the time, I figured that they must be using their sense of smell, but I didn't pursue the matter."
When the neuroscientist began seriously studying the common moles' sense of smell last year, he discovered that it was even more remarkable than he had expected.
He created a radial arena with food wells spaced around a 180-degree circle with the entrance for the mole located at the center. He then ran a number of trials with the food (pieces of earthworm) placed randomly in different wells. The chamber was temporarily sealed so he could detect each time the mole sniffed by the change in air pressure.
"It was amazing. They found the food in less than five seconds and went directly to the right food well almost every time," Catania said. "They have a hyper-sensitive sense of smell."
After observing dozens of trials, he noticed a general pattern. When the mole first entered the chamber, it moved its nose back and forth as it sniffed, but then it seemed to zero in on the food source, and moved in a direct path. This was pretty remarkable, and made Catania reconsider the idea of stereo sniffing. Although there is evidence for this ability in stationary rats trained to detect flowing air, no one had shown how this might work for a natural behavior.
To further investigate the moles' sense of smell, Catania blocked one of the moles' nostrils with a small plastic tube. When their left nostrils were blocked, the moles' paths consistently veered off to the right and when their right nostrils were blocked, they consistently veered to the left. They still found the food but it took them significantly longer to do so.
Next, the researcher designed a chamber where the food was always placed in the same position, directly across from the entrance. Moles using both nostrils went almost directly to the food, but the path of those with their left nostrils blocked was consistently to the right of the direct path and that of those with the right nostrils blocked was consistently to the left.
"This is strikingly similar to a landmark study of hearing in barn owls performed in 1979 by Eric Knudsen and Mark Konishi at the California Institute of Technology, who found that blocking one of the owl's ears caused them to misjudge the location of a sound source," Catania said.
The definitive evidence that the moles rely on stereo sniffing came from yet another test. Catania inserted small plastic tubes in both of the moles' nostrils and crossed them, so the right nostril was sniffing air on the animal's left and the left nostril was sniffing air on the animal's right. When their nostrils were crossed in this fashion, the animals searched back and forth and frequently could not find the food at all.
As for humans, Catania remains skeptical. "In humans, this is easier to test because you can ask a blindfolded person to tell you which nostril is being stimulated by odors presented with tubes inserted in the nose." Such studies suggest it is only when an odor is strong enough to irritate the nostril lining that humans can tell which side is most strongly stimulated.