The common mole is nearly blind, but it makes up for its poor eyesight with a nose that can smell in stereo, a new study says.
Most mammals, including people, see and hear in stereo. For instance, stereo vision means that we see an object in three dimensions.
But only a few mammals have been confirmed to have a stereoscopic sense of smell. That means that each nostril operates independently of the other, sending different signals to the brain that are then computed to determine the direction of the odor.
A previous study showed that rats could be trained to tell whether an odor came from the left or right. But the new study in moles is the first to show a mammal using this ability as part of its natural foraging behavior.
Biologist Kenneth Catania showed that the common mole—also known as the Eastern American mole—relies on the ability to distinguish between subtle differences in the intensity of smells at each nostril to locate food. (See "Secrets of Smell: Different Nose Parts for Stinky, Sweet.")
"I came at this as a complete skeptic. I didn't really think it was possible, partly because the nostrils are pretty close together," said Catania, who is at Vanderbilt University in Nashville, Tennessee.
The Dirt on Mole Vision
In one experiment, Catania placed a mole into a circular arena with food wells spaced around a 180-degree circle. Pieces of chopped earthworm were placed randomly in different wells. Catania then temporarily sealed the chamber so that he could detect each time the mole sniffed by the change in air pressure.
When a mole first entered the chamber, it sniffed the air, and in five seconds or less would zero in on the earthworm pieces. "It would wiggle its nose ... and then just go straight toward the food in what looks like a straight line," said Catania, whose research appears February 5 in the journal Nature Communications.
In another experiment, Catania blocked one of the moles' nostrils with a small plastic tube. When the left nostril was blocked, the moles consistently veered off to the right, and when their right nostrils were blocked, they veered to the left.
The results are strikingly similar to a landmark study of hearing in barn owls performed in 1979, which found that blocking one of the owl's ears caused them to misjudge the location of a sound source, according to Catania.
Catania also inserted small plastic tubes in both of the moles' nostrils to cross them, so that the right nostril was sniffing air on the animal's left and the left nostril was smelling air on the right. When the nostrils were crossed, the confused moles searched back and forth repeatedly. (See "Moles, Shrews Can Smell Prey While Underwater, Study Suggests.")
Although it took them longer than usual to locate the food when their nostrils were blocked or crossed, the animals usually managed to find it after a while. Catania thinks that's because the moles aren't relying solely on their stereo sense of smell to find food.
The animals are also simultaneously sampling the intensity of an odor in different locations, comparing the intensities, and heading toward the stronger smell—a strategy that most mammals, including people, would use.
Stereo Smell Helps Avoid Enemies
Upinder Bhalla, a neuroscientist at the National Center for Biological Sciences in Bangalore, India, confirmed in 2006 that rats can smell in stereo.
Bhalla said that the ability to smell in stereo is useful for animals that have to make rapid decisions.
"If you can locate the direction of a cat in one sniff rather than two, you would be more likely to survive," Bhalla, who was not involved in the mole study, said by email.
"Similarly, if you are more accurate at tracking an odor to its source, especially in the rich mix of odors in the wild, you are more likely to find it and find it quickly."
Bhalla suspects that the ability to smell in stereo could be very common among mammals—including us. For instance, a 2010 study by Noam Sobel, a neuroscientist at the University of California, Berkeley, suggested people also use both nostrils to locate odors. (Also see "Better Smellers are More Sympathetic, Study Says.")
"Even humans can do this if pushed to it—though we rarely are," Bhalla said in an email. For example, for the 2010 study, experiment subjects were asked to get on their hands and knees in a grassy area and track the smell of chocolate.
"I would certainly expect all the strongly [smell-dependent] animals—which is a large fraction of them—can use stereo."
Study author Catania agreed that many other mammals can sniff in stereo, but "for humans, I remain very skeptical—and lean toward no," he said.