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Eastern Mole. Photo by Ken Catania.
ScienceThe Loom

Stereo Mole Noses

Never underestimate a mole.

It’s easy to have low expectations for these creatures. My own experience with moles is limited to the sight of their carcasses at my doorstep, laid there by our triumphant and generous cat. Stretched out on the cold bluestone, they look like little more than eyeless pouches of fur.

Ken Catania of Vanderbilt University has gone a long way to redeeming the mole. In the 1990s, he did some of the first studies of star-nosed moles, a species with 22 oddly finger-shaped tentacles flaring from its nostrils. He discovered that those tentacles are an exquisitely sensitive touch organ with 25,000 sense receptors. It uses this bizarre star to rapidly scan its tunnels for food, sweeping back and forth a dozen times a second. It takes a fifth of a second from encountering a piece of food for the star-nosed mole to eat it, making it the fastest eater on record in the animal kingdom. (Here’s a story I wrote about Catania in the New York Times, plus a wonderful illustration of its fast-acting anatomy.) Catania’s research landed the star-nosed mole in Guiness Book of World Records.
Guiness Book of World Records.

Recently, Catania thought it would be interesting to compare an awesomely sensitive creature like the star-nosed mole with a pathetically equipped relative. He chose the Eastern mole, the common species that I so often find dead on my doorstep.

Scalopus aquaticus seems, on paper, like a supremely mediocre mole. It has no star on the end of its nose. Other mole species that lack a star still have lots of touch sensors on their faces. But these sensors are delicate bits of tissue, and Eastern moles, adapted to life in drier, harder ground, have lost them. They are blind, and their ears are only sensitive to low-frequency sound.

So Catania started observing eastern moles search for food. Instead of stumbling around, they turned out to be, in their own way, just as impressive as their star-nosed relatives. Despite being starless and blind, they consistently headed straight for the food.

Catania decided to study the moles formally, by building a special box. One half of the box was a holding chamber, in which he could put a mole. It was divided from the other side by a wall with a closed doorway in the center. On the other side of the box, Catania drilled 15 pits, arranged in a semi-circle at equal distances from the door. Into one of those pits Catania would drop a chunk of earthworm, and then he would opened the door. The mole would squirm through and explore the new part of the box. Every time, the mole headed straight for the hidden food.

The only way that Catania could imagine the mole could manage this was by smell. But the mole’s sense of smell would have to do more than just tell it that there was a worm somewhere in the box. It would have to direct the mole to the precise spot where the worm had been placed.

How animals smell their way to food is still fairly mysterious. One method that’s been well supported by experiments is to play the hot-cold game with your nose. If you get closer to food, its smell will get stronger. If you move away, it gets weaker.

Another possibility is that the moles can smell in stereo. They can compare signals from their two nostrils to judge how far to the left or right a smell is. We use stereo for our hearing, using the differences between what our left and right ears detect to pinpoint the source of a sound. Perhaps moles can do the same with their noses.

Other scientists had found some suggestive evidence that rats could smell in stereo, but Catania was not entirely convinced. After all, ears are separated on either side of the head, but nostrils are right next to each other. How they could get a different signal across such a small distance was hard to fathom.

So Catania did a series of experiments. He used an air pressure sensor to record when the moles took a sniff. As you can see from this video (sniffs marked by white spots), sniffing was a vital element of their search.

Catania then manipulate the noses of the moles in various ways to see if he could alter their performance. In one experiment, he plugged up one nostril. Now, instead of finding their food with 100% accuracy, the moles consistently failed. And they failed in a consistent way. If their left nostril was blocked, they veered off to the right. And if their right nostril was blocked, they veered to the left.

This reliable error suggested that they did, indeed, rely on both nostrils, each providing information about the smells on each side. Catania then ran another experiment in which he inserted tubes into the mole noses. The air flowing into each nostril crossed over to the other one, where it was detected by the nerve endings there.

Now the moles got hopelessly lost. Instead of simply getting biased information that steered them off course in a consistent way, the moles were overwhelmed by confusion.

Catania concludes that moles probably use both stereo and hot-cold strategies to find food. From a distance, hot-cold works well. Closer up, where smells can get much stronger over even a short distance, the moles switch to stereo.

It will be interesting to see if other animals show such clear evidence of smelling in stereo too. Catania has some evidence that star-nosed moles aren’t so good with stereo smell, which he’s now following up on. It’s possible that eastern moles took a capacity found in many mammals and evolved it into a much more powerful form to adapt to their particular kind of life in the hard ground. I’m not too eager to have pipes shoved up my nose, but I’m awfully curious to find out if we humans share the gift of the mole.