New York Times,
December 26, 2006Link
You may enjoy the smell of coffee in the morning, but it is not the coffee itself that you are smelling. The human nose can only smell molecules that escape liquid, float into the nostrils, and dissolve into the thin layer of slime coating the olfactory nerves. Stick your nose directly into the coffee, and you will be too busy coughing and sneezing to enjoy the bouquet.
Scientists have long assumed that smelling underwater, like smelling under coffee, was impossible for mammals.
“It was something that mammals couldn’t do,” said Dr. Kenneth C. Catania, a biologist at Vanderbilt University. But Dr. Catania has discovered, much to his surprise, that moles and shrews can do it. They did not evolve a radically new nose, however. They just starting blowing bubbles.
Dr. Catania’s first clues came in the 1980s, when he was a graduate student studying star-nosed moles kept at the National Zoo. The moles hunt for prey underground and underwater. Dr. Catania noticed that when they were underwater, they sometimes released a stream of bubbles, at about two bubbles a second. If the moles were holding their breath, it did not make much sense for them to be leaking.
“It seemed unusual,” Dr. Catania said. “I couldn’t think of any good reason for an animal to do this.”
Dr. Catania made his original observations with the naked eye, but in recent years has been recording star-nosed moles with high-speed video. Using this technology, he noticed that the star-nosed moles were actually producing up to a dozen bubbles each second, but most of the bubbles never detached from their noses. Instead, the animals sucked the bubbles back in.
The bubbles resembled the puffs of air the moles used to smell objects out of water, Dr. Catania noticed. He also observed that the animals released their bubbles only when they were investigating objects.
“The bubble comes out of the nostril and spreads over the thing they’re exploring, and then gets sucked back into their nose,” Dr. Catania said. “So it’s essentially an underwater sniff.”
Dr. Catania speculated that odor molecules crossed from the water into the bubbles, which the moles sucked into their noses to smell. To test that idea, he ran an experiment.
The moles could choose between two paths, one of which led to food. On the food trail, Dr. Catania laid down earthworm scent. To prevent the moles from using their touch-sensitive stars to get any clues, he covered the bottom of the paths with a mesh.
The moles, Dr. Catania discovered, followed the scent accurately 85 percent of the time on average, with one mole getting a perfect score. “That’s pretty astounding,” he said.
When Dr. Catania used a mesh that was too fine for bubbles to pass through, the animals chose correctly 50 percent of the time — no better than chance.
Dr. Catania wondered if any other mammals could sniff underwater. He tested water shrews, which are known to swim for their prey. In the scent trail test, he found, the water shrews scored about as well as the star-nosed moles.
Bubble-sniffing is a striking example of how evolution tinkers with bodies, rather than rebuilding them from scratch. Like most fish today, our aquatic ancestors had noses that were just holes above their mouths through which water could flow. As vertebrates moved on land, the ancestral nose was modified to capture odor molecules from the air. Frogs and other amphibians still have two-chambered noses. They can seal off the front chamber to smell in water, and use the back chamber in air.
Mammals, which became more terrestrial, lost the ability to smell in water. Rather than reinvent a fish nose, however, moles and shrews have modified their air-sniffing anatomy to sniff bubbles.
“One of the things I would guess right off the bat is that there’s some sort of carefully controlled valve to keep it from inspiring water,” Dr. Catania said. “The timing and the behavior is probably carefully controlled as well. But the evolution of these minor adaptations does not seem to be a huge leap.”
Dr. Catania hopes that the discovery will bring some new respect to moles, shrews and other small mammals. “People used to classify them as primitive mammals with simple behaviors and an uncomplicated brains,” Dr. Catania said. “And looking at them is showing the complete opposite.”
Copyright 2006 The New York Times Company