The star-nosed mole may take the prize for the most extreme adaptation. Its eponymous nose, which looks like a fleshy pink starfish sticking out of its face, is the most sensitive organ of any mammal's on Earth.
The mole is about the size of a mouse and the front of its face has 22 appendages, which gives it a bizarre look and makes it exquisitely sensitive to its environment. In fact, its nose is so sensitive, that scientists have not been able to measure the lowest threshold of force that will activate its nerve endings, says Ken Catania, the Stevenson Professor of Biological Sciences at Vanderbilt University.
“There are 25,000 tiny ‘touch domes’ on the star, and there are 100,000 nerve fibers supplying them,” Catania says. “To give you a good comparison, a human hand, which we know is a very sensitive thing, has about 17,000 touch fibers. The star is only the size of your fingertip. So, imagine five times the acuity and sensitivity of your hand compressed to the size of one fingertip.”
Perhaps a better analogy is to the retina, Catania says. “They use the star the way we use our eyes, moving around this little sensitive area for high resolution, and scanning with the other parts of the star,” he explains. “It’s analogous to a visual system in that behavior, so it’s kind of a unique animal in that sense, as well.”
The star-nosed mole also holds the Guinness World Record for eating speed.
“They can decide to eat something, snap it up and continue searching for the next thing in about 200 milliseconds — a fifth of a second,” Catania says.
In addition to its amazing sensitivity to touch, the mole can use its nose to smell underwater, which nobody had believed was possible for a mammal.
“We filmed them underwater, thinking maybe they would be less efficient, [but] it was completely the opposite,” Catania says. “We discovered that they’re actually sniffing by pushing out air bubbles onto things and re-inhaling the air bubbles as a way to collect odorants — as a sort of workaround to use olfaction underwater.”
Studying the star-nosed mole has given scientists a peek into the way the human nervous system is mapped in the brain. “The brain is a really interesting area in this species, because one of the big goals of [figuring out] how brains are organized is to look at how sensory maps are laid out,” Catania says. “Usually, that’s a pretty difficult thing to determine, even though we have a lot of technology to do it these days.”
“[But] in star-nosed moles, you can actually see a star in the neocortex,” he continues. “So, where the star information projects into the neocortical layers, there is a star pattern there. When we saw that, we realized there was one part of the star pattern that is disproportionately large, and that turned out to be a touch fovea.”
Star-nosed moles live in much of the northeastern United States, primarily in wetlands. Catania and his colleagues collect most of theirs in Pennsylvania.
“They are fairly common, but rarely seen,” he says. “Imagine if you had moles in your lawn and you wanted to go see one in the morning. That would be pretty hard. Translate that to a swamp and you get the idea of why it’s so challenging to see them. But you might see one sometime — most likely if your cat brings it in, unfortunately.”