The Star-Nosed Mole Takes Adaptation To The Extreme
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.”
Ken Catania is the Stevenson Professor of Biological Sciences at Vanderbilt University in Nashville, Tennessee.
IRA FLATOW: This is science Friday. I’m Ira Flatow. A little bit later we’ll be talking about the pain of commuting when infrastructure is in bad shape. Have you been stuck in a highway congestion or on a delayed train recently? Give us a call to complain. Our number, 844-724-8255, 844-SCI-TALK, or tweet us at Science Friday, @scifri.
But first, you can say that my next guest is an expert in extreme animal behavior. He’s researched the snap of a crocodile’s jaw, he has studied the voltage discharge of electric eels, he even knows about something called worm grunting. But among his favorite research subjects, the star-nosed mole takes the prize for most extreme adaptations. We’re talking Guinness Book of World Records level stuff. For example, it’s eponymous nose, like a fleshy pink starfish sticking out of its head. You have to look at this to see it. And it’s the most sensitive organ of any mammal ever, and studying them has given us a peek into the way the human nervous system is mapped in the brain.
Ken Catania is Stevenson Professor of Biological Sciences at Vanderbilt University. Welcome back to Science Friday, Ken.
KEN CATANIA: Hi, it’s great to be here again.
IRA FLATOW: This is one of the more odd-looking creatures. Describe to our listeners what they look like.
KEN CATANIA: Sure. So this is about a mouse-sized little mammal, so it’s not all that big, but the front of its face has these 22 appendages on it, so that gives it this really bizarre look. And since it’s a mole, it’s also got these big clawed forelimbs. So those two things put together really give it a bizarre appearance.
IRA FLATOW: But a lot of moles have tiny eyes. They rely on touch. How did the star-nosed mole come to have this specific adaptation?
KEN CATANIA: Yeah, that’s a really interesting story, and it has two sides to it. One would be how did this thing evolve, and the other is why did this thing evolve. The how did it evolve is really interesting, because it looks like the evolutionary sequence is played out in the development. So if you look at the embryos, they are amazing-looking morphology, where the star unfolds off the face almost like a sort of a flower petal coming off the sides. And that suggests, and we have other evidence for this, that this thing evolved from sensory strips on the side of the face rather than growing out as straight appendages.
The how it evolved is, in the wetlands, there are a huge resource of little soft-bodied things that this animal can eat, and in order to do that, because these things are small, it has to have high sensitivity, high acuity, and be really, really fast.
IRA FLATOW: We know that its nose, as I said before, is the most sensitive organ of any mammal. Give us an idea of how sensitive it is.
KEN CATANIA: Well, that’s interesting, because we actually have not been able to measure the lower threshold for force that will cause activation of these nerve endings, so they are extremely sensitive. There’s a couple of ways you can think of sensitivity. One is compression that would cause a nerve to fire. Another way is to look at the receptive field size, and they have microscopic receptive fields. And receptive field is basically what’s the size of the skin region that one nerve will respond to. So that’s sort of like the pixels in a camera. It’s an incredibly high-resolution touch organ, so it gives them a great picture of what’s going on in their world.
IRA FLATOW: How much resolution is it? Can you give us an idea?
KEN CATANIA: Well, there’s 25,000 of these little tiny touch domes on the star, and there are 100,000 nerve fibers that are supplying these things. So to give you a good comparison, a human hand, which we know is a very sensitive thing, has about 17,000 touch fibers. And the star is only the size of your fingertips. So imagine five times the acuity and sensitivity of your hand compressed to the size of one fingertip.
IRA FLATOW: Wow, wow. So it’s like your retina? Close to that kind of sensitivity?
KEN CATANIA: Yeah, I think that’s–
IRA FLATOW: So it’s like a retina at the front of its face.
KEN CATANIA: Absolutely, I think that’s a great analogy. The star is just completely made of sensory organs stuck out there on the nose. I use the word amazing probably too much when I talk about this animal.
IRA FLATOW: When you look at the animal, there’s nothing short of amazing about it. And they hold a world record, too, don’t they?
KEN CATANIA: That’s right. And we were originally studying their behavior to look at how they move the star around. And in order to do that, we had to get a very high-speed camera and film at 1,000 frames per second just to see what was happening. And when we published these results, I got this phone call and we discussed how quickly they were eating. And a little bit later, I realized this was the Guinness Book of World Records, and they sent me this nice certificate in the mail. Of course, I sort of joked. I took my PhD off the wall and put the Guinness Book of World Record certificate up there instead, because it’s just so much fun to have that.
So they can eat something in about– they can decide to eat something, snap it up and continue searching for the next thing, in about 200 milliseconds, so a fifth of a second.
IRA FLATOW: Wow, wow. And then the other thing they use these special noses for is they can smell underwater. Isn’t that unique, too?
KEN CATANIA: Yeah. It’s just one surprise after another with this animal. So we filmed them underwater, thinking maybe they wouldn’t be as– because they dive, they live in wetlands. And so we thought maybe this would be sort of less efficient underwater. Well, it was completely the opposite. We discovered that they’re actually sniffing by pushing our air bubbles onto things, and re-inhaling these air bubbles as a way to collect odorants, as a sort of a workaround to use olfaction underwater, which nobody had believed was possible for a mammal, so really interesting discovery.
IRA FLATOW: Well, it does sound like there’s a lot of stuff yet to be discovered. And you’re the guy to do it, I think.
KEN CATANIA: Well, thanks.
IRA FLATOW: What other things would you like to know? I mean, you’re looking at it because I’m sure the nerve fibers are interesting to you, and how the brain works, right?
KEN CATANIA: Yeah. So the brain is a really interesting area in this species, because one of the big goals of looking at how brains are organized is to look at how these sensory maps are laid out. And usually that’s a pretty difficult thing to determine, even though we have a lot of technology to do this now these days.
In star-nosed moles you can actually see a star in the neocortex. So where the star information projects into the neocortical layers, there’s a star pattern there. And as soon as we saw that, we realized there’s one part of the star pattern that’s disproportionately large. And that turned out to be a touch fovea. So 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. It’s very analogous to a visual system in that behavior, so kind of a unique animal in that sense as well.
IRA FLATOW: I know I mentioned something earlier that I have to get to, because I have to know. I have to ask you about another area of your research, which is worm grunting. Worm grunting. So our listeners heard me say it correctly.
KEN CATANIA: Yeah. It’s not something the worms do. This is a bait-collecting technique that has been done in the southeast for decades, and it’s always been a mystery. So basically what is done is a wooden stake is pounded into the ground and you rub it with a piece of metal, and literally hundreds or thousands of big earthworms come out of the ground. It’s an amazing thing to see. And so the question has always been– this is one of those mysteries that I love– why on earth would these earthworms do this?
And all the way back to Darwin, there was speculation that maybe they interpreted vibrations as digging moles and this was an escape response. And so, sure enough, I went down there and hooked up with some of the worm grunters, Gary and Audrey Revell, who are really wonderful people, who are doing this as a way to make a living. And together we explored this question, and it’s very clear that the moles in the area– these aren’t star-nosed moles, these are more common moles– are creating vibrations. And, rightly so, the earthworms are terrified of them and come to the surface. It’s a lot like the arms race between bats and moths, which you may have heard of, but it’s happening underground.
IRA FLATOW: We have a lot of videos on our website. We have the moles, we have the worm grunting on our website at sciencefriday.com/mole. Is there anything else you want to know about this mole? What I want to know is can I find one in my backyard if I want to look at this mole? How common is it?
KEN CATANIA: Well, they do live in much of the Northeastern United States. They live in large areas of New York state. We collect them mostly in Pennsylvania. They are fairly common, but rarely seen. So 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 some time, most likely if your cat brings it in, unfortunately.
IRA FLATOW: So they’re sharing the underground real estate with the other moles and rodents that are under there.
KEN CATANIA: Yeah. Yeah, so when we do the field work, we set out hundreds of traps. And more often than not, we’re catching other animals in the same tunnels that the star-nosed moles live in. Little shrews that have tiny body size, maybe three grams, and tiny brains. We get weasels, which are the top predator in the area, that are down– those are beautiful animals, and they’re down there eating the moles and anything else they can catch. Voles, sometimes snakes and salamanders. So it’s a fascinating ecosystem that, honestly, we know almost nothing about what goes on in terms of the pecking order and how they interact. So that would be a really interesting thing to expand on.
IRA FLATOW: Are you always– do you think there’s stuff yet to be discovered underground?
KEN CATANIA: There’s always something new to discover. And I have to say, I can never predict what it’s going to be. I usually discover something completely unexpected, in whether it’s a star-nosed mole, an electric eel, worm grunting, and so forth.
IRA FLATOW: I know you’ve been on Science Friday before, and we’ve always enjoyed having you on.
KEN CATANIA: Yeah. The last time I was on, I should say they told me I really needed to get a landline, and we were traveling for the holidays. And so the idea was to stop at a hotel. And so I walked up to a somewhat stern elderly woman behind the front desk at the time, and I said, with my wife next to me, hey, I need a hotel room for an hour. And that really did not go over very well.
IRA FLATOW: We don’t do that kind of thing here.
KEN CATANIA: That’s basically what she told me. There was no recovering. I had to go to another hotel and start off differently and say, well, I have a radio interview to do. So that’s one of my favorite stories about that experience.
IRA FLATOW: Yeah we’re always happy to provide you with any excuses you need if you’re out hunting, or collecting research, because we believe in that kind of stuff. Well, find something new, Ken, and come on back and tell us about it, because you always got some great stuff to share with us.
KEN CATANIA: Well, thanks so much. It’s in the works already. I’ll give you a hint that it will be shocking. It will be shocking.
IRA FLATOW: It will be shocking. We’ve had you on about something shocking before.
KEN CATANIA: Yeah, there’s more to come.
IRA FLATOW: There’s more to come. Can’t wait. Ken Catania is Stevenson Professor of Biological Sciences at Vanderbilt University. If you see a guy on the ground looking for stuff, that will be him. Thank you, Ken.
KEN CATANIA: Thanks so much.
IRA FLATOW: As I say, we have the mole video and the worm grunting there down at sciencefriday.com/mole. We’re going to take a break. When we come back, have you been commuting lately? Have you been going through the nightmare of the commute? We’re going to be talking about our aging infrastructure, and what it’s doing to our attempts to just get around. So go to our website and tell us about your experience. Also, send us a text, or get on the line and tell us if you had a nightmare today or this week. We’re glad to hear from you. Stay with us. Be right back after this break.