What's a Mammal?

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You think you know what a mammal is, right? Sure, but you might be suprised to learn that the dictionary definition you learned in school may not be all it's cracked up to be. Just as astronomers struggle to define a planet, biologists are trying to decide what makes a mammal a mammal.

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DEBBIE ELLIOTT, host:

In this week's Science Out of the Box, we take a closer look at something we all learned back in grade school, or thought we learned: the definition of the word mammal.

You might think you know all the things that make a mammal a mammal, but as NPR's Nell Boyce recently discovered, you don't know the half of it. What's more, the word mammal has a surprising history. Nell started her investigation by giving a little pop quiz to visitors outside the Smithsonian Institution in Washington, D.C.

NELL BOYCE: Everybody knows what a mammal is, right? It's easy. I mean come on. We're mammals, and we all learned in school what is a mammal. You think you know, right?

Unidentified Man #1: A mammal? Let's see, alive young.

BOYCE: Ah, but the duckbill platypus lays eggs. Come on, you learned it in school.

Unidentified Woman #1: Fur?

BOYCE: Fur, very good. That's one of them.

Unidentified Woman #1: Yes.

BOYCE: What's another one?

Unidentified Woman #1: I have no - I really don't know.

Unidentified Man #2: Hair.

Unidentified Girl: I think it's hair and then something else.

Unidentified Boy: Milk.

BOYCE: Milk.

Unidentified Boy: They have nipples.

BOYCE: Nipples, all right. Thank you very much.

(Soundbite of kids cheering)

BOYCE: Okay, close enough - fur and milk. Fur and milk is basically what we learned. That's what you get in school, in filmstrips, in songs.

(Soundbite of song)

Unidentified Man #3: (Singing) We got milk to give our young, to help them grow and make them strong. Another thing that we all share is the world's supply of hair.

BOYCE: And so imagine my surprise when I was just like coming to Smithsonian Museum of Natural History and I come to see their new Mammal Hall, and when you walk in, the first thing you see is this sign. And the sign is so weird that I actually got the person who sort of created this exhibit - her name is Sally Love - and I said, will you just come in here and read this sign?

Ms. SALLY LOVE (Smithsonian Institution): We have mammals equal hair plus milk plus special ear bones.

BOYCE: Special ear bones.

Ms. LOVE: Uh-huh.

BOYCE: No one ever told me special ear bones in school.

Ms. LOVE: It's a very distinguishing feature.

BOYCE: Mammals have these weird little three-bone structures that only mammals have. It turns out ear bones is just one of many, many things they could have picked, because mammals have a million distinct features. Well, maybe not a million, but you know, a lot.

It turns out that we also have special kind of teeth, we have a single bone in our lower jaw, we have like a special brain region called the neocortex. The diaphragm, very special. In our heart we have this weird aortic arch that goes to the left. There's like special blood cells, red blood cells. And the list just goes on and on and on and on and on.

(Soundbite of song)

Unidentified Man #3: (Singing) So the warm blood flows through the red blood cells like the nuclei to the large heart chamber, maintaining the very high metabolism rate they have. Mammals, mammals.

BOYCE: Then I started wondering, well, wait. Mammals have all of these different traits, and yet we're named after one of these things: milk, mammaries, the breasts, which if you stop for even like a moment and think about it, it's like the worst possible one. You know, it's the worst possible one because half of this group - namely the males - don't have mammaries. They don't produced milk. They have like little shriveled organs there, you know, nipples. But as I did some more reading, I found out that not all males even have that.

Ms. JULIE ROVNER (NPR): Hey, Hopper.

(Soundbite of horse)

Ms. ROVNER: Want a carrot?

BOYCE: I have a colleague, Julie Rovner, and she's got this male horse. And I just told her, look, next time you go to the barn, take a look under there, see if you see anything.

Ms. ROVNER: I see a lot of dirt and a lot of orange hair, but nothing that looks like a mammary or a nipple or anything.

BOYCE: Nothing, just absolutely smooth. It turns out not only horses but even rats and mice, the males have nothing. And so it just made me think even more that the term mammal is kind of uniquely bad. It's sort of so unsuitable. Why couldn't they have some other kind of name?

I'm not the only person to think about this. There's a historian - her name is Londa Schiebinger - and a few years ago she got together with a classicist, and they sat down to think of other names that would be just as legitimate as mammal.

Ms. LONDA SCHIEBINGER (Historian): We could be called pallosa(ph), the hairy ones. We could be called aracovega(ph), the hollow-eared ones.

BOYCE: Now, the reason she was doing this kind of weird little exercise is that when she had her first baby and was breast-feeding, she got interested in where the word mammal came from. What she learned is that for the longest time, from the days of Aristotle, all the way up into the 18th century, the word mammal didn't exist. Creatures were called something else. They were called the four-footed ones, or quadrupeds. That was just a standard term, until this guy Carl Linnaeus came along, and he developed this new name. The reason he focused on milk and not some other trait is that the guy was obsessed with breasts, but not for the reason you're thinking. He was interested in something called wet-nursing.

In the 18th century, women tended not to nurse their own babies. They sent their children out to midwives in the rural areas to be nursed. This was very common for rich women or even middle-class women like merchant women. Now, wet-nursing had some problems. The wet nurses usually would take on a lot more children than they could feed. There was a lot of deaths. And Linnaeus was a physician, and he was totally opposed to the practice of wet-nursing.

Ms. SCHIEBINGER: He wrote a political tract called The Mercenary Wet Nurses. He didn't in that particular tract talk about his term mammalia, but it was written at nearly the same time that he was coining the term, and therefore these things were on his mind.

BOYCE: And what's more, with this term mammal, he seems to have hit a kind of popular nerve. You see, in the 18th century, there was a lot of social upheaval. There were revolutions in France, in America, and women were taking the opportunity to say, hey, we'd like more power. We'd like to be doctors or lawyers or go to university or have the vote. Now, this made a lot of people very uncomfortable, and this was the context in which people were looking at this new term, mammal, and thinking, huh...

Ms. SCHIEBINGER: When Linnaeus connected women to nature and made it seem natural that women, all females, suckle their children, this was the very moment when Western societies decided that women would not be citizens but that they would be compassionate wives and nurturing mothers in the home.

BOYCE: There were other scientists trying to promote alternate terms, but you know, this mammal thing, it really caught on.

Nell Boyce, NPR News.

(Soundbite of song)

Unidentified Man #3: (Singing) Mammal, mammal. Their names are called, they raise a paw. The bat, the cat, dolphin and dog, koala bear and hog. The fox, the ox, giraffe and shrew, echidna, caribou.

ELLIOTT: Next time I see a dog or cow, I'm going to think - ear bones? Three's more Science Out of the Box at npr.org.

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