Noticing Tue, 08 Dec 2015 23:37:45 +0000 en-US hourly 1 Meet the Bug That Breathes Through Its Butt Tue, 08 Dec 2015 14:00:08 +0000 In our last post, you were a beetle with a breathing problem. In this one, you’re underwater. And because you’re in the water, you’ve got what looks like an unsurmountable, frightening, oh-my-god-I’m-going-to-die predicament.

The post Meet the Bug That Breathes Through Its Butt appeared first on Noticing.


mayfly larva underwater

Image: Macroscopic Solutions. License: Creative Commons / Flickr

This tiny insect, barely larger than a grain of rice, is a baby mayfly (a mayfly nymph). It spends the first years of its life entirely submerged, then crawls up into the air, and flies off. We couldn’t help wonder, how does it breathe down there?

If we opened our air passages (our nose and mouth) in a tub of water we wouldn’t last a minute. We’d drown. In some (less friendly) circles, this is called ‘waterboarding’, but you can’t waterboard a mayfly nymph. Dunked, it breathes easily. It’s solved this problem. But how?

The solution is looking right at you. If we move in a little closer, notice what looks like a set of feathery objects, protruding out on this animal’s left and right, down toward its butt…

mayfly larva wide

Image: Macroscopic Solutions. License: Creative Commons / Flickr

Move in closer still, and you’ll notice there are copper-colored branching tubes inside each of those feathery folds. They look like veins in a leaf. Those are its breathing tubes.


Image: Macroscopic Solutions. License: Creative Commons / Flickr

You can see them clearly here.


Image: Macroscopic Solutions. License: Creative Commons / Flickr

We keep our breathing parts inside us (our mouth, nose, windpipe, lungs).  So do all land animals, including insects. But underwater breathers don’t. They stick their breathing tubes out into the water. That’s what those feathery objects are; they’re gills. Mayfly nymphs extend their respiratory system out into the surrounding water, effectively rearranging their insides to be closer to their outsides.

And they breathe this way for a very compelling reason: oxygen in water is harder to find. 21 percent of the air is oxygen. In water? Oxygen is less than 1 percent.

Yup, that little.


So animals in the water don’t dare wait around for oxygen to come to them. They have to find ways to get out and meet more oxygen molecules. Sticking body parts further out is a help. That’s all gills are, a way for a creature to get its air-breathing insides closer to any passing oxygen.

Another Problem: Oxygen’s Stuck

But even with gills, there’s another big problem that underwater breathers have to solve – what little oxygen there is in water, barely moves.


In our last post, we took a clump of air, and found that oxygen (that red dot) is so hemmed in by other molecules, it gets kind of stuck, like a heavy metal fan in a mosh-pit. In air, an oxygen molecule bumps into its neighbors 6 billion times every second (that’s a pretty epic mosh-pit).

That’s air.

In water, an oxygen molecule is even more hemmed in, an astonishing TEN THOUSAND TIMES MORE. It has 60 trillion collisions every second, random zigs, zags, sometimes up, sometimes down, sometimes left, sometimes right. It’s so stuck in place, it barely gets anywhere.

Move or Die

Which is why gill-breathing creatures have no choice. They’ve got to keep moving, or figure out a way to stroke the water, to draw in a fresh supply of oxygen. Because consider what happens if you’re a gill breather and you find yourself in still water. Let’s say you decide to hang out for a while in the same spot.

Illustration: Robert Krulwich

Illustration: Robert Krulwich

Oh dear.

Because oxygen molecules in water are basically stuck in place, you can actually use up your local supply and die. That’s why fish keep gulping water – to keep a steady stream of oxygen rich water flowing past their gills.

So when in still water, gill breathers need to ensure there’s a flow. Most do this by constantly moving around, leaving a trail of oxygen depleted water behind them, like goats mowing through a field of grass.

Some insects take a more laid back approach to breathing. Mayfly nymphs, who we met earlier, don’t need to chase after oxygen molecules. Instead, they stay in place and fan their feather-like gills to and fro, creating little currents of water that bring in a fresh oxygen supply.

mayfly gills

Image: Jon Sullivan. License: Creative Commons / Flickr

But there’s one amazing insect that rises to the challenge of finding a fresh oxygen supply in a rather unusual way. Instead of moving itself through the water, it moves the water through itself.

Darner dragonfly nymph (Aeshnidae)

Image: Jan Hamrsky.

The dragonfly nymph has its gills inside its rectum, and it breathes through its butt-hole. Yup, you read that right. In ‘inhales’ by sucking water into its butt, and ‘exhales’ by squeezing the water back out.

Whaaaat?“, said Robert on first hearing this.
Robert: …this is true?
Aatish: I’m not making this up.
Robert: But we just said gills are for sticking OUT, now you’re saying these stay IN?
Aatish: Whatever works, I suppose.
Robert: But how would this work? You say the gills are up this animal’s butt?
Aatish: I did.
Robert: The same butt it poops out of?
Aatish: The same.
Robert: Well, how do I say this delicately. Wouldn’t it be a little… what’s the word?… unhygienic to breathe and poop through the same orifice?
Aatish: It would, which is why dragonfly nymphs have evolved their own biodegradable garbage bags – a thin membrane that wraps around their poop to keep it from polluting its surroundings.
Robert: Come on.
Aatish: No, really, these creatures swim around with their own built-in waste disposal system.
Robert: Have you seen these packets?
Aatish: I haven’t, but I’ve read about them. They’re called peritrophic membranes, and lots of insects use these poop baggies to clean up after themselves.
Robert: Well, I’m going to try to imagine them…
Aatish: Don’t…


Robert: …is this what you imagine?
Aatish: I was trying NOT to imagine them.
Robert: Oh.

Having gills inside (rather than outside) the body gives the dragonfly nymph a few terrific advantages. Not only does it attract fresh oxygen, there’s a totally neat side-effect – jet propulsion!

Darner dragonfly nymph (Aeshnidae)

Image: Jan Hamrsky.

By squeezing water out of its butt, the dragonfly nymph can propel itself in the opposite direction, rocketing itself safely out of harm’s way, or launching it towards its lunch (hooray for Newton’s third law). As far as we know, dragonfly nymphs are unique among insects in this ability.

dragonfly nymph rocket

Illustration: Robert Krulwich

But the thing that makes this little baby dragonfly even more spectacular is what those rocket blasters appear to do for its self confidence.

As you’re about to see, it uses its rocket blasters not only to move about the pond, but the same water squeezing system powers a grasping arm on its face – it’s a terrifying stretchable mouth part, that flings outward and takes food into a death grip.

Darner dragonfly nymph extending its labium (Aeshnidae)

Image: Jan Hamrsky.

In this video you will watch it effortlessly consume 6 little insects in a row, most of them baby mosquitos (good riddance!) but then it gets all cocky and goes after an impossibly large passerby – which makes no sense. It would be like one of us trying to bite a cow, and yet, if you’ve got rockets in your butt, apparently you become an optimist.

We should end here.

You can’t top a dragonfly nymph’s breathing system – or can you? We can’t help ourselves. We’ve decided to add one more addendum to our addendum. “Noticing” does this to us. So, if you come back in a little bit, we promise you three little breathers who get their oxygen in surprisingly unexpected ways, by breaking through, breaking in, or hanging on. They’re guaranteed to… [Aatish: Don’t do it, Robert. Don’t go there.] take your breath away. [Aatish: Noooo!]


We first heard about bewildering butt-breathers and other curious aquatic insects from Professor Gilbert Waldbauer, who describes these delightful characters in A Walk Around the Pond. Waldbauer is an entomologist and a keen noticer, and his book is an eye-opening look into the surprisingly varied and interesting world of tiny critters.

And big thanks to expert insect photographer Jan Hamrsky, who gave us permission to use his images of dragonfly nymphs. Lose yourself in his stunningly beautiful collection of insect photographs on his website Life in Fresh Water.

Also, thanks to Cynthia Berger, for sharing some astonishing dragonfly facts with us. Her book on dragonflies is a delight.


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Gasp! A Breathing Puzzle Tue, 10 Nov 2015 20:00:34 +0000 Most animals not only don’t breathe like we do, they do it so differently, we still haven't quite figured out what they’re doing. What we do know is the way these creatures take in air and get their oxygen is nothing short of astonishing. So that’s our topic: mysterious breathers.

The post Gasp! A Breathing Puzzle appeared first on Noticing.

We want to talk to you about breathing — something you do all the time (if you don’t, we have no idea how you’re reading this.) Breathing, of course, just happens. You don’t have to think about it much. But if you do, you’ll discover there are a vast number of animals on this planet, animals you know well, animals who buzz, bite and crawl about in plain view — who not only don’t breathe like we do, they do it so differently, we still haven’t quite figured out what they’re doing.


We’re going to start with the familiar — with you. Take a look at this pulsing “how-we-breathe” diagram, created by the wonderful illustrator Eleanor Lutz.

As you can see, when we humans breathe in, we lower that muscle just below our lungs — that’s our diaphragm. When it pulls down, air gets sucked in; when it pushes up, air gets squeezed out. So in effect we’ve got a pump in there, pulling oxygen into our body, sending carbon dioxide out.

But what if you’re a bird? Birds, (we didn’t know this), do it differently.

They don’t have a diaphragm, instead they pump air by inflating and deflating pouches called air sacs, and this creates the suction they need to draw air through their lungs.

But now let’s check out a grasshopper. Do you notice anything unusual?

Yes, it has air sacs to help it breathe, but notice it doesn’t use its mouth, nose, (or ears or anus) to let air in or out. It has no obvious breathing holes. How’s that possible? Where does the air come in?

Well, here’s the weird answer: A grasshopper, it turns out, breathes with its entire body. Eleanor illustrates this in her graphic by making the whole grasshopper flush yellow. And it isn’t just grasshoppers we’re talking about.

Insects (which means the majority of animals on Earth) don’t have lungs. In a sense, they are lungs. You could think of every insect as a walking, flying, hopping lung.

Here’s how it works. If you take a magnifying glass and inspect the surface of an insect, any insect, you’ll find their outsides are punctured by little holes called spiracles. Take this caterpillar, for example.

indian moon moth caterpillar

Image: Dean Morley License: Flickr / Creative Commons

See those little orange ovals along its middle, that look a little like eyes? Let’s get up close to one of them.

indian moon moth spiracles

Image: Dean Morley License: Flickr / Creative Commons

And closer still…

Image: Dean Morley License: Creative Commons

Image: Dean Morley License: Flickr / Creative Commons

That’s actually a kind of air valve, called a spiracle. The caterpillar can open or shut it this valve, depending on whether it wants to let air in or out. If you had X-ray vision, or a dissecting knife (or if you were lucky enough to chance upon this totally see-through transparent caterpillar) you’d discover that these holes open into a maze-like network of tubes called tracheae that extend into the insect’s body.

Image: Jim Cordoba, Enio Cano

Image: Jim Cordoba, Enio Cano

Oxygen wanders in through these spiracles (you can see the openings — that line of glowing dots that look like subway stops along the length of the caterpillar), and then drifts into a labyrinth of tubes that branch out into smaller and smaller tubes, until finally, at the teeny-tiny tips, the oxygen reaches the end of its branching journey, arriving at the insects cells.

In contrast, our bodies have a circulatory system pumping blood to get oxygen from our lungs to our cells. But in insects, there’s no blood involved in oxygen’s journey. Instead, the oxygen just floats all the way, right up to the cells’ doorsteps.

To ventilate their insides, bigger insects must actively breathe in and out, pulsing their abdominal muscles, as you see here.

But tinier insects barely budge. They breathe in a lazier way. Instead of pulsing their body, they just open their pores and sit there, like opening windows in a living room. Then they wait for the air to just… drift in.

Wait a second! WAIT A SECOND!!!

We’re writing an essay here about breathing. Breathing feels like it should be a physical act, something your body does, not just opening a body hole and thinking “come on in.” It can’t be that passive.


What’s more, says Aatish (he’s the one of us with a PhD in physics), if you know a little about the physics of air, you have good reasons to find this drift-in style of breathing – just open your pores and let the air in – more than a little puzzling. Which is why we should have…

A Short Conversation About Oxygen

(Readers with physics PhD’s should feel free to skip to the next section.)

Aatish: OK, Robert, I want you to close your eyes.
Robert: Why?
Aatish: Just do this.
Robert: Ok. They’re closed.
Aatish: Now I want you to imagine the oxygen floating in the air around you.
Robert: Ok…
Aatish: …and tell me what you imagine.
Robert: Well…
Robert: I see a molecule. Two little O’s linked together, and they’re whizzing around, like from from the window to my neck, and then… I don’t know… they bounce off my neck and ricochet from me to… to you, to your ear.

aatish ear bounce

Aatish: Ah.
Robert: Ah, what?
Aatish: So you’re picturing oxygen molecules whizzing through space, bouncing off walls, and every so often they might collide with not just my ear, but with other molecules in the air.
Robert: Yeah, that’s what I was thinking…
Aatish: Well, no.
Robert: No?
Aatish: The reality is totally different. If we magnified the air around you right now, what you’d find is…


…an immensely crowded space. The air is so jampacked with molecules, our poor oxygen molecule can barely budge. Whenever it tries to move, it hits a neighbor, bounces randomly back or forward or up or down, then hits another neighbor. Can you guess how many collisions an oxygen molecule makes in a second? Just one second?
Robert: I have no idea.
Aatish: Are your eyes still closed?
Robert: Yes.
Aatish: 6 Billion.
Robert: Whaaaat?
Aatish: Yes! More than six billion bing-bangs with the neighbors. That’s so many collisions, a free-floating oxygen molecule barely gets anywhere. I read that an oxygen molecule can travel only 80 nanometers — that’s 8 millionths of a centimeter (3 millionths of an inch) — before it bumps into another molecule and goes careening off in a totally random direction.
Robert: So —-?
Aatish: So air isn’t empty space. Quite the opposite. At the molecular level, it’s more like a thick smoothie. And just like you need suction to slurp down your drink, common sense says you can’t just wait for oxygen to drift in — you need to pull it in.

If that’s so, then how do these little guys breathe?

We know they do…

Ants, mosquitoes, beetles can breathe without sucking, pulling or grabbing air. But how do they breathe without pulsing their bodies?

“The question you are asking is one that greatly troubles me,” says Jon Harrison, a scientist who’s spent years researching how insects breathe and grow.

The truth is, we haven’t quite figured it out. Harrison and his colleagues are still trying to understand how much insects breathe by oxygen drift (passive breathing) and how much by pulsing their sides (like a grasshopper).

Jon thinks that all insects can breathe passively if they absolutely have to; he says most insects can go totally without oxygen for hours at a time and not die. He’s seen animals go from active to quiet to what looks like totally dead, then miraculously bounce back. (If you want to hear some stories, click on this nearly dead beetle.)

Lazy Beetle

Illustration: Robert Krulwich

Click for more

The evidence that insects can breathe without moving, says Jon, “is that you can put an insect in an atmosphere with absolutely no oxygen and, like a mammal, it appears to kill them. They’re completely paralyzed. But then when you put them back in normal air, they will all come back to life.” “Most insects can survive 2 to 6 hours without oxygen.. it’s very cool.”

“When that’s happening, we now know from X-rays, that they are completely paralyzed inside. So the heart’s not beating and nothing’s moving inside, but yet they will recover. And so we know under those circumstances that they’re getting enough oxygen, to restart the system at least, by pure diffusion or drift — a passive process.”

Click for more

Lazy Grasshopper

Illustration: Robert Krulwich

Click for more

Jon’s lab has even done experiments where they inject insects with an anesthetic, so they’re temporarily paralyzed and can’t move, and yet they can breathe just fine.

BUT, and here’s the important caveat, says Jon, most insects don’t breathe in this passive way most of the time. There aren’t many studies on how very small insects breathe, and those few studies that have been done found that, indeed, tiny insects DO pulse their insides. In other words, Jon believes that while insects are all biologically capable of breathing without moving (and can do so for hours if they’re trapped in a room without oxygen, or if they’re paralyzed), most don’t choose to live like that.

“It can be extraordinarily difficult to answer a really interesting broad question in biology”, says Jon. “There are hard problems. We need young kids to get interested in them and help us figure them out.”

But when little insects breathe passively, he thinks he knows how they do it. Let’s take, for example, this blue blister beetle.

blister beetle back

Image: Macroscopic Solutions License: Flickr / Creative Commons

If you looked really, really closely along the beetle’s sides, you’ll find tiny holes that puncture its hard, shiny exoskeleton, just like we saw before in the caterpillar.

blue blister beetle spiracles 1000px

Image: Macroscopic Solutions (disfigured by us to label the spiracles) License: Flickr / Creative Commons

Those are its spiracles, the air-holes through which it breathes. Air passes through these holes to enter the beetle’s breathing tubes (its trachea).

Here’s what one of these spiracles looks like when you’re right next to it.

blister beetle spiracle

Image: Macroscopic Solutions License: Flickr / Creative Commons

We can get in closer to see what happens on the inside. So let’s shrink down to the size of air molecules and wander inside this microscopic cavern.

Breathing is just a way to get oxygen to a hungry cell on the inside. The cell can then use this oxygen to tear apart food molecules and get the energy it needs.

So here’s our trachea. Air is coming from the left. A little ways in, we’ve placed a hungry cell looking to gobble an oxygen meal. Half an inch deeper, there’s another cell, just as hungry. The cells are waiting. We’ve painted the oxygen molecules red.

spiracle zoom in

Beetle image: Macroscopic Solutions License: Flickr / Creative Commons

As we’ve seen already, air is a thick soup of molecules. As the fresh oxygen supply drifts into the beetle’s body, the red dots meander about randomly, each second bumping into about as many neighbors as there are people alive on Earth. So it’s no surprise, then, that they make slow progress.

If there were nothing in its way, an oxygen molecule could travel half a kilometer in a single second. But bumping through this crowd of molecules, it manages to cover just one centimeter in that time.

OK, enough talk, it’s time for lunch! Watch what happens.


Illustration: Aatish Bhatia

Perhaps unsurprisingly, the cell closer to the surface gorges itself on oxygen. But the cell that’s deeper inside gets far fewer oxygen meals. The molecules just couldn’t reach it in time. Sad to say, the deeper cell is likely to starve. Then die.

Ah! So Breathing Affects Size

This explains something you see everywhere you look.

Animals that breathe passively with their whole bodies are always small. Think about wasps, bees, ants, houseflies, gnats, mites, centipedes, beetles. Have you ever seen a six inch housefly? Never.

insect size barrier 1 1000px

insect size barrier 2 1000px

Illustration: Robert Krulwich

None of these guys are much larger than a few inches. Why?

Because they can’t be big. Because air molecules have such a hard time getting around, an insect’s passive breathing system can only work over a very short distance. So insects are fated to be short themselves. If they got much bigger, their insides would starve from lack of oxygen.

If humans were to breathe like insects do, with trachea instead of lungs, we’d be covered with air-holes. And because oxygen wouldn’t get very deep in us, we’d have to be much, much smaller.

Man with Trachaea

Illustration: Robert Krulwich

So how creatures breathe constrains their size. And this, of course, explains why you will never, ever bump into a situation like this.


License: Public Domain

Except — and this we didn’t know — if you had been on Earth some 300 to 360 million years ago during the Carboniferous era, you would have met scarily — and we mean SCARILY large insects. Not as big as this ocean-going monster housefly, but there are fossils of Carboniferous dragonflies. They look like today’s dragonflies, with the same wings, same body shape, but back then, they measured more than two — nearly three feet across!

person with giant extinct dragonfly

Illustration: Robert Krulwich

This isn’t a joke. Take a look at this fossil…


License: Wikimedia / Creative Commons

The animal in question was called Meganeura and it not only had giant wings, it had a body the size of a modern seagull. It was that big.

There were also spider-like creatures back then with leg spans (you may scream now) nearly 20 inches across; scorpions that measured over two feet — nearly a dozen times as big as scorpions you might’ve encountered — and maybe the craziest of all was a millipede, (yes, a millipede) called Arthropleura, that could grow to eight and a half feet long!

But relax, it didn’t eat meat. It liked plants.

How could these critters grow so big? It turns out the air was different back then. The world was dense with forests and swamps, so oxygen levels rose to an astonishing 35% (compared to 21% of the atmosphere today). So if you were a little spiracle on the outside of a giant millipede, there was so much more oxygen drifting down your tube. Here’s what it might look like inside you.


Illustration: Aatish Bhatia

There’s now more oxygen molecules streaming in, and so, just by chance, more of them could penetrate deeper. Which means the first cell gets all the oxygen it needs, but now the second, deeper cell gets fed too! It doesn’t die. So the animal can now afford deeper insides. Which makes it bigger.

So that explains why so many animals at the same time became giants. They got more oxygen lunches.

Think about this for a second: How weird and wonderful is it that a seemingly arbitrary pattern in nature, that insects are all small, can be explained in part by imagining the crowded, frenetic, jiggling motion of invisible air molecules — that what’s in the air shapes what’s on the ground.

Who’d have guessed?

We’re going to end now. But here’s a promise. There’s one dangling chapter of this story that so delighted us we decided to turn it into a post all its own. We found a cast of tiny characters that employ some totally ingenious strategies to get their oxygen fix. There’s even one unusual insect that can breathe through its butt (we won’t show you the details yet)…

creature redacted

Illustration: Robert Krulwich

…and wait till you find out what else this animal can do with its remarkable rectum / amazing anus / bewildering butt-hole. The mind, (or rather, the body) boggles.

That’s coming next. So come back.

The post Gasp! A Breathing Puzzle appeared first on Noticing.

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Why Elephants Don’t Explode: How Nature Solves Bigness Wed, 02 Sep 2015 14:15:40 +0000 Shrews are very odd animals. They don’t slow down. They barely sleep. Like tightly wound springs, they unleash crazy amounts of energy, and then, out of some deep need, or some deep necessity, they release more.

The post Why Elephants Don’t Explode: How Nature Solves Bigness appeared first on Noticing.

Oh dear. There’s something not quite right with this animal.

It’s obviously cold, and yet this teeny little fur ball is dashing frantically across the ice, probing, sniffing, darting, never pausing, never resting, it just barrels on, ferociously hunting, hunting, hunting. That’s what shrews do. This one’s a short-tailed shrew. Shrews are very odd animals. They don’t slow down. They barely sleep. Like tightly wound springs, they unleash crazy amounts of energy, and then, out of some deep need, or some deep necessity, they release more. They can weigh less than a penny, and yet they must eat all the time, move all the time, hunt all the time. No other mammal behaves this way. Just consider…

Etruscan Shrew sits in a thumb

Image: Trebol-a License: Wikimedia / Creative Commons

This little guy is the lightest mammal known to science. It’s also a shrew, a species found in the Mediterranean and in parts of Asia, called an “Etruscan” shrew, that, like our video-shrew, is constantly moving — on average, 13 times every second. Think about doing anything (even blinking) 13 times in a second. That’s beyond frantic.

To keep up its pace, its heart beats faster than a hummingbird’s (going as high as a record-breaking 1500 beats per minute). And so, shrews must burn a lot of fuel. Which means they have to constantly gulp air and gobble food. Every gram of this shrew’s body uses 67 times more oxygen than we humans do, and it eats nearly twice its body weight every day — just to stay alive. Twice its body weight!

If this shrew goes without food for just four or five hours, it’ll starve.

So by necessity the shrew spends its life furiously focused on hunting, snatching, biting. If you go to YouTube and type in “Shrew vs”, you’ll see them battling a crazy array of scary animals (scorpions! snakes!) And while we aren’t necessarily recommending a YouTube shrew-fighting binge, let’s just say that we wouldn’t bet against the shrew.

shrew vs

Something’s Not Right Here…

Now — there’s something uneasy-making about these shrews (other than the alarming ease with which they can take down a garter snake). They’re such an extreme life form.

Just to give you a sense of how crazy their appetites can be (and it’s not just shrews we’re talking about, it’s a bunch of teeny mammals), let’s do a quick exercise. Just below you will find two mammals, one big, one little. The top one is a vole (a small furry rodent about as big as a mouse), the bottom one an African elephant, the largest land animal in the world. Both voles and elephants love to eat grass. So in front of each, we’ve place a freshly cut pile of lunch.

Your job? You are a portion-adjuster. You decide the amount of grass each animal needs to eat every day. Just follow your intuition and move the toggle switch to the right sized meal. Go ahead, give it a go.

Not what you’d figured, right? We both thought the vole would need less — being so little. But no, the vole eats about 80 percent of its weight in food every day. The elephant, just 5 percent of its weight. Which means, relative to size, the vole has a much bigger appetite than the elephant. Every ounce of a vole needs 16 times as much food as an ounce of an elephant.

Why? Something odd is going on here. What if… (and we’re just wondering at this point..) what if big animals and little animals are built differently? Literally. From the bottom up.

The Big Idea

If you hold a tiny creature like a mouse in your hand (don’t try this with a shrew, because it’ll bite your finger), you can almost feel it whirring, brimming with energy. Clearly, lots of little animals (mice, chipmunks, squirrels) behave like this. On the other hand, the biggest mammals — elephants, whales, rhinos — seem to live at a quieter, slower pace. So there’s an obvious surface difference between big and little animals, but how deep does that difference go?




Big versus Little – Profoundly Different?

Well, here’s another clue. Take a look at the graph below.

It’s a collection of dots. In this case, each dot is a different animal. Going from left to right, it starts with the littlest animals (see our Etruscan shrew? It’s the very first dot at the extreme left because, as we know, it weighs less than a penny), and moving to the right you’ll see a mouse, a squirrel, a rabbit, a fox, a lion, a tiger, and so on. Way, way on the right is our pal the elephant.

This graph is measuring appetite. Not for grass. But for oxygen. The graph is asking, which animals are running so furiously they consume the most oxygen per body weight? That is, if you took a small chunk of shrew, fox, lion and elephant, each chunk the same weight, which chunk (which animal) is consuming the most oxygen?

Check it out! It’s just like our “Lunch Puzzle”. The pattern is the same. The littlest animal (the shrew) gobbles oxygen at a high, high rate (that’s why it’s high on the chart), just like the little vole gobbled grass. But as we move on to the bigger animals, their proportional need for oxygen goes down. You can see the dots dropping lower and lower as you move to the larger animals on the right.

Just as every ounce of a vole needs 16 times as much food as an ounce of an elephant, a similar pattern holds true for the air they breathe. When compared ounce for ounce, a vole gulps down about 11 times more oxygen than the elephant.

And so, an elephant isn’t at all like a giant vole. Per ounce of body weight, a bigger creature has a lower metabolism — its gulps less air, burns less fuel, and releases less heat — than a small creature.

And it must be this way. To see why, we’re going to do a little ‘thought experiment’. This isn’t a real experiment, and in this particular case not a particularly nice one either, involving as it does, a self-combusting elephant.

alarmed elephant


Note to Elephant: Remember, you are an imaginary elephant. Being a fantasy, it is not possible, we feel, for you to experience any actual harm. In fact, we wish to remind everybody that no actual elephant would EVER be harmed here at “Noticing.” We love elephants. But, moving on…

The Mystery of the Exploding Elephant

So, we’re going to take our big mammal and place it next to a teeny mammal, in this case, a mouse. As you can see, our elephant is MUCH bigger than the mouse.


How much bigger? Well, the mouse weighs just under an ounce (about 20 grams); the elephant 11,000 pounds (5,000 kilograms). In other words, that elephant weighs 250,000 times more than the mouse does.

OK, so that’s pretty big.

But there’s a problem here. If you go back to our last post, we saw that as a creature grows in size, its insides grow faster than its outsides (i.e. its volume grows faster than its surface area). If a little mouse were to grow 250,000 times bigger to become an elephant, once it gets elephant-sized, it has trillions and trillions of nice, warm cells bunched together in its torso, its legs, so it’s pretty hot in there. And, because it’s grown bigger, it also has more surface area — more skin — to let the heat out.

So it’s got a big problem. While the elephant has trillions more hot cells on its inside, it hasn’t got nearly enough surface area to let that heat out. It’s got 250,000 times more volume than the mouse, but only a piddling 5,000 times more surface area (more skin) on its outside, which leaves a crazy amount of heat – trapped, locked in – with no place to go.

If an elephant burned fuel at the same rate as a mouse or a shrew, its insides should get so impossibly hot, that at some point, it should just…..

exploding elephant


But elephants never do that.

And our elephant friend, puzzled by its continuing existence, says, “Isn’t that what my huge ears are for? To vent my extra heat?”

elephant displays it ears

“Isn’t that what my huge ears are for?”

Indeed, elephants use their large, thin ears to dissipate heat, in much the same way that a car’s radiator does — a large surface area is great at radiating heat. But unlike a car radiator, elephant’s ears don’t have nearly enough surface to get rid of the amount of heat we’re talking about here.

In fact, if an elephant somehow had enough skin to release all of this the extra heat that we’re imagining, it would end up with great crinkly folds of skin, like a giant dimpled golf ball, or, says Princeton Professor John Bonner, like “a monstrous walnut”.

increasing surface area to volume ratio of an elephant

Here’s what happens to our imaginary elephant if we increase its surface area, while keeping its volume fixed

So how come real elephants don’t go up in flames?

We already know the answer — elephants don’t burn fuel at the same rate as mice do. That’s why they have such small appetites for their size.

Looking Deep Down

If you burrow down — all the way down — to a typical cell in an elephant, and then compare it to a typical cell in a mouse — amazingly, the two cells behave differently.

Elephant cells aren’t lazy. They’re always working, but compared to mouse cells, elephant cells typically do their job a little more slowly, burn less fuel to get the job done and, being more efficient, they run cooler.*

So that’s why elephants don’t spontaneously combust (and neither do we, much to Calvin’s relief.) An elephant is built from cooler stuff than a mouse. Even though an elephant has many, many more little heaters packed inside its body, each heater runs at a much lower setting. Says John Bonner,

A larger animal could not even exist unless its cells had a reduced rate of metabolism. It would either starve or burst into flames, or both.

John Bonner

Shrews, we now know, are at the other extreme. A shrew is built from hungrier, warmer stuff. Shrew cells are like Mexican jumping beans in a cocktail shaker.

mexican beans in a cocktail shaker

Without its internal heaters turned up to-the-max, a shrew would leak out all of its heat and freeze. And this explains its furious appetite. Shrews are so insatiably hungry because, well, their cells are. If a shrew can’t find food, then, says John Bonner, it’ll suffer “irreversible internal damage after a few hours”. It’ll starve.

Psst… want to find out how a shrew powers its extreme lifestyle? How does it work at a cellular level? Click here to find out.

All the way down to its cells, a shrew’s body is built for a high-energy lifestyle. If you peer closely, really closely, at a shrew’s muscles, you’ll find that the cells inside are like tiny gas guzzlers — they’re hungrier, greedier, and more energetic…

Muscles come in two kinds, ‘slow-twitch’ and ‘fast-twitch’ (if you’re a meat-eater, you might know these as ‘dark meat’ and ‘white meat’). We all have a mix of both types of muscles. So for example, a marathon runner’s muscles have more of the energy-efficient ‘slow-twitch’ fibers that keep pumping out energy slowly and consistently. Meanwhile a sprinter’s or a weightlifter’s muscles have more of the gas-guzzling ‘fast-twitch’ fibers, that provide short bursts of furious energy. Think a Prius versus a Formula One car.

An Etruscan shrew’s muscles are all of the fast-twitch variety — there isn’t a single slow-twitch fiber in its muscles — putting this extreme creature in a category beyond even the fastest human sprinters. Its cells are jam-packed with mitochondria, the microscopic battery-packs that power the cell. These cellular superpowers are the reason this shrew’s muscles can fire faster than any other known creature, breathing 15 times a second, moving 13 times a second, and shivering a blazing 60 times a second. And they’re also what makes the shrew such a hungry, hungry animal.

Small is Different

And so, all the way down to the level of the cell, the metabolic rate of each creature — how much oxygen it gulps, energy it burns, heat it releases — is carefully tuned through evolution to meet the challenges of its size. Big creatures aren’t giant versions of little ones, instead they’re built from cooler and calmer parts. Their internal fires must burn at a lower rate.

As far as we know, this wide-ranging rule holds true for most animals on the planet, not just mammals, but also birds, fish, crustaceans, snails, amphibians, reptiles, insects, and more. Thanks to this rule, our mammalian ancestors could grow from the twitchy, frenetic, shrew-like creatures that burrowed beneath the world of the dinosaurs into the lumbering giants that roam our world today.

And thank goodness for this scaling difference. Think what would happen if the bigger animals on earth had the temperament, the appetite, the needs of a ferociously hungry shrew? That would be so horrible, so inconceivably terrifying that it would be — of course! a Hollywood movie!


Made in 1959, “Killer Shrews” is the story of handsome boat captain Lorne Sherman (James Best) who lands his supply ship on an isolated island inhabited by a geneticist and his beautiful daughter Ann (Ingrid Goude). Ann’s dad has created a new breed of “blood curdling, horrifyingly poisonous” giant shrews, who must eat “three times their own weight in food” every day or starve. And then, all of a sudden, a hurricane cuts them off from civilization, just as the hungry shrews discover the warm taste of human flesh…

Uh-oh. The script doesn’t go there explicitly, but we know what poor Ann and Lorne are thinking as the shrews chew their way through the adobe walls of the lab, sniffing human food: “It’s dangerous to tinker with the gently scaling gifts of cellular biology,” or as Lorne prefers to put it, SHOOT! NOW!



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Who’ll Freeze First? A Puzzle About Size and Staying Warm Thu, 27 Aug 2015 01:00:19 +0000 It’s 20 below freezing in a Tennessee forest, and biologist David George Haskell takes his clothes off. All of them. On purpose.

The post Who’ll Freeze First? A Puzzle About Size and Staying Warm appeared first on Noticing.

It’s 20 below freezing (that’s -11C), a bitterly cold wintry day in a Tennessee forest, and biologist David George Haskell takes his clothes off. All of them. On purpose.

Haskell Naked Shivering

In Professor Haskell’s book, The Forest Unseen, he says he wanted to “experience the cold as the forest’s animals do, without the protection of clothes,” but the moment he’s naked, his body pulses with pain, his head hurts, and while he’s standing there shivering, he notices a little gang of Carolina chickadees gathered right next to him, perched on trees. They seem totally comfortable, feathered as they are. He looks at them, they at him, and then, embarrassed at his growing numbness, he decides, ‘enough’, puts his clothes back on, and on his way home, totally casually, as if everybody knows this, he says something that made both of us go, “What??”

The Chickadee Mystery

Chickadees, he says, do an odd thing. They change size as they move north and south. Not the individuals. But if you go 600 or so miles south, to Florida, the chickadees down where it’s warmer, tend to be smaller. Tennessee chickadees are, on average, about ten to twenty percent larger than Florida chickadees.

Chickadees Going North

But reverse direction and head 600 miles north, up to Maryland, or further up to Massachusetts and even Maine, and you’d find a closely related species, the black-capped chickadee, which, says David, “is ten percent larger again”.

And this isn’t just a chickadee thing. It’s true of bats, rabbits, hares, goats, gazelles, deer, moose, hyenas, dogs, cats, mongooses, possums, kangaroos, monkeys. If you see a squirrel outside your window, and then imagine seeing that same species of squirrel some 500 miles north, chances are it’s bigger up there, smaller down south (assuming you are reading this in the northern hemisphere).

This pattern isn’t universal. It governs only about two-thirds of mammal species, and bird experts argue how well it applies to birds. But it isn’t a fluke either. It seems that for some reason, as animals move toward the poles, nature likes them bigger. As animals head to the equator, nature likes them smaller.

With so many different-sized, different-shaped, different-looking critters all around us, it’s a little surprising to imagine that nature has a hidden rule or secret pattern that governs so many of them. Why, a chickadee might ask, would this be?

Puzzled Chickadee

We wondered too.

Turns out scientists have a name for this phenomenon: “Bergmann’s Rule”, after the German biologist who described it in 1847. To see how it works, let’s go back to our wintry forest, but this time instead of a naked biologist and a chickadee, we’ll feature two mice. One big, one little. (Please excuse Robert’s drawings. His mice are never quite alike, and sometimes look alarmingly like bears. We like to think of them as mice drawn by a moody artist.)

Two mice meeting in forest

Just as before, it’s 20 degrees below freezing, and both mice are very, very cold (though for some mysterious reason they’re unable to burrow underground to get warm.)

Two Mice Shivering

And, being of a somewhat morbid mindset, these mice begin to wonder, which one of them is going to freeze to death first.

The Question

The big mouse thinks it’s not going to be him.

“You are smaller and more fragile,” he said to his little friend, “so you will freeze before I do. What’s more,” he said, trying not to look too pleased as gazed down at his round rump, his chubby legs, his big, soft tummy, “I have more meat.”

Mouse Big & Small

“And all that meat is made out of trillions of cells, each one quietly burning food and giving out heat, like a tiny furnace. So overall, I’ve got a bigger furnace than you. I’m going to stay warmer for longer, and so I will last longer. I will outlive you.”

The little mouse looked up at him and said, “I think that you’ve forgotten something. You may indeed have more meat, but you also have more skin, and skin leaks heat. Because you are warm blooded, inside you it’s 98 degrees. But outside you it’s 20 below freezing and snowing,” and then resting his head on the bigger mouse’s back, he said, “this humongous back of yours is like a leaky window. It’s radiating heat…”

Mice Like a Leaky Window

“…and while it’s true you have a bigger heater on the inside, because you’re large on the outside, you will leak so much more than me. You, my big friend, are going to freeze first. I’m afraid that’s how it’s going to be.”

The two mice stared at each other. Which of us, they wondered, was right? Will the little mouse run out of heat first? Or will the bigger mouse, with more surface area, leak even faster?

The big mouse rubbed his pudgy paws on his warm pudgy stomach, thought for a while, and then smiled. With his paw, he traced out some square shapes in the snow.

The Answer

“I think I figured this out,” he said quietly, casting a sad glance at his little friend. “It’s true that being bigger I have more surface than you do. But I think I can show you that though you are teeny, you are leakier than I am.”

“Let’s, for the purposes of analysis, think of ourselves as simple shapes, and this,” he said, drawing a cube, “is you.”

Mouse equals cube

The little mouse had never thought of himself as a cube before. “As for me,” said the big mouse, “I’m going to be twice your size – twice as wide, twice as long, and twice as tall”, and he drew a second, bigger cube.

Mouse with Ruler Drawing

“Judging by a ruler, the big cube is twice the size of the little one. But there’s another way to measure bigness. Suppose we put away the ruler and ask instead, how much paint would you need to paint the surface of the big cube?”

Mouse Painting Surfaces

“Well…” says the little mouse, “If the big cube is twice as wide, we’d need twice as much paint to cover it, right?”

“No, that isn’t right,” says the big mouse. “Look closer…”

Mouse Twice As Tall drawing

“See, every face of the big cube has four little faces hiding inside it. So if you’re measuring how much paint you need cover it — its surface area — it’s actually four times as much as the little cube.”

“Hmm… OK, I’m with you so far,” said the little mouse. “But doesn’t this just prove my point? If you have four times as much surface area as I do, you must be leaking four times as much heat. That doesn’t bode well for you, my friend.”

“Well,” said the big mouse. “Here’s the thing. We can measure bigness in yet one more way.”

“There’s another way?” said the little mouse, feeling colder than ever.

“Imagine each cube as a fleshy, warm piece of meat. How many of those warm little cubes of meat could you pack into the big cube?”

Mouse Cubing the Cube

The little mouse looked at the big cube and counted the units with his frosty paw, and discovered that when you double the length of a cube, the inside grows… there they are, plain as day… to 8 units of warmth.

“EXACTLY!”, says the big mouse, who can no longer contain its excitement.

So when an object grows twice in size (as measured by a ruler)…

…its outside surface grows four times bigger,

…and its inside volume grows eight times bigger.”

“That means that even though I — like the big cube — lose four times as much heat as you from my skin, I more than make up for it, because I have eight times as much hot meat inside!! My leaky window might be four times as large as yours, but my furnace is eight times bigger.”

“And so, it’s not me that has to worry about the cold after all, it’s you!” cried the bigger Mouse.” And sad to say, (for all of you who root for little guys), Biggie was right.

The Lesson

The lesson here is very simple: when a creature grows bigger, its insides grow faster than its outsides. (Or, as the math folks say, bigger things have smaller surface area to volume ratio.) You can see this for yourself. Move your cursor over our mouse friend here, and see what happens as it changes size.

So Bergmann’s Rule now makes sense. As animals move into colder latitudes, every extra bit of bigness produces a lot more inside — and the more inside you’ve got, the better your chances of surviving the cold.

As for the fate of our two mice friends, you’ll be happy to learn that they used their new-found knowledge to find a way out of their predicament. They realized that if they both huddle together into a ball, they’ll drop their ratio of surface to volume even further – combined, their heaters outstrip their windows. And so, our brave mice settle their differences, they hug, they cuddle, and together they weather the storm.

Mouse cuddling for warmth

Small is leaky.

You can see this idea play out all over the place. When you leave a batch of cookies out to cool, the small ones cool the fastest. Babies have a harder time staying warm than we do, because their surface to volume ratio is much larger than yours or mine. Small things (whether cookies, mice, or babies) lose heat more easily. And this explains why warm-blooded animals are bigger in colder climates.

Now That We Know Bigger Keeps Us Warmer, Turns Out Nature Has Another Strategy. Just As Sly.

There’s another route to warmth. Instead of changing size, what if, instead, you changed your shape? To see how this might work, let’s go back to our cube for a moment. We can rearrange the blocks in a cube into a more slender tower. Both these piles have the same number of blocks, but it’s easy to see that the slender pile has a greater surface area than the squat one.


Credit: Iacopo.lea License: Public Domain via Wikimedia Commons.

In the squat pile, more of the ‘meat’ is protected on the inside, while in the skinny pile, more of it is exposed. The slender pile leaks more heat than the squat one, while their furnaces are the same size.

So as we go to colder climates, in addition to finding larger animals (Bergmann’s rule), we should also expect to find animals with rounder, stockier bodies and shorter limbs. This latter idea is known as Allen’s rule, after Joel Allen, a 19th century American zoologist. According to Allen, birds and mammals in warmer climates should have longer limbs, ears, tails, snouts, beaks, and so on, the added surface area helping them cool down, while those in colder climates should have shorter limbs, to hold on to their heat.

Rabbits’ ears are a nice example of this rule. As you make your way across North America from Mexico to Alaska, you tend to encounter rabbits and hares with increasingly shorter ears. So in the deserts of Mexico and the Western United States, you’ll find the black-tailed jackrabbit sporting these enormous ears.


Jackrabbits have long ears. Image: Pschemp. License: CC BY-SA 3.0 via Wikimedia Commons.

Meanwhile, much further north, up in Canada and Alaska, there lives the snowshoe hare, a creature with shorter ears than most other hares, just as Allen predicted.

Snowshoe Hare

And snowshoe hares have small ears. Image: D. Gordon E. Robertson. License: CC BY-SA 3.0 via Wikimedia Commons.

Birds offer some of the clearest evidence for Allen’s rule. A study looking at measurements of 214 bird species found that their beaks get shorter as you go to higher latitudes. So for example, Antarctic penguins tend to have a shorter bills than South African penguins. Toucans high up in the Andes, where it’s cold, have shorter bills than toucans down in the rainforest, where it’s hot. A similar pattern holds for seagulls, wild turkey, and so on.

The explanation for this is that birds use their beaks to lose heat, and a longer beak leaks more heat, a fact that might have been obvious to us if only our eyes could see infrared light. Just take a look at these infrared photos of bird beaks.

birds radiating heat

An infrared camera reveals how bird beaks leak heat. Next to each bird is the temperature difference between its beak and its surroundings. Image: Matthew R. E. Symonds and Glenn J. Tattersall / The American Naturalist.

Or check out this infrared video of a toucan, and you can easily see the beak glowing as it radiates heat.

Bergmann’s rule (big vs. small) and Allen’s rule (slender vs. compact) aren’t capital R rules that apply in all situations. Instead, they’re simplified models. The real world is more complex (and more interesting) than the world of cubical mice. In real life, mice run short of food, face droughts, get diseases, get hunted by armies of cats, and can burrow underground for protection. A creature’s size may depend on all these factors, and not just on warmth. Bergmann’s rule has its fans and has its critics, but it’s been used by scientists for 175 years, and is still being used today.

Hey! What About People?

That being said, there’s one mammal we haven’t considered as yet, and that’s us: humans. So do these rules also apply to us as well?

You’d expect that, if these patterns held in humans, people living closer to the tropics should weigh less than people towards the poles (Bergmann’s rule – less mass equals a smaller heater). And their bodies should be more slender, with proportionately longer limbs (Allen’s rule – a larger surface leaks more heat).

In a recent study, researchers surveyed the sizes of 263 different modern human groups believed to have stayed in the same place for (at least) the last 500 years (because recent migrations would scramble these patterns). And they found that, just as you’d expect from Bergmann’s rule, people tended to get heavier as they moved towards the poles, agreeing with previous studies of human size. Meanwhile, many other studies have found evidence for Allen’s idea in humans.


“A Sami indigenous northern European family in Norway around 1900.” Public Domain.

So while these patterns are noisy and hold only on average, there’s evidence to suggest that as humans moved to colder climates (and stayed there for a long time), they developed a heavier and more compact body shape, driven by the need to stay warm. Human populations living close to the poles, like the Inuit, the Aleut people or the Sami people tend to weigh more than people who live at lower latitudes, consistent with Bergmann’s rule. And they have somewhat broader trunks and shorter limbs, consistent with Allen’s rule. In contrast, people adapted to life in the tropics tend to be leaner, with longer arms and legs relative to their size, which helps them stay cool.


Image by Bjørn Christian Tørrissen. License: CC BY-SA 3.0 via Wikimedia Commons.

What About Pygmy people? (click here to find out)

Groups of pygmy people have independently evolved towards small size in the rainforests of South America, Africa, and South East Asia. Although this fits with Bergmann’s rule, it still raises a question. Why did pygmy people evolve towards being smaller than other equatorial people?

One possible explanation, according to researchers, is that in hot and humid rainforests,  the usual way that we stay cool on hot days — by sweating — doesn’t really work. That’s because sweat doesn’t evaporate easily in the high humidity (as anyone who has spent time in a rainforest can attest).

And so, their small size may have evolved as an alternate strategy to stay cool in a super-humid environment, because smaller individuals produce less heat and leak proportionately more of it. Another notion is that pygmy people evolved towards small size because food is scarce in rainforests, and a smaller body means that you can survive on fewer calories.

Yes, there are lots of exceptions, and you can probably think of a few. Once again, we are not describing a universal law here, it’s just a pattern that shows up so regularly, so often, that despite the exceptions, it bears noticing. The world is full of differently-shaped, differently-sized, differently-sorted animals, and yet, in that jumble of difference there is a pronounced pattern: higher latitudes seem to select bigger and more compact bodies, lower latitudes smaller and leaner ones, and that may be because towards the poles you need to keep yourself warm, towards the equator you need to cool off, and bigger, compact bodies are better when it’s cold out.

The guy who taught us this, Professor David Haskell of Tennessee, could run his own little experiment. If some of his kids were to leave Tennessee for the Arctic and hang out there for another 10,000 years, would they gradually become more compact and thicker? Would the Haskells who stay in Tennessee stay lanky? It would be interesting to find out.

But then again, if the next 100 generations of Haskells are as curious as David, if they do cockamamie things like strip naked on sub-freezing days “to experience the cold as forest animals do,” we’re guessing their chances of lasting ten thousand years become a little… umm… dicey.

Curious animals are a special category. We love them. But try not to include them in long term studies.

Naked Haskell in forest


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Welcome to Noticing! Wed, 26 Aug 2015 04:00:52 +0000 Hi there. We’re Robert Krulwich and Aatish Bhatia, and this is our little patch of the web, which we’re calling “Noticing”.

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Noticing Sharing Question

Hi there.


noticing hands small


woah hmm aha small


On some days.


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