The sombre hummingbird, a tiny bird from Brazil, has the highest maintained body temperature
of any animal we know of.
If your body was that temperature, you’d experience convulsions, brain damage, or maybe
just die.
A human with the metabolism of a hummingbird would need to eat about 80,000 calories a
day to survive.
That’s because staying warm by making your own heat takes a ton of energy.
And that brings up a paradox, because creating your own body heat is incredibly expensive.
The largest animals on Earth have to spend most of their day just eating enough to keep
their heat engines running.
Most animals on earth just use the heat outside their body.
It’s easier.
If you ate like a crocodile, whose body temperature hovers around 32 degrees, you’d have to
eat less than 50 meals a year.
And that works just fine for Crocodiles.
So that got me thinking, If making our own heat takes so much effort and is so costly,
why do we do it?
[MUSIC]
Hey smart people, Joe here.
Every organism on Earth, from the smallest bacterium to enormous elephants, break complex
molecules into simple ones to harness the energy to sustain life.
And using energy to move bodies and run the machinery of cells produces heat.
Endothermic creatures are special because they create enough internal heat to keep their
body temperature above their environment.
We humans are endothermic.
What you might call “warm blooded.”
But it’s more accurate to say that we create our own heat, inside our body.
And we’re also something else, homeothermic.
We regulate that heat at a steady temperature.
For most of us, that’s between 97 and 99 degrees fahrenheit.
Most animals aren’t this way.
Endothermy is only found in mammals, birds, and a few special exceptions.
Most animals are ectothermic, they don’t produce enough internal heat to stay warmer
than their environment.
And just like you have to constantly feed wood into a stove to heat a house, we have
to shovel fuel to our own heat making machinery.
And that means food.
A LOT of food.
Most of the energy from what we eat is released as heat, and about 10% of your daily calorie
intake is dedicated just for regulating your body temperature.
You might not realize it, but you think about making heat all day long.
Or at least, that happens deep in the core of your brain.
So deep that you aren’t aware of the processes controlling your thermostat.
Your body’s thermostat lives here.
The hypothalamus.
If your body temperature goes down by just 5°C, your brain starts getting fuzzy.
Your heart loses its normal pumping rhythm.
Which is pretty bad news for staying alive.
So our bodies work hard to stay warm from the inside out, and maintain a narrow, safe
temperature range.
If you’re too cold, your body does things like shivering, to create muscle warmth.
You stop sweating.
Your blood vessels constrict to keep heat from radiating off the surface of your skin,
and you get the goosebumps.
Which might not seem that helpful now, but to our hairy ancestors, it was a way to trap
air in our furry insulating layer.
But if our bodies get too far above their narrow optimal range, you basically turn into
this egg.
Heat unwinds the very structure of proteins in your cells, and you could die.
So if you’re too hot, your hypothalamus triggers your blood vessels to open up, and
all those little hairs lay flat.
And you start sweating, drawing heat from your body through evaporation, the same way
blowing on hot soup cools it off.
Your body is constantly turning these cooling and heating systems on and off to keep your
temperature steady.
Most animals that make their own body heat regulate their temperature in a similar way,
but the ideal temperature for that thermostat can vary a lot.
Compared to other endotherms, I’m pretty cool.
Just sayin.
What I mean is that human body temperature isn’t that warm.
The hottest 20 percent of endothermic mammals keep their bodies higher than 37.9 C. And
birds are even hotter.
They typically run between 40C and 44.4C.
And warmer animals have to eat more.
A 10 degree Celsius increase in body temperature means 2 to 3 times higher metabolism to maintain.
When heat is added to a body, it speeds up the kinetic energy of its molecules.
It’s like pouring more water over a water wheel.
Everything made of molecules speeds up and so do the reactions inside your cells.
Carbs, proteins, and fats are used up faster, and so is ATP, your body’s battery power.
So you HAVE to eat to replenish that molecular fuel.
For us that means more trips to the fridge, but to any animal that has to hunt or forage
for its own food, that means spending even more energy just to keep your thermostat steady.
In the eye of evolution, keeping a body warm is expensive and risky.
So there must be a good reason to do it.
Why do we go through so much effort just to keep ourselves warm from the inside?
Why do most reptiles, amphibians, fish, and insects, get to live without spending so much
energy to be alive?
That story goes back about 315 million years.
And like a game of blindfolded hide and seek, it’s a story that starts cold, and gets
warmer.
At first, all four legged vertebrates were ectothermic, their body temperature changed
according to their surroundings.
And the first step to changing that was a literal step.
When amphibians first wiggled their way out of the swamps and walked onto land.
The move to land brought new challenges.
Near the end of an era called the carboniferous, earth started to cool.
We call it “the great drying.”
Water got locked up in glaciers, the forests shrank.
And drier habitats popped up in their place.
To survive in this drier world, animals had to evolve new adaptations.
Things like holding your body up without the aid of buoyancy, saving your water, and not
drying out under the hot sun.
These all required higher metabolism.
This is also when we think animals first started chewing, allowing them to draw more energy
out of the plants and prey they consumed.
If you run your tongue on the top of your mouth, you’ll feel another innovation on
the path to warm bloodedness.
The secondary palate.
It’s what separates the nasal cavity from the mouth.
What does that have to do with being warm blooded?
Unlike early amphibians and reptiles, you can breathe through your mouth or your nose.
Meaning you can breathe and eat at the same time.
The secondary palate opened up more time for chowing down because they could breathe while
you did it.
Here in the Permian, we also start to see the first evidence of walking on two legs.
And moving around on two legs turns out to be a lot easier if you’re warm-blooded.
The big muscles required for bipedal motion require more energy, and to sort of rev up
your body’s engine that high, you need to be able to make your own heat.
So if I didn’t make my own heat, I couldn’t do this.
At least not for long.
No animal exists on earth today that can run fast, far, whose temperature is less than
30 C. Cooler muscles just can’t move as fast.
There’s only one modern reptile that can even run on two legs, the jesus christ lizard,
and it’s a sprinter than a marathon runner.
Then, about 252 million years ago, Earth experienced its worst extinction ever.
The Great Dying.
About 70% of the species on land went extinct when volcanoes released carbon dioxide and
methane into the atmosphere.
Temperatures skyrocketed, and oxygen plummeted.
The animals that survived and took over afterward were ones that could grow fast.
And in large part, that meant animals with a higher metabolism, and higher body temperatures.
We used to think dinosaurs were more like lizards today, lazy, scaly things more likely
to be basking on rocks in the sun, but we now know that dinosaurs were much more like
birds and mammals.
They grew fast, and moved in ways that suggest they could stay warmer than today’s reptiles.
Some large dinosaurs might have handled that just by being huge.
Think about it: A swimming pool isn’t going to lose heat as fast as a cup of tea.
So the largest ancient reptiles could have stayed warm even if they weren’t entirely
warmblooded.
Consider that the Komodo dragon, the largest lizard alive today, is able to maintain a
relatively constant body temperature compared to its smaller relatives, just by being big.
But it’s harder for smaller animals to stay warm compared to big animals, because they
have to work harder, pound for pound, than a big animal to maintain the same temperature.
Which is why we think full endothermy or warm-bloodedness first arose in something small.
And that happened, because this happened.
[BOOM]
About 66 million years ago, the sky basically fell down.
Most big animals died, because they couldn’t duck or find.
cover, and got roasted.
Small animals were more likely to survive, and a lot of them were capable of making their
own heat.
Animals like shrewdinger.
Yeah.
This is our best guess at what the first placental mammal looked like.
Shrewdinger was small, about the size of a rat.
And the bird line of dinosaurs made it too, because they were small.
Over time, they got faster, better at running, and eventually took to the skies.
And flying takes a lot of energy too, so being endothermic would have helped.
We tend to think of endothermic and exothermic animals as two sides of a coin, as if you’re
one or the other.
But we’re discovering new exceptions all the time.
Some sharks can maintain a body temperature higher than the water they live in.
So can other fish like the opah.
The tuna can too.
And the hedgehog tenrec, an adorable little mammal found in madagascar, is essentially
ectothermic or cold-blooded when it’s not breeding.
Their temperatures can plummet to -2 C without lasting damage.
So warm and cold blooded is a spectrum, not a red and blue binary.
But this still doesn’t solve our paradox.
Being endothermic is a lot of work.
So much work that many endothermic species spend most of their time eating just to feed
this engine of heat and survive.
You would think that evolution would have pushed us large animals back in the cold-blooded
direction.
But it hasn’t.
Some of the answer might just come down to chance.
When a big rock fell from the sky in the last major extinction, the things that survived
that apocalypse just happened to be the things that were on their way to being totally warm-blooded.
Those species were able to take over the Earth that we live on today, because they were what’s
left.
But even though creating our own heat is incredibly expensive, it can have some unexpected benefits.
Without it, well, we might not be able to survive in as many habitats and climates on
Earth.
And though cooler bodies might not get sick as often, since our warm bodies are great
germ incubators, getting sick would be more deadly, since the heat we are able to create
during a fever is a key part of our immune system.
We also couldn't move around as much, and we’d be smaller.
And some scientists think if we weren’t warm-blooded we wouldn’t have enough extra
energy to raise and nurture our young.
I mean, crocodiles aren’t winning any parent of the year awards.
So being this hot is hard work.
And to be honest, exactly how the history of life led to warm-bloodedness, there’s
still a lot more we need to learn to answer that question.
Evolution doesn’t have a destination, it’s the journey that’s important.
It’s a good reminder that there’s still plenty of big mysteries of life to solve.
Stay curious.
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