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. You know what feeds the fire of this highly
metabolic YouTube channel? Patreon. Thank you to everyone who supports the show
on Patreon. We couldn't do it without your support, if
you'd like to join our community, check out the link down in the description, and we'll
see you in the next video.
