I have recently really enjoyed reading Nick Tennick‘s series on how animals keep cool during the summer months. I’ve often found the difference between warm- and cold-blooded animals quite fascinating but somehow I’ve never really understood the evolutionary reason behind why warm-blooded animals, are in fact, warm-blooded. Warm-blooded mammals (which we as humans fall under) use so much of our energy trying to stay warm or cool, whereas we could easily just use external sources, like cold-blooded animals. So why have mammals evolved to be this way?

The largest mammals on earth must spend most of their day eating enough just to keep their heat engines running. Elephants have to eat about 5% of their body weight daily to sustain themselves. For a fully-grown bull, that could mean eating around 250-300 kg of vegetation every day.

Endothermy versus Ectothermy (Warm vs Cold-blooded)

Most animals are ectothermic; they don’t produce enough internal heat to stay warmer than their environment, also known as cold-blooded. Endothermic, or warm-blooded, creatures are special because they create enough internal heat to keep their body temperature above their environment. Endothermy is only found in mammals, birds and a few special exceptions.

Most animals use the heat outside of their body. It’s easier. If you ate like a crocodile, whose body temperature hovers around 32°C (90°F), you’d have to eat less than 50 meals a year. And that works just fine for crocodiles.

Just like you have to constantly feed wood into a fire to heat a house, we have to shovel fuel into our 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 caloric intake is dedicated just to regulating your body temperature.

The Sombre Hummingbird, a tiny bird from Brazil, has the highest maintained body temperature of any animal that we know of, at about 44.5°C (112°F). As a human, if your body was at that temperature, you’d experience convulsions, brain damage, and possibly die. A human with the metabolism of a hummingbird would need to eat around 80 000 calories a day to survive. That’s because staying warm by making your own heat takes a lot of energy.

When heat is added to a body, it speeds up the kinetic energy of its molecules. Everything made of molecules speeds up and carbs, proteins, and fats are used up faster, and so does 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 food, that means spending even more energy just to keep your thermostat steady. This is where the paradox lies: mammals use more energy to try and get more energy. This paradox didn’t make much evolutionary sense to me. So it got me thinking:

If making heat takes so much effort and is so costly for mammals, why have we evolved to do it?

The hottest 20% of endothermic mammals keep their bodies higher than 37.9°C (100), and birds are even hotter. They typically run between 40°C and 44.4°C (104°F and 112°F). Warmer animals, like birds, have to eat more. A 10°C (50°F) increase in body temperature means a 2-3 times higher metabolism to maintain.

Evolutionarily Speaking

Keeping a body warm is expensive and risky, so there must be a good evolutionary reason to do it.

That story goes back about 315 million years. And 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.

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?

I will go through three major events in evolution that could be the reason for the rise of endothermic mammals. It may be that we are the way we are by pure chance resulting from these events.

1. The Great Drying

Near the end of an era called the Carboniferous era (around 359 to 299 million years ago), the earth started to cool. We call it “The Great Drying”. Water got locked up by freezing into glaciers, ocean levels decreased and forests shrank. Drier habitats popped up in their place.

To survive in this drier world, animals had to evolve new adaptations. Things like holding their bodies up without the aid of buoyancy, saving their 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 is what separates the nasal cavity from the mouth. What does that have to do with being warm-blooded? Unlike early amphibians and reptiles, mammals can breathe through their mouth or nose. Meaning we can breathe and eat at the same time.

A giraffe bull feeds on the flowers of a knob-thorn tree. It’s able to do this for long periods of time because of the secondary palate.

The secondary palate opened up more time for eating because we could breathe while we did it.

2. The Great Dying

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 afterwards were the 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.

3. The Cretaceous-Tertiary Extinction Event

Finally, about 66 million years ago, an asteroid that was as much as 10 kilometres (6 miles) in diameter hit the Earth. It was the huge amount of thermal heat released by the meteor strike that was the main cause of the extinction. Only really mammals in underground burrows and aquatic environments were protected from the brief but drastic rise in temperature. About 70% of species went extinct.

Most big animals died because they couldn’t duck or find cover. Small animals were more likely to survive, and a lot of them were capable of making their own heat. And the bird line of dinosaurs made it because they were small – flying takes a lot of energy too, so being endothermic would have helped.

But This Still Doesn’t Solve Our Paradox

I’ve mentioned it a lot, but 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 answers as to why we are warm-blooded 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 smallest things that were on their way to being warm-blooded. Those species were able to take over the Earth that we live on today because they were what was left.

The smallest mammals were able to survive the last extinction because of their ability to hide in burrows – much like the ones built by dwarf mongooses.

But even though creating our heat is incredibly expensive, it can have some unexpected benefits. Without it, 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 can 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. Some scientists also think that 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.

No animal exists on earth today that can run fast, and far, whose temperature is less than 30°C (86°F). Cooler muscles just can’t move as fast. A cheetah’s body temperature is approximately 38.3°C (101°F) but increases to 40°C (105°F) after sprinting.

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 are still plenty of big mysteries of life in nature to solve.