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?
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.
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.
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?
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.
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.
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.
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.
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.
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.
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.
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.