#ectothermy

“Cold-blooded” and “warm-blooded” are the colloquial terms for it. They’re... not exactly wrong, but incomplete. A “cold-blooded” critter simply does not have internal workings devoted to keeping it inside a specific band of temperatures No Matter What. (Though some may in fact be able to keep parts of their body much warmer or cooler than their surroundings - see the swordfish, among others, with their rete mirabile of blood vessels to keep their eyes warm in the sea to spot prey.) On the other hand a “warm-blooded” critter (birds and mammals being our key contenders) not only has the inner workings to do so, it absolutely must, or perish.

If you’re not sure what your critter should be, consider that these are two different ways of adapting to life with distinct advantages and disadvantages. Sort out which of those fit your critter best, and you should know what to pick.

First, let’s take ectothermy. As have most animals in the history of the world. It’s the most common way of life for a reason: it works. Since ectotherms rely on the outside world to keep them warm or cool, they don’t need to spend any energy on maintaining a constant internal temperature. This makes needing to eat much, much less of a worry; critical if you’re in a low-prey environment. Full-grown Nile crocodiles can pretty much live on only two wildebeest a year... and guess what? The herd’s migration usually takes them across particular rivers twice a year. Convenient!

(Herbivores prey on plants, algae, etc. Those, too, can be in short supply.)

Three distinct disadvantages, though. First, you have to make proteins that function at a wide range of temperatures; this costs extra resources. Second, your eggs are usually more temperature-sensitive than you are. Finding the right environment for them is critical. Third... if you really really need a fever, say to kill off a bacterial or fungal infection, you have to figure out how to get your environment to do the work. You have to give yourself a behavioral fever. Alligators, crocodiles, and their relatives will all do this by basking in the sun longer and more often than normal.

So. Endothermy. Apparently pounced on by both proto-mammals and some of the dinosaur lineages. This is a very resource-intensive strategy. You have to eat; you have to eat frequently. And if you get just a little too hot or cold inside, you die. A snake that drops down to 50 F inside may be perfectly fine, though it probably doesn’t want to freeze. A human whose internal temp drops to 85 F? That is nightmare territory and heading toward death. Why would any creature do this?

Look at the three disadvantages of ectothermy. An endotherm doesn’t make proteins that function across the temperature spectrum; it doesn’t have to. All its building blocks can be finely tuned to the temperature it’s supposed to be, with a little wiggle room in case of, you know, a smidge too cool or too hot. This saves a lot of resources. Sensitive eggs? Can be kept with you, either in a nest you keep warm or even - gasp - internally. (Mammals are so metal.) Fungus and bacteria? Get hit by a blazing reception of fiery fevers and fine-tuned immune systems. They can kill us, but numbers-wise, ectotherms are at far greater risk.

And evolution, very often, is a numbers game. It doesn’t take much of an advantage to pull ahead. It only takes a small, constant advantage over time.

Of course, there are exceptions. Like hibernating hummingbirds (some let their temp drop to 40 F every night) and pythons shivering to keep eggs 5-10 degrees warmer than outside air (as the Everglades snarl in frustration). And then there are paleontologist speculations that the really big sauropods and some other dinosaurs had mesothermy, which doesn’t seem to exist on the planet today because the (probably asteroid impact) disaster at the K-T boundary wiped out every critter past a certain size, and you had to be sauropod-size to make it work.