Warm-blooded vs. cold-blooded animals


OK we aren’t in kindergarten anymore. What does warm-blooded actually mean metabolically? Which version is older? If I spent the night naked in the desert, at 40 degrees all night, I’d probably die.

What about a naked lizard? Would it just slow down but not die? Naked lizard seems better. What exactly is so good about warm-blood approaches?

Also, what the fuck are birds?

Tiny dinosaurs, seem kind of like reptiles. Are they cold-blooded? I have questions.

In: 0

Warm-blooded animals can maintain (within limits) a constant internal temperature. Cold-blooded animals cannot. Modern birds (which are part of the theropod group of dinosaurs) are also warm-blooded and it’s likely that at least some non-avian dinosaurs were.

Body temperature is more than just what temp you happen to sit at. To a certain degree it’s a measure of the energy available to your body’ assorted systems.

Warm blooded creatures attempt, even if they’re not always successful, to maintain a high level of energy by “burning” food. This allows them to stay active even in cold environments. That gives them plenty of energy to actively find food or pursue prey, evade predators, move to more favorable areas and so on.

Cold blooded creatures don’t have to waste that extra energy “keeping the engines warm” so to speak, but that also means that their ability to move and act at any given time is limited by ambient temperatures. That makes them more efficient and able to get by on MUCH less food, but it also limits their ability to acquire food or evade predators in cold circumstances. Your desert lizard, for example, MUST find cover when it’s cold at night because he’s not going to be able to run away from any hawk or coyote that happens by looking for a snack. This is also why cold blooded predators trend more towards ambush or stealth attacks than pursuit, think crocodilians and many snakes for example, and only really engage in active pursuit for any period when they’re in very warm climates.

It’s also notable that it’s something of a spectrum. Many creatures aren’t fully homeostatic, meaning they don’t try to maintain the exact same temperature at all times like the human body always striving towards ~98 degrees, but they ARE able to burn extra food to keep themselves a little warmer than their surroundings and thus a be a bit more energetic than a fully cold blooded creature. As an example you see that with cold water predatory fish like tuna and some shark species.

Birds are warm blooded, like mammals with some differences in heart structure and circulation. Dinosaurs…are somewhat up for debate. There were almost certainly a lot of species that were warm-ish blooded, like the tuna we mentioned above, and on their way to developing full warm-blooded status as birds. The sheer SIZE of many famous species also meant that they would be able to generate and maintain heat much more easily than smaller animals so likely many of those species were at least somewhat closer to homeostatic than traditional reptiles even if they weren’t on their way to adapting into birds. Exactly how far that went is the subject of a lot of research and debate as we look at bone structures related to the circulatory system, a few soft-tissue imprint fossils and so on to better understand their setup.


Hopefully that provides some context while still being ELI5-ish