I am designing a mammal living in a warm environment (like a warm desert) which has developed a switchable metabolism, so that during the day it is cold blooded and during the night (when it is colder) it goes on warm blood mode.

This switching roughly halves its energy demand (half of a day it doesn't need to warm itself up, heat is provided by the environment) and gives it an advantage in an environment with scarce resources.

Would this be possible, or do I need to make some other assumptions?

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    $\begingroup$ It is not only that poikilothermic animals have much lower metabolic rates than homeothermic animals; the big problem is that poikilothermic animals lack the metabolic mechanisms to regulate their internal temperature. For an example of lifestyle consequences see estivation. $\endgroup$
    – AlexP
    Feb 2, 2017 at 8:20
  • $\begingroup$ Didn't see it was your question before editing, normally for mods and other high level users I make suggestions vs just doing it. I just separated out your paragraphs and added a couple tags. Rollback if you wish. $\endgroup$
    – Cyn
    Apr 25, 2019 at 16:44
  • $\begingroup$ @Cyn, moderators and other high level users are not immune from errors. No need for preferential treatment, I'd say. $\endgroup$
    – L.Dutch
    Apr 25, 2019 at 17:33
  • $\begingroup$ @L.Dutch Sure. I mean if you misspelled something I'd probably fix it. But this was paragraph markers and tags. I did the former because you have line breaks but not double ones (or double spaces). But you know the tags better than I do. Just saying... $\endgroup$
    – Cyn
    Apr 25, 2019 at 17:36

4 Answers 4


Why not? Some lizards have ways of warming themselves from within when necessary. This can evolve into something more like what warm-blooded animals use rather than cruder mechanisms, but still keep the overall cold-blooded details so it doesn’t require the furnace to be on all the time.


It should be possible. Leather-back turtles are capable of being endothermic at will, as a function of activity and quantities of brown adipose tissue (very common in mammals, not so common in other animals).

Us mammals and birds OTH, we are endothermic but retain a high metabolic rate at rest. So in essence, those cute and big bad leathery boys of cold seas can flip a switch and go endothermic.

Bluefin tuna is also capable of being endothermic according to its needs. It has specialized "red" muscles at its core that allows it to regulate its temperature even when experiencing temperature swings that would stop a human heart.

I could see a mammalian-like being able to switch to a cold-blooded/ectotherm modus operandi when there's excess ambient heat.

Many mammals in Madagascar have developed the ability to go into torpor during the hottest days of summer. So it wouldn't be impossible to think a mammalian-like being being able to do that on command as well.

I don't think we see mammals having this ability to go ectotherm because the Earth has never given mammals regular drastic temperature challenges on a daily basis.

Imagine an Earth-like world with, say, a 72 hour day/night cycle causing many regions to go experience a 90F (32C) temperature swing from midnight to midday.

If the night extreme is in the freezing, but midday temperatures are what we consider "normal", probably mammal-like beings (and other beings for that matter) would develop the ability to hibernate on demand or have "anti-freeze" agents in its fluids. Some crickets in New Zealand allow themselves to be frozen at night.

OTH, if it is the midday temperatures that are extreme (extreme from our POV), then animals would/could develop on-demand ectotherm behavior (or go into a state of torpor.)

I think your hypothetical being to be quite possible.


What you're describing seems a lot like torpor, which is a sort of middle ground between sleep and hibernation. There are plenty of animals which do that including mice, bats, and birds. https://en.wikipedia.org/wiki/Torpor

This article also explains the difference between sleep, torpor and hibernation: https://www.sciencenewsforstudents.org/article/explainer-how-brief-can-hibernation-be

"True hibernators might drop their metabolism by 94 percent or more. In contrast, Geiser notes, animals that experience daily torpor drop theirs by only some 65 percent."

  • $\begingroup$ Not sure why this question appeared in my recent question feed as it's over 3 years old. I'll check that date next time! $\endgroup$
    – David258
    Jun 18, 2020 at 9:27

You could have a creature that is solar powered. During the day, it uses sunlight to create and store chemical energy, which it then uses at night. During the day, you could have it just continue to use the same chemical process, or somehow directly use the sunlight for energy.

This is incredibly similar to something you may have had for breakfast today. Basically, it's photosynthesis.

Granted, it's not a switchable thermal cycle, but it serves the purpose, in my opinion.

  • $\begingroup$ Interesting idea. But to execute photosynthesis the being need to harvest minerals from the ground, so it should have roots, which are not really handy if it wants to wander around. $\endgroup$
    – L.Dutch
    Feb 2, 2017 at 8:06
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    $\begingroup$ And it’s also nowhere near enough power. And that doesn’t answer anything about its thermoregulation! Basking in the sun saves energy needed to warm itself, if it was in warm-blooded mode. $\endgroup$
    – JDługosz
    Feb 2, 2017 at 8:16
  • $\begingroup$ I didn't say it wouldn't also eat. Perhaps the occasional rock for lunch, to keep the minerals balanced. $\endgroup$
    – Carmi
    Feb 2, 2017 at 8:32
  • $\begingroup$ While an interesting idea this doesn't actually answer the question... $\endgroup$
    – Tim B
    Feb 2, 2017 at 10:23
  • $\begingroup$ Sorry, I'm new around here. What's the local etiquette? Should I delete the answer (since it doesn't really answer the question) or leave it up for the sake of the discussion it may generate? $\endgroup$
    – Carmi
    Feb 2, 2017 at 11:34

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