I'm creating a planet inhabited by beings who have a body temperature that is colder than humans. What conditions on the planet would require them to have evolved this trait? Thanks

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    $\begingroup$ Lots of creatures right here on Earth have this already. $\endgroup$ – Kilisi Jan 25 '18 at 19:54
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    $\begingroup$ Yeah, every single cold blooded animal ever. I seem to recall a species of arctic fish that keeps antifreeze (or a biological equivalent) flowing in its circulatory system so its blood doesn't solidify. $\endgroup$ – Jakob Lovern Jan 25 '18 at 19:56
  • $\begingroup$ define colder than humans, 36 degrees, 20 degrees, 5 degrees? a few degrees requires no change at all. $\endgroup$ – John Jan 25 '18 at 21:36

Not much, actually

To start off with, let's just see if there's any creatures with a lower core body temperature (I'm probably gonna use that term a lot, so I'm gonna acronym it CBT.) than humans. According to 'Biology, Medicine, and Surgery of Elephants', elephants have a CBT of 96.8-98.6F (or 36-37C for anyone not living in the backwards imperial system.) There you go, a creature with a lower CBT than ours...ish. Sloths are even lower, with a CBT of between 80 and 93 degrees, according to wikipedia. I also found this riveting article from the scholarly journal Functional Ecology that discusses this exact question. Apparently, mass significantly impacts the CBT of some families of animal. I'd read through the article 4 or 5 times just to digest the highly scholastic language. Also take a look at some of its references. They seemed to contain some interesting (though not necessarily useful) information.

Or take most animals that aren't either birds or mammals. Backyard lizards, for example. They're cold-blooded animals, which means that they don't keep a consistent internal temperature. Boom, just make sure your world doesn't ever get up to 98.6F.

On the other hand:

There's a few good reasons that our CBT is where it's at (or near enough, we fluctuate constantly.) This article, which appears to be written by a hardcore phi conspiracist, claims that a lot of the important temperatures in our body are phi away from the freezing and boiling points of water. Interesting, but without a reason why1, we can't change it.

Let's disregard that for a moment. Our CBT is actually pretty critical to a lot of processes. The chemistry of a lot of the enzymes used in our body works such that they work best at... 98.6F. Some of them work better at fever temperatures, but that's an extreme case with immunological reasons. Specifically, the reason we're not living at the same temperature we do when we are sick is that most of our enzymes don't work as well so hot.

Possible solution: Handwave some alternative evolution in there. All good.

There's a good reason that the CBT is higher than the surrounding air: Movement. If we were isothermal with the environment, we'd overheat whenever we did... anything. (Fish and reptiles are different.2) Keeping ourselves comfortably above the environment temperature means that we can shunt off excess heat after strenuous exercise through sweating, etc.

What would you have to change?

To start with, you're not gonna have a human when you're done. Accept it and move on.

Solution 1: Lizardmen

According to numerous sources,dubious these already exist and they control the government. How would they work, though?

Cold blooded creatures stay at the same(ish) temperature as their surrounding environment. On the one hand, this means that they don't need to spend as much energy on maintaining a CBT. On the other... The metabolism of reptiles is a lot slower than that of mammals of equal size. Since they have to work over a greater range of temperatures, they can't work as efficiently, which means that they're not as active. Your lizardmen would move slowly, with quick bursts of speed.

Lizards spend a large portion of their day sunning themselves to increase their temperature. In part, this feels really good.3 More importantly, though, the increase in temperature speeds their metabolism and allows them to digest things faster.


  • Reduced resting metabolism: Because cold-blooded animals don't have to keep a stable temperature, they're able to survive much longer without food. A crocodile, for example, can survive six months without a meal.
  • IR invisibility: Warm-blooded animals show up pretty bright on thermal cameras. In contrast, since cold-blooded animals are the same temperature as their surroundings, they're harder to detect on IR cameras.
  • Longer (resting) anaerobic respiration: With a low metabolism comes a slow energy consumption. The internet places the lung capacity of a snake at 30 minutes of air.


  • Reduced speed and mobility: Most reptiles hunt with a 'sit-and-wait' strategy. Because they can't use energy as fast as mammals, they're not very good when it comes to running down their prey.
  • Reduced cranial mass: Most cold blooded animals... Aren't that smart. (Remember that velociraptors were dinosaurs, which are in a grey area.) The reasoning behind this is fairly simple: Brains take energy to run, and lots of it. In fact, an estimated 20% of human energy consumption goes to the brain. Reptiles, being inactive for most of their lives (sit and wait predators) simply need enough intelligence to carry out the four 'F's of life:

    • Fighting
    • Feeding
    • Fleeing
    • Mating

    All that brain stuff is extra.

Conclusion/Possible Evolution:

Lizardmen probably wouldn't have enough brain capacity to become the dominant species of your world. They'd be slow creatures who would be highly adept at building traps to catch their prey. Social interaction, if any ever developed, would likely be limited to necessary functions. If complex villages developed, then their leading government would either be some sort of tyrannical dictator system (dinosaur pun intended) or it would be a weeks-long village council system.

Social dynamics could be generated through sexually driven evolution (That's how a lot of the coolest and weirdest things we've seen are evolved: lyrebird vocal cords, peacock feathers, etc.) In essence, the most sociable lizardman gets all the mates.

As intelligence and sociability increase, prey catching ability would also have to increase to fuel it. This, in turn, would need to be backed by an increase in average body temperature. Since what you want is low-temperature creatures, they'd need to evolve in a geothermally active region and harvest the latent heat from that until some genius learns to make fire. If this active region was near a volcano that periodically erupted, you could make a case for highly fertile soil that yields crops.

Solution 2: Sloths

Body temperature is directly correlated with metabolism (to an extent,) which in turn is directly correlated with level of activity. Sloths have legendarily low activity levels, being slow enough that algae is able to grow on them.4 Anyways, sloths do have some some pretty cool superpowers. For one, they're able to increase and decrease their body temperature. (We can't really ask if it's at will, though. I'd expect that it's subconscious or maybe instinctual.) They're also able to hang on branches (yes, hang. They've got some pretty cool curved claws that wrap around it, and hunters have reportedly shot sloths dead, only to be disappointed as the corpses...stayed there. Slow even in death.

Sloths (being the ferocious leaf-rending predators they are) can't do much. Their level of activity, though, shows the lower limit for body temperature of warm blooded creatures. Worse yet, sloths aren't able to sustain higher thinking because they've gotta keep their metabolism low.

Enough of my sloth-bashing. How could we take them and make them a apex predator, while still retaining the low metabolism?

As with the solution involving lizardmen, slothmen would necessarily need to be patient people. High activity isn't really allowed when you're not using lots of energy.

Since sloths are herbivorous, I'd expect that they would make the jump to eating bugs first. A little extra protein in their leaves can only be beneficial, right? Sloths begin to create simple traps to grab (conveniently highly nutritious) gnat-like insects. Simple in this case is probably something like 'rub your algae covered fur over a leaf and bugs get stuck in it; you can then eat the leaf for extra food.' Trap making like this isn't all that hard to evolve: look at how successful spiders are!

Once a source of energy greater than leaves has been established, you can begin working on brain capacity. Wikipedia says that sloths have low visual acuity, so building the brain really needs to be based on other senses.

(I'm gonna handwave that away, though.)

Once the pathway towards higher brain function starts, it'll snowball as the benefits pile up. You'll have to be careful, though, because the larger your slothman's brain is, the more energy it'll use. I'd take a cue from the lizardman section and begin to lay ever larger traps, catching increasingly nutritious prey. I know I've really been stressing the idea of 'trap your food,' but it's really the most efficient way of attaining energy. The slothman has to set up the trap once per kill, and it's much more energy-efficient to trap your food than to run it down.

Even food trapping can lead to more and more intelligence. If there were another species of animal that the slothman hunted, then you could build in an evolutionary arms race as the slothman got better at trapping and the prey got better at escaping. Well... Until the slothman begins using too much energy.

Solution 3: Alternative Biological Basis

Note that, for the most part, most people aren't aware of a lot of these facts. A spoonful of handwavium can put your creatures' temperature at wherever you feel like. Careful not to put too much in, though.

Using unobtanium instead, you can rebuild basic biology from scratch. If your creatures weren't carbon/water based (There's plenty on worldbuilding.SE you can use to research this) then only a few experts would be able to effectively argue against your biological setup. Unfortunately, I'm not one of them.

I'd probably start by seeding your world with a non-natural composition. I'd theorize that carbon, being the lightest element with 4 valence electrons, was most common in early Earth. If, instead, the world had large amounts of silicon or sulfur... Well, all bets are off at this point. Honestly, if you felt like it, you could rebuild chemistry itself by substituting electrons for muons. There's not enough research in this area, though, so be careful. Very careful


  • Highly mutable biology means that you can do basically whatever you want.


  • Might not be physically accurate in the least.

Final conclusion

While it wouldn't be hard to shift a creature's CBT by a few degrees (You do this every day, even!) it's not likely that you would be able to maintain normal human physiology and reduce it much lower. Switching to an ectothermic environment would allow you to reduce it to the level of your environmental temperature, but it might risk making your creatures too difficult to connect with. Keeping in a warm-blooded type of animal risks breaking the budget as it increases in intelligence. Building an alternate biology would be a huge amount of work, but it would afford you a huge amount of leeway in terms of structure.


  1. As with phi-related things, it's often evolutionary advantage that converges to that point. Phi is sort of a 'local maximum' for a lot of different processes. An... interesting... three-part youtube set by Vi Hart on the subject can be found here, here, and here.

  2. Again unverified, but there seems to be a link between size and heat regulation. All the large (and living) animals I can think of are mammals. Snakes can grow pretty long, but in the end, they're still not as thick around as a cow. Even whales are mammalian. There's significant debate over whether dinosaurs were warm- or cold-blooded, but it looks like they did at least some heat generation.

  3. Ok, I can't actually speak for reptiles on this point, but have you ever laid down in front of a window when there's a bunch of sunlight streaming in and pulled out a book and just soaked in the sun? Yeah, exactly.

  4. On the subject of sloth algae... Apparently, its not just algae that grows on them. Every single sloth in existence is its own ecosystem. There's a ton of collaborative and parasitic critters that live in its fur.

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    $\begingroup$ While "I read it on the Internet" certainly isn't good enough for an actual citation, you can improve your references considerably by simply linking to the sources you found, thus allowing the reader to judge the credibility of the sources and their claims... $\endgroup$ – user Jan 25 '18 at 21:20
  • $\begingroup$ Sloths find a middle ground, being self regulating but lazy about it, tolerating 10's of degrees F variation. $\endgroup$ – user25818 Jan 25 '18 at 21:46
  • $\begingroup$ I'll go ahead and do a bunch of research on this new information, then implement it. Thanks for the tip! $\endgroup$ – Jakob Lovern Jan 25 '18 at 22:07
  • $\begingroup$ This is a great and in-depth answer, but it is a little snarky for my tastes. $\endgroup$ – kingledion Jan 26 '18 at 2:06

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