I need my race to be highly affected by climate and nature changes. And they are more like a reptile - Humanoid. So making them Cold blood seems good for me.

But they are an intelligent race too. They live like our ancient civilizations: hunter-gatherers. They have their own language, country, religion, myths, and all.

I know that cold blood will affect metabolism, and our brain is something that required a lot of energy. So how can it make possible for a cold-blood race to have such an intelligence level?

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    $\begingroup$ Hello Sangeetha, welcome to WB. Your question is very broad and violates several of the Help Center's restrictions (see help center and help center). Do you have a specific question? Why can't you simply have intelligent lizards in your world, just as Star Trek did? (If you're asking if intelligent lizards can exist in real life, they don't, and it's impossible to predict the evolutionary future... but nothing's stopping you from having intelligent lizards.) $\endgroup$
    – JBH
    Commented Jun 13, 2022 at 6:07
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    $\begingroup$ "Cold blooded" simply means that the creature lacks a mechanism to maintain a constant internal temperature. Cold blooded creatures by definition have very little endurance, because they overheat and die. A human brain uses about 25 watts, which somehow needs to be dissipated; humans can do it, because as warm blooded creatures we do have a mechanism to keep our internal temperature constant. $\endgroup$
    – AlexP
    Commented Jun 13, 2022 at 10:32
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    $\begingroup$ Biologists no longer use the term, not only because they can be warm, but because they often have mechanisms that do affect their body temperature, just not so much that it does not vary regularly. $\endgroup$
    – Mary
    Commented Jun 13, 2022 at 12:20
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    $\begingroup$ Octopodes (octopuses, octopi, whichever) are cold-blooded, and among the most intelligent creatures in the sea. Certainly the most intelligent invertebrates. (Dolphins and whales may be smarter, but it's hard to measure that sort of thing.) $\endgroup$ Commented Jun 13, 2022 at 18:44
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    $\begingroup$ @AlexP overheating is typically related to how good the cooling mechanisms are, which is independent of whether there is an internal heating mechanism. $\endgroup$
    – toolforger
    Commented Jun 14, 2022 at 11:06

6 Answers 6


Their brains would be in low-power mode when cold and only reach the full intelligence when warm. They would operate mostly on instinct in low-power mode, just like real reptiles. But when heated by the sun, they could be as smart as a human. Maybe they can go even smarter when needed, at the risk of overheating.

This can make for fun cultural habits. Chess gets a new layer of strategy where you try to stall until the shadow of a tree falls on your opponent. Kids pack a thermos full of hot tea for exams. Discussion of the weather gets another dimension. "Man, it was so cold yesterday I was too dumb to remember my wife's name."

  • 44
    $\begingroup$ Not to mention a fun variant of the insanity plea: "Your honor, the defendant committed this act because he was cold." $\endgroup$
    – PipperChip
    Commented Jun 13, 2022 at 16:30
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    $\begingroup$ @PipperChip "CSI Miami's Horatio Gator": You can say this was... [Puts on Sunglasses] a cold-blood murder... YEAAAAAAAAWWWW $\endgroup$ Commented Jun 13, 2022 at 21:50
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    $\begingroup$ I'm reminded of the Trolls in Terry Pratchett's Discworld novels, which have silicon brains and (like computers) tend to function better in cold environments. One Troll character gets locked in a freezer at one point and becomes a genius while slowly freezing to death.. Trolls generally live on mountains for this reason, and the ones that come down to warmer climates get very very stupid. $\endgroup$
    – Ruadhan
    Commented Jun 14, 2022 at 11:54
  • $\begingroup$ there is no such thing as low power mode for a brain, a given mass brain's energy usage barely changes no matter what. $\endgroup$
    – John
    Commented Sep 22, 2023 at 0:33

Cold-blooded is not what you think

There is essentially endotherm and ectotherm.

Endothermic organisms have internal heat sources, ectothermic rely on external ones.

Brain is a function of available power, not of body temperature. If the brain cells are built to work at low temperature given enough chemical energy, they don't dumb down at low temperatures.

Various world-building consequences from these premises:

  • These reptiles come from a region where there's little temperature change, so developing internal heat sources was less of an evolutionary advantage than developing a better brain.
  • They could develop in an area with temperature change. Many ectotherm animals still regulate their body temperature via behaviour: level of body activity, seeking out warmer (or colder) places would be the options that come to mind. A better brain might help with locating and successfully devending hot spots - more so if the spots are small and you need to compete (low-temperature planet with volcanic activity?), even more so if the spots are large enough to suit a group but not everyone (social interaction is a massive intelligence booster: you need to predict what the other person will be doing to make cooperation work, and the smarter the average of your friends are, the more brain power you need to predict them - that's a never-ending spiral).
  • Once you have something like a civilisation, wealth will be defined by your ability to be warm. The poor ones will be given just enough heat to be able to work, kings, officials, and leaders will always have enough warmth to function. You'll have a pretty literal implementation of "dumb masses". Now as heat sources become cheaper due to technological advances, some societies will decide to keep the masses dumb, others will distribute warmth more evenly and have more distribution conflicts but in the long term, those societies that manage to solve the distribution conflicts in a peaceful manner will get a small but steadily growing technological advantage (with all the shenanigans of brute-but-strong-military-wise empires attacking the smarter ones to get their technology, smart scientists being demoted to stupidity because they're now subservient). Lots of potential for social conflict, and the storytelling that comes with it.
  • $\begingroup$ "Brain is a function of available power" Doesn't the metabolism that provides that power slow down, too? (Though human brains use only a fraction of the energy the body can provide. So it might not be a problem.) $\endgroup$
    – Caesar
    Commented Jun 14, 2022 at 5:42
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    $\begingroup$ @Caesar I was trying to make the point that the energy to keep the brain at efficiency could be chemical instead of heat. It would likely be a different cell chemistry, so that's a bit flaky... to avoid that route, one could also make the brain endothermic without making the rest ectothermic, mixed-thermic organisms do exist on Earth after all. $\endgroup$
    – toolforger
    Commented Jun 14, 2022 at 7:23
  • $\begingroup$ I don't believe endotherm and ectotherm are the real difference; all organisms generate heat through metabolic processes and are all influenced by external heat sources. Rather, 'warm blooded' creatures have biological processes that are well tuned for a small range of temperatures (and thus have mechanisms to keep the body temperature within that range); 'cold blooded' creatures have looser biologic processes that work acceptably well over a much larger range (and thus lack those potentially expensive temperature regulation processes). $\endgroup$
    – poncho
    Commented Jun 16, 2022 at 12:47
  • $\begingroup$ @poncho Well, you can look it up in Wikipedia and find it's the terminology used in biology. I don't know the details behind the decision to use these terms (and not others), but my best guess is that endothermic organisms have significant mechanisms specifically for generating heat (e.g. shivering), while endothermic ones do not. As always in biology, you'll find organisms in-between the extremes, but that does not make the classification completely useless. $\endgroup$
    – toolforger
    Commented Jun 16, 2022 at 17:17
  • $\begingroup$ @Caesar Actually, if I have heard correctly, the human brain uses a large part of the power the body provides. I've heard numbers in the 30 percent range. So quitte significant. $\endgroup$ Commented Jun 17, 2022 at 7:14


A large brain requires a lot of energy. It doesn't make sense for a cold-blooded creature to have one, since the point of being cold blooded is to conserve energy. This is why snakes and lizards only eat every few weeks rather than every few days.

However it sounds like you only want the drawbacks of cold-bloodedness and not the benefits. So there is no inherent problem here. Except it is hard to imagine why your species would evolve in the first place. I leave that part to your imagination.

So you have a species that is smart like a person, scaly like a reptile, functions poorly (but does not die) in the cold. But still needs to eat every few days.

They eat less than people since they don't waste energy heating their body, but more than lizards because they still use energy maintaining the big brain.

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    $\begingroup$ One could conserve energy exactly to expend it on the brain rather than the heat. $\endgroup$
    – Mary
    Commented Jun 14, 2022 at 0:15

Body Temperature and Intelligence

It is entirely feasible that the pressures to have intelligent, tool using, social creatures can happen with cold blooded as well as warm blooded. It just hasn't really happened on earth (to my knowledge).

We are world building SE, though, so I am going to take my shot at making this plausible. IMHO, we just don't know enough about the genesis of our own species to know all the ramifications of fiddling with biology and ecology to know for sure how things will change.

A Plausible Path: Tree-Dwelling Lizards

Tree dwelling, generalist lizards could have taken similar paths as those bipedal mammals did. This includes thumbs for grasping, social behavior, tool use, moving out of trees... All those steps to get from a rat to a human could potentially have happened for a lizard to lizard-man with some changes.

A big change I recommend is raising the temperature of their world. This makes their cold bloodedness a benefit. Hypothermia still happens, just not as easily or often as it would here, and plenty of environments are still deadly without fire and tool use.

Additionally, there may be greater emphasis on trapping than active hunting. Endurance hunting may not be their cup of tea, as they will likely not evolve sweat. Trap making is much more lazy way of getting a meal, and ambush hunting may work for them, too!

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    $\begingroup$ I think a smaller variation in temperature would make cold blooded intelligence more feasible. Have a warm planet with a strong greenhouse effect and little to no axial tilt (so seasons are much less pronounced or non existent). Day and night differences are minimal as heat from the sun is easily trapped in the atmosphere and surface where it then radiates back out. Our lizard aliens are still a little more sluggish at night, but most of the impact can be easily mitigated with clothing and heat sources. $\endgroup$ Commented Jun 13, 2022 at 17:05
  • $\begingroup$ @Crazymoomin Very Yes! As anyone who has read the Mistborn trilogy has seen, getting the correct amount of heat is a tricky process... Which is why I simply left it at "raising the temperature". $\endgroup$
    – PipperChip
    Commented Jun 13, 2022 at 18:34
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    $\begingroup$ @Crazymoomin Maybe lots of geothermal energy and a short day/night cycle could keep the temperature more consistent, too. If surface temperature is very consistent, then it would make sense not to develop thermoregulation, because why would you develop something you don't need? Probably every life form on the planet is cold blooded. Then if something changes so they start getting temperature fluctuations, it would be very dramatic for the whole planet. $\endgroup$ Commented Jun 16, 2022 at 14:58

As an interesting side effect, they brains would be able to warm up on they own while thinking, because the working brain consumes significant energy. Hence, the wise proverb for them would be, "after good thinking, think again!"

  • $\begingroup$ Interesting idea, and I like the proverb. But this feels more like a comment than an answer. $\endgroup$
    – Llewellyn
    Commented Jun 15, 2022 at 20:33

The energy your brain uses doesn't relate to intelligence

Firstly, the human brain isn't the biggest user of energy. Estimates put it at 20-25%. That's significant, sure, but it's not the biggest thing compared to our muscles.

In children it can be up to 60% (same link), but we don't say that children are necessarily more intelligent. And there are other animals with similar relative energy usage in the brain to humans.

Even on an absolute level, women typically expend less calories in the brain than men, but women are not less intelligent than men.

Brain structures are basically the same

Lizards certainly tend to work by instinct - but so do most wild animals. A study shows that there isn't a fundamental difference between the structure of lizard brains and other animals, only a difference in the degree of development. This means there's no obstacle at all to random mutations allowing evolution to take this path, providing there's an ecological niche which could be filled by a smarter lizard.

Why is "powering down" at night a problem?

Sure, lizards may snooze when the sun goes down. So do most mammals and birds though. Humans certainly do (or at least will do naturally). This isn't an indicator of intelligence either. And in warmer conditions, reptiles often operate at all times of day - in fact many species are at least crepuscular or fully nocturnal. If you can't regulate your internal temperature, too much heat can be worse for you than too little.

Basically, I'm afraid you have a number of wrong assumptions which are going to screw up your world-building. If you want to world-build with intelligent reptiles, you may need to learn more about both reptiles and intelligence first.

  • $\begingroup$ the energy your brain uses is determined by mass and density which is directly tied to intelligence. 20-25% is the biggest when you consider it makes up less than 2% of a persons mass. powering down is an issue since the brain needs constant energy, sleep doesn't really reduce the energy needs of the brain. growing a brain is even more calorically inessive which is why energy usage goes up for children. large brains are just ridiculously expensive organs. $\endgroup$
    – John
    Commented Sep 22, 2023 at 0:40
  • $\begingroup$ @John I gave links to evidence that all your claims are incorrect. In particular, your assumption that mass or density are an indicator of intelligence is simply not true. The most obvious way to disprove that claim is that women are not less intelligent than men. :) $\endgroup$
    – Graham
    Commented Sep 23, 2023 at 7:23
  • $\begingroup$ @John And as one link says, we have the same relative energy usage in our brain as a treeshrew. They aren't as intelligent as us, so clearly there's more to it. $\endgroup$
    – Graham
    Commented Sep 23, 2023 at 7:34
  • $\begingroup$ correct there is more too it, brain size scales allometrically so smaller animals need proportionally larger brains to get the same function and as you may notice tree shrews are tiny. to be as smart as a human a tree shrew would need a vastly larger brain than they have. Its not absolute mass it proportional mass, women expend fewer calories because their overall body mass is smaller thus the brain is smaller but the proportion mass is the same. $\endgroup$
    – John
    Commented Sep 23, 2023 at 11:47
  • $\begingroup$ Also please reread my claim, mass and density are indicators of how much energy a brain consumes, percentage, that is proportional mass and density are indicators of intelligence. your own links support me here. your third link states both my claims. read the subsection titled "the hungry brain" this is also why powering down at night is a problem, the brain doesn't need less energy when you sleep so the body needs to supply that energy 24/7 $\endgroup$
    – John
    Commented Sep 23, 2023 at 11:51

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