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It's a fairly well-known fact that reptiles are ecotherms, meaning that they require outside sources of heat to stay warm and metabolize. However, being too hot can cause heat sickness (not fun), and being too cold can trigger hypothermic states where metabolisms slow down, and movement becomes difficult.

Now, size and shape affect how well creatures retain heat, which also influences how much heat they retain and how well they can retain it. Since the creatures in question are human-sized, humanoid creatures, how would you determine the upper and lower temperature limits?


Related: This question is similar in that is discusses temperature ranges, but it's looking more for answers on how to change the range of thermoregulation for endotherms, rather than figuring out the existing range for ecotherms. This question is also similar, but assumes the knowledge of temperature ranges for their creatures. I am looking to find those numbers.

This is a graduated question from the Sandbox

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    $\begingroup$ I think this might help you en.wikipedia.org/wiki/Komodo_dragon if you need a human sized reptile. $\endgroup$ – shieldedtulip Oct 5 '17 at 0:29
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    $\begingroup$ Incoming heat is received through your skin, the surface of your body. One possible approach would be to assume that the temperature flux must remain constant. So you'd take the temperature range for some reptile and divide it by the approximate surface area of the reptile. That would get you a flux range. Then, to get the temperature range for a reptile of any other size, you simply multiply the flux range by your reptile's surface area. I don't know enough biology to say that this assumption would be correct, though. $\endgroup$ – Phiteros Oct 5 '17 at 0:33
  • $\begingroup$ @Phiteros There's surface area to consider, but there's also the total mass and shape of that mass to keep in mind since taller, thinner creatures cool off faster than shorter, rounder ones. $\endgroup$ – Pleiades Oct 5 '17 at 1:45
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    $\begingroup$ @Pleiades Well that's in part because the taller, thinner creatures have a larger surface area than the shorter, rounder ones. But I agree, there are certainly more variables to consider. I think my method might work as a very rough first-order approximation though. $\endgroup$ – Phiteros Oct 5 '17 at 1:46
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    $\begingroup$ American alligators are the most cold tolerant of modern large "reptiles" and can tolerate temperatures as low as 7 degrees celsius for long periods with no apparent discomfort. . $\endgroup$ – John Oct 5 '17 at 3:01
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I suppose the lower limit would be somewhere around the freezing point, where ice formation would cause tissue damage. But the reptile would be very, very comatose. If you want the creature to be functional, then it must be remembered that even reptiles generate heat from muscle activity. If the reptile had an unlimited food source, or huge amounts of internally stored energy, I can imagine that it could be active through internally generated heat right down to tissue damage. Just because an organism is an ecotherm, doesn't mean it can't store energy (fat) or have a thick layer of insulation. Being an ecotherm just means it doesn't have to generate heat just for the sake of generating heat. However, it also means that it can not maintain optimum efficiency for optimal performance. Ecotherms are EXPECTED to have a broad range of functionality and performance.

The upper limit, of course, would depend on the reptile's ability to release heat to the environment. Some reptiles have fin structures that they can open up, like fan radiators, to disperse heat. Again, just because they are an ecotherm does not mean they can not have mechanisms to disperse excess heat. Their blood can still move heat to the extremities. The upper absolute limit, I suppose, would be their bodily fluids boiling. The next limit would be that temperature at which their tissues actually bake (or, more specifically, their tissues going through chemical transformations due to the application of heat).

The complexity of the answer is compounded if you make the reptiles sentient, and aware of their surroundings, so they can actively take measures to seek heat sources or to cool their bodies.

I have this vision of a frog wearing a sweater.

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    $\begingroup$ A frog breathes through its skin when above water so putting on a sweater may kill it :( $\endgroup$ – theindigamer Oct 6 '17 at 11:47
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    $\begingroup$ @theindigamer Humph. No hand-knitted sweaters from Grandma Frog at Christmas, then. $\endgroup$ – Justin Thyme Oct 6 '17 at 15:53
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The simplest way would be to look at reptiles that roughly approximate humans in size... alligators, crocodiles, larger turtles or monitor lizards. All tend to be found in tropical or semi-tropical environments, as the very large reptiles are not known to hibernate during cold periods.

In general, the larger the reptile, the warmer the climate it lives in... very large snakes are also restricted to tropical climates. It appears that their large muscles need an average temperature not far off what warm blooded creatures maintain.

The fossils of an enormous snake dubbed Titanoboa, reaching 50 feet in length, have been found and dated to the Paleocene time frame, when the climate of the earth was known to be warmer than it is today, averaging around 90 degrees F, presumably hotter in the equatorial areas.

There is one advantage to being cold blooded that you might factor into your human reptiles: because they don't burn off energy maintaining body temperature, their food needs are considerably lower.

At the same time, you can consider two fish that are semi-warm blood, in that they maintain a body temperature around 10-15 degrees higher than ambient temperatures... the great white shark and bluefin tuna. Both are very powerful creatures, the bluefin tuna can achieve speeds of around 50 mph. Both are also very large, and tend to be found in cooler waters. It is possible that your reptile humans could make this same adaptation.

Curiously enough, this adaptation has not been seen in reptiles.

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  • $\begingroup$ It is actually seen in leatherback turtles. $\endgroup$ – Pelle Lundkvist Oct 10 '17 at 23:05
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If you look at modern lizards for comparison then have about a 15-20 degree celsius temprature range they are active in, this is a bell curve so activity drops off on the ends. Now an intelligent species would be on the wider scale since they can apply technology.

Now you have a bit of play in where this range sits, in colder adapted reptile it may start as low as 15 degrees, in warmer adapted ones temperatures up to 45 degrees are acceptable. So you have to ask yourself what kind of climate are my creatures adapted to cold or hot.

You also have a small issue with the word "reptile", Crocodillians in general handle a wider range of temperatures than most other "reptiles". The term "reptile" can be applied to endothermic organisms, so you should decide if your creatures are actual ectotherms or not.

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There's way more to this than size and shape

First, as for warm-blooded, it's not just warm-blooded and cold-blooded.

The real root and determinate factor to this question isn't size and shape but a species' genetic adaptations.

Metabolism (/ecotherms as you referenced) is but one genetic adaptation a species has selected as a response to the thermal challenges of its environment. Metabolism can be handled in many ways on a chemical level. There are plenty of other adaptations that can be employed in response to this challenge, either singularly or in combination.

  • Hair can add varying levels of insulation to resist the cold. Density, construction, disposability of it can all variably impact the response to cold. Even color can be used dispense excess heat.
  • Sweat can lower an organisms body temperature in response to heat, though this requires access to water.
  • Hibernation. Some animals just hibernate when it gets too cold. Some frogs (amphibians) allow themselves to be frozen in ice to pseudo hibernate and thaw out in the spring. Saharan crocodiles bury themselves in mud pits during the dry season and enter a state similar to hibernation.

Your reptile could have unique adaptations that meet this challenge similarly to the above but still appear reptilian. For instance it could have insulating scales that trap heat in, allowing it to survive colder temperatures. It could sweat to reduce its temperature. It could have reflective scales to reflect excess sunlight. It could store fat and become warm-blooded if it senses it has become too cold. It could just hibernate and encase itself in ice.

The bottom line is these limits you want are governed purely by the genetic adaptations of your species and not just its size and shape.

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With the right biochemistry the lower limit is frozen solid, thawed and carried on, but that's a "life limit" not an activity limit, modern crocodiles and alligators are about the biggest reptiles on Earth they remain relatively active down to about 13 degrees Celsius but they stop feeding at as high as 21 degrees. I believe 56 degrees is the point at which most reptiles have to seek shelter or they risk organ damage but I can't think where I heard that number.

So those temperatures could be a guideline for you, I would think that an upright biped would in fact suffer greater complications at both high and low environmental temperatures, the human head is more exposed relative to the body than it would be in a quadrupedal body form so at low temperatures it suffers greater heat lose but as it is at the top of the body and heat rises it would also cause issues at high temperatures.

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Instead of searching for an answer based on size, which habitats/climates would fit your story best? There is actually plenty of possibilities of adaptations to temperature within reptiles of the size your looking for. I will mostly look at the lower temperature border in this answer.

Have you considered mesothermy? Mesotherms are animals who because of high metabolic rates generate enough heat to have a constant body temperature despite changing surrounding temperature. One extant example is the leatherback turtle, who has even been found within the Arctic Circle! High muscular activity, counter-current heat exchange, thick fat covering and incredibly something similar to brown fat tissue found in mammals for generating heat without shivering.

But even more extreme were Dinosaurs! There has been a lot of debate about the metabolism of dinosaurs and some scientists believe that some speceis were in fact warm-blooded. You can find a lot about that in this exhausting wikipedia article. Some dinosaurs species have lived in very cold conditions and the article gives some temperatures:

Dinosaur fossils have been found in regions that were close to the poles at the relevant times, notably [...] the North Slope of Alaska [...], so polar dinosaurs and the rest of these ecosystems would have had to cope with the same extreme variation of day length through the year that occurs at similar latitudes today (up to a full day with no darkness in summer, and a full day with no sunlight in winter).

Studies of fossilized vegetation suggest that the Alaska North Slope had a maximum temperature of 13 °C (55 °F) and a minimum temperature of 2 °C (36 °F) to 8 °C (46 °F) in the last 35 million years of the Cretaceous (slightly cooler than Portland, Oregon but slightly warmer than Calgary, Alberta).

The possibility of migration is disregarded:

But a round trip between there and Montana would probably have used more energy than a cold-blooded land vertebrate produces in a year; in other words the Alaskan dinosaurs would have to be warm-blooded, irrespective of whether they migrated or stayed for the winter.

So to answer your question, maybe something around 2 °C (36 °F) can be the lower limit for year round human-sized reptiles. With hibernation you can even have it colder: the farthest north extant terrestrial reptile is the Viviparous lizard although you will have size-constrains for your human-sized reptiles...

For the upper limit, similar approaches are possible. If you want your species to live in hot deserts, think about heat exchangers like the neural spines of Spinosaurus, bright skin colour, behaviour adaptations like night activity or borrowing/living in caves. Compare for example here.

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