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I'll keep this one brief. I have an alien from a planet which has extremely cold nights and extremely hot days (the day and night cycle may be short or long, I haven't decided).

When the alien gets too cold, it becomes comatose, and its heart beat slows right down, like a strangled alligator. When the sun comes up, it gets heated up, and it returns to normal. In the day, its body focuses on trying to prevent overheating, and the creature spends a lot of its time going between caves to not waste energy in the heat. These creatures are pretty large, by the way, they weigh about four hundred pounds (at 1G gravity), and are terribly strong.

The question is, what organs and biology are best for this temperature control? Giant ears won't work so well with the dust storms, and they can't sweat because water is too precious and scarce, so I wondered if they'd have five hearts or something.

Perhaps more details are necessary?

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  • $\begingroup$ What is the environment? Sandy Sahara style desert? $\endgroup$ – Bellerophon Oct 20 '16 at 20:04
  • $\begingroup$ Mostly rocky and cavernous, with some deserts. $\endgroup$ – J. Doe Oct 20 '16 at 23:28
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Shutting your body down when it gets too cold is called torpor. It is a bit like hibernation but occurs daily, rather than for weeks at a time. All sorts of animals from lizards to hummingbirds go into torpor, so that's entirely plausible.

The main problem with torpor is that it normally occurs in small creatures. It takes energy to warm up again. Warming up a rat is like boiling enough water to fill a tea cup. Warming up a cow is like boiling enough water to fill a jacuzzi. So your alien will have to have high energy foods readily available to constantly replenish its energy supplies. Normally that means lots of fat or lots of sugar. Either it instantly snacks on nectar like a humming bird or bee. Or there is a better time of year where it can build up huge fat reserves and live off those in the bad times.

Brown fat (brown adipose tissue) is a special type of fat designed to generate lots of heat when it is metabolised. Human babies have lots of it becasue they are unable to shiver when they are first born. You creature should have lots of the alien equivalent of brown fat.

Fur/feathers, thick skin and a fat layer (white adipose tissue) can all act as insulation against both heat and cold.

Shedding heat. You mentioned giant ears. A structure like elephant ears, or the sail on the back of a Dimetrodon, are good for radiating away heat (or absorbing it when the sun comes up). But given the harsh nights, you'll need to make the structure retractable or able to roll up or fold away. Otherwise it'll shed too much heat at night AND get frost bite in all that exposed tissue.

Perhaps it has 'balloons' it can inflate, like a male hooded seal's 'hood', or something that unfurls like a manta ray's cephalic fins.

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  • $\begingroup$ This is a very well thought out answer. Thank you. The creatures do have extremely thick skin, tough, armoured, leathery skin, which as it happens is very heat resistant. I might give them ear-like frills that can fold up. The aliens are very carnivorous, but they could gorge themselves on prey then digest it for a week, like many predators do. The creatures probably should have a brown fat reserve they can burn to warm up quickly and to give them energy to hunt and return to their food stores. Thanks again, this is good material. $\endgroup$ – J. Doe Oct 23 '16 at 19:24
  • $\begingroup$ @J.Doe You may have to invents some new, non-Earth biochemistry for them if they are big carnivores in a desert. Earth biochemistry uses water to break the nitrogen bonds in protein. If water is in short supply, this is a problem. (It is also a problem when there is plenty of water, but it is all in the form of ice & snow, since eating ice will cool you down - hence polar bears eat the brains & blubber of seals, but hardly touch the meat). I don't know enough biochemistry to suggest a water-free way of breaking down the protein when digesting it. $\endgroup$ – DrBob Nov 1 '16 at 12:34
  • $\begingroup$ I think they should have a large enough source of water in underground caverns where the melted snow and rains collect. They'll have to make trips to known water sources, and drink a lot to store it. $\endgroup$ – J. Doe Nov 2 '16 at 1:12
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You can cool water down a few degrees by throwing common salt inside. Of course you can somehow inverse that effect - to gain warmth. Your Aliens could use that system to build a kind of fridge inside themselves. That way they cool during the day. They reverse that effect during the night. That would warm them up. They would loose some energy during this process but it could help them to keep some important body parts at a steady temperature. Also the energy they lose would warm you during the night.

There are many other chemical reactions you could use instead throwing salt into water. One good example is aggregate state change from solid to liquid. Which they use would depend upon the temperature they want to keep. If they have a normal body temperature of around 0°C for example they could use ice. It takes a lot of energy to melt ice.

Additionally they should have a good isolation. That would include fat and something like hair or plumes. Additionally to keep their warmth there should be as little circulation of fluids as possible. You want the warmth/cold to stay where it is and any liquid would change that.

Also since they want to keep their warmth and cold equally, they would want to consist of something with a high specific heat capacity - such as water. That would mean that it takes the sun a long time to heat them up and the night a long time to cool them down.

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  • $\begingroup$ Hmm, interesting points. I'll have to look into possible chemical reactions they might be able to use. The planet may have the substances necessary for such. I wonder if any animals use a system like this. For the creature's biology, they're very strong, with very thick, heat resistant hide, which may become very hot to the touch if they're in the sun for too long. Their insides could be plenty watery, though. $\endgroup$ – J. Doe Oct 21 '16 at 1:52
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During the Day

As @AndreiROM said, cold-bloodedness will help manage heat during the day most effectively. If your creature generates its own heat, it will very likely overheat and die quickly. Your idea of cave-dwelling seems ideal so that these creatures can spend more time gathering food and less time cooling down.

During the Night

Consider copying some physiological features of the North American Wood Frog. These creatures can survive for months being completely frozen by creating a supply of anti-freeze-like compounds and circulating them through their bodies' systems. This prevents ice damage; dormancy during the night would solve issues of being too cold to do tasks.

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  • $\begingroup$ In they're case, there isn't enough water they need to worry about freezing externally, but there is the possibility of their blood freezing, so having some anti freeze compounds may be a wise idea. $\endgroup$ – J. Doe Oct 21 '16 at 1:53
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I don't see how more hearts would make for more efficient temperature control. There are many creatures in our oceans which must stave off hypothermia, or oxygen deprivation, for example, and do so by managing their blood flow away from their peripheral systems and to their main organs (seals, etc.)

Such a fine-tuned circulatory system could reroute blood closer to the main organs at night, and more along the peripherals during the day, in order to shed as much extra heat as possible.

Another possible solution is having this thing be cold blooded. Snakes also "hibernate" when cold, and soak up heat in the sun.

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  • $\begingroup$ I was considering more hearts might be able to pump blood more efficiently, and so generate less heat when pumping. I also considered increased blood flow through extremities might be helpful, but I don't expect that is the case. Finally, the hearts would be able to generate a lot of heat if the creature was getting too cold. I don't know much about how blood vessels move in the body, but I know they do, so it may be best as you say to enhance that process. $\endgroup$ – J. Doe Oct 21 '16 at 1:46

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