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Aliens are very rarely portrayed as being furry, at least in proper xenobiology worldbuilding. They're virtually never depicted with feathers, and only sometimes with scales.

Currently, there are three known integument structures which endothermic animals are known to use or have used; hair (in mammals and some other therapsids), feathers (in dinosaurs) and pycnofibers (in various archosaurs). Cicadas are also endothermic, and they insulate themselves with setae, but I guess that falls under the hair category. Scaly dinosaurs are thought to have used scales to trap heat, but that's not exactly confirmed.

There are endothermic animals which have no hair, feathers or any kind of filamentous integument - such as cetaceans, tuna, and mosasaurs, but they're all aquatic, and since heat works slightly differently in water, I'm going to leave that aside for now. Moving onto hypothetical forms of thermoregulation, I could see three-layered exoskeletons (refrigerated by tracheae), skin folds, air-filled shells, and radiators such as dewlaps working, but, here is my rather difficult question;

Could a land animal with naked skin keep warm and cool down by its own means without radiators? Perhaps insulation with fat and fat alone would work, but how would it cool down without skin folds or heat radiators? Would sweating work, or would it be too water-wasting?

Edit: For further clarification; fire, clothing, frequent bathing, and only inhabiting warm areas are also not allowed.

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    $\begingroup$ The answer "yes; humans -- they're animals, too -- have been doing it for 200,000 years" is so obvious that your question seems flawed. $\endgroup$ – RonJohn Oct 1 '18 at 18:37
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    $\begingroup$ Just because this seems like cheating, do you include simple clothing in that 'etc.'? Could the animals stick cotton/other critter's pelts to their bodies for warmth, or large leaves for shade to stay cool? $\endgroup$ – Giter Oct 1 '18 at 18:37
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    $\begingroup$ @Giter: People in their natural state don't wear clothes if the climate is warm enough... South American and Australian aborigines come to mind, as well as African pygmies. $\endgroup$ – AlexP Oct 1 '18 at 20:12
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    $\begingroup$ @AlexP: The question seems to be about keeping both warm and cool, so the hypothetical animal might have to worry about overheating. In 120f/48c degree temperatures, there's only so many clothes a person can take off. $\endgroup$ – Giter Oct 1 '18 at 20:25
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    $\begingroup$ @SealBoi Does this question only deal with species that do not have modern technology to maintain temperature? If so, that would invalidate the answers that give humans as an answer. $\endgroup$ – John Locke Oct 5 '18 at 13:31
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So several person have mentioned the obvious fact that members of Homo sapens, and perhaps members of other closely related species, have been mostly hairless for hundreds of thousands of yeas at least.

But they have omitted mentioning that members of some endothermic land dwelling species have also lost so much of their hair that it plays no role in temperature regulation.

Species of elephants, rhinos, and hippos that dwell in tropical regions have lost so much of their hair that what is left has no ability to help regulate temperature.

Note that some related species that lived in temperate or arctic regions retained dense coats of fur, like the wooly rhino and the wooly mammoth.

It is certainly possible that if the tropics of a planet are warm enough, mammal equivalents a lot smaller than hippos, rhinos, and elephants might also lose their hair equivalents.

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    $\begingroup$ I'd say that naked mole-rats qualify for "a lot smaller than elephants". $\endgroup$ – AlexP Oct 1 '18 at 20:10
  • $\begingroup$ Elephants do use their ears as radiating surfaces for temperature regulation, however. $\endgroup$ – Thucydides Oct 1 '18 at 23:34
  • $\begingroup$ Rhino are your best example, but they all still have hair just not as much, and their young are downright hairy. $\endgroup$ – John Oct 2 '18 at 2:01
  • $\begingroup$ Elephants have heat radiators and skin folds, hippos have this kind of sun screen thing, and all three spend much time cooling off in water/mud baths. $\endgroup$ – SealBoi Oct 2 '18 at 7:21
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While the answer "humans have no hair either" seems to be a common thread, I suspect the OP is really asking for ways a creature could do thermoregulation without external surfaces:

Could a land animal with naked skin keep warm and cool down by its own means without radiators? Perhaps insulation with fat and fat alone would work, but how would it cool down without skin folds or heat radiators? Would sweating work, or would it be too water-wasting?

Elephants use their ears as external radiators, so that is out.

Although this may not fully satisfy the OP's intent, I would suggest that the way birds breath is a good way to fulfill the requirement. Birds have very complex lungs, and their body is filled with air sacks that serve to both lighten the bird's skeleton and increase the surface areas of the lungs to facilitate the most efficient air exchange.

enter image description here

Sample Avian respiratory system

This massive surface area is also filled with a network fo capillaries, providing a huge surface area for heat transfer to occur as well. The birds effectively carry their radiators internally.

Now in order to make this work more closely to the OP's question, the creature may evolve a more granular control of the mechanism. During cool days or when activity is limited, the creature can regulate the flow of blood to the air sacks, or there may even be sphincters or valves to limit the amount of airflow in and out of the air sacks.

Warming is a different matter. In an endotherm, the issue is heat being generated from within, and in the cold, you either eat more calories to ramp up the metabolism, or shut down and hibernate. If the climate is very extreme, the creature might evolve a large and elaborate sinus system to pre-warm and moisten the air before it is brought into the lungs. Although unlikely, it may be possible to have "air to air" heat exchangers in the form of galleries inside the sinuses where warm exhaled air is used to prewarm inhaled breaths.

The animal may have a large protruding nose, or perhaps a crest like some forms of Hadrosaurids for air to flow through while being conditioned.

enter image description here

Hadrosaurid skulls: two species of Parasaurolophus

So if we take the OP's meaning to have a hairless animal with no visible or external radiating surfaces, then the trick is to expand the lungs and respiratory system to take the role instead.

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There are endothermic animals which have no hair, feathers or any kind of filamentous integument - such as cetaceans, tuna, and mosasaurs, but they're all aquatic, and since heat works slightly differently in water, I'm going to leave that aside for now. Moving onto hypothetical forms of thermoregulation, I could see three-layered exoskeletons (refrigerated by tracheae), skin folds, air-filled shells, and radiators such as dewlaps working, but, here is my rather difficult question;

Could a land animal with naked skin keep warm and cool down by its own means without radiators?

Look in the mirror for your answer. Or at a picture of Dwayne "The Rock" Johnson.

No hair, and he survives just fine. You don't have much hair, and you survive just fine, too. In fact, all humans -- manifestly land animals -- are pretty darned hairless, and we survive just fine.

enter image description here

And for those of you who think that humans require clothes, fire and other technologies to keep their body temperatures regulated... tribesmen from Papua New Guinea.

enter image description here

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  • $\begingroup$ +1 simply for the inclusion of that picture. $\endgroup$ – Joe Bloggs Oct 2 '18 at 6:36
  • $\begingroup$ The asker has specified that "[The] question is referring to species without technology." (see the comment on the question) This answer specifically uses humans as an example, and because humans have developed technology such as heating/AC as well as clothing to protect them from the elements, this is not a valid answer. $\endgroup$ – John Locke Oct 5 '18 at 16:49
  • $\begingroup$ @JohnLocke shall I change the picture to one showing the mostly hairless tribesman of Papua New Guinea who wear nothing but grass skirts? $\endgroup$ – RonJohn Oct 5 '18 at 16:54
  • $\begingroup$ @RonJohn Clothing is an example of technology for protection from the elements, so I don't think the answer would be applicable either way. $\endgroup$ – John Locke Oct 5 '18 at 17:50
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Humans have evolved to become increasingly hairless due to the fact that technological advances mean that we no longer need it. If we are discovering other species of aliens, then they are likely sufficiently technologically advanced that they have no need for hair or feathers to regulate temperature.

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    $\begingroup$ We have lost our fur way earlier than we have developed civilization. $\endgroup$ – L.Dutch Oct 1 '18 at 19:01

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