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Technological approaches to refrigeration tend to involve either high compression ratios of fairly exotic (from a biochemical point of view, anyway) volatile substances, or high-power, low-efficiency solid state electronics.

Neither of these seem particularly practical paths for a living organism to take for lowering its temperature.

What earthling biology has come up with is the equivalent of a swamp cooler--i.e., sweat--which dumps heat into the energy of vaporization of a consumable volatile substance (water). But if the air is already saturated, or you get dehydrated, you're screwed--a swamp cooler or sweat gland can't function anymore.

So, is there a plausible mechanism by which an organism could evolve to actively pump heat into the environment (i.e., refrigerate itself) in a closed-loop system that does not require wasted consumables like sweat?

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    $\begingroup$ "Actively pump heat" == input work to move heat in a direction or at a rate that it would not flow passively. $\endgroup$ – Logan R. Kearsley Aug 26 at 18:20
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    $\begingroup$ Only when the energy is generated somewhere else. the heat generated to run the system is greater than the heat it can "consume" You can't have a generator running a closed loop air conditioner and cool both below the ambient temprature. $\endgroup$ – John Aug 26 at 18:29
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    $\begingroup$ @John That's not true. Commercial heat pumps can have thermal efficiencies of over 300%. The amount of heat rejected is larger than the amount of heat extracted from the cold reservoir, because it includes the waste heat from input work, but that does not prevent a net reduction in cold-side temperature. $\endgroup$ – Logan R. Kearsley Aug 26 at 18:33
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    $\begingroup$ An absorption refrigerator works using a solution of ammonia in water (nothing exotic) and operates at constant pressure (15-16 atm in domestic applications). $\endgroup$ – AlexP Aug 26 at 21:35
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    $\begingroup$ @LoganR.Kearsley Define 'passive'. Standing outside a hive beating air into (or out of) it, seems pretty 'active' to me! $\endgroup$ – Strawberry Aug 27 at 17:19
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Look at what desert animals do to stay cool: they cannot afford wasting hard gained water, thus they increase the surface with which they can exchange heat with air.

This results in larger ears, for example, and more slender bodies (while the animals living in colder climates tend to be more spherical).

Just compare a fennec fox with an arctic fox, and the difference will be striking:

fennec fox

arctic fox

those large ear lobes are nicely engineered radiators, which the fennec uses to keep its thermal balance without losing water.

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    $\begingroup$ As far as I am aware, however, those are all passive radiators; they depend on the animal's body temperature being higher than that of the surrounding air, and merely improve the speed of heat flow. Or do they in fact have some mechanism for specifically pumping heat into the ears to raise their temperature above the body core? $\endgroup$ – Logan R. Kearsley Aug 26 at 18:04
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    $\begingroup$ So, that's a "no" then--they are not actually operating a refrigerating heat pump. $\endgroup$ – Logan R. Kearsley Aug 26 at 18:12
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    $\begingroup$ @LoganR.Kearsley You can't move heat against the gradient, without consuming energy. You can't raise the heat of the ears above the body core unless the ears are generating even more heat thus warming the core, that is just thermodynamics. You can use a chemical or phase change but then you are back to using a consumable, or getting really low material efficiency which in biology gets you more heat faster than you can get rid of it. Air conditioners work because the heat generated to run the system is done somewhere else. $\endgroup$ – John Aug 26 at 18:24
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    $\begingroup$ @LoganR.Kearsley No refrigerators and air conditioner do HALF of it, the energy to run those systems is done somewhere else and that heat that generates is dissipated separately. You cannot cool a closed loop system below ambient temprature. If you could you would then be able to use the differential to generate more energy, you would have a perpetual energy machine. $\endgroup$ – John Aug 26 at 18:49
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    $\begingroup$ @John The energy to run a refrigerator is dissipated in the machinery of the refrigerator. The inefficiencies of production are irrelevant. You can run a refrigerator just fine off of a battery stored in the refrigerated compartment. Or, heck, for a less ridiculous example: you can air condition a car with an air conditioner that is powered by the car's own engine. $\endgroup$ – Logan R. Kearsley Aug 26 at 19:29
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Suppose you had an animal with a second "heart" that could be used as part of a pulse-tube cooler. This heart would act as the pump to compress and relax a closed chamber, with an ear or a backbone "fin" as the heat exchanger.

Or, There's a cooler system the name of which escapes me, in which air is forced into a tube. Partway along the tube, the diameter is increased, which causes the air to cool due to expansion. This tube then turns back on itself, acting as a heat sink for the incoming air. In this way the part of the machine in contact with the first part of the tube gets quite cold over time. So all your animal needs is to be able to exhale through such an arrangement. Presumably there'd be a muscle-valve to shut off this alternative exhalation port when not wanted.

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    $\begingroup$ I believe the term you are looking for is "vortex tube". They are generally extremely low efficiency. But I had not heard of pulse tubes before; that looks promising! $\endgroup$ – Logan R. Kearsley Aug 26 at 19:26
  • $\begingroup$ The vortex tubes don't "turn back" on themselves. $\endgroup$ – JRE Aug 27 at 13:20
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    $\begingroup$ @JRE I do not recall the name of the gadget I wrote about, but I've used a version which was a single 'grooved' object where the expanded part of the tube encircled the pre-expanded part (both were multi-row) in the manner of a heat exchanger. $\endgroup$ – Carl Witthoft Aug 27 at 14:43
  • $\begingroup$ Hm. Counter-current heat exchanger plus expansion cooler? Don't know of a specific name for that... $\endgroup$ – Logan R. Kearsley Aug 27 at 17:10
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Yes, there are at least two plausible mechanisms. Compressor-based heat pumps don't seem very plausible for a biological process. However absorption refrigeratoration, mentioned in a comment by AlexP, may be plausible. What I find even more plausible, though, is something like "rubber band refrigeration". This mechanism relies on having a material that heats up when stretched.

No, the second law of thermodynamics isn’t in jeopardy. The secret is in the molecular structure of rubber bands. The bands are made of long polymer chains. A relaxed rubber band’s chains are a tangled mess. Stretching the band causes the chains to untangle and line up in an orderly fashion. By stretching the band you are decreasing its entropy. The energy of the molecules in the band don’t change, but entropy does. All the work one does to stretch the band has to go somewhere, and that somewhere is heat.

Such polymer chains can plausibly be created by biological processes, in the form of stretchy webbing/skin that the organism stretches out then allows to cool back to ambient temperature. After subsequently relaxing the stretched-out material, it's cooler than the original body temperature and absorbs heat out of the body. Then the process repeats. The only question is whether it could plausibly be made efficient enough to be of practical benefit.

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  • $\begingroup$ This puts me in mind of bullfrogs and the like. I wonder if the expansion and relaxation of their chin pouches is for cooling using this principle? $\endgroup$ – Wildcard Aug 27 at 14:56
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    $\begingroup$ @Wildcard: It seems unlikely to me, if nothing else just in that such a process is unlikely to evolve without a strong evoluationary pressure to do so (needing to remove heat when the environment exceeds safe body temperature, precluding passive cooling). Frogs are more likely to need to retain heat than remove it. $\endgroup$ – R.. Aug 27 at 17:17
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    $\begingroup$ An evolutionary pressure to develop such a trait, rather than just evolving to survive at higher temperatures, might be first developing in a cooler climate bordering a very hot and humid one (perhaps in the mountains) then discovering a resource (obvious one: plentigul food supply) that requires venturing into the climate that would otherwise be unsurvivable. If the organism already had stretchy parts with accidentally-right thermodynamic properties, evolution could easily drive the improvement of those to provide active cooling. $\endgroup$ – R.. Aug 27 at 17:18
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In addition to L.Dutch's answer involving ears, it is believed by some scientists that the plaques on the back of stegosaurus served the same purpose, i.e.: the beast pumped blood to the plaques were they could exchange heat with air, much like the heat sink on top of a computer processor. Those plaques had grooves which seem like the space for blood vessels.

Steggie

Steggie plate

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    $\begingroup$ Although it turns out stegosaurus plates match a blood supply for a keratin bone interaction (literally horny covering) than specialization for cooling. But there are plenty of extinct animals with large cooling surfaces, Dimetrodon for instance, and Stegosaur plates would still have had some thermal effects. link.springer.com/article/10.1007/s00015-010-0021-5 $\endgroup$ – John Aug 26 at 20:52
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Deep loop heat sink organ.

Giving up heat to the air, yes, yes. How droll. Been done and done and done.

Squirrels and dogs scrape the earth and lie belly down, depositing heat in the cool dirt. I propose taking this one step further.

The hot animal has below it a very long erectile organ which it pushes deep into a yielding substrate like sand or dirt. The temperature is lower at depth and for this purpose deeper is better until you get to a couple of km down and things starts heating up again. So not that deep. That would be silly, anyway.

enter image description here

I envision something like this grasshopper ovipositor.

https://biologydictionary.net/ovipositor/

This buried organ then serves as a heat sink. Hot blood is pumped down into this organ. It deposits heat at depth and cooled blood returns to the animal.

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Thermoacoustic refrigeration requires only a resonant chamber and a source of high amplitude sound, both of which have precedents in biology.

https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine

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    $\begingroup$ Actually, I'd argue that this is an answer to the question, which seeks "a plausible mechanism by which an organism could evolve to actively pump heat into the environment". It might benefit from a bit more exposition; for example, an Edit to list a few different species that have each of the two adaptations mentioned, and a summary of how the process works, would almost certainly improve this answer greatly. While links to external resources are encouraged for reference, it's better to use such links only for additional material, not material essential to understanding the answer itself. $\endgroup$ – a CVn Aug 27 at 18:30
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One could also imagine the use of a phase change material. Like having cells or vesicles filled with a wax that melts inside the body thus cooling it in the process. The vesicles would be transported to the surface where they solidify and release heat. This would also only work if the outside is cooler than the body temperature. This seems to have no advantage over just pumping blood to the surface. However, it could be an advantage if there are large temperature changes between night and day. The process could be used to keep the organism cool during the day and to restore over night.

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