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In a world inhabited by small grassland herbivores, night-time is a very scary time indeed. With no moon or other substantial light-source to illuminate the plains, they are in great danger of becoming an easy meal for the planet's apex predator, the Stracnor: large nocturnal lion-like creatures with an amazingly soft step, allowing them to move within meters of their prey without detection.

Over many thousands of years, the Stracnor's prey has evolved long-range heat sensitive pits, similar to those on snakes, that serve as an early warning system — able to detect the infrared radiation emitted by nearby creatures and giving it a chance to escape if being stalked.

But the Stracnor has a clever trick of its own, able to cool its exterior to an extent where its infrared signature matches that of the surrounding landscape perfectly, hence rendering it undetectable to the specialised senses of its prey — leaving them utterly defenceless once more.

Is it possible for this "tactical cooling" to actually happen, and if so, how can it be achieved?

EDIT: Ambient temperatures fluctuate from around 20–25˚C during the day, down to 10–15˚C at night.

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    $\begingroup$ Compare polar bears. IIRC they are among the best insulated animals on Earth, and they are nearly undetectable using IR imaging equipment. $\endgroup$
    – user
    Oct 2, 2016 at 12:35
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    $\begingroup$ Not many people in the answers below have covered that fact that the Stracnor has to breathe, and the heat/scent of that breath will be a dead giveaway, despite any insulation for the body. For this, the Stracnor needs to hold it's breath, but this limits it's ability to scent it's prey. $\endgroup$
    – user10945
    Oct 3, 2016 at 7:24
  • $\begingroup$ @Pete, a good point indeed. $\endgroup$
    – Harry David
    Oct 3, 2016 at 7:26
  • $\begingroup$ Surely the animals would evolve to just come out in the day instead? $\endgroup$
    – djsmiley2kStaysInside
    Oct 3, 2016 at 11:25
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    $\begingroup$ Make your predator cold-blooded, and it'll always be the same temperature as its surrounding environment. That will cause serious problems for any prey animals that are reliant upon thermals to detect incoming threats. It also solves the breathing problem. $\endgroup$
    – aroth
    Oct 3, 2016 at 11:49

10 Answers 10

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If it has a mane like a lion, this could be your heat shield.
If it flaired out like a large umbrella and was made of a thin membrane or possibly fur, then it would block its body heat from whatever prey is directly in front of it, while allowing the heat to radiate normally from its body.

Chlamydosaurus

Prey seeing it from the side would have no trouble knowing it was there, but if the predator is that careless then it'll go hungry and have to do better tomorrow.

Its face would be the only real problem, and it could just have a lot of thermal insulation to minimize its signature.
Give the attentive prey a sporting chance.

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  • $\begingroup$ This is exactly the right shape! It should be made from feathers as suggested above, and augmented by a bushy mane as suggested by another answer above. The only thing that cannot be concealed like this would be the eyeballs. They should either be actively cooled, or they should use a periscope type of system with a mirror at ambient temperature, made from a substance which does not reflect long IR radiation very well. $\endgroup$
    – hexagon
    Oct 2, 2016 at 21:27
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    $\begingroup$ There are many animals that have transparent membranes that can cover their eyes that serve a wide variety of purposes. It would not be far fetched for the membrane to help insulate the eyes while stalking. $\endgroup$
    – Anketam
    Oct 2, 2016 at 23:49
  • $\begingroup$ The problem with this shape is that all the sensory organs and respiration are on the wrong side of the shield. The creature would have to close it's eyes and hold it's breath for the final approach. $\endgroup$
    – Jason K
    Oct 3, 2016 at 14:31
  • $\begingroup$ @JasonK As Anketam pointed out, the eyes could be covered by a membrane like a second eyelid that was thermally protected. It could have breathing slits like gills on its neck. You might not get perfect masking, but it doesn't have to be perfect, just good enough to get close enough to pounce. I'm also not suggesting that OP copies this creature exactly, as that would be silly. The picture is an illustration of what I'm talking about. $\endgroup$
    – AndyD273
    Oct 3, 2016 at 15:15
  • $\begingroup$ @AndyD273 Almost makes you wonder just how forward the umbrella shield can be. Could it be IN FRONT of the mouth? Perhaps it has a blowhole, remnant from when it was an aquatic mammal, and the mouth is just for eating. Do dolphins breathe through their mouth? Maybe the inside of the umbrella membrane can be lined with a reflective membrane like the tapetum lucidum (eyeshine in cats) to really reduce any possible IR leakage forwards of the predator. $\endgroup$
    – Jason K
    Oct 3, 2016 at 15:35
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Is it possible for this "tactical cooling" to actually happen, and if so, how can it be achieved?

Fur

Infrared is surprisingly easy to block. You can walk past an infrared alarm using nothing but a bed sheet. So long as the bed sheet is not warm, you're concealed.

The trick is insulating the fur from the body heat so the fur doesn't heat up.

Actually feathers might work best for this. Feathers on a wing that can be held away from the body and allowed to cool to room temperature should work exactly like the sheet without any fancy extra insulation. Just flare out the wing and stand behind it.

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    $\begingroup$ The problem is that all the body heat still has to go somewhere. If the fur is a perfect insulator, the animal overheats and dies. $\endgroup$
    – Salmoncrusher
    Oct 2, 2016 at 14:48
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    $\begingroup$ Which is why feathers would again be appropriate. They can be dynamically opened at will, or tightly packed together. $\endgroup$
    – Spooler
    Oct 2, 2016 at 17:05
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    $\begingroup$ That'd work -- it could close up and go into 'stealth mode' for brief periods of time, but if it takes too long sneaking up on its prey it has to choose between potentially dying from overheat or giving itself away... $\endgroup$
    – Shadur
    Oct 3, 2016 at 7:46
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    $\begingroup$ This would be a lot like "going dark" in Elite: Dangerous. You shut all of your heat vents and go totally cold to external sensors..but rapidly build up internal heat, which must be either dumped into metal slugs and ejected or vented, both of which make you very visible. $\endgroup$
    – etherealflux
    Oct 3, 2016 at 18:35
  • $\begingroup$ Or just stand behind the cold wing. Hold it out far enough and the body won't heat it up. Your heat will only be visible from behind the wing. $\endgroup$
    – candied_orange
    Oct 3, 2016 at 22:23
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Another method that would combine with fur/hair methods is vasoconstriction. This is a constriction of the blood vessels in the skin in cold conditions (or in response to drugs, but that's not relevant in this case) to reduce heat loss.

If vasoconstriction evolved to be partly voluntary, then the predator could use it on the parts of its body facing towards the prey, reducing its heat output on that side. Heat-detecting pits are a fairly limited sensor, and the combination of this and fur should be able to get the predator within range for an attack.

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  • $\begingroup$ Nice answer, I can easily envision such an adaption being successful B) $\endgroup$
    – Harry David
    Oct 13, 2016 at 5:49
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While most of the posters have spoken of using insulation, the issue of dealing with trapped heat remains.

In earthly animals, the general adaptation to remove excessive heat is to create large radiating surfaces. In the modern era, elephants have large ears which are used as radiating surfaces. Desert Foxes also have a similar adaptation. This is actually a very old adaptation. Dimetrodon, a creature living in the Permian era some 295 million years ago, had a large "sail" on its back which is thought to be a radiating surface (turned face on to the sun to rapidly warm the creature, and edge on to the sun to cool off). It most likely served double duty as being a sexual display as well (something like a peacock's tail or deer antlers).

enter image description here

Dimetrodon

This suggests some possibilities.

The simplest one would be the creature looks something like a male lion, with the insulating "mane" surrounding the head and shoulders, but almost bare skin along the flanks and underbelly to radiate heat.

The next idea would be to give the creature neck "frills". Evolving for sexual display, they also allow the creature to rapidly radiate heat away after a chase, or to preemptively cool late at night before setting out on a hunt at dawn.

Alternative structures like a "sail" along the spine, or vertical plates like a Stegosaur might be problematic in terms of camouflage, but could provide the ability to cool if the blood vessels are under some sort of voluntary control. Once again, the best use would be to radiate heat at night and hunt at dawn before warming up too much.

The final solution contradicts the description of the creature, but growing very large (like T-Rex size) will allow the creature to act as a "heat sink" for a short period of time, with the hot blood being pooled inside internal organs while blood flow is constricted to the skin. This will be difficult to achieve, especially since the limbs will need a great deal of blood for the final part of the hunt. The behaviour will be rather different as well, the creature needing to remain still while the hot blood is pumped out of the internal organs and into the skin to shed heat.

All these solutions would mean that while the creature has infra red camouflage at some times, it will also radiate like a beacon after the hunt.

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As others mentioned, it's not that hard to block heat - but heat has to go somewhere, or your creature will overheat and die. Neck-frills or other heat-blockers would disguise the front, but sometimes, it's just not possible to attack something head-on. Worse, these frills are visually obvious; prey with decent vision will see the frills easily.

Instead, I suggest a three-part process that will make your creature absolutely invisible:

  1. Fur. Polar-bear-like fur traps heat before it ever reaches the surface; no heat detection for you. This fur covers the creature from head to toe; only the pads of the foot are left un-furred.
  2. Switchable vascular system. When at rest, the creature's single heart will pump blood through the body as normal. However, when in stalking mode, certain blood vessels constrict, stopping blood transfer between the "inner" system, around organs and muscles, and "outer" system, a thin mesh just under the skin. The inner system pumps as normal, but is shut off from the outer system. Additionally, it helps the fur hide the creature's body heat.
  3. Tail-fan. While at rest or otherwise not stalking prey, the tail is furled and looks, well, like a tail. It helps with balance, etc., but is otherwise a normal tail. However, when stalking, overheated, or trying to woo the ladies, the tail unfurls into a large fan. The fan is covered in capillaries. In stalking mode, the tail is waved quickly back and forth (or up and down, depending on how you want it oriented); the musculature of the tail wraps around blood vessels, so when it waves, it pumps blood. Air passing over the tail cools it, and is fanned away, distributing the heat signature. Blood in the tail is quickly cooled, then circulated through the outer vascular system, cooling internal temperature.

If the creature is overheated (maybe it's a particularly warm day), it can gently wave the tail to lose heat; in this case, since the body is not in "stalk" mode, it cools the blood a little more directly.

Finally, the tail makes a great mating indicator. The larger or more rosily colored (rosy color = good blood flow) the tail, the better it is at cooling; better cooling means better hiding, better hiding means more kills, more kills means a better mate.

This system allows the creature to finely control its own body temperature. If it gets very cold, the creature can furl its tail and trap in body heat; if it's a real scorcher of a day, it can unfurl the tail and quickly cool off. It also allows the creature to keep its body heat behind it, out of sight in even short grass; even on dry earth, it can fan its own heat-signature away.

There is one downside: injury. The tail would be fairly fragile; tearing or cutting the tail would reduce its effectiveness, while a large injury has the possibility of quickly bleeding out. Luckily, the dual vascular system would have a built-in safeguard to keep blood loss to a minimum: clench the tail muscle (as would happen when in pain), and the blood flow stops.

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  • $\begingroup$ I particularly like your reasoning for a fan-like tail, thanks for that :) $\endgroup$
    – Harry David
    Oct 13, 2016 at 5:47
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Cold Blooded

I never heard you say that the predator had to be warm blooded. An extremely powerful ambush predator that could never chase prey down sounds a lot like a crocodile. A croc would not have too much trouble maintaining a low body temperature at night with which to sneak up on prey.

Slow Metabolism

On the other hand, if you want a mammal, a sloth can maintain a body temp down to 24C. Granted sloths are...sloth-like, but you can afford a slower metabolism if you sleep all day and ambush hunt at night. Either of these features could be combined with insulation for effective infra-red camouflage.

Pedantic side note

The word 'stracnor' is pronounced with a voiceless stop (k) followed by a nasal consonant (n). That is a VERY unusual combination in English, and it will immediately strike your readers as an awkward thing to pronounce. You may consider either a. changing the pronunciation, or b. explaining that it comes from a foreign language. Reading unusual words can break an immersive fantasy experience...Tolkien's languages were one the reasons for his success!

There are three common voiceless stops (k [or c], p, t), and two common nasals (n, m). Do you know how many words have a voiceless stop followed by a nasal? I do; I looked it up. Not counting compound words (fitness, footmen, topmast, milkmaid, etc), medical terms and words with a silent letter (pneumatic, knife, etc), there are exactly 6 (utmost, hypnosis, shrapnel, picnic, acne, and acme).

Sorry! I was compelled to write it!

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  • $\begingroup$ :'D Thanks for the linguistic analysis! I was going to ask what people thought of the name (back to the drawing board I guess...) $\endgroup$
    – Harry David
    Oct 3, 2016 at 6:48
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    $\begingroup$ Pedantic side reply: K followed by N considered hard to pronounce? Take note, look now, I'll give you some quick negatives... :) More generally, I've never heard anyone have problems with "fitness", "footman", "topmast", "milkmaid", "stripmine", "leftmost", etc. If we can pronounce these as compound words, there's no reason they should be a problem within a "normal" word. It certainly sounds slightly un-English, but in a sci-fi setting, a word without familiar roots can actually be a good thing. $\endgroup$
    – Graham
    Oct 3, 2016 at 12:38
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    $\begingroup$ Back to the subject though... Reptiles certainly can manage a low body temperature during the night, but they can't do very much whilst they're cold. Muscle contraction is a chemical reaction, and that chemical reaction is very much less efficient at lower temperatures. Crocs need that sunbathing in order to warm their blood to a temperature where they can function as a predator. So something other than haemoglobin for blood (or an enzyme which still catalyses the reaction at low temperatures) would seem to be in order. $\endgroup$
    – Graham
    Oct 3, 2016 at 12:47
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    $\begingroup$ kingledion has obviously adopted the hunting strategy of the stracnor, since nobody saw that coming.....;-) $\endgroup$
    – Thucydides
    Oct 3, 2016 at 17:59
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    $\begingroup$ @kingledion I don't feel dirty saying "fitness". I may feel dirty after getting sweaty doing it though, but that's a different thing. (And likewise, I don't feel dirty saying "milkmaid", but... ;) $\endgroup$
    – Graham
    Oct 4, 2016 at 8:17
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It won't work without lots of problems the way you described. However, it is possible. Your predator should have thick thermal insulator skin on the sides of its body. Since the climate is warm, body wide thermal insulation will cause issues; so the rest of its skin, the parts that will not be detected by the prey, should conduct heat to allow creature to cool.

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  • $\begingroup$ or the parts that face the prey are thick and don't radiate and the parts that are thin don't face the prey and do radiate? $\endgroup$
    – Marky
    Oct 2, 2016 at 6:03
  • $\begingroup$ Wing-like appendages could open up in the day to vent and close at night to shield. $\endgroup$
    – SRM
    Oct 2, 2016 at 6:03
  • $\begingroup$ Wings would be troublesome as it is a huge leap in an evolutionary sense. Thicker skin + hair to conceal front facing heat dissipation could be justified easier. $\endgroup$
    – Cem Kalyoncu
    Oct 2, 2016 at 7:02
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Install tactical cooling fans on your predator.

enter image description here

It will keep your predator cool and composed, ready for its next search.

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  • $\begingroup$ More importantly: utilize surface area, and airflow to decrease temperature. $\endgroup$
    – Alex Johnson
    Oct 3, 2016 at 14:11
  • $\begingroup$ I like it. [Small grassland herbivore wakes up] "Mum? Is it just me, or can you hear a giant mac-book coming toward us?" $\endgroup$
    – Harry David
    Oct 4, 2016 at 1:42
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Evolution primarily changes what is pre-existing much faster than entirely new feature

If such a predator was originally warm-blooded then it would need a constant body temperature to maintain organ function and not die of hypothermia. It would take tens or even hundreds of millions of years for such a radical change to its biology, and would take far too much energy to be practical, even if it was only the surface.

However, that does not exclude other parts without specific requirements for temperature. Long hair is the most feasible adaptation it could develop to conceal body heat, especially if it already had hair to begin with.

If its hair (especially around the face) were to become fluffier and more reflective, it would stay relatively cool and prevent it's prey from seeing the few patches of exposed skin that would have given it away before.

If you want to get a bit more complex, it could groom its hair into a compact shape to shield the majority of it from the sun, or lick it so evaporative cooling (the same process in sweating) actively keeps the hair cool, further increasing its "invisibility" at night.

These are relatively mundane solutions to the problem, but probably the most plausible.

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I just came across this question and I have a fairly simple answer.

Your predator takes a bath before it hunts.

The predator jumps in the water or in the mud and drastically cools their skin and\or fur. At the same time, surface vessels constrict to conserve body heat. Thus you end up with a very cold skin on the predator. If you view it in infrared it just looks like a big dark spot. On top of that there will be temperature variations across the surface, breaking up its infrared signature like stripes on a tiger.

Viola, nearly invisible predator.

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