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Remember the Xenomorphs from aliens, how they didn't show up in heat vision? I was wondering how you could recreate that effect biologically... a creature which doesn't pop-up on IR (at least, not obviously), because its surface temperature is about equal to its surroundings.

It may not always be in this state, but rather it may be a defence mechanism to prevent detection from creatures with IR.

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Just as I was contemplating this, L.Dutch posted an answer saying in part that one of your two options would be to

1. tune your surface temperature to that of the environment/background (unpractical if you are out of tropical zones and want to stay alive)

I'm going to go out on a limb and say that it's not so unpractical; especially so if you are happy to make this a temporary defense against merely IR sensing.

There are two obvious ways that I can think of:

  1. Make your creature cold-blooded and able to regulate its metabolism down to a very low level. Many Earth reptiles fall into this category at least to some extent. You could probably make it even more extreme if the creature only uses it as a temporary defense mechanism.
  2. Give your creature very good insulation. IR sensing presumably works by detecting the difference against the ambient environment. If the outer surface layer of your creature is at or near ambient temperature (whatever exactly that is), it will be very hard to detect by such means. A fair number of Earth species that live at extreme latitudes (near the poles) fall into this category, to varying degrees. For an extreme example, look at polar bears, which are very difficult to detect in IR because their fur insulates them so well that their outer temperature is only very marginally above that of the ambient environment. You'll have to figure out a way to keep them from overheating without giving themselves away, but again if you only need it for a period of time then ability to reduce metabolism might work.

Both options are viable in that there actually exist real-life creatures on Earth which use them. You will need to tweak those approaches slightly, but given some appropriate selection pressure, neither would seem completely unreasonable.

As a bonus, you can now have creatures which are difficult to detect in IR both at high and low latitudes! (Near the poles and near the equator, respectively.)

Whichever way you go, keep in mind that this would be very specialized against something hunting by IR. The instant you face a predator that hunts by scent, or sound, or sight, your defense becomes a huge liability. Since different predators in your world would likely evolve different hunting strategies, you should plan accordingly if you don't want your species to go extinct very quickly.

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    $\begingroup$ Good answer, but just a minor point. As I understand it, cold blooded animals CAN'T regulate their metabolism as it's regulated directly by the ambient temperature. Therefore, in the cold they get sluggish, whereas warm blooded creatures don't but need more energy. IR doesn't work on cold blooded creatures for that reason. Therefore, any creature which doesn't get picked up on IR can be defeated by leading them into a large fridge, where their core temperature drops and they slow down. As such, option 2 is better. :) $\endgroup$ – Tim B II Dec 22 '17 at 8:54
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I got your room temperature animal right here.

http://coolcosmos.ipac.caltech.edu/image_galleries/ir_zoo/alligator.html alligator in infrared

Reptiles are the same temperature as the ambient environment: they are ectotherms. The human holding this alligator is hot in infrared but the alligator is the same temperature as the rest of the room, and so blends in.

If your animals are cold blooded they will not show up on infrared. If they are warm blooded but can cut blood supply to the skin and let that cool to ambient, or maybe cool it with evaporation those areas will not show up very well on infrared: an example - this cat has a cold nose.
cat with cold nose on infrared http://www.robotroom.com/Flir-Infrared-Camera-3.html

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The emitted IR radiation is proportional to the 4th power of the absolute temperature AND to the emissivity of the surface. You have then two options:

  1. tune your surface temperature to that of the environment/background (unpractical if you are out of tropical zones and want to stay alive)
  2. tune your surface (aka skin) emissivity. I.e. a low emissivity surface like an aluminum mug would look less IR bright when compared to an high emissivity surface (a black potter mug) at the same temperature (both filled with hot coffee).

Point 2 can be achieved by modified melanocytes which can modify the emissivity of the skin to match the background, more or less what octopuses do with their color.

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