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I'm trying to design new animals. One animal I came up with is a type of nocturnal & carnivorous predator that is shaped like a very large wolf. This predator sees in infrared light instead of visible light. The advantage of seeing in infrared light is that you can see the heat radiating off of lots of object. This makes tracking heat sources and warm-blooded organisms in nighttime or stormy/foggy weather or underwater or that are camouflaged much easier. There is a reason infrared is used for night vision. The disadvantage of seeing in infrared is that it is harder to pick out precise shapes and colors in infrared vision. That and objects that don't give off heat like plants or rocks or ectotherms are harder to see.

So would this giant wolf creature be an effective hunter in Eurasia for instance or are there very good reasons why no animal can see only in infrared?

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  • $\begingroup$ Infrared vision wouldn't be great in storms or underwater because water absorbs infrared light quite well. Night vision would be an advantage but plenty of predators, such as owls, manage just fine by starlight. $\endgroup$
    – Cadence
    Jan 4, 2023 at 4:11
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    $\begingroup$ Warm blooded animals cannot see in thermal infrared, because their own eyes are sources of thermal infrared. It would be like mounting the flash light inside the photo camera. $\endgroup$
    – AlexP
    Jan 4, 2023 at 7:59
  • $\begingroup$ That rather depends on it's prey and it's other senses .. if the world only has cold blooded animals, this is it's primary sense and it's other senses are rudimentary at best then no it won't be a very good predator 🤗 $\endgroup$
    – Pelinore
    Jan 4, 2023 at 8:25

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TL;DR: there's more than one kind of infrared, but there aren't any kinds that are particularly good for biological vision due to sensor noise. The thermal kind is definitely not suited for vision, especially in a warm blooded predator.


You seem to be conflating two different kinds of infrared here... near- and short-wavelength infrared, ~1ÎĽm ("night vision") and far-infrared, ~10ÎĽm (thermal cameras).

Only being able to see in near-infrared isn't a particularly big deal. If you had several different kinds of infrared receptor, you might even have an equivalent of color vision.

A series of three portraits of the same person, in visioble light, near IR (750-900nm) abd short-wavelength IR ((1500-1700nm). Hair and skin "color" in near and short IR wavelengths changes quite dramatically, with skin appearing dark instead of light, and hair appearing light instead of dark

(Image credit Nick Spiker via Wikimedia)

Longer wavelength IR gives you thermal vision:

A person with their arm covered in a black plastic bag taken in visible lightThe same image in long wavelength IR, showing that the person's glasses are opaque in this wavelength, appearing black, but the plastic bag is quite transparent and their arm is clearly visible

(Image credit: NASA/IPAC via Wikimedia)

I've talked about near-IR sensitivity in an answer to another question here: How to get cool night-vision without lame drawbacks?. The salient link is probably Why animals don't have infrared vision.

"For a long time, people assumed that light and heat had to trigger via different mechanisms, but now we think that both types of energy, in fact, trigger identical changes in the pigment molecules," says Yau. Moreover, since longer wavelength pigments have higher rates of false alarms, Yau says this may explain why animals never evolved to have infrared-sensing pigments.

Basically, even short IR receptors are triggered by the animal's own body heat, which causes "noisy" images resulting in poor, low-resolution imaging. The cooler the animal the better its dark vision will be, but cool animals have slow metabolisms and make for rather unenergetic predators.

The problem only gets worse with long-wavelength IR, which on a warm-blooded creature would be a bit like trying to see whilst having bright flashlights shining onto your face. Some cold-blooded predators have things like pit organs, but these don't "see" so much as "sense warmth". Pit organs aren't eyes, and so these snakes don't get predator-style heat "vision".

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No

Predators hunt prey within an environment whose features matter.

Your predator must navigate the physical realm in its hunt. If part of that realm goes undetected, your predator is slated for a rude awakening. To make matters worse, prey that are not crippled by attenuated senses will have a major advantage because they are cognizant of parts of the environment that are unknowable by their hunters.

  • Adding heat vision: advantage
  • Substituting heat vision: disadvantage

You've got remember that this contest is predator vs prey: the sensory capabilities of prey are vitally important. If prey can sense something their predator can't, the prey gain an advantage. On Earth, 7 hunts out of 8 are failures (source: BBC Earth series); making things worse is practically apocalyptic.

I should mention that this thinking is much less relevant in spaces that are overwhelmingly homogeneous, such as the ocean or outer space. (Although a creature who only sees infrared will have trouble near the sea floor.)

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    $\begingroup$ Agreed. There are many animals that possess infrared vision, but they can see visible light as well. Mosquitoes, bed bugs, snakes... $\endgroup$ Jan 4, 2023 at 4:00
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    $\begingroup$ I would add that bats can also see infrared. This gives them great sight during dawn/dusk but they have notoriously poor vision in broad daylight. This gives them a tactical disadvantage: they are blinded by bright artificial light. $\endgroup$ Jan 4, 2023 at 4:02
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It depends on the quality of your infrared vision. At the end of the day, any two objects will somewhat differ in the intensity and frequency composition of infrared light they are emitting. If your predator can make use of this information, it should not only avoid bumping into stones at full hunting speed, but also be capable of seeing as much as a comparable predator using normal vision.

This can be very roughly compared to wearing strongly tinted glasses that filter a specific colour: Even though some details are more difficult to make out, you can still navigate the world without considerable detriment. And that’s even though you haven’t trained seeing like this all your life.

However, there are a few problems with high-precision infrared eyes:

  • Regular vision is primarily based on a single light source, mostly sunlight (and similarly moonlight, starlight, or artificial light). We see things because sunlight is reflected by them. Also through reflection, sunlight gets to indirectly illuminate things not directly reached by it. By contrast, infrared vision would be primarily be based on what objects emit and gets reflected less.

    Now why does that matter? A big challenge of vision is to adapt to different intensities, since hardly any measurement device is capable of measuring with decent accuracy at different orders of magnitude without any adjustments.
    For example, the light intensity of a bright noon and a minimally lit room differs by orders of magnitude. Yet, thanks to the adaptiveness of our eyes, we can decently see in both situations. However, the adaption mechanisms we have either take several minutes or inherently affect the entire eye (pupils). Thus we cannot see at both intensities at the same time – and we don’t need to since the given light source sets a common intensity level for all things we can look at the same time.

    For infrared, no such standard exists. Thus the adaption needs to depend on the region of the eye: It has to be capable of seeing high-intensity sources (warm things) and low-intensity sources (cold things) at the same time.
    That would require a much more sophisticated adaption mechanism.

    Without this, navigating through nature would be comparable to a human navigating through an obstacle course in a dark room, where every surface is deeply black (absorbing any light), except for some very bright light sources here and there.

  • Related to the above point, the predator is an emitter of infrared light itself. This is particularly relevant to the eyes themselves. To avoid this, the sensory cells would have to be located close to the surface of your predator, which in turn means that they are exposed.

  • It is not that evolutionary plausible.

    The vast majority of vision evolved around what we call visible light because we got plenty of it (sunlight). While some creatures lost the capability to see over evolution, these usually do not need to see at all and are certainly not predators who need to move around quickly. As I see no reason for your predator to lose regular vision, it needs to have never have evolved it in the first place. But that only makes sense if your predator’s evolution happened mostly in a dark environment, which is some stretch for a Eurasian land animal. And even then, I find it more plausible to evolve low-light vision instead of infrared vision (due to the first point).

  • Finally, what would you gain? As mentioned initially, the predator’s infrared vision would have to compare to regular vision in terms of quality and be able to distinguish small differences in infrared information. If that’s the case, you also cannot do anything interesting with it anymore, such as confusing your predator by shielding of your heat: You would just turn from hot-blooded prey to cold-blooded prey, but the predator can see you all the same.

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