The Predator: A vision science perspective

There is a real argument to be made that Predator is perfect. I don’t mean that it’s objectively a good movie (though it is) but that I can’t imagine how or why you would change a thing about it. It was a sleepover standard during the 80s and 90s, mostly because it’s hard to find a movie that is awesomer than Predator. I know -  “awesomer” is not a word. But what else are you going to say about a movie that features two governors (Jesse “The Body Ventura”, 38th Governor of Minnesota & Arnold Schwarzenegger, 38th Governor of California - weird that they’re both 38th, but there it is), a killer alien with every weapon you can imagine, and guys shooting guns that are meant to be mounted on helicopters? 

The plot of the movie is…well, look it’s not really that kind of movie, folks. There’s a bunch of commandos who helicopter into the jungle and it turns out that there’s an alien there who just came by Earth to hunt muscle-y guys with unclear ties to the military. These particular Earthlings are armed to the most ridiculous degree you can imagine: There’s a scene in the movie where the commandos fire everything they’ve got into the jungle in a panic, leveling what seems like acres and acres of greenery. They straight up kill a rainforest! To paraphrase Bruce Lee, however, trees don’t shoot back (Clouse et al., 1969). The Predator does, though, and makes mincemeat of the team one-by-one. What’s a mercenary to do about an enemy with a cloaking device, homing lasers, shoulder missiles, and God knows what else? In the end it comes down to Arnold by himself taking on The Predator with log traps, fire arrows that he makes with his wits and some loose brush, and a good bit of cold mud. It doesn’t get much awesomer than that, my friends.

Among the things that makes Predator a great movie is the fact that we don’t just get to see the alien doing all kinds of scary, deadly stuff. We also get to see his point-of-view as he stalks our intrepid commandos through the Costa Rican wilderness, a classic tactic that easily builds tension by making you aware of danger that the protagonists don’t know about. What I think everyone remembers about The Predator though isn’t just that you get these glimpses of his POV, but what his POV looks like. We don’t just see our characters roaming about from behind a tree or hidden in some vines - we see them in some kind of rainbow, low-fi visual display that makes them stand out from their surroundings like beacons. They’re decked out in nine kinds of bad-ass camouflage each, but they may as well be wearing those reflective vests that road construction crews wear on the interstate. 

If you didn’t think these guys were toast before, these shots sealed the deal. For one, they’re just completely exposed apparently, so it’s obvious that The Predator can just pick them off at his leisure. Besides that, however, this view raised a question that even 10-year-old pre-vision-science me couldn’t help but wonder about: What is going on? What is he even seeing? How does he do that? My 10-year-old self just wrote it off as an instance of AwesomeVision and watched the rest of the movie, but my 40-something-year-old self now has some thoughts about these shots. Specifically, they’re data. Usually, I try to understand how the visual system works by asking people to tell me about things they’re seeing in the experiments in my lab. Which of these textures looks more shiny to you? Would you call this material stone or wood? Do you think this face looks like someone you know? I can’t crawl inside anyone’s head and get access to what they see, so I use button presses, response times, brain responses, and other proxies for their actual visual experience. This, though? This image up here? This is a gold mine. I actually do get to climb inside The Predator’s visual system and see what he experiences. The question is, what does it tell us?

Mostly I think it tells us some interesting stuff about what his visual system, specifically his retina, does to measure light. To be more precise, I think we can say a good bit about how The Predator’s retina achieves wavelength encoding in a way that’s different from what our retina does. By itself that’s pretty neat, but it also gives us some potential insights about what kind of visual world this alien comes from and how it differs from Earth. I’ll refer to this as the visual ecology of his environment. To understand what might be happening in the Predator’s retina, however, we’ll start by talking about what happens in ours to give us an experience of color in visual scenes. 

How do we see color with our retina?

To begin thinking about why things look the way they do to the Predator based on our glimpses of his visual experience, we need to know why things look the way they do to us. Specifically, how do the earliest stages of vision make it possible for us to measure light in the environment and how do their functional properties also constrain what we can see? I’m going to focus this discussion on the nature of color vision at the level of the retina, which we sometimes describe more formally as wavelength encoding (Wandell 1995). That latter term is a fancy way of asking how our visual system makes it possible for us to estimate what kinds of light we’re looking at, and these estimates are what leads different parts of our visual world to have a different color or lightness than others. 

Light waves - amplitude and wavelength, intensity and color

The idea of different kinds of light might seem a little strange to you, but you’re also probably familiar with several different varieties of light in your environment. When we’re thinking about your vision we’re only talking about one small slice of a much larger spectrum of different kinds of electromagnetic energy, all of which are considered “light.” This small portion of electromagnetic energy you can measure with your eyes is referred to simply as visible light, but the full spectrum is populated by radio waves, microwaves, X-rays, gamma rays, and more. What makes these different kinds of light different from one another? In a word: wavelength.

See page for author, CC BY-SA 3.0 <http://creativecommons.org/licenses/by-sa/3.0/>, via Wikimedia Commons

See, one way to describe the nature of light is to think of it as a wave. You can think of a water wave if you like, but the idea is more general. Waves in the physical sense are a phenomenon by which energy is transferred from one place to another by means of some kind of oscillation - moving band and forth. If we’re talking about water, you can think about ripples in a pond that radiate out from where you tossed a rock in. If you’re holding a jump rope or a Slinky, shaking one end up and down so that you send a bump down the line from one end to the other is also a wave. All waves can be described in terms of the same set of properties, which means we can use a common mathematical language to talk about them all. Two of these that are especially relevant to the human visual system are the wavelength and amplitude of light waves. If we think about those ripples in a pond, the amplitude of those waves is the word we use to describe how tall the peaks are in each ripple: A big rock will lead to a big splash, and water that ripples up and down with a large amplitude. The wavelength of those waves is the distance between ripples. If the ripples are very far apart, we’d say that the wave has a long wavelength. Ripples that are very close together mean that the wave in question has a short wavelength. These two properties of light waves are related to two different aspects of our experience of light: intensity (amplitude) and color (wavelength). Most light that you see is a mixture of many different intensities and wavelengths. Light sources like lasers are an exception to this, but pretty much everything you see around you is illuminated by light that has many different wavelengths of light in it and reflects light towards your eyes that is also a mixture of different wavelengths and amplitudes. Part of what your retina does is turn that incoming light into a visual experience, and color is the way in which you experience light that differs in its wavelength ingredients. If things look like they are different colors to you, then it means that they must have different wavelengths of light in them. The converse of this is not true, however, but we’ll get to that in a moment. First, we want to describe how your retina provides information about the wavelengths that you are seeing.

Measuring wavelengths with photoreceptors

Your eye is a wonderful optical device with a bunch of parts that contribute in their own unique way to the tasks of image formation and transduction. Image formation refers to the process by which the light from 3D objects in the environment are projected onto 2D patterns in the eye. Transduction refers to the transformation of those patterns of light into an electrical signal that the nervous system can receive and transmit to the cells in your visual pathways. We’re interested in this process because The Predator’s experience of the visual world gives us some clues regarding the way he must be transducing light - but how do you do it?

If you were to look at your retina very closely, you could see the cells that transduce light for your nervous system. These cells are called photoreceptors, and they come in two main varieties in the human retina: The rods and the cones, so named for the shape of the outer segment of each cell. Rods have a more cylindrical outer segment, while cones have a tapered shape. You’ll find rods and cones in different parts of the retina, with cones heavily concentrated in the central part of the retina (the fovea) and rods more concentrated in the periphery. Despite having different shapes and different positions on the retina, both kinds of photoreceptors do essentially the same basic job. Specifically, these cells each have a photopigment on them which will bleach when exposed to light. That bleaching is the outward indicator of a chemical process in which the incoming light changes the shape of some molecules, setting off a cascade of events in the cell that ultimately lead to electrical signals that are propagated to the next stages of visual processing. The details of this process aren’t really our concern, however - what we’re interested in is how these cells end up giving you an experience of brightness and color. 

The thing you have to know about the rods and cones in your eye is that they accomplish transduction by absorbing incoming light, but they don’t just absorb any old photons that come their way. Instead, rods and cones differ in terms of what kinds of light, specifically which wavelengths, they absorb best. If you go back and look at that depiction of the full electromagnetic spectrum up above, you’ll see that visible light is just a little sliver of the full range of different kinds of light. Inside that sliver, which spans wavelengths from approximately 400nm to 700nm, the rods and cones each absorb an even narrower band of incoming light. The strength of the signal a photoreceptor produces depends on how many photons get absorbed, which means that these preferences for some wavelengths over others lead to differences in the strength of the signal each cell generates to incoming light of different kinds. In turn, those signals are the only tool you have to start estimating things about the light you are seeing. So: Because anything outside that 400nm-700nm isn’t going to get absorbed, you don’t get to see it! No signal, no sight. Inside that range, things get more interesting, though. Consider the rods by themselves for a  moment: They absorb 500nm wavelength light the best, but are capable of absorbing light between about 400nm and 600nm. That means that while they’re excellent for producing a strong signal to 500nm light, they can still produce a big response to 400nm or 600nm light if there’s enough of it. If all you want to do is detect the presence of light, this isn’t so bad, but what if you want to do something more? What if you want to not only detect light, but have some way of distinguishing one kind of light (different wavelengths) from another? You’ve got yourself a problem if you’re only going to listen to your rods because you won’t know if the signal you get from them is the result of a lot of light that’s poorly absorbed or a little bit of light that’s absorbed very well. This bit of confusion is usually referred to as a consequence of the principle of univariance in vision science, which just means that any one photoreceptor only gets to vary the strength of its signal, but there are two different things about the light that make it change. Trying to figure out what the status of both of those things happened to be from just the one number the cell gives you is a bit like knowing that two numbers multiplied together got you a value of 120, but being asked to guess what the two numbers were. 

So what to do? If listening to just one photoreceptor is the problem, then listening to more than one photoreceptor is the solution. The rods are not alone of course, and while the cones do transduce light with a similar basic mechanism, they differ in terms of which kinds of light they absorb best. Even better, there are 3 sub-classes of cone in the human retina (unless you’re colorblind), each with a different profile of what wavelengths of light they like to absorb best. You can see a graph of those preferences, along with that of the rods, in the figure below. 

What this means for us is that incoming light doesn’t just produce one signal: It produces 3 signals if we’re just listening to the cones. We’re still going to be blind to anything outside the 400nm-700nm range that the cones’ absorption spectra span, but we can do something new: We can use those 3 signals that come from the long, medium, and short-wavelength cones to ask if different incoming lights have the same wavelengths in them or not. Any one of the cones will have the same problem with univariance that the rods have, but even if one of those signals from the cones is the same for two different incoming lights, it could be the case that the signals from one or both of the remaining cones will differ, giving us a way to tell those lights apart based on the wavelengths that are in them. 

I say could be the case because this is far from a perfect guarantee! There are still lights with different wavelengths in them that will lead to the same 3 signals in all 3 different kinds of cones, meaning we can’t hope to tell them apart. Sets of light with that property are called metamers and they are directly related to how the absorption functions of the cones overlap with one another - and they do overlap, after all! That overlap is actually quite important - without it, you would probably have a larger group of metameric colors. Imagine that they didn’t overlap at all for a moment: Now each cone has an independent range of wavelengths it responds to, but inside that range you can’t tell different lights apart based on wavelength because of the principle of univariance! That would mean that the best you could do to tell lights apart would be to say whether or not there was something inside each of 3 different ranges, with a simple “yes” or “no” for each interval. OK, it’s probably not quite as bad as all that, but in the limiting case that amounts to about 8 or so different kinds of signal you could distinguish which is a lot of metamers. The overlap is the thing that allows you to make more fine-grained distinctions between lights with different wavelength ingredients, so it’s a really important feature of your photoreceptors. 

We’re almost back to The Predator, but I want to make this as clear as possible before we go on to our alien visual system: Distinguishing light based on wavelength is what leads to your experience of color - your experience of light having different colors to it is the subjective signal that lights have different wavelength ingredients. That experience depends on (1) Having more than one photoreceptor to get signals from, and (2) Having photoreceptors with different preferences for absorbing wavelengths of light. Changing either how many photoreceptors you have or how their absorption spectra differ from one another will change your abilities to tell light with different wavelengths apart. This is precisely what is different about the color vision systems of humans with color-blindness, by the way: Some color-blind individuals lack one or more of the cone sub-types, while others have absorption spectra that overlap by different amounts than we find in typical color vision. Which cones you’re missing and which spectra are shifted in which direction determine the set of metamers a color-blind person has that someone with typical color vision will not. This leads us directly to our questions about The Predator’s visual system: What can we conclude about his photoreceptors, both in terms of their number and their absorption spectra based on our glimpses of his subjective experience of color? 

How does The Predator see color with his retina?

Alright, back to our favorite hunter-killer. We want to use the data that we get from the 1987 film (not the sequels - I’m not current, unfortunately) to draw some conclusions about The Predator’s photoreceptors. How many photoreceptors do we think he has anyway? This can vary a lot across species: Dogs have just two sub-classes of cones, but the mantis shrimp (below) has 16 of them! What can we conclude about The Predator’s vision based on our snippets of his vision?

How many photoreceptors does The Predator have?

The first thing I think we can safely conclude is that The Predator is not a monochromat. That is, he doesn’t just have one kind of photoreceptor. Why? Remember that any one photoreceptor doesn’t allow you to distinguish light based on wavelength. Instead, all that you’d get to do is experience either strong or weak signals from your single class of photoreceptor. The consequences of this for your visual experience are that you probably don’t have an experience of color, but are instead limited to an experience of light intensity. Because The Predator’s vision appears to vary both in intensity (there are bright bits and dark bits) and color (there are red, blue, and green bits), I’d say that the dimensionality of his visual experience isn’t just one - he’s doing some work to distinguish light based on wavelength. 

Now there is a caveat to this - just because you only have a signal that varies in one dimension doesn’t have to mean that you only experience grayscale vision.. This is for the dataviz nerds in the crowd, but you can assign any colormap you want to a one-dimensional scale, sometimes with disastrous results for various kinds of information visualizations. There isn’t anything stopping you or your visual system from deciding that low intensity would be experienced as blue and high intensity would be experienced as red, for example. We’re going to guess that The Predator doesn’t have the same color look-up table as the default Matlab plot, though, meaning multiple photoreceptors are probably in play.

What wavelengths do Predator cones absorb best?

Our color vision isn’t just constrained by the number of photoreceptors we have, though - remember that it matters a great deal how the absorption preferences of those cells are distributed. What wavelengths do they absorb and which ones are they blind to? How do the absorption spectra of different cones overlap? The former helps determine the range of light you’re sensitive to at all and the latter helps determine your ability to tell light with different wavelengths apart. 

Let’s start with the easier question - what kinds of light can The Predator see? We have some pretty clear hints about this from the way that our soon-to-be-lasered soldiers look as they’re walking through the forest. They are obviously sending way more of some wavelengths towards The Predator than the surrounding terrain. The simplest explanation for this massive difference in their brightness vs. that of the environment that they’re in is that The Predator has what you probably think of as heat vision. But what does that mean in terms of wavelength? To put it simply, thermal radiation (also known as heat) is light emitted by warm objects and it is longer in wavelength than the visible light we can detect with our eyes. If you’ve heard the word infrared before, that’s what we’re talking about here: The longest wavelengths of visible light look red to us on their own, and infrared light has a wavelength that is longer than that. Because our cones can’t absorb this kind of light, we don’t get to see it unless we use some kind of sensor that can detect it. The Predator, on the other hand, may have photoreceptors with a different span of wavelengths than ours. 

We get an especially nice clue about this property of his visual system after former Gov. Schwarzenegger stumbles frantically away from The Predator, out of a river and into a sort of muddy alcove. Coated in mud and desperately trying to wrap himself around some vines and branches in a last-ditch effort to hide…somehow the creature looks straight at him and then walks away. In case you, the viewer, missed the import of this stunning moment, the script blesses us with the line “He didn’t see me.” But why didn’t he see him? That mud is probably pretty cool and is likely doing a decent job absorbing the thermal radiation coming from Arnold’s skin, meaning that as far as The Predator is concerned there isn’t anything to see here. This tells us not only that his photoreceptors must span wavelengths into the infrared region, but also that he may not be doing a ton of work to absorb light in the medium to shorter-wavelength region that the human eye detects. The other problem that he may be having, which is also evident in our first figure depicting his vision, is that he appears to have pretty poor abilities to distinguish light based on wavelength. That is, he appears to have some fairly broad categories of metamers - colors that are physically different in terms of wavelength, but lead to the same signals from the photoreceptors. The fact that there are only a few distinguishable colors in his POV shots suggest to me that he may have cones that overlap very little in their absorption spectra, giving him the sort of yes/no code for colors that I mentioned earlier. This scheme may make him pretty great at quickly detecting the presence of some wavelengths, but pretty poor at making fine-grained distinctions. Pretty useful for killing mercs from the treetops, but apparently much less useful for killing one who managed to get himself coated in something cold and slimy. 

But wait - there’s more!

So far, we’ve been able to make some pretty decent guesses about what must be going on with The Predator’s visual system from just a few short examples of what he can see. There’s a twist, however, and I think it’s actually a pretty big one: This is all stuff that he sees with his helmet on. Yup, The Predator is all kitted out with some serious armor, including some kind of techno-helmet that clearly has a bunch of gear on it. In fact, we know that these POV shots we’re getting are being filtered through some kind of AR display because we get to see targeting shapes superimposed on his field of view a couple of times. All this leads to a key question: If what we’ve been getting so far are the properties of his helmet-based vision, what about his actual visual system? 

Luckily we get some (limited) data about this too. Late in the movie (it’s not a film), The Predator takes his helmet off slowly to show off the work that the prosthetics team did on some killer mandibles and such. It’s totally gratuitous in terms of the plot (like I said, it’s not that kind of movie), but it’s fantastic from a horror-movie standpoint and clearly also exists to fully elaborate our analysis of The Predator’s visual system. I say this because there is absolutely no reason we need a POV shot as the helmet comes off, but we get one anyway. This is clearly there for the community of sci-fi comparative vision researchers, which to my knowledge is just me. So, what do we get? What’s the difference between helmet vision and true Predator vision?

Predator vision without the helmet is terrible. In particular, everything is washed out in red tones and most of the image is actually pretty dark. The exceptions seem to be the parts of the scene that are straight up on fire. What does this imply? I think it implies that for the most part The Predator’s own retina is pretty poor at absorbing most of the visible light coming from objects and surfaces on Earth. Further, I’m interpreting the fact that everything looks kind of red to him under these circumstances to mean that our range of visible light is largely compressed in what his visual system considers the long-wavelength part of his visible spectrum. What does that mean in terms of what his photoreceptors absorb best? I think it means that his visual system is much better at absorbing short wavelengths of light, probably with peaks in what we would call blue to violet light, into the region that we call ultraviolet. Ultraviolet light arguably isn’t that great to invest in on Earth’s surface because it’s in comparatively limited supply- just check out the falloff at short wavelengths in this figure. There are species with some sensitivity to UV light, though - reindeer are one of my favorite: Snow and ice reflect a ton of the incoming UV light from the sun, but terrain that doesn’t have snow is much worse at reflecting it. This ends up meaning that stuff reindeer can eat stands out against the snowpack as a very dark spot, which makes it easier to find food in a snowy, icy, field. Fish, birds, and butterflies all see in the ultraviolet range, too, and the latter strike me as particularly funny when we think about The Predator. The guy may be built more or less like a human, he may be armed like some kind of laser tank from the future, but he’s got the eyes of a butterfly.

Other features of Predator vision: What’s with the flashing eyes?

Now that we’ve tried to characterize the Predator’s retina in terms of wavelength encoding abilities, there are a few more little details about the way he’s depicted in the movie that I want to talk about. In both cases, it’s probably true that the visual effects team did this stuff to make the Predator even awesomer, but that doesn’t mean we can’t try to think about them in terms of vision science. 

First, I want to talk about one of our first views of the Predator as he stands watching our doomed commandos step warily through the rainforest. Because of his invisibility cloak, we get to see at best a sort of rippling, shimmering outline that only provides hints of the killing machine waiting right behind the soldiers. But then, just as one of the men turns to look at him, his eyes flash for a moment: Just a glimmer of greenish-yellow light before all Hell breaks loose.

It’s awesome. I mean, it’s just intimidating as all get out, right? The guy’s just hanging out being completely invisible with his superior tech, but then decides screw this and breaks cover with some laser eyes because it’s not like he has to bother hiding anyway. He has a point, but you also might be wondering a few things about this phenomenon. Specifically, (1) What’s making his eyes glow exactly? And (2) Is this really just a flex, or is there a good reason for the glowing eyes? 

If you have pets at home, the image of the Predator above might actually look familiar to you. It’s not at all uncommon to take a picture of your little house beast, especially if it’s a cat, and end up with a strangely demonic image of your furry companion staring at the camera with glowing eyes. Though it’s tempting to assume this means that cats are secretly just waiting for their moment to strafe us all with eye lasers, there is an interesting difference between their vision and ours that explains what’s happening. Cats, along with a number of other animals, have a bit of tissue in their eye called a tapetum - tapetum lucidum, if you want to be fancy about it. The tapetum is a bit of reflective tissue that sits behind the retina, where it serves as a retroflector. Light passes into the eye, where some is absorbed by the photoreceptors. Some of what doesn’t get absorbed passes further into the eye, where it can then bounce off of the tapetum and back through the photoreceptor layer again for another chance to be absorbed. Some of it also doesn’t get absorbed on this second trip, and can escape the eye through the pupil, landing eventually on the sensors in your camera. The green color in cats has to do with the composition of their tapetum, which is rich in riboflavin as opposed to the zinc that makes up the tapetum in other animals.

 This double pass through the photoreceptors means that cats and other animals with a tapetum get another chance to absorb incoming light, which makes them more sensitive to low light levels than organisms without one. Humans are one such organism that lacks a tapetum. What we have instead is a bit of tissue called the pigmented epithelium, which acts as a backstop for light. Instead of reflecting unabsorbed light back through the photoreceptors, the pigmented epithelium just soaks that light up so it isn’t just bouncing around in there. The downside is that we have neither amazing night vision nor glowing Predator eyes, but the upside is that the lack of reflected light bouncing around in our retina may make our vision a little sharper than it would be without this layer of cells. 

The Predator thus may have a tapetum lucidum of his own, and a cat-like one with riboflavin to boot. That’s all well and good, but we still haven’t figured out why he shows off his eye-shine right before murdering a guy while otherwise invisible. There’s a very good reason for this though! All this talk about light reflecting around in the retina and being absorbed by either photoreceptors or pigmented epithelium cells highlights a really important point about invisibility: An invisible eye can’t see! To be invisible like the Predator is in this scene, you have to find some way to stop light from reflecting off of you towards an observer, and you also have to stop light from being absorbed by you if you’re between the observer and the light source. Light has to pass through you with as little interruption as possible, otherwise those interruptions will lead to light coming from your vicinity to look different and that makes you detectable! Modern limited attempts at invisibility attempt to play this trick mostly by moving light around an object so that it comes out on the other side doing most of what it was doing before it got to the thing we’re trying to hide. A very simple example of this is the Rochester Cloak, which uses a series of lenses with specific focal lengths and at specific distances to create a zone of invisibility where you can hide an object from view. You can buy your own kit for making one without spending too much money at all.

The Predator’s invisibility cloak seems to be doing something like this. I especially love the sort of rippled effect it has at the edges as though the light is being distorted just a bit as he moves around. That’s consistent with him having some sort of neat optical material on his suit that does a pretty good job of bending light around his body, but also suffers from some distortion if he moves too fast. Remember though, he can’t absorb light while he’s invisible or else you’ll see him. But how does a retina work to sense light? It absorbs it! I think this means that when the Predator is fully invisible, he can’t see anything either. The suit is thus pretty good for hiding out, but maybe not so great for action. What’s a murderous alien to do when it’s time to make some precise laser shots and take out a future governor then? Let your eyes absorb some light just for a moment so that you can see again (which also lets some light out of your eye from your tapetum) and then shoot your shot. You can see again, you look properly intimidating, and it’s a win-win for everybody - everybody except for the poor guy you’re shooting at, of course. 

Glowing blood, biofluorescence and visual ecology

There is one last odd detail about his physiology that stands out as more than a little odd given his species’ goals: why on Earth would he have glow-in-the-dark blood? 

Yes, it turns out that the Predator bleeds (and thus that Jesse Ventura can kill it) and when it bleeds it oozes a sort of Mountain-Dew-colored electric glowing substance. This turns out to be an amazing boon to anyone trying to track his movements through the jungle because if a blood trail is useful for anyone stalking prey, a blood trail that you could read by at night is even better. But doesn’t this mean that this has to be about the least adaptive thing for any species to have? Leaving a glowing trail of your blood behind you if you’re injured seems like almost the perfect recipe for ensuring that your species doesn’t make it. Ancestors of the Predator with glow-in-the-dark blood should have been successfully tracked and eaten by…well, something or other, and been unlikely to survive to reproduce. Predators with blood that wasn’t so detectable would be less prone to being tracked and killed, so this should have been the phenotype that persisted. So how exactly would any apex predator (or Predator) end up with this? 

To answer this question, we have to think about the physics of biofluorescence. While it makes the Predator impressively other-worldly, there are plenty of examples of Terran species that biofluoresce too. But how do they do it? Where does the energy come from to make something like a Marbled Scorpion or a Virginia Possum glow? First, we need to make an important distinction: bioluminescence refers to the generation of light by means of a chemical process. Angler fish for example famously have a dangly sort of headlamp that glows in the depths of the ocean by means of chemicals called luciferin and luciferase. The key thing about bioluminescence is that you don’t need to put light in to get light out. Biofluorescence is different from this in that now we’re talking about light being transformed from one form to another - light in turns into light out, leading to the glow we see when these critters are illuminated by the right light source. 

I think the Predator’s blood is most likely biofluorescent - it glows when light shines on it, and that light is then transformed into another kind of light that radiates outward. In general, this involves light with shorter wavelengths (and thus higher energy) being absorbed by the biofluorescent material and being radiated back out with a longer wavelength (and thus lower energy). This, as they say, explains everything! Remember how I concluded above that the Predator probably had a visual system that was adapted to an environment rich in UV light? Ultraviolet light has a shorter wavelength than visible light and is good for making biofluorescent materials do their thing. What this means is that the Predator’s blood is a terrible thing for him on Earth, but it’s an excellent thing on his homeworld. In a UV environment populated with critters that presumably are also most sensitive to short-wavelength light, the Predator’s blood soaks up the light in the environment that’s most easily detected by other organisms and radiates it back out as light that is detected very poorly. An injured Predator thus doesn’t leave a glowing blood trail back home, but leaves dark, difficult to see blood behind. Earth, unfortunately for him (but maybe fortunately for us), is rich in medium-wavelength light and many land-based organisms are much more sensitive to that light than to ultraviolet light. This means that the Predator’s blood does exactly the wrong thing for him: It turns light that we couldn’t see well into light that we detect very easily! The lesson here is that visual ecology matters, shaping aspects of physiology that we might not have immediately linked to vision.

Conclusion

So there you have it - The Predator offers not just a tour-de-force of 80’s action movie magic, but also serves as a nice xenobiological study of color vision. Though I sincerely doubt that all this was on the design team’s mind as they were putting everything together, I sort of love that it all coheres in a neat way that combines different aspects of graphics, biological and artificial sensors, visual ecology, and optics. To put it more simply, I think it highlights one of the great things about perceptual science, which is the fact that your experience is the data. One way to describe what vision science is to people is to say that I want to know why the world looks the way it does. Underneath that statement is the fact that the way the world looks to us reveals a ton about how our minds and our brains work, even if they’re from a faraway planet that’s populated by murder-aliens.