Welcome to Science of the Scare! Every month I will dissect a Big Science Question from a horror movie and talk through it in (mostly) easy-to-digest terms.
Science and horror have a wild, entangled history and have left us with loads of questions to ponder. Deep, important questions like: just how many ways could we have a zombie pandemic? Is genetic engineering always a slippery slope to monstrosity? This month’s Big Science Question:
Would coating yourself in mud really hide you from the Predator?
In John McTiernan’s Predator (1987), we got our first glimpse of a terrifying alien hunter that was not only physically superior — able to leap deftly into trees and one-handedly rip a human’s spine right out of its body — but also armed with incredibly sophisticated technologies. Squaring off against it in the jungle, merc Major Alan “Dutch” Schaefer (Arnold Schwarzenegger) is able to hide temporarily by coating himself in mud. But could that actually work? Can you actually hide in plain sight from a creature built to be the ultimate hunter?
First let’s talk about the Predators themselves, actually an alien species known as the Yautja. While the most recent instalment in the series, The Predator (2018), implies that the Yautja are hybridizing with other species to give themselves and all-around biological edge, it’s clear that so many of their adaptations point to a preferred desert, or at least hot-climate, environment. Their skin has a reptilian look to it, and the scaliness helps to reduce water loss, and even though they can tough it out in the extreme cold of Antarctica, most of the movies suggest that they definitely prefer to be warm. They might even be cold-blooded ectotherms: the novelization for AVP: Alien vs. Predator (2004) explains that the Yautja’s suit contains a thermal netting to keep its body warm.
And then they also have heat vision.
A number of real reptiles, snakes such as pit vipers, can also pick up infrared signals, so we can use these real examples to try and understand the Yautja. Pit vipers don’t actually perceive infrared heat signatures with their eyes. Instead, snakes that can perceive infrared have holes in their faces near their eyes called pit organs (the “pit” part of the pit viper’s name has nothing to do with a hole in the ground). The pit organs are lined with a membrane that can detect infrared radiation off warm bodies as heat, not as visible light. According to a 2010 study, the membranes activate when a threshold temperature is sensed (for some rattlesnakes, this temperature was measured as around 28˚C/82˚F, or around body temperature if you’re a mouse). The snakes can then assemble this heat information to “see” their prey up to one metre away, just not in an actual visual way. This heat detection is especially useful at night when the warmth coming off preys’ bodies can stand out more against the cooler night air. A really early study into infrared receptors from 1956 found that snakes with pit organs can detect a human hand better in a refrigerator than at room temperature.
Just like pit vipers, Yautja thermal vision works better when there’s a big temperature difference between its prey and the environment (and unlike pit vipers, the Yautja are actually seeing infrared images with their eyes). Luckily—well, not so luckily for the prey, they have helmets that enhance the wavelength-range of their vision and help them detect subtler differences in temperature, not to mention that they can spy humans from much farther off than one measly metre. So, while Dutch might technically benefit from the hot and humid jungle masking his heat signature, the Predator hunting him down can refine what he sees to track him down even in the daytime.
“The thing with using mud as a camouflage from infrared vision is that there are so many factors working against you.”
Here’s where mud camouflage comes in. Mud has historically had a role in visual camouflage for people who want to hide in the wild, but that’s for when we are trying to hide from others who see in a similar spectrum as we do. When Dutch coated himself in mud and guck, he was less relying on this camouflage technique and more drawing inspiration from animals like pigs who take mud baths to cool down. Pigs don’t have sweat glands to cool them down, but mud has the same effect: the water trapped in the mud cools their skin and evaporates more slowly than a straight-up water bath would.
As a mini-experiment, I borrowed an infrared camera to test how well mud could shield my skin’s heat signature. I compared two stripes of mud, one heavy stripe of mud (left in the second image) and one thinner, more evenly spread strip of the same mud (right in the second image). Both applications of mud managed to cool my skin down a few degrees, from almost 36˚C/97˚F pre-mud to 33˚C/92˚F for the heavy coat of mud and 34˚C/93˚F for the lighter coat of mud. After a few minutes, the thin coat of mud had dried completely and had warmed back up to my skin’s original temperature, and the heavier stripe was warming up, but much more slowly.
After about 20 minutes, the heavy mud stripe had only warmed up about one degree Celsius to 34.6˚C/94.3˚F. I got bored of watching mud dry on the back of my hand at this point, so I didn’t wait to see how long it would take for a heavy coating of mud to warm back up to my original skin temperature. On the bright side, the back of one of my hands is very soft now.
The thing with using mud as a camouflage from infrared vision is that there are so many factors working against you. First off, whether you’re applying mud with a light touch or a heavy hand, it will lower your skin temperature slightly, but nowhere near to the temperature of my environment. If I were outside in a jungle at a part of the day when the air is closer to my body’s temperature, I might have had better luck, but as you can see in the photos, there’s a huge temperature contrast between my body and the inside of my house (a difference of close to 10 degrees Celsius), and the mud’s cooling powers just aren’t enough to hide me.
Secondly, your mud is really only useful while it’s wet, so you’ll want to coat yourself with a thick layer to buy you more time. The trouble is, you’ll also want to be really good at covering yourself in mud as evenly as possible, because any thinner patches might reveal you as an unusual shape worth investigating, especially since the water will evaporate more quickly from areas where the mud is thinner and so will warm back up to body temperature faster.
“Getting coated with mud worked for Dutch, and it might work for you in a pinch if you find yourself trying to hide from a Predator.”
Finally, some parts of your body are warmer than others, and some parts are pretty much impossible to hide without extra equipment. If you’re lucky, it might not be a problem, but hot air being exhaled through your nostrils or out of your mouth will show up super clearly via infrared. Holding my breath and keeping my mouth closed didn’t keep my (admittedly un-muddy) mouth from glowing like a beacon with infrared imaging.
Getting coated with mud worked for Dutch, and it might work for you in a pinch if you find yourself trying to hide from a Predator. The good news is, you probably don’t even need mud to hide because plenty of materials part of your everyday life are much better infrared insulators. One great one is glass. If you ever wondered why biologist Casey Brackett (Olivia Munn) was basically able to hide in plain sight in The Predator while a Yautja was rampaging within a military lab, or why other unnamed scientists seems untouched in the background, it was probably because there was a pane of glass between them and the alien. Actually, the fact that the Yautja managed to notice Casey at all through the glass without its sense-refining helmet runs contra to what we know about Yautja biology.