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Living Motion Detector

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"Keep absolutely still. Its vision's based on movement."
Dr. Alan Grant, Jurassic Park

A Living Motion Detector is a person, monster, or robot that can only detect you if you're moving. Depending on its level of intelligence, it may forget about you when you stand still, or it may go looking for you. A common tactic in combating one is to somehow make objects around you move while you yourself stay still, causing it to attack the moving object instead of you.

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Examples:

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    Anime & Manga 
  • Highschool of the Dead: In explaining why their zombies are different, the manga posits that, since the brain and the optical nerves are dead, zombies detect through vibrations instead of visual signifiers. This can be useful for distracting zombies, as a wet cloth thrown against a locker will cause a horde to turn away from the human standing right in front of them.
  • JoJo's Bizarre Adventure:
    • Notorious B.I.G., Carne's Stand in Golden Wind, has this trait. Trish manages to confuse it by using her Stand to shake the chairs on the plane she was riding, leading it to attack them instead.
    • Diego's raptor form, achieved via his Stand; Scary Monsters from Steel Ball Run works like this, similar to the T. Rex in Jurassic Park.
  • My Hero Academia: Mustard's quirk while also being an emitter for Deadly Gas, can also detect movement if anyone passes through it.

    Films — Animation 
  • How to Train Your Dragon: Gobber tells his students that Deadly Nadders have a blind spot due to the horn on their snout. Unfortunately, the don't have a "deaf spot", as Ruffnut and Tuffnut find out after their arguing over the blind spot clues the Nadder to their location.
  • Ice Age: Dawn of the Dinosaurs: When the T. rex appears, Manny tells everyone to stand perfectly still. Unfortunately, one hedgehog doesn't take the hint, and his screaming and running away alerts the T. rex.

    Films — Live-Action 
  • The Deadly Spawn has very toothy grinning aliens who are blind but hunt by sound. Be very quiet and it won't notice you.
  • Dinosaur Island (2014): When confronted with a T.rex, Lucas tries standing still to avoid being spotted. Kate calls him out saying it's ridiculous and won't work ("Where did you get an idea like that?"). Sure enough, the T. rex sees them just fine.
  • Jurassic Park: The Tyrannosaurus rex has vision based on movement. Unlike the book, this is portrayed as a natural feature of the actual species, not a side effect of filling in some genes from frogs. Which is, of course, an example of Artistic License – Paleontology. Actual Tyrannosaurs not only had great eyesight, they might have been better at spotting their prey than eagles.
  • In the film Metamorphosis: The Alien factor, the alien that used to be human presumably worked like this since it had no eyes, and no visible olfactory organ.
  • Pitch Black: A variant, in that the monster can't see you if you stand between its eyes, because of a blind spot caused by their armored heads.
  • Python: The titular giant snake uses motion-based vision. On one occasion, the Mad Scientist who created it and the guy running the black ops are both face to face with it. The scientist stays still and survives, while the team leader gets impaled after trying to draw his gun.
  • Tombs of the Blind Dead: In a variant, the Blind Dead can only track you down if you make a sound. Staying still helps. It can't save you forever, however — they can hear your heart beating.
  • Tremors: The underground Graboids can only detect you by the vibrations caused by your movement — as long as you stay still or keep concrete between you and the soil, they can't tell where you are.
  • The Ugly Dachshund: Invoked, as Mr. Toyama plays dead after he mistakes Brutus for a lion.
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    Literature 
  • Dune: Hunter-seeker drones use an anti-gravity field that obscures their sensors, making it nigh-impossible for their operators to see anything that isn’t moving.
  • Jurassic Park: Played straight in the first novel, the T. rex is explicitly stated to be unable to see Dr. Grant frozen in terror. Unlike the film adaptation, however, it is implied that it was the result of amphibian DNA as other dinosaurs didn't notice Grant and the kids when they're not moving. In the second novel, likely in response to the distilled film explanation, the T. rex's vision problem was explained as an absurd junk theory based on similarities between dinosaur skulls and those of some modern frogs. T. rexes can see things that were standing still perfectly well, as one man fatally finds out. The behavior of the T. rex in the first book is retconned as not being hungry.
  • In Kea's Flight, the engine room robots have weak sensors. They can detect color and shape up to one meter away and motion up to five meters away. When the protagonists are sneaking through the maintenance layer, they have to watch out for robots so they can move a safe distance away and then hold perfectly still until the robot leaves.
  • In On a Pale Horse, by Piers Anthony, Zane faces off against a demon that looks like a 5-meter-tall preying mantis. He realizes that it can only perceive him as prey if he moves. (And he can even do that as long as he doesn't move in a way that resembles the motion of prey.)
  • In Shadows for Silence in the Forests of Hell, moving too fast during the nighttime will draw the attention of the shades. If their eyes are closed, you can move slowly and they won't notice you. If their eyes are green, then holding perfectly still may cause them to lose interest and close their eyes again. If their eyes are red, you'd better run, because only getting behind silver will save you.

    Live-Action TV 
  • Arrowverse: The Black Flash is completely blind except for things moving at Super Speed.
  • Angel:
    • The assassin in "Blind Date" is blind but has learned to detect "other parts of the electromagnetic spectrum." The net result is that she's a human example of this trope, at least when she's fighting someone like Angel who doesn't generate body heat.
    • Also:
      The Groosalugg: I once happened upon a herd of Bur-beasts and As You Know, engorged Bur-beasts will couple with Anything That Moves. I was forced to stand perfectly still for eleven days and nights.
  • Doctor Who:
    • The Raston warrior robot from the episode "The Five Doctors" tracks its targets by detecting movement.
    • Possibly the hand-mines from "The Magician's Apprentice"/"The Witch's Familiar", assuming they're living bioweapons and not machines; they respond to both sound and movement, meaning that they can still spot a perfectly still person if they're making noise.
  • Ultraman Tiga: The monster Silvergon could only see movement, leading to a hilarious scene where Tiga strikes at it, then freezes up as soon as it turns to face him. This gets especially silly when he's forced to hold a precarious crane stance. Eventually Silvergon gets fed up and starts blindly running around, lashing at the air.
  • Squid Game: The giant doll from the "Red Light, Green Light" challenge is equipped with motion camera scans on her eyes. Should she detect someone move when she isn't singing, she sends a signal to the guns adobe her to execute said person at the spot.

    Tabletop Games 
  • Dungeons & Dragons has the shrieker, a giant underground mushroom that reacts to light or movement. Its sole purpose is to alert and attract monsters; in-universe, this is due to it having developed a symbiotic relationship with the game's various subterranean terrors — the shrieker gives the other monsters an early alarm system and in turn the remains of the meals identified this way nourish the shrieker itself.
  • In Warhammer 40,000, according to the 5th edition Tyranid Codex, it was until recently believed that Raveners couldn't see anything that wasn't in motion. It was later proved that they can see stationary objects (and people) just fine; they are compelled to chase opponents, when a biologist studying them ordered his men to remain still when attacked by them.

    Video Games 
  • The Closer: Game of the Year Edition: When the party infiltrates Twitter HQ to salvage the title character's media reputation, they must sneak around employees to ascend the building. True to the trope, some of the employees only send you down a floor if they see you moving. Otherwise, you're practically invisible to them.
  • Deep Sleep Trilogy has the bottom feeders, who will devour you if you move the mouse too far too fast.
  • Detention: In order to avoid detection from the Lantern Specters, you need to turn your back against them, stand completely still, and hold your breath at the same time. Failing to do any these will cause the Specters to pick you up and strangle you to death.
  • Emily Wants To Play:
    • In the first game, Mr. Tatters the Monster Clown will play a game of "Red Light, Green Light" with you. He will turn his back against you every time he appears. You will be killed if he ever finds you moving when he says "uh, uh, uh" like he is trying to find you. Moving the camera also counts. And if Chester appears in the same room as him...
    • In the sequel, Mr. Tatters patrols the rooms instead of spawning out of nowhere. Once he spots you, he will initiate a game of Red Lights, Green Lights and the environment will be illuminated with alternating red or green lights. Moving in his red state will have you killed right on the spot while you can move freely in his green state. Idling too long or attempting to run away from him will only lead to your demise.
  • Fallout: New Vegas has Blind Deathclaws, which have high perception despite their blindness, and which will alert other Deathclaws when they detect you.
  • Five Nights at Freddy's: Sister Location: Funtime Foxy attacks the player based on their movement, leaving them alone if they stay still. Night 3 requires the player to cross a pitch-black hallway and avoid moving when Funtime Foxy is nearby. The return trip isn't as fool-proof, however, but Foxy doesn't kill you after the jumpscare.
  • Gears of War: The Berserker is blind, but can detect where you are through your footsteps, as well as your smell.
  • Half-Life
    • Half-Life has the massive but completely blind tentacles that detect you by noise. You can either crawl past silently, or set off an explosion and run past while it strikes at the louder noise.
    • Half-Life: Alyx has Jeff, a Xen mutant that stalks the brewery in City 17. He's blind, but as a fellow Resistance member explains, he's "got an ear like Mozart". Almost any somewhat loud noise, be it a broken bottle or squeaky valve handle, will set him off and he'll stomp over to investigate; if he finds anything living, Jeff will barf out a stream of Hollywood Acid that'll instantly melt the target. Much of the chapter named after him is based around performing tasks and puzzles while he's around, as he's Immuneto Bullets and impossible to fight directly as a result.
  • Left 4 Dead: Witches don't seem to care about you until you a) get really close, b) make a ton of noise in their direction, c) blind them with your flashlight or d) damage them or knock them back in any way. You'll know how angry they are getting by how much they're snarling in your direction. During daytime maps they get up off the ground and start wandering around; during this period, they're far more lenient towards any provocations and can usually be run past safely.
  • The Legend of Zelda: Breath of the Wild gives us the four Moldugas of Gerudo Desert. If you move at all in the sand with one of these beasts nearby, you can bet it'll sense your vibrations and attempt to zero in on your location. Fairly quickly if it confirms your location.
  • Metro 2033: While calling them "living" is a stretch, the Dark Stars anomalies are mobile balls of lightning that can sense movement. Anything that attracts one too close will get flash-fried to a burning crisp by its electric discharges, be it a swarm of rats, a mob of nosalises or an unwary player.
  • Metroid Dread: The E.M.M.I. are equipped with motion detection. Even if Samus is nowhere near them, they'll detect her movement nearby, and will do a thorough search of the area even if Samus tries to hide with the Phantom Cloak.
  • Minecraft has the Warden, a boss-like mob related to the Sculk. It's blind as a bat, but very sensitive to vibrations like non-sneaking footsteps and thrown objects like snowballs and eggs. Just standing still doesn't help either, as it'll sniff the air to try and locate what provoked it into appearing.
  • Mr. Hopp's Playhouse 2: One section involves a corridor which Esther can only walk through when none of the Big Bad Triumvirate of the killer toys' giant heads are watching from the darkness. Even if they haven't completely left, moving too soon immediately results in getting caught.
  • Penumbra includes the enormous Grey Rock Worms. They fit this trope to a T, judging by the research papers left behind, are very hard to escape from, and they charge like bulldozers towards their prey. It's implied that the Tuurngait virus is what made them grow so large, as nobody factored in how big they could grow when the mine was still active.
  • RATUZ: Near the end of the game, Prisoner Five has to go through a labyrinth that's home to a mole rat-like monster. When it eventually finds Five, it will immediately eat him whole if he makes a movement, but if Five stays still, it will sniff the area around him before leaving. An NPC that you helped earlier even warns Five to stay still if he comes across a creature in the labyrinth.
  • Resident Evil 4: Los Garradors are blind and hear your movement if you're running, as well as your commands to Ashley. Sadly for them, you fight them in the vicinity of bells — one strategy for defeating them involves shooting the bell and then shooting its weak point while it's distracted. Another is simply walking, which they can't hear.
  • A Story About My Uncle has a beast that's essentially a worm with a big eye and lots of teeth. Long before you encounter it, there's a wall carving that warns not to move when the eye is open.
  • Undertale: Doggo can only see moving things. This is largely so he can serve as a Justified Tutorial for "blue" attacks that can't hurt you if you stand still.
  • Until Dawn: The Wendigo have motion-based vision due to deteriorated senses, at one point even attacking each other due to being the only beings moving in the area, and deeming one another a threat by default.
  • Yoshi's Island: One of the enemies is Boo Man Bluff, a blindfolded Boo that homes in on Yoshi whenever the cute little dinosaur makes a sound. That includes jumping, throwing eggs, or even touching coins; the ghosts have good enough hearing to hear the obligatory video game blips and beeps. To add to the fun, the ghost can only home in on the place where you last made the sound, and looks quite clueless and confused when you aren't there.

    Web Comics 
  • Girl Genius: The "Fun-sized mobile agony and death dispenser" clank in this comic reacts to movement, including that of one's lips and chin.
  • Stand Still, Stay Silent: The in-universe Rule #1 after which the comic is titled says it all:
    If you come across a Beast, a Troll or a Giant, do not run or call for help but stand still and stay silent. It might go away.
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    Web Original 
  • Critical Role: In the first episode of the second campaign, when Jester approached her and Caleb, Nott stated to not move because Tiefling eyesight was based on movement. Jester stated that was true (even though it is not true in any edition of D&D) but also noted that she could still hear Nott speaking.
  • Tales from My D&D Campaign: The Kua-Toa can see invisible things only if those invisible things are moving.
  • The SCP Foundation has a variant in the form of SCP-372, the Peripheral Jumper. It's not so much the overall motion it detects. Rather, it's where you're going to look, so it always moves out of your view. It's called the Peripheral Jumper since the only recorded visual of it is it showing up in the corner of the image as it darts out of the way, or rather, the peripheral view. If there's literally no way for it to jump out of somebody's view (like say, having multiple people looking at it from many angles), then it will make one, often by gouging somebody's eye out and using the ensuing pain and chaos to slip out of sight.

    Western Animation 

    Real Life 
  • In case you were wondering, the real Tyrannosaurus rex completely and utterly averted this. Some scientists have examined the structure of its eye sockets, and suggested that the animal's vision was at least as good as a human's or even an eagle's and likely possessed superior binocular vision, par for the course given its existence as a complex, large predator. In fact, T. rex and its kin had the best vision of all the carnivorous dinosaurs; other giant theropods like Allosaurus and Giganotosaurus had very poor vision and mainly relied on their sense of smell, while raptors like Deinonychus and Velociraptor were better, but still inferior to humans and T. rex.
  • While giant trevally have excellent sight, they are capable of calculating the motion of their prey, and remarkably, one population in the Seychelles have figured out how to catch birds in this way, expertly predicting and following their flight paths and instantly seizing any unlucky enough to land on the surface of the water for too long.
  • Frogs are extremely nearsighted, but capable of detecting movement at a distance.
  • Many insects. The compound eyes present in many species are awful at making out detail, but excellent at detecting even minute movement.
  • As well as many fish.
  • Dogs are visually more attuned to motion than color.
  • Some strains of the Siamese cat have poor eyesight due to genetic problems. These cats react better to motion than stationary scenes or objects as a result. Some will move their heads to produce the necessary movement in their field of view so that they can determine where everything is.
  • While many people think that bulls hate red, they're actually enraged by the motion of the cloth, not the color.
  • Deer, although they can certainly see you standing there, aren't particularly alert to stationary humans. As they're also not good at judging distances visually, the best way to get close to one is to wait until it lowers its head to browse and then quietly walk straight towards it, freezing when it looks up. So long as it doesn't see lateral movement, it won't catch on that you're coming nearer. This is (part of) the source of the phrase "deer in the headlights": a deer that wanders onto a highway will take note of oncoming cars, but won't register that they're coming closer until it's too late.
  • An item that is camouflaged to blend in with its current surroundings is generally a lot easier to pick out if it is in motion rather than stationary.

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