Some times there is a reason for a lust for pink matter: the monster thinks it will get the powers/knowledge/memories of the unfortunate brain owner. Other times... well, maybe brains just taste good.
If there is a brain eater on the scene, expect to see a scooped out head at some point, if for no other reasons that it looks really terrifying. If this is a comedy, there will nearly always be one person 'immune' to the problem, due to not having any brains to begin with.
The trope may be of particular saliency now because we no longer tend to believe that the essence of one's individual humanity, call it a 'soul', resides in (say) one's heart or (as the ancient Greeks supposedly thought) the liver; most of us would tend to place any such thing in the brain, with the notable exception of Stephen Colbert, who prefers the many neurons found in the gut. This gives the trope the impact of some other cultures' urging the victorious to eat their vanquished enemies' heart, or of the torture of Prometheus by Zeus' eagle's daily meal being the Titan's liver, which regrew each day for torment's sake.
The subtrope of brain-eating zombies seems to have originated with 1985's Return of the Living Dead with the famous character Tarman shouting "Brains" throughout the film, and most of the time when it appears it is either a reference to that film, or unknowingly influenced byother references to that film. Before Romero, zombies were not expected to eat people at all, except in some fringe Haitian legends which included anthrophagy as part of a general rampage on the part of the normally passive zombies whose controlling voudun priest had died.
It should be noted that most zombies of the Romero sort just eat whatever meat off the person until they get bored. Zombies of the Russo mold are more inclined to eat people's brains, and since Removing the Head or Destroying the Brain is the general way to kill many such zombies, new zombies tend to arise through infected survivors of zombie attacks rather than merely being killed by the zombies like in Romero's work. As you'll read down the page, you'll find most examples to be zombie free.
A subtrope of Picky People Eater. See also: Zombie Gait. Not to be confused with Brainless Beauty (although being one may have its advantages.)
For the other type of Brain Eater (which primarily shows up among creators), see Filibuster Freefall.
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A set of HULU ads have various celebrities encouraging people to watch more television, because it softens your brain so they (being, of course, aliens in disguise) can eat it.
Anime and Manga
The first villain in the Gunnm manga has to eat brains to get his endorphin fix and brains have a lot of endorphins. In general, Yukito Kishiro loves finding excuses to draw internal organs.
One of the numerous serial killers in MPD Psycho uses brains as plant food for his favorite flowers... by cutting open their skulls and using them as flower pots. While they're still alive.
In one of the weirder scenes in the film, the apes in Princess Mononoke ask San to give Ashitaka's (unconscious) body to them so that they can eat his brain. They believe that, in eating a human's brain, they will become as strong and as smart as the humans are.
In Hunter × Hunter, the Chimera Ants eat any flesh they can find but prefer brains. Particularly human brains. The Chimera Ant King is even pickier and enjoys the brains of Nen users.
The Venom Symbiote in Spider-Man must eat human brains, which sometimes doesn't agree with his human host. Sure, he could find the same chemical he obtains from the brains in chocolate but where is the fun in that?
It's chocolate. If the Venom symbiote isn't willing to get vitamins from chocolate it must be a horrible, horrible person - oh yeah.
Amusingly enough, there really is a chemical found in the human brain and chocolate: phenethylamine.
What the Symbiote really feeds on is adrenaline. (As revealed in a retcon of Venom's origin.) Because Eddie Brock was dying of cancer, which caused his adrenaline glands to overproduce the hormone, that's what first attracted it to him. (The craving for brains, along with other organs of victims at times, may just be out of pure malice.)
Of course, this all changed when Mac Gargan (the Scorpion) became the new Venom. This version was more insane than Brock ever had been, and became a true cannibal, not picky about it in the least.
On the subject of Venom, the Xenophage is a bug-like alien predator that preys on Symbiote. It doesn't just devour brains, consuming the Symbiote and its host, but it tends to go for the brain first. As a result, during the Alien Invasion of Symbiotes during Venom: The Hunted, Venom is initially blamed for the deaths it causes.
Rhona Burchill, the Mad Thinker, in Ultimate Fantastic Four devised a way to increase her brain's "processing power" by grafting someone else's brain tissue onto her brain, so she killed her little brother because he "wasn't using it anyway" — the implications are roughly equal that he was genuinely mentally deficient and that he simply had a lower IQ then his already-a-genius-but-psychotic sister.
Jack Of Fables introduced us to Lady Luck during his stint in Vegas. She lost a good chunk of her powers due to a soured romance and regains some of it when she eats the brains of high-rollers.
Enigma features The Head, a lizard monster that sucks peoples brains out through their noses.
The first zombie movie that used the brain-eating element was Return of the Living Dead. Here, the zombies eat brains because they give off endorphins that kill the pain of decomposition and rigor mortis.
In the movie Planet Terror, the zombie-like mutants eat Fergie's brain, prompting the gag "Looks like we've got a no-brainer." Similiarly, a repeated line throughout the film is "I'm gonna eat your brain and gain your knowledge," first said as a small boy is playing with his action figures, although by the time Josh Brolin starts saying it, it gets creepy/hilarious.
The Brain Bugs in Starship Troopers, who use a rather straw-like proboscis to stab through the skull and suck out the juicy brainmeats within.
The Kothoga monster from the film The Relic requires certain chemical from human brain to sustain itself.
Hannibal has a scene with a man literally eating his own brains.
In the Japanese movie Gamera Vs Guiron (AKA Attack of the Monsters), two alien women from the planet Terra plan to eat the brains of two children. It actually has a scene where one of the kids is secured, with his head shaved, about to become the main course. And it's a kids' movie.
Funnily enough, at least in the dub, this doesn't seem to be part of their culture or usual plans. Just some random idea that one alien sprang on the other.
Brain Damage features a fast talking brain-eating worm named Aylmer and his current host Brian. The whole movie is a metaphor for drug addiction because in exchange for feeding him brains the parasite injects addictive blue fluid that makes him act as though high.
The evil alien in the Dolph Lungren film I Come in Peace (also known as Dark Angel) injected humans with overdoses of heroin in order to harvest Endorphins, that could be rendered into a powerful recreational drug on his home planet.
Faces of Death had people eating monkey brains right after having to kill the monkey by beating it on the head with mallets which resemble a pestle.
There's a moment in Blood Sucking Freaks where a depraved doctor uses a straw to suck some woman's brains out.
The Brainiac revolves around a magician who was to be executed in 1661, who then used his magic to get on a passing comet and returns to earth 300 years later as a brain-sucking monster. It also brought us the page image.
The Outpost (aka Mind Ripper) had a mutant who fed on brains to get his fix of sterols.
In the original The Hills Have Eyes, Papa Jupiter threatens to eat the brains of the protagonists' kids.
In the anthology film Body Bags, the hair growth experiment in the "Hair" segment is actually part of a scheme by wormlike alien invaders to take over human hosts and eat their brains.
The Mad Scientist in the schlock horror film Blood Relations experiments with brain transplants to give old, decrepit people young, healthy bodies. He also takes one of the brains stolen from a victim and fricassees it on the stove, making a ghastly joke about "food for thought."
An important clue in... well, Clue. The dish that everyone ate in the beginning was Monkey Brain Soup, popular in Cantonese cuisine... and a favourite of Mrs Peacock's, tying her former cook to her in the blackmail scheme. It identifies her as the murderer in 2 of the endings.
An old children's joke is to place a hand on the head of another child and undulate the hand, and ask, "You know what this is? ("What?") A brain-sucker. You know what it's doing? ("What?") Starving."
In Hannibal, Lecter stirfried up some guy's brain. Thomas Harris lovingly describes how Lecter prepared it, table side, fresh from the source (he literally scooped out the slices of brains from the victim's skull at the table, in front of Starling). He gently coated it with breadcrumbs then sauteed it, serving with caper-berries and black truffle brown butter sauce. Just the way that Harris described it is enough to make the reader hungry, especially since he is a foodie and the recipe would actually be delicious (except the obvious part, of course).
Deadhead, from the Wild Cards novels, absorbs the memories of creatures (and people) whose flesh he eats, getting the clearest 'read' from the brain.
In the short story A Midwinter's Tale, eating brains passes on the memories of the dead creature.
In Nation one of the cannibal chiefs mentions he would like to eat Daphne's brains. It's sort of a compliment, it means he thinks she's very intelligent.
From the Star Wars Expanded Universe, you have the Anzati, a species that eats the "soup", also called the "luck" or the "Sea of Memory" of sentient beings. They're humanlike, but with a sort of proboscis in each cheek which unfurls to jam through the victim's nostrils and into the brain, then suck it out. One Anzati claims that the brain-eating thing is the only way to get at the "soup", and apparently lucky individuals are sensed as such and are more desirable. They also have incredibly long lifespans and no natural biorhythm - no pulse, no body heat - and tend to have a hypnotic ability that makes victims submit. To hammer the point home, there are myths about Anzati victims becoming Anzati themselves, and the irreverent slang term for the species is "snot vampire".
Mbwun from The Relic. Human brains aren't its first choice, though, it prefers to eat the plants from the Amazon used as packing material in some specimen crates (which have much higher concentrations of the hormones and such it needs). The events of the novel happen because the crates are moved to a more secure area of the basement after a curator notices they've been broken into, forcing it to search for alternatives (read: brains).
The zombies in Ryan Mecum's Zombie Haiku eat brains. One of the haiku is:
brains brains brains brains brains
brains brains brains brains brains brains brains
brains brains brains brains brains.
One of the short stories in the book My Zombie Valentine focuses on a Zombie Apocalypse originating on the set of a zombie apocalypse film. The Alpha Bitch seems to be immune to zombie attack, and they speculate that she doesn't have any brains. However, it turns out that she's wearing a perfume that repels zombies.
In the Starfleet Corps of Engineers series, the creators of the monster shii on the planet Sarindar fed on brain matter. A now departed race which colonized the planet centuries ago, they built artificial constructs in the shape of shii beasts to collect the heads; these decapitated victims in order to gather the brains.
In Sharper Than A Serpent's Tooth, a group of sentient zombies raised by the Big Bad and sent to slaughter the living reference this trope, debating whether or not they should eat brains and, if so, whether condiments will be required.
The Rodents of Unusual Size from James Herbert's The Rats and its sequels have a preference for brains, and often chew their way through the eyes of their human victims to reach this tasty treat.
Mogworld: Averted and lampshaded. The zombies in the game just don't eat. Using the trope to terrify the nearby populace certainly makes for a fun time on your day off, though.
In The Shahnameh, the evil king Zahhak is cursed by Ahriman with two brain eating snakes on his shoulders, necessitating the daily sacrifice of two innocents to prevent them eating his own.
In Terminal World, by Alastair Reynolds, there are bio-mechanical creatures called "Carnivorgs" that survive by eating the brains of the people they capture. The front of their heads end in a drill, which they use to punch through their victims foreheads. The worst part? This process doesn'tkill the victim.
In Spots The Space Marine both the "Violinists" (allies of humanity) and "Crabs" (enemies) do this to their own kind to transfer memories. Violinists do it to pass on knowledge to the next generation as they have very short lifespans, while Crabs do it to share intel on their enemies, once the squad learns about this they are ordered to kill using headshots. The Crabs also do this to captured human marines but their Fiddler attache assures them that the biochemistry is incompatible.
Lampshaded in City of Devils with the zombies, who turn humans into zombies by eating their brains. And can only communicate using the word "brains" with slightly different inflections.
The Neanderthal Clan Of The Cave Bear practices this as an Older Than Dirt trope, sacrificing a man to the totemic giant cave bear, after which the shamans adjourn to a private shaman soirée deep in the caves, where they pass the victim's skull around and ritually munch his brains.
In The Vampire Chronicles, a witch can absorb the residue of a dead person's soul by eating the brains. As it turns out, the technique also works on living vampires.
Live Action TV
The Psirens from Red Dwarf, Series VI episode I. They look like man-sized insects and will project false visions to their victims. They're perhaps the only brain-eaters to use a straw.
Brain-eating by humans and aliens alike is a constant motif in the first season of Lexx.
Sylar from Heroes steals the powers of others by removing their brains and doing something unspecified with them. Fans love to joke that he eats the brains. This seems unlikely, though, as on the few occasions when he's shown just after he's stolen a power, he's had blood all over his hands, but none on his face.
Word of God says that yes, he was going to eat them, but they realized how ludicrous that would sound, so they left it vague. It remains to be seen whether they'll ever come up with a different explanation.
Molly does make mention that he eats brains in the season one finale. To be fair though, she was just a little girl and probably didn't understand what Sylar was doing while poking around in her father's brain.
There is also a suggestion that Sylar's original power was in being able to understand how things worked, which implies that he might merely need to take the brains out to study them and somehow make the required changes to his own brain. How he would do that is another matter entirely...
This is actually lampshaded in the first episode of Season 3: we cut back to Sylar and Claire after last having seen Sylar cutting the top off of Claire's head, and he is indeed doing something - poking at Claire's brain, occasionally stopping as though examining something. Claire asks, "Aren't you going to eat it?" His response?
Sylar: (incredulous) Eat your brain? Claire, that's disgusting.
Does it really matter what he does with the brain? He's still ripping it out of your skull!
In the Angel episode "Bachelor Party", a demon tries to eat Doyle's brain as part of a ceremony before said demon marries Doyle's ex-wife.
When Data has nightmares in a Star Trek: The Next Generation episode, one of his visions is of Dr. Crusher drinking from Riker's head through a straw. It's unclear if she's supposed to be drinking his blood, cerebrospinal fluid, or (liquified) brains.
Played for Laughs in "Bottle Bash" episode in which Adam speaks to a Jello-brain: "You remember when I hit your head with a beer bottle?" (eats spoonful of Jello out of the brain) "No you don't!"
"Mm, it's a delicious memory!"
On their 2013 Zombie Special — after one test was concluded, Adam placed platefuls of Jello brains on a table and called their test zombies over for a snack.
An episode of Series/Dexter has him discovering a serial killer who eats people in general but has a special liking for the brain, marinading one as a delicacy (which also clears him from being a different serial killer who mutilates brains in order to remove specific pieces).
Averted in The Walking Dead. Those zombies tend to go for limbs and necks instead.
Jonathan Coulton's "Re: Your Brains". It's told as a memo from an office worker who's become a zombie to one who is not yet zombified, and the chorus says "All we wanna do is eat your brains. We're not unreasonable. I mean, no one's gonnaeat your eyes."
And the Lovecraftian coelocanth Aboleths want to eat your brain and gain your knowledge.
Inverted by the Chuuls, who are allergic to brains and will instead peel the flesh and bone from around it and eat that. In an aversion of Gameplay and Story Segregation, this often means that Illithids will train them as garbage disposals for brainless corpses.
The Intellect Devourers eat you brain and then nest in your skull.
Though, in reality, the brain mole doesn't eat the brain, merely absorb psionic energy from it. There's little to no physical harm.
Should be noted that brain-eating is one of the Illithids' greatest weaknesses, because killing a humanoid per illithid per month (any less and they'll die, and more is preferable) draws a LOT of attention. Some sources indicate that illithids can survive on animal brains (and in one case on a kind of moss)... the problem is that the sustenance they gain is proportional to the sapience of the target, and it the lass sapient the target, the less tasty it is, too. A human brain can sustain an illithid for a month — the brain of a bear can sustain it for a few days. Some illithids solve this problem by allying themselves with monsters that eat the rest of the corpses, not just chuul, as noted above, but often grimlocks.
Actually averted by zombies for a change; they don't eat people at all, just kill them because as mindless undead they hate all living creatures. Ghouls and Ghasts fit Hollywood's conception of cannibalistic undead better, but they just eat flesh in general, and don't seem to have any particular fondness for brains.
In Ravenloft, one strain of vampires feeds on cerebrospinal fluid, which is close enough to this trope.
The fan-made Old World of Darkness game Zombie: The Coil had zombies as playable characters. They gained Viscera and sustained themselves by eating flesh. Eating the target's brain granted them the maximum amount of Viscera (though the game noted it takes effort to open the skull).
All Flesh Must Be Eaten, a game entirely about zombies, has "Braaaaaaaains" as an option for a zombie's diet. Because this takes effort and means the zombie needs to eat more people to stay active (as opposed to "All Flesh Must Be Eaten", where they can eat any part), it reduces the zombie's power level.
In White Wolf's Werewolf: The Apocalypse, one secret society of werewolves has discovered that they can gain spiritual power by eating the brains of other supernatural people. Doing so rapidly makes them insane pawns of the Wyrm, however.
In Myriad Song Morphir begin life as ordinary carnivorous plants. But if someone feeds them the brains of sapient beings they become sapient and mobile, as well as produce buds that contain memories from those they ate, often smoked as a drug.
Inspired by Dungeons & Dragons as discussed above, the game NetHack includes Mind Flayers which can suck out your player's intelligence and/or memory of levels you've already visited. The intelligence drain is merely deadly; having to re-explore previously-visited levels is downright annoying. Fortunately, you can turn the tables and possibly gain intelligence points by eating Mind Flyer corpses. The game even announces a success with "That was real brain food!"
By beating them into a fine paste and absorbing it with his Cthulhu-tendrils. It is awesome.
Combine Advisors from Half-Life 2 take the memories and knowledge of their victims by consuming their brains. Or at least we assume that's what they're doing. They ram their tongue in through the brainstem. Of course, they are psychic so it could just be some form of alien data transfer rather than NOMNOMNOMKNOWLEDGE.
The Jonathan Coulton song quoted at the top of this page is one of the songs that will play on the jukebox in Left 4 Dead 2. Despite the fact that the game's zombies don't seem to eat brains, the song's chorus ("All we wanna do is eat your brains...") summons a horde of them to attack the Survivors.
Word of God explains it. The zombies have an advanced form of rabies. Think rabies symptoms x10. This induces insanity, and loud noises and bright lights give them extreme pain. This is why they attack when car alarms are set off, as well as why they attack pipe bombs. It's their way of saying "SHUT UP/TURN THAT LIGHT DOWN, IT HUUUUUUUUUUUURRRRRRRRRRRRRRRRTTTTTTTTSSSSSS".
Zombies want to eat your brains, so in order to stop them you... set up a yard full of plants as a defense? The basic concept of brain eating zombies is also played with a bit in the closest thing the game has to a theme song, "Zombies On Your Lawn".
In addition, the game features a take off of Pop Cap's Insaniquarium, where the fish are replaced by snorkel zombies, and the fish food is replaced by brains.
Parodied in Dragon Fable by the Bread Zombies in the Necropolis' cafeteria who want "GRAINS...GRAINS..."
In Office Zombie, the Zombie leans back and says "Braaaaaains!" as one of his taunts. Also, several brains-related voice clips play in the background like intercom announcements, such as "Brains, line 1" or "Need brains in cafeteria."
In Kingdom of Loathing, During the Zombie Slayer challenge path (in which you yourself are a zombie), you are only able to eat brains. These are not a naturally-occurring food in the kingdom (for the most part; the Zombie Chef drops them now and again), but as it happens, monsters drop brains while you're on the path. You can also increase the drop rate by learning the Skullcracker skill - which means, naturally, you learn how to skip the middleman and get to the grey matter yourself.
In Team Fortress 2, there's a special Halloween skin that you can apply to your characters that invokes this trope, although whether they're really dead or just in very convincing costume isn't clear. For example;
Scout: It ain't original, but its true. I love brains.
Reality On The Norm has the zombie Michael Gower, who, before his soul was restored, was a mindless killing machine with an appetite for brains. In one game (I Spy III), the protagonist has a nightmare about the zombie chasing her to slurp her brains while moaning about various parts of the brain, like "Cerebellum!" and "Medulla oblongata!"
Parodied in Dawn Of The Dragons. One group of zombies moans "brains..." but a friendly necromancer informs the heroes that the zombies are merely telling them they need to destroy the zombies' heads to stop them because they can't stop themselves. Later, a zombie in your army says "Brains..." behind one of your companions which she ignores. Until she slips on some splattered brain on the floor. The zombie promptly says "Told you."
Parodied in Narbonic when Zombie Dave keeps saying "Braaains", even though Helen informs him there's no reason he needs to eat brains.
In The Order of the Stick, there was a time when Elan got separated from the party and encountered a Mind Flayer. He then asked it if it was going to eat his brain. The Mind Flayer smelled him and said "No, thanks, I'm fine". Later on, when the rest of the party caught up, the Mind Flayer jumped over Roy. Vaarsivus immediately complained, since they were the party's mage and therefore had a much more succulent brain.
Zombies in OOTS also seem capable of saying nothing but "Brains...". When Xykon re-animates his dead dog in one of the prequel books, the first thing it does is kill a bird and eat its brain.
In Sluggy Freelance, one of Gwynn's jobs in recent arcs was as a waitress at Zomblebees. The daily special? "BRAAAAAINS".
One of her co-workers was revealed to be an actual zombie. She explained that while zombies don't have a particular preference for brains, you are what you eat, and that to keep command of her mental faculties, that's exactly what she eats.
This is later explained as that the zombies have to eat what they're losing. Decomposing brains = need to eat brains.
An earlier villain was a Mad Scientist who infused her body with Nano Machines and one of the abilities they gave her was to absorb knowledge by eating brains. Biting into Sam's brain later left her feeling stupider (being a vampire who doesn't much use his brain, he felt just fine.)
Something Positive has a rather funny scene where Davan, contemplating the effect of authors' prejudices upon his ability to enjoy their work, imagines John Gardener returning and asking for "Braaiiins...preferably heterosexual braaaiiins..." A nearby male-male couple indignantly respond, "We're here, we're queer, and our frontal lobes are delicious, asshole!"
In Skin Horse, on the St. Charlie train of mad scientists, there is even a Brain-O-Mat for all the zombies onboard.
Unity gets considerably smarter and rational after binging on brains, briefly. It might have something to do with being a swarm of nanobots inhabiting a patchwork corpse rather than a traditional zombie. As another zombie who was smart to begin with gets dumb and lethargic after eating a similar quantity of grey matter.
Parodied and subverted in My Gym Partner's a Monkey episode "That Darn Platypus", where a new platypus student keeps rubbing into everyone's faces that he is an alien. Adam refuses to believe this, but the animals have reason to believe from various signs, such as when he keeps mentioning he enjoys brain juice. Turns out it was just a nutritional fruit juice. And then got inverted towards the end where it is implied that it really is made from brains.
The REAL Alien is the hippo.
In the Invader ZIM episode "Backseat Drivers From Beyond the Stars", ZIM is simultaneously trying to keep his Robo-Parents from eating a child, pilot the (incredibly large) ship of his alien leaders to come to Earth to see his "progress" while trying to keep his rival Dib from crashing said ship, and keep a creature in a containment chamber's status stable. The creature is a giant brain-sucking parasite. No reason is given as to why it sucks the brain right out of your skull or even how ZIM managed to obtain such a thing in the first place (but this is Invader ZIM after all...)
Comedy Central series Ugly Americans features many zombie characters, all of whom seem to have a natural desire to eat human flesh, especially brains. This is Played for Laughs, since the civilized zombies regard brains in much the same way that recovering alcoholics regard booze.
Played with in the Halloween Episode of The X's, the Xs (sans truman) are turned into zombies and moan "Brains", but the zombified Tuesday moans "Spleens" because she's allergic to brains.
South Park did this with Kenny as a zombie.
Eating human brain is (or was) a practice of several cultures, either as a funeral rite or just part of eating people. It's usually for the purpose of taking knowledge, gaining immortality, or whatever.
Women of the Fore tribe in Papua New Guinea ritually ate the brains of the tribe's dead. Which caused the spread of a prion disease called kuru.
Brain (of animals) is actually a delicacy in several countries, which probably makes humans kind of monstrous to them.
Note that the current edition of The Joy Of Cooking omits all recipes that contain animal brains, and states that this is due to concerns over mad cow disease (see above).
If you do feel like eating some, like say scrambled pork brain, keep in mind that this type of food is amazingly high in cholesterol and fat; a little over a 100 g can easily exceed 1000% of the recommended daily cholesterol intake.
There's an account of a rogue baboon killing people and eating their brains.
A cryptid, the Nandi Bear, is alleged to do this as well (and may have actually been inspired by that baboon).
Some accounts of the thylacine, otherwise known as the Tasmanian tiger, alleged that it could bite clean through the skulls of sheep or dogs, then consume the contents.
Most predators will eat brains, assuming they can crack the skull. This means that small prey like mice and lizards, which can be eaten whole or nearly whole, often have their brains consumed, while larger prey like bison tend to have their brains eaten by scavengers instead of the animal that actually killed them (often by small scavengers who simply crawl through the eye sockets). Some predators also take out smaller prey with a bite to the head, as crushing the skull is a quick way to stop your dinner from fighting back.
Models of human evolution that depict early hominids as part-time scavengers often cite the brains of dead animals as an essential food source, as brain tissue contains lots of fats that were otherwise hard to come by on the ancient plains of Africa. As few predators could break into the skulls of their prey, hominids who could beat the hyenas to an abandoned carcass could bust one open with a rock and obtain precious nutrients to feed their own evolving brains' growth.
While planarian worms don't normally eat each other's brains, a certain scientist named Dr. James McConell got the clever idea to teach some worms to run a maze, then fed the trained worms' entire bodies (including brains) to some untrained worms. Amazingly, the untrained worms learned the maze much more quickly, as if they had actually gotten some information by eating worms who knew what they were doing. Apparently, Mega Manning is a thing if you're a planarian.
However, this turned out to be too good to be true, when other scientists eventually realized that the untrained worms learned the maze more quickly because they were following the mucus trails of the previous worms they had cannibalized.