My Instincts Are Showing

"Covered with towels? Ack! My instincts are showing. I just buried a Tupperware bowl."

- Florence Ambrose, Freefall

A non-human character tries to live amongst humans. Or maybe a human character is made half-non-human. He does a good job of hiding his non-humanness, but occasionally his species's normal (and non-human) behavior comes out.

Whatever the reason for a transformation may be, this is almost guaranteed to happen during or after Shapeshifting. This may include food preferences unusual for their current form or other stereotypical behavior.

In cases of shapeshifting, it's similar to The Mind Is a Plaything of The Body. For non-shapeshifting cases, it's similar to Fee Fi Faux Pas. Compare also with Furry Reminder.

Anime & Manga

 * Happens a few times to Katy in The Fantastic Adventures of Unico when she gets turned into a human girl.
 * Greed's chimeras in both versions of the Fullmetal Alchemist anime are subject to this, most notably the snake girl.
 * Later, when Heinkel (a lion chimera) is fighting Pride, Pride notes that Heinkel isn't even hesitating to attack someone who looks like a little boy. Heinkel says that his animal instincts are screaming at him to kill the "boy" before he gets killed first.
 * Satsuki, the werewolf in Millennium Snow, actually fetches a stick thrown by another character.
 * In Tokyo Mew Mew, Ichigo gets infused with feline DNA and suddenly finds herself snatching fish in her teeth.
 * She also takes cat naps, much to her distress, especially when it happens in class.
 * Animals turned human or humanoid in Princess Tutu usually act like humans, but almost all of them have at least one tell. For the protagonist, it's a tendency to waddle when she walks.
 * In chapter 30 of Ratman, Unchain gets a variant. He has the power to absorb the DNA of things he consumes, which have mostly been normal and genetically engineered animals. When fighting Ratman, every single cell in his body screams that he can't possibly win, that Ratman is about a hundred steps higher on the food chain than him. Considering that he considered himself the top of the food chain (which is probably also where his name comes from), its something of an ironic defeat.

Comic Books

 * In Sam and Max, one comic story has Sam (a dog) incapacitate a crook by biting his arm. It works, but afterwards, Sam says to the audience that he's never done that before, and he's really embarrassed.

Film

 * In An American Werewolf in London, before David turns into a werewolf he starts having dreams of hunting, killing and eating a deer in the forest.
 * In the Cat People remake, the first time Irena comes close to transforming, she strips off and hunts a rabbit in the bayou. Earlier, when frightened, she leaps to the top of wall. Her brother Paul often climbs trees and after a transformation, will eat the leftover skin clinging to his body.
 * In The Black Cauldron, Fflewddur Fflam keeps getting turn into a frog and back again by witches. Moments after he's turned back to normal for good, he catches a fly with his tongue and eats it.

Folklore & Mythology

 * One of Aesop's fables involves this. Two of the gods have a bet as to how much people can change themselves. One of them turns a cat into a human woman, and sets her up to marry. The other god demonstrates that some things never change by causing a mouse to run through the wedding ceremony, causing the cat-bride to embarrass herself when she tries to chase and eat it.
 * Ambrose Bierce does his usual piss-take on it by having the cat-turned woman not try to chase it, but rather to scream and carry on so much that the human she was falling in love with gets fed up.

Literature

 * Werewolves in Discworld have some doggy instincts even in human form. They tend to wince at the word B.A.T.H., for instance. Angua is the only one who tries to suppress these instincts.
 * Some Vampires have almost as much trouble, particularly with the reversed-names thing.
 * Played horrifyingly straight in Witches Abroad with the Big Bad Wolf.  tried to "enhance" a wolf so that it would think and speak like a person... which left the beast a tortured mess torn between human logic and animal instincts.
 * Happens in the Animorphs series. Once, while morphing seagulls, they found it hard to maintain a train of thought because they kept greedily eyeing every dumpster with a half-finished bag of chips sticking out as they flew over.
 * Actually, happens at least once a book, what with it trying to be educational and teach kids about different animal instincts.
 * Inverted in the Belgariad. A sorcerer who takes the form of an animal will find that they start to acquire that animal's instincts and some of its behaviour. This gets worse the longer you spend in that form, and as Belgarath notes when the time comes to change back to your natural form you might not want to. However, as a side benefit you instantly acquire the language of the species you change into and retain it when you change back, because all animal languages are the result of brain patterns not culture.

Live Action TV

 * All the transgenics from Dark Angel are subject to this occasionally. Dog-based Joshua growls, snuffles interesting things, and acts like an excited dog around food; cat-based Max and Alec show varying feline behaviors like distrusting dogs, fastidiously bathing, acting predatory around prey animals, picking the people who least want to see them at the moment and sitting right next to them...

Tabletop Games

 * GURPS calls this Stress Atavism. Most templates for uplifted animals have it.
 * This is common in Changeling: The Lost, as shown in the Curses for changelings of the Beast and Elemental Seemings. Beasts spent so long as wild animals that thinking non-instinctively is hard for them, resulting in a penalty of rolls requiring Intelligence. Elementals, likewise, spent so long as a primal force that interacting socially with other humans is a rough affair.

Theater

 * Nick Bottom in A Midsummer Nights Dream is an especially funny example; he isn't aware that he's got an ass's head, he just knows that he's in the mood for oats, dried peas and a bottle of hay.

Video Games

 * In the Black Velvetopia level of Psychonauts, Raz gets the bulldog painter to help by asking "Who wants to go for a walk?" The painter struggles against, but gives in to his "stupid dog brain."

Webcomics

 * All over the place in Freefall, the Trope Namer.
 * Florence the wolf is the main target.
 * She puts a bowl of leftovers in the fridge covered in towels, as mentioned in the page quote.
 * Smacks her nose against the screen when a mouse goes by.
 * Is unable to avoid barking at a cat in an early strip.
 * Has trouble trusting Sam because she thinks he's acting like a low-ranking pack member.
 * And this (explanation).
 * Frisbee!
 * Weaponized! By Sam!
 * Sam is prone to falling back on his scavenger-instincts too, claiming that "Food tastes better when you steal it from a predator", among other things (though often he does this deliberately).
 * Now, watch Sam and Florence's instincts crash into one another; what Florence the social predator sees as deferent behavior from a lower-ranking pack member is considered to be nigh-heroic alpha male behavior by Sam's species (provided you do it to your enemies; Sam got chased off his homeworld when a stunt like this got out of hand and ended up seriously inconveniencing his own royal family).
 * From Girl Genius comes Krosp III, Emperor of All Cats - basically, a cat with human-level intelligence, speech, thumbs, and the ability to walk erect... he'd like to claim that his 'cat-instincts' have no hold over him, but Agatha rather enjoys proving otherwise. Using a piece of string.
 * Eerie Cuties is on a gleeful rampage with this.
 * Kade's reaction on Ace is an example.
 * Brooke used it.
 * Nina as a chocolate-loving vampire has her instincts rather... haywire.
 * Also, the unfortunate story of Brooke and her pet mouse, Skittles.
 * In Homestuck, after  he is annoyed by his new dog instincts
 * And now  can't keep from barking at a cat.
 * Even after gaining a genius-level intellect, Scratch Fury, Destroyer of Worlds still succumbs to his feline instincts on occasion.
 * Head Trip once Discussed the missing (and rather inconvenient) side of Catwoman.

Web Original
"Capt. Storm: Instinct. What a love-hate relationship."
 * In the rebooted Furtopia RP of ~Darwin's Soldiers~, Captain Kayle Storm (an unspecified lupine soldier) Lampshades, subverts and plays this trope straight. When his rifle jams, he clubs a terrorist with it and latches his jaws onto the terrorist's face and proceeds to maul him. Subverted in the fact that he spends a few seconds vomiting after he does so.

Western Animation

 * Brian from Family Guy lives this; his family members even occasionally invoke the trope to distract and/or humiliate him. For example, he will chase balls, is attracted to the sound of dog food in a bag, and is terrified of the vacuum. He also has a tendency to bark at strangers, especially black strangers (subconscious racism which he is ashamed of).
 * Probably best represented in an early episode where Brian goes off to see the world. We see him digging for fossils (with a trowel), uncovering a bone and being proud of his discovery. Then he immediately re-buries it (with his hands/paws) and sits on the spot, looking around to make sure nobody tries to take it.
 * Every now and again Stewie gets a "baby reminder" (being unable to see Peter when he covers his face with his hands, for example), just to point out that he's still technically an infant.
 * On Fosters Home for Imaginary Friends Mr. Herriman, a rabbit, has an addiction to carrots that he's ashamed of.
 * A common gag in T.U.F.F. Puppy. Most often in the form of Dudley chewing his own butt. Kitty acts on her instincts much less often than Dudley, but give her a cat toy and...
 * The Chameleon often blows his cover because whenever he sees a bug he has to eat it.