In perhaps the most graphic form of Kick the Dog, writers often display how vicious, ravenous, or just plain evil a predatory species is, by having the strong among them literally prey upon the weak. While cannibalizing outsiders or eating sentient creatures is nasty enough, doing so to members of one's own social group is indicative of betrayal as well as cruelty, suggesting an evil or Horror Hunger so indiscriminate that no one is exempt.
With non-sentient creatures, Monstrous Cannibalism conveys how voraciously hungry the predators are, and may indicate that their species as a whole is starving and desperate. For sentients, it demonstrates a ruthlessness that bodes ill for any other species they interact with: if their instincts or culture make eating their weaker fellows a standard practice, what hope might a stranger have of being spared? In either case, seeing them turn upon their own for food is often what pushes a species over the Moral Event Horizon into being monsters in the eyes of the audience, not just beasts or antagonists.
Subtrope of I'm a Humanitarian; often a supertrope to Cannibal Tribe, if the tribe eats its own as well as enemies/strangers. Compare To Serve Man, in which victims from outside one's social group are preyed upon, and No Party Like a Donner Party, in which cannibalism is an abhorrent last resort. Contrast No Zombie Cannibals and Ape Shall Never Kill Ape.
May result in a Food Chain of Evil within the group. A common way for the Monster Lord to establish its dominance over its underlings. Sometimes overlapped with Always a Bigger Fish.
The intelligent undead of Marvel Zombies are quite open about the fact they'd like to devour one another, but are balked from doing so by the fact that zombie flesh tastes unbearably awful to them. A few stubborn ones try anyway, but have to spit out any bites they take out of a fellow-zombie.
In Reign of Fire, the gigantic male dragon pounces on and catches one of the smaller female dragons, presumably to eat. The humans who witness this conclude that the dragons must be starving.
In Pitch Black, when the last survivors are running to the shuttle, Alien Blood starts falling all around them from the flock of creatures above the canyon. It appears that the ravenous creatures have begun killing and eating one another in midair.
In Lord of the Rings, orcs will resort to cannibalism if other meat isn't available. The Two Towers has a scene in which an orc captain executes one of his soldiers for disobedience, then declares "Looks like meat's back on the menu, boys!" as the rest of his men tuck into the corpse. (Averted in the books, where orcs are disgusted by the thought of eating orc flesh, though they have no taboos about eating humans.)
In the Lost in Space movie, the space spiders immediately eat any of their number who are injured. This turns out to be a Chekhov's Gun when Future!Smith (who's been mutated into a Half-Human Hybrid by being bitten by one) is eaten alive after being stabbed.
"They eat their wounded."
In Galaxy Quest, when the team comes across a group of blue teletubbie-esque creatures, one of them is injured so is offered some water by its friends, until the others immediately jump it and start to eat it.
In Pandorum, when Bower, Manh and Nadia kill one of the Hunters, the rest of its pack show up and immediately start to devour its corpse.
At the end of "The Lurking Fear", one of the odd-eyed subterranean monstrosities turns on a weaker companion and eats it. To the narrator's eyes, it looks like a routine practice.
In The Dream-Quest Of Unknown Kadath, it's mentioned that ghouls traditionally eat their own wounded after a battle, although Pickman has been discouraging the practice.
Taxxons from Animorphs suffer from a Horror Hunger so extreme that they will dive down and eat anything wounded in the area, be it humans, other Taxxons, and of course...
Jake: <Is it dead?>
Ax: <In a matter of speaking. One half of it is consuming the other half.>
In Chess With A Dragon, the mantis-like alien race's culture incorporates cannibalism of one's rivals or inferiors, to such a degree that "gracing the table" is a standard penalty for anyone that loses badly while gambling. Two of the ones involved in the novel's conspiracy actually formed their alliance by working together to ensure another's loss at the gaming table, purely because they think the targeted player looks tastier than the others.
Deconstructed/subverted in Snuff. Goblins are known to eat their young, a fact repeated by the average racist (basically 99% of the Disc's population) to prove they're "evil beasts". What is not widely known is that goblins only do this when faced with famine that would guarantee the death of both mother and child, and that the mother then makes a special pot to contain the child's soul until it can be reborn, hopefully under better circumstances.
Referenced in Unseen Academicals, in which history depicts the race of orcs as savage warriors that devoured their own dead after a battle. Whether this really happened is unclear, although Nutt's ability to revive when stabbed suggests that fallen orcs vanishing from battlefields because they got back up again may have been misinterpreted as proof they'd been eaten.
When junior postman Stanley tries to parley with Mr. Grylle in Going Postal, suggesting he use the post office to send a letter to his grandmother, Grylle matter-of-factly replies that he ate his grandmother. The impression conveyed is that wild banshees don't consider this unusual.
In Beasts of Gor Tarl and his friend are being hunted in the Gorean Arctic tundra by a pack of wild sleen, tenacous hunting mammals with six legs. Tarl's friend sets a trap: a knife with a piece of meat, its hilt frozen in a large slab of ice. One of the sleen bites the meat off the knife, slicing its mouth in the process. It then tries to lick the blood off the knife, slicing its tongue further. The other sleen, smelling blood (literally), kill it and eat it.
In The Wheel of Time, it is often said that Trollocs "will eat anything, so long as it's meat." They prefer humans, but will eat their own if it's convenient - trollocs are very lazy. This actually works to their advantage in war, because they need no supply lines: after a battle, they just cook up their own fallen.
In C. S. Lewis' "The Pilgrim's Regress", John and Virtue come upon a dragon hoarding its gold. In the dragon's poem is a line like "At times like these I wish I hadn't eaten my wife." He did so not out of hunger, but out of greed and paranoia regarding the gold.
On Primeval, the team catch a baby raptor in a shopping mall, then use it as bait to set up an ambush for an adult that's still on the loose. They expect that, being from the same pack, the adult will come to the crying hatchling's aid, but it pounces on and eats the leashed juvenile instead.
The Hundjager, from Grimm, are said to consume their mother from inside the womb. They have been shown to be cold, unmerciful, and relentless trackers and assassins who usually work for the Verrat.
This trope applies to many evil Dungeons & Dragons races, but is a particular hallmark of sahuagin culture, in which the maxim "Meat is meat" is used to justify their preying upon everything from other races to deformed sahuagin hatchlings to their own dead.
Subverted in depictions of trolls from the Mystara D&D setting. While they readily chomp on one another whenever they're feeling peckish, their regeneration powers ensure that any troll who's nibbled upon will quickly and painlessly recover. It's implied that some man-eating trolls are actually inoffensive by nature, but simply too stupid to grasp that non-trolls they try to make friends with don't like having their arms or legs bitten off.
The vampire bloodline called the Noctuku are apex predators to apex predators, preying on other vampires' blood and flesh. If they drink all of their vampire victim's blood they commit diablerie by consuming their soul. While Noctuku are normally loners, they're known to form Cannibal Clans called clutches that systematically prey on other vampires. If their activities are ever discovered, they're hunted by vampires unironically enforcing the double standard that only humans should be hunted and killed.
Another bloodline, the "En", are also cannibals... but of the megalomaniacal king variety. They gain power over a city and give free reign to their dark appetites, creating children only to feed on them later (unless their child gets the drop on them).
In Warhammer40000 the Tyranids consume all lifeforms, including their own units once they're no longer needed.
Arcanum: The chapter of the manual dealing with Orcs notes that they prefer to cannibalise rather than bury their dead. It's also an accepted orcish custom for an orc mother who gives birth to triplets to present one of the newborns to the father, who eats it in celebration of his own fertility.
It is implied that Seekers of the Name from Fallen London will eat other humans when their Horror Hunger is out of control. Your character can do it too, if you're a Seeker.
In Elona, cannibalism causes one to go temporarily insane, unless one undergoes a mutation that makes them OK with eating other people. From that point on, they'll kill and eat anyone they encounter, each other included.
The Primal Zerg in StarCraft kill and eat other creatures in order to absorb their genes and become more powerful. They gain the most essence by eating one another.
In Mass Effect 3, you encounter a new type of mooks called 'Cannibal'. Its 'special move' is to eat their own kind in order to restore health, showing how much of an eldritch abomination the reapers have turned them into.
When the Kvrk-chk are introduced in Quentyn Quinn, Space Ranger it's stated that they devour their dead on the spot, at social gatherings it's considered common courtesy to tear off a limb as a gift to the host, and their youngest adults have to disperse far from home every thirteen years to avoid being eaten by their younger siblings. To say nothing of their attitude towards other sapient races.
Stated to be the case in Drow Tales; Drow (especially homeless lowborn) will eat each other, although the Drow are on dire straits by the time the comic takes place. All the soft tissues usually pink on a healthy elf are purple from malnutrition; and even clan matriarchs have this.
In Australia, the decimation of rodents' natural predators in a farming community caused an explosion in the rat population. In desperation and starvation, the stronger rats preyed on weaker ones until they virtually died out.
Ant colonies regularly "recycle" injured or dead colony members as food. Averted if a dying ant suffers from an infectious disease its fellows can detect by smell, in which case it's killed and dumped outside the nest to prevent sickness from spreading.
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You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.
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