Biology of Ghouls

This information was gathered by myself as a constant work in progress document. Information may not be 100% correct. Questions asked in comments will be answered to the best of my ability, and possibly added to the information shown here.

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What are Ghouls?
Ghouls are typically defined as undead beings present in various cultures and folklore. The sort of ghouls that will be described in this document, however, are not undead. They are living beings, like humans. The key differences between ghouls and humans is the possession of kakugan and kagune, along with the requirement of human flesh for sustenance. Ghouls and humans are nearly identical to one another and have no immediate characteristics that would differentiate themselves upon first glance; their differences are in their diet, mentality, and inner biology. While ghouls and humans generally cannot produce offspring, if certain specific conditions are met, ghouls and humans can produce offspring (see: one-eyed ghoul and half-human) As ghouls are defined as predators of humans, they are commonly hunted by the Commission of Counter-Ghoul (CCG).

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Biology While humans and ghouls are nearly identical to one another, ghouls possess kakugan and kagune, which can be controlled at will, and a higher RC cell count.

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RC Cells Both humans and ghouls carry RC cells (AKA ‘Red Child’ cells, as they appear as a red human fetus) in their bloodstream. Ghouls carry a higher RC cell count than humans. Human’s RC count generally appears in the hundreds, while ghouls carry an RC count usually over one thousand.

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Kakugan
Kakugan are one of the main identifying traits of a ghoul. These are located in the eye(s), and change the eye color of the ghoul by activating RC cells. When a kakugan is activated, it turns the sclera black, with a red iris, and red veins through the eye, and onto the skin around the eye. This occurs when a ghoul is excited, starving, or during activation of their kagune. There has been no known purpose of the kakugan. In a normal ghoul, the kakugan is activated in both eyes. If it is only in one eye, it is a one-eyed ghoul.

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Kagune

(image above: placement of kagune sacs labelled by type)

Ukaku
In an Ukaku type, the kagune sac is located behind or above the shoulder blades of a ghoul. Ukaku types rely on quick movements and speed, however, their faults lie in their lack of stamina. These ghouls can overcome the rounded nature of a Bikaku by using their signature long-ranged attacks, however, they are at a disadvantaged when up against a more durable Koukaku.
Koukaku
In a Koukaku type, the sac is located in the central area of the back. Koukaku types lack in speed what they gain in durability. These kagune typically appear in a shield-like formation and wraps around the arm. They succeed against the quick Ukaku with their durability, but lose against the more offense-built Rinkaku.
Rinkaku
A Rinkaku-type kaune sac is located in the lower back. These kagunes typically appear as tentacles, varying in number. They are built for brute-force, powerful attacks, although the kagune itself is rather frail. This kagune overwhelms the slower Koukaku, yet the more rounded Bikaku is able to sever the fragile Rinkaku.
Bikaku
The Bikaku-type sac is located in the lower-lower back, around the tailbone. These kagune often appear as one or more tails, and have no clear nor distinct weaknesses or strengths, often used in quinques for beginner investigators. These kagune are often used as a sort of “trump card” in battle. These kagune are more durable than the Rinkaku and can shatter the more fragile Rinkaku.

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Diet Due to a particular enzyme their body produces, ghouls are incapable of digesting any food other than humans and other ghouls (see more: kakuja). Their sense of taste is different, making any other food taste disgusting and uncomfortable, causing a powerful urge to vomit. If ghouls are forced or willfully attempt to digest such food, it will weaken their physical condition considerably.

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One-Eyed Ghoul
One-eyed ghouls are ghoul-human hybrids, that lean more to the ghoul side. They possess the same abilities as a ghoul, and appear as ghouls in scans. These rare specimens are often treated as an urban legend or unheard of to normal ghouls. These hybrids get their name from the single kakugan they possess.

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Half-Human
Half-humans are ghoul-human hybrids, but differ from one-eyed ghouls by leaning more to the human side. They do not carry kakugan nor kagune, yet register as ghouls in scans. These hybrids have higher physical capabilities in combat and experience accelerated aging.

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Kakuja
A kakuja is a rare phenomenon that is created when a ghoul repeatedly cannibalizes other ghouls. This occurrence is more likely in wards that have a higher ghoul-human ratio. Through consumption of both humans and ghouls (the latter most importantly), a ghoul absorbs RC cells, increasing the RC cell count present in their body. If enough cells are present, it may trigger a mutation that forms a second kagune. These transformations start off appearing as human-like, but can develop into a larger, more monstrous form. Kakuja can be incredibly dangerous, as they carry a second kagune that functions as armor, amplifying their strength and skills. The user may also become insane while using it. The reason for this is unknown, as some ghouls are able to remain sane while fighting with a kakuja kagune. The appearance that the kakuja creates may be affected by the user's psychological state.

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Origins There is no hard evidence as to how ghouls came into existence, as their origins are disputed even by the CCG. There are two main theories for the existence of ghouls.
Evolution Theory This theory suggests that ghouls were previously creatures that began to prey on humans centuries ago. With the rise of human civilization and technology, these creatures began to evolve to fit almost seamlessly into human society, capable of developing a form almost identical to humans.
Parasitic RC Cell Theory This theory suggests that ghouls are simply humans infected by RC cells long ago. As RC cells are present in the bloodstream, this theory says that select humans were infected by these cells long ago and are carried through the generations. These cells affected the brain and created a differing mentality (i.e. quicker desensitivity, more susceptible to devaluing human life) and different needs to survive. RC cells take over the body and are carried down to the offspring.

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Canterlot Avenue Ghouls