- A Colonel: "Those aren't men...they're animals...you can't communicate with an animal."
- Vanessa: "They're not animals. I might not be able to reverse the full transformation yet but the side effects, the lost time, the aggression."
- — A very heated discussion between a Colonel and Vanessa.[src]
Human-Animal Physiology is the presence of animal features (DNA, traits and abilities) within a human being. Those with this physiology are referred to as Beasts or Super Soldiers (a occupational name).
This physiology is the direct result of gene-splicing, which is the act of injecting genetically engineered animal DNA into the body of an ordinary human, changing their DNA, which in turn progressively changes their original physiology. In this case DNA from Proto Beasts were used.
The most drastic aspect of this physiology is the physical change their cells undergo when large amounts of adrenaline is released into the body. giving their body more animalistic features along with a sizeable increasce in physical abilities on top of their already enhanced physiology. Depending on the individual, this may be seen as either a gift or a curse. They can also exhibit animal-like traits, such as loud growls, roars, and even heightened aggression, even towards those to whom they are close.
This physiology also gives them unusual abilities, far beyond the normal. Their senses of sight, smell, and hearing are all superhumanly acute. They can see with perfect clarity at greater distances than an ordinary human, even in near-total darkness. Their hearing is enhanced in a similar manner, allowing them to both hear sounds ordinary humans cannot and also hear to greater distances. Their sense of smell is so acute they can gather information about something or someone as well as track targets by scent alone. However, this sense is too weak to pick up scents eroded over time.
They are physically quite strong, more than any man and most animals, and have been known to overpower their adversaries in both human and beast form. They generally kill their victims by ripping them apart with their bare hands (tearing apart muscle in the process). They are able to lift a grown man up and throw him across the room with one arm. Unfortunately their strength bear emotional underpinnings in that it's fueled by rushes of adrenaline; the more intense their feelings get the more powerful they become, which makes it hard to control the force of their impacts.
They are also incredibly fast and agile, as well as being quite durable and resilient; one was able to walk away from a hard impact with concrete without so much as a scratch, although bullet wounds to the heart or head will kill them as easily as normal men. They also have considerable survival instincts, since they will attack anyone they view as a threat.
A Beasts physiology grants them longevity, meaning they age very slowly. For exampel the Proto Beast Liam has after 260 years barely aged.
The longer they live the more abilities they develop. For example, ten years after he gained this physiology, Vincent Keller develop Night Vision. The use of certain abilities can cause their eyes to glow. A good example of this is Night Vision. Sometimes, Vincent's eyes glow when uses this ability.
A male Beast is able to sexually reproduce with a human woman and the offspring will be born a beast. A case of this is Tori Windsor.
Since Proto Beasts have existed prior to the muirfield experiments it can be assumed that they are able to reproduce.