Board Thread:Speculation House/@comment-14138255-20141017020106/@comment-9090085-20141118192250

Shadow at Morning wrote:

The Grimm are sufficiently different from our understanding of biology that all of our biological principles come into question when dealing with them, since most of them apply to conventional life, which the Grimm clearly aren't. We can make precious few assumptions about entities which seem to casually disregard thermodynamics. Someone else can make the points again; I've not now the time. I can do that. What we know about the Creatures of Grimm serves to highlight just how unnatural they are:

All of this paints a picture of the Grimm as something other - They are not animals, they are monsters - Creatures of Destruction. It's the reason why theories of the Grimm being weapons are relatively common: They're effectively "unliving"(by Remnant's standards), and seem to exist to kill humans. Whether they're the creations of ancient Humans; a vengeful spirit of the world; or have some other origin is up in the air. But that's shaky. Heck, as Natedog5105 pointed out above, the Grimm might be the manifestations of a philosophy - anonymity and conformity - which oddly parallels the "longest war in Remnant's history".
 * All living things in Remnant have light and darkness in them, even animals; the Grimm have only darkness.
 * Grimm don't need to feed or kill - they chose to.
 * A Grimm's sole purpose in life seems to be killing humans(/faunus) and bringing their creations to ruin. Even wiser Grimm like the Goliaths are like this - they're just waiting for the right time to strike.
 * Grimm are immortal, and continue grow throughout their lives.
 * Their bodies evaporate on death in a mater of minutes, leaving nothing behind but the destruction they caused.
 * Grimm are attracted to the negative emotions of people; not, say, the scent of sweat.

Short answer: the Grimm don't adhere to the rules of real-life animals; they appear to just have a coincidental similarity in appearance.