Summary: alien movement and attacks make no noise.

Observation: "You almost never see this in combat, and thank Jeezus for that. You'll be standing with your squad, watching all the entry points, when someone'll get hit. And you won't know from where. Then someone else will get hit, so you'll all dive for cover. You'll be sitting there in dead silence, listening to each other's breathin', when you see the bugger glide down behind your squadmate, quiet as a hologram. Before you can yell out, it's slammed him against the wall and ripped him up."

Thesis: an alien that can hunt and attack without being noticed is deadlier, and requires less energy. This is the "stealth" model – probably requires shock-absorbent cartilage between all joints, lubricating fluids in weaponry, and a corresponding change in behavior. Humans can be very quiet if they move correctly – these aliens suddenly learn similar techniques.