Summary: alien becomes tireless; its energy renews rapidly.

Observation: "The 'army' of aliens that pinned down a squad of marines with a never-ending hail of spikes, turned out to be one extremely annoying Lerk." Or: "the normally hyper Skulks started literally bouncing off the walls, like a birthday party of three-year-olds who found Mommy's amphetamines."

Thesis: The aliens rely on their own physiology for their abilities and weapons. Every spike they fire, or extreme physical act they commit, drains them. The adrenaline evolution is probably an internal overhaul – an upgrade of all internal organs and systems, in favor of stronger and more efficient ones. The resulting body can recover very quickly from exertion, and will not tire as fast. As with other abilities, these modifications prohibit other evolutionary choices – namely, celerity and silence.