"He became quicker of movement than the other dogs, swifter of foot, craftier, deadlier, more lithe, more lean with ironlike muscle and sinew, more enduring, more cruel more ferocious, and more intelligent. He had to become all these things, else he would not have held his own nor survived the hostile environment in which he found himself."
Source : "The Playboy Interview: Stan Lee on Superheroes, Marvel and Being Just Another Pretty Face". Interview with David Hochman, playboysfw.kinja.com. March 31, 2014.