In chapter 3, Jack is becoming obsessed with hunting. As the chapter opens, he is tracking a pig by himself. First he is described as a sprinter, a comparison that portrays him as goal-driven—ready to pour himself into an exhausting effort to reach the finish line—which for him is killing a pig. Next he is described as crawling on all fours "yet unheeding his discomfort." This also shows how focused he is; even physical pain does not deter him from his quest.
Golding describes his physical appearance: his hair is becoming bleached blond by the sun, and it is getting shaggy. He wears only shorts, and his back is sunburnt and full of freckles. He carries a sharpened stick for a spear.
Although Jack at first uses his mental abilities for tracking the pig, he begins relying more and more on his innate, animalistic senses, especially his sense of smell. His blue eyes "in this frustration seemed bolting and nearly mad." This also shows that his reason is giving way to instinct. When a bird startles him, his reflexes make him catch his breath, and he jumps back in an "ape-like" motion.
Golding's description of Jack in this section shows his obsession with hunting and his becoming less governed by reason and more governed by his five senses and instincts. This change makes it harder for Ralph to see eye-to-eye with him, especially when Jack can't seem to remember what rescue is.