At the start of the chapter Jack is on the hunt and most of the descriptions regarding his actions equate him to an animal on the prowl - a predator seeking its prey. In the first paragraph we read:
Jack was bent double. He was down like a sprinter, his nose only a few inches from the humid earth...
...Then dog-like, uncomfortably on all fours yet unheeding his discomfort, he stole forward five yards and stopped.
The highlighted words indicate that he has assumed the position of an animal. In paragraph two it is suggested that even his body seemed like that of some creature:
...his bare back was a mass of dark freckles and peeling sunburn.
He is described as naked (except for his shorts) and that his nostrils were flared - a description usually used to describe an animal.
To further emphasize Jack's animal-like behavior, we read in paragraph three that:
...he stole forward and cast this way and that over the ground.
And, in paragraph four:
Jack himself shrank at this cry with a hiss of indrawn breath, and for a minute became less a hunter than a furtive thing, ape-like among the tangle of trees...
The terminology makes it clear that Jack had become like an animal. In the first instance, he was casting his eyes around, as a predator would when seeking its prey. The highlighted words in paragraph four are self-explanatory.
Even after he has completed his unsuccessful hunt, Jack does not lose his bestiality, as illustrated by the following extract:
Jack took up a coconut shell that brimmed with fresh water from among a group that was arranged in the shade, and drank. The water splashed over his chin and neck and chest. He breathed noisily when he had finished.
He drinks as an animal would.
He later declares that he has an instinctual sense which he shares with the animals:
“There’s nothing in it of course. Just a feeling. But you can feel as if you’re not hunting, but—being hunted, as if something’s behind you all the time in the jungle.”
“Only I know how they feel. See? That’s all.”
His actions in this chapter foreshadow Jack's actions later when he rebels against rules and order, abandons civilized behaviour and adopts savagery. He and his hunters become savages and form a tribe. They are later primarily responsible for Simon's death and kill Piggy. They also capture others, such as Sam and Eric and hold them captive. Eventually, they hunt Ralph and plan to kill him.