Jack showed the rest of the boys a shallow gash on his arm caused, he said, by the tusk of the boar as he had tried to grab hold of it.
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I presume that you are referring to the start of chapter 3: Huts on the beach where Jack is alone in the forest. The answer is that Jack is trying to hunt pigs.
The boys hunt for Ralph as an order of Jack.
He is camouflaging himself so he can hunt without being seen
Jack made his announced his own tribe in Chapter 8: Gift for the Darkness. He said "I'm going off by myself. He can catch his own pigs. Anyone who wants to hunt when I do can com too." In the end of he chapter Jack tries to show his power and steal Piggys specs.
because they saw the beast and he wants to hunt it down and thought Ralph said that his hunters were not hunters