What is the symbolism of Jack's hunters killing a nursing sow, butchering it, and putting its head on a stake in Lord of the Flies?

This episode occurs in Chapter 8 "Gift for the Darkness,"
and it is related in such a way that we react with horror and disgust.  Sexual, or more
accurately rape-like, diction is used in this scene to show the devolvement of the boys
into savagery.  We see here no noble man versus beast conflict; instead, we have a
nursing, bleeding sow portrayed as the victim of the boys' frenzied
bloodlust:

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Here, struck down by the heat, the sow fell and
the hunters hurled themselves at her.  This dreadful eruption from an unknown world made
her frantic; she squealed and bucked and the air was full of sweat and noise and blood
and terror.

This episode is
one of a series of parallel scenes in which a killing of an animal is involved.  The
first killing of a pig is barely mentioned as Jack and his tribe kill this one just as a
ship passes by the island.  The second scene involves the killing of a boar.  This is
the third one, and the most savage.  The next hunt, of course, is a manhunt with Ralph
as the prey. 

The "trophy" of this hunt is the pig's head
that is placed on a stick.  It left to appease the beastie, in another savage ritual. 
This pig's head becomes a major symbol of evil and temptation in the novel, as it is
this pig's head that talks to Simon urging him to "play" like the others.