Originally posted by SiEgE:

Eh.. sadly, you don't really have to. You can kill stuff without it being much of a threat to you, or your base.

This is true until after you've beaten the third boss. You MUST put a roof over your livestock before you defeat the third boss or they'll be sniped from the air like my two star boar were. Now I only have 1 star boar :(

P.S. You can move livestock from a pen to a barn by building a fenced in path between the two and scaring them along it with a torch.