ezo
Hero
Many animals are aggressive, so protecting its territory is certainly part of that, but IMO the only reason an animal intelligent creature might fight to the death is defending their young. There are certainly animals who will see one of their young taken off to be eaten if it keeps themselves alive and protects the rest of their young. Few animals will actually die themselves to keep their young alive.An animal intelligence monster will only attack under those circumstances you mentioned anyway(as well as protecting its territory)
So, If the PCs don't encounter them, then the animal intelligence monster had no reason to attack.
Animals give up territory and would rather continue starving (and try to find food elsewhere) than actually die trying to get food (i.e. the tasty PCs). So, really, the more I think about that, animal intelligent creatures (other than undead/constructs) should really just about always flee instead of fighting to the death.
Now, that isn't to say the PCs might just kill an animal before trying to get to its young, as the young are easy to take if the parents are dead... but in my games "going after the young" would not really be something I would tolerate.
Of course, if we examine animal intelligence creatures which are not actually animals (or again undead/constructs) we are left with an assortment of plants/oozes, a couple fiends I think, and mostly some monstrosities.
Notably I think the exceptions arise with the monstrosities. Gorgons, hydras, bulettes, etc. might not be smart enough to realize they are about to die and due to their aggresive natures might fight to the death.
Frankly, I miss a codified morale system.