Creatures bumping into each other - What?

What do you think of creatures bumping onto each other ? It is bad IMHO. To solve that we could either a) add a physics to our game ( add a barrier for each ) or b) use spatial tree ( like mine quadtree c) use spatial subdivisions. f.e. divide map into size R grid squares. then for each square, compare the content of the square to its surrounding squares and do n^2 check on all elements in those buckets for collision