Animal Boundaries

[Bazile]

I watch a lot of nature programmes

It's frequently mentioned about the territories of animals - for example being X square miles

How do these animals know where the boundaries of their territory starts/ ends ?

[Buenchico]

See here:
https://en.wikipedia.org/wiki/Territory_(animal)#Advertising_the_territory

[Chipchopper]

I have Muntjac deer that come into my garden to mark their territory.
They do this by rubbing their antlers on the bark of small trees. The scent glands are at the base of the antlers, so they can cause damage to the bark, if the trees don't have tree guards around them.

[Marky101]

They "pace " their boundary out.