Like many animals, the chipmunk claims an area as its own territory and chases off others of its own kind. But there seems to be some sort of agreement between them whereby they sometimes allow each other passage.

From "Chipmunks On The Doorstep", by Edwin Tunis, page 50:

"What has been well established in the case of birds and other territorial animals is evidently true of chipmunks, too. Chippy can dominate any other chipmunk invading his claim, regardless of size or sex, but his aggressiveness fades rapidly at the border. If the trespasser is (as is usual) the next-door neighbor, the situation reverses itself at the boundary. As soon as he crosses the line, the defender becomes an invader. The neighbor's courage then builds up at once, and he runs the erstwhile defender back home.

What complicates this further is an arrangement perfectly understood by chipmunks but confusing to humans. Trespass regulations are completely suspended for workers crossing a territory to get supplies from a distant source. Such a one, whether going empty to a nut tree or returning with bulging pouches, may cross a dozen territories unhindered, and hardly noticed, by their owners."

Edited to add: I wasn't sure what forum to post this in. It isn't about companion animals, but it's not about how humans treat animals either.