They are so fun to watch (and, granted, I don't live anywhere near where they do, so I don't have to worry about them destroying my fields and pastures). When someone approaches, the sentinel lets out this funny little, "Yip!" and raises his hands. It's an adorable gesture—they look somewhat like cheerleaders when they do this.
They also often perform a gesture that looks remarkably like a kiss. They hold each others faces and appear to even "make out" for long minutes. How cute is that? Lots of zoos have special viewing colonies set up where you can look into their complicated tunnel villages and see how they interact. But I once spent a very happy day observing them in their natural habitat in South Dakota.