How do meerkats take turns standing guard?

Out in the dry red dirt of southern Africa lives a tiny detective with a very big habit: standing up tall and watching everything. A meerkat. While the rest of the family digs and snacks, one of them climbs up onto a mound, stretches to full height, and squints at the sky and the horizon. This is the sentinel — the lookout. And the curious part is how the family decides whose turn it is.
First, the danger. Meerkats are small, and plenty of bigger animals would happily make a meal of them — eagles overhead, jackals across the sand. So while everyone is busy poking their noses into the dirt looking for beetles and grubs, somebody has to keep their eyes UP. A foraging meerkat with its head down is a meerkat that can't see trouble coming.
