Meerkats are primarily insectivores, but also eat lizards, snakes, scorpions, spiders, plants, eggs, small mammals, millipedes, centipedes and, more rarely, small birds. Meerkats are immune to certain types of venom, including the very strong venom of the scorpions of the Kalahari Desert, unlike humans. They have no excess body fat stores, so foraging for food is a daily need. Meerkats forage in a group with one "sentry" on guard watching for predators while the others search for food. Sentry duty is usually approximately an hour long. A meerkat can dig through a quantity of sand equal to its own weight in just seconds. Baby meerkats do not start foraging for food until they are about 1 month old, and do so by following an older member of the group who acts as the pup's tutor. The meerkat standing guard makes peeping sounds when all is well. If the meerkat spots danger, it barks loudly or whistles.