Warthogs respond to the warning calls of other mammals and birds, particularly the oxpeckers which often search them for ticks, as they are short-sighted and short-legged. They usually use abandoned aardvark burrows as their shelters, after enlarging and modifying them, and line them with grass before giving birth. They will also use shallower aardvark holes as temporary shelters. When running for shelter, young warthogs will scamper into their burrow head first, but adults do a remarkable about-turn at the entrance, usually accompanied by a cloud of dust, and reverse in, so as to present their formidable tusks to an attacker. Family groups, numbering between five and ten, avoid other warthogs that may stray into their home ground, and maintain group contact with soft grunts. The female will drive off the offspring from her previous litter just before giving birth to a new litter: if she loses a number of the new litter, the group may be re-united, particularly her female offspring.