The plains zebra is a highly social species, forming harems with a single stallion, several mares and their recent offspring; there are also bachelor groups. Groups may come together to form herds. The animals keep watch for predators rather than attempting to hide; they bark or snort when they see a predator, and the harem stallion attacks predators to defend his harem. Individual zebras can apparently recognise each other by their striping patterns.[17] The stripes may also serve as visual cues for grooming.[12] In addition, they could serve to help zebra groups stay together when they are fleeing. A 2012 study suggests that stripes may have developed to discourage biting flies. The plains zebra's major predators are lions and spotted hyenas. Crocodiles are predators with river crossings. Wild dogs, cheetahs, and leopards prey primarily on the foals.