The skin of the rhinoceros is extremely thick, nearly hairless in most species, and deeply folded in some. The horns, arising from the skin, are made of keratin, a fibrous substance. The legs are stout and short and end in broad feet, each with three toes. Rhinoceroses are herbivorous, browsers or grazers according to the species. Most live near water and like to wallow in mud; all swim well. They have poor vision but good hearing and a good sense of smell. Mostly solitary animals, they feed by night and in the early morning and evening; they rest in shade during the heat of the day. They are often accompanied by small tickbirds (oxpeckers) that feed on parasites in their skin and, by their cries, alert them to danger. Although most rhinoceroses are placid animals, mothers fiercely protect their offspring.