The rhinoceros family is characterised by large size (one of the few remaining megafauna alive today) with all of the species capable of reaching one ton or more in weight; herbivorous diet; and a thick protective skin, 1.5–5 cm thick, formed from layers of collagen positioned in a lattice structure; relatively small brains for mammals this size (400–600g); and a large horn. They generally eat leafy material, although their ability to ferment food in their hindgut allows them to subsist on more fibrous plant matter, if necessary. Unlike other perissodactyls, the African species of rhinoceros lack teeth at the front of their mouths, relying instead on their powerful premolar and molar teeth to grind up plant food. The horns of a rhinoceros are made of keratin, the same type of protein that makes up hair and fingernails. Rhinoceroses have acute hearing and sense of smell, but poor eyesight. Most live to be about 60 years old or more. The word "rhinoceros" (ρινόκερος) is derived from the Greek words rhino, meaning nose, and kera, meaning horn; hence "horned-nose". The plural can be rhinoceros, rhinoceri, rhinoceroses, or rhinoceroi. The collective noun for a group of rhinoceros is "crash". The five living species fall into three categories. The two African species, the White Rhinoceros and the Black Rhinoceros, There are two living Rhinocerotini species, the endangered Indian Rhinoceros and the critically endangered Javan Rhinoceros and endangered Sumatran Rhinoceros.