The dugong is a medium-sized marine mammal. It is one of four living species of the order Sirenia, which also includes three species of manatees. It is the only living representative of the once-diverse family Dugongidae; its closest modern relative, Steller's sea cow (Hydrodamalis gigas), was hunted to extinction in the 18th century. The dugong is the only strictly marine herbivorous mammal, as all species of manatee use fresh water to some degree. Manatees (family Trichechidae, genus Trichechus) are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. They measure up to 13 feet long, weigh as much as 1,300 pounds, and have paddle-like flippers. Manatees are herbivores and eat over 60 different freshwater and saltwater plants. The main causes of death for manatees are human-related issues, such as habitat destruction and human objects. Natural causes of death include adverse temperatures and disease. Taken from "Johnson's household book of nature, containing full and interesting descriptions of the animal kingdom, based upon the writings of the eminent naturalists, Audubon, Wallace, Brehm, Wood and others" edited by Hugh Craig, published 1880.