Although the Sirenia have a streamlined body form like other marine mammals, they are the only marine mammals that feed primarily on plants. The unique feeding niche is the key to understanding the evolutionary background of this order. This was the ancestor of the modern manatees and dugongs. There was a fifth species in this order, Steller s sea cow, but it was exterminated by humans in the mid 1700 s. It was adapted to cold water areas of the northern Pacific, feeding on kelp.
Sirenians expend little energy compared to other mammals of the same size.
Slow movements because of few predators and a warm constant environment contribute to the low metabolism. Another reason that Sirenians expend low energy while grazing is because they have few competitors for food in contrast to the complex division of resources in terrestrial habitats. The only other large marine grazers are sea turtles. Life span is long, but unknown. Reproductive rates are low.