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.
Females give birth to a single calf after about a years gestation.
Calves stay with the mother for one to two years and sexual maturity of the calf comes at 4-8 years. With long life spans, low renewability of food resources and few predators, rapid reproduction brings no advantage. Each species of the order is geographically isolated from the others. Each can occupy both saltwater and freshwater except the Amazon manatees that only inhabit the Amazon river and it s tributaries. Manatees are distinguished from dugongs mainly by their large, horizontal, paddle-shaped tail.