As with most large mammals, manatees have a low reproductive rate. Manatees are not sexually mature until they are about five years old. On average, one calf is born every two to five years, and twins are rare. Intervals between births range from two to five years. A two-year interval may occur when a female, or cow, loses a calf soon after birth. The gestation period for female manatees is about a year.
This is why I think that Manatees should not have been de-listed as endangered last week. They breed too slowly and die too quickly at our hands. It is believed their lifespan is about 60 years if they aren't killed first.