Sirenian Life History

Abstract

Sirenians are unique among the marine mammals in that they are herbivores. The manatees, as their common names suggest, are distributed in the coastal tropical and subtropical regions of the Atlantic Ocean. The dugong is found in the South Pacific and Indian oceans. The manatees and the dugong are considered endangered or threatened under various national laws and international conservation schemes. The Steller's sea cow is the exception: not only is it extinct but it inhabited the islands in the western reaches of the Aleutian archipelago in the North Pacific Ocean. Detailed age estimation studies have only been done for the Florida manatee and for the dugong. Female Florida manatees reach sexual maturity between 2.5 and 6 years of age. Male Florida manatees reach sexual maturity between 2 and 11 years of age based on the presence of active spermatogenesis in the testes. Even though the estimated maximum life expectancy for the dugong is similar to that of the Florida manatee, dugongs appear to mature at greater ages, and the age at sexual maturity may vary among populations. In both manatees and dugongs the male plays no apparent role in the care of the young. After mating, males and females go their separate ways. Florida manatee calves average about 120 cm in length at birth, but viable calves may have a birth length ranging from about 80 to 160 cm. This, along with variable individual growth rates, results in a highly variable length at age distribution. A key factor in assessing fecundity is the gestation period, which, despite numerous conceptions and births of Florida manatees, is not known for any sirenian species. Even though the Florida manatee and the dugong have been studied intensively since the 1980s, detailed data on many aspects of their life history are only beginning to be elucidated and there is considerably less information on the other species of sirenians.