The sirenians (manatees and dugongs) are more closely related to elephants than to marine mammals such as whales and dolphins. They belong to an ancient radiation of African mammals known as the Afrotheria. This superorder includes seven groups of animals thought to have shared a common ancestor around 100 million years ago. These animals have little superficial resemblance to each other. Together with the sirenians and elephants, this group also includes the aardvark, hyraxes, sengis (or elephant-shrews), golden moles and tenrecs.
Sirenians are conventionally divided into four families: two, the Prorastomidae and the Protosirenidae, are extinct and known only from the Eocene, while the Dugongidae (dugongs, including Steller’s sea cow) and Trichechidae (manatees) survive today.
The fossil record of the Sirenia extends over 50 million years. The sirenians are thought to have branched off from the Proboscidea (elephants) during the Palaeocene, and quickly dispersed to the New World. The earliest known representative of the group was a pig-sized prorastomid from Jamaica from the early Eocene. This four-legged animal could walk on land, but probably spent much of its time in water. It had an extremely dense skeleton, which acted like ballast to keep the animal submerged in shallow waters. By the middle Eocene, protosirenid sea cows were abundant in North America, Europe and Asia. While the early sirenians had narrow snouts that suggested selective browsing, the muzzles had by this time broadened and became more downturned to ‘mow’ the carpets of seagrass on the ocean floor. The Dugongidae first appeared in the mid-late Eocene of the Mediterranean region. It was among these forms that the loss of the functional hind limbs took place. The manatees (Trichechidae) are the dugong’s closest living relatives. They are thought to have diverged from the Dugongidae at some time between the late Eocene and mid Oligocene.
There are three species of manatee alive today: the Amazonian manatee (Trichechus inunguis), the American or West Indian manatee (T. manatus) and the African manatee (T. senegalensis). The Amazonian manatee is thought to have become isolated when the Andes mountain range uplifted in the Pliocene 1.8-5 million years ago, changing the river drainage of the Amazon Basin from the Pacific to the Atlantic Ocean. In 2008, Dr Marc van Roosmalen proposed that there was another species, called the 'dwarf' manatee (Trichechus pygmaeus) because it lives in shallow, fast running water and feeds on different species of aquatic plants than the Amazonian manatee. It has been suggested that because of its small range it is critically endangered, but this is not currently recognized by the IUCN.
Individuals usually travel alone, or in pairs consisting of a female and her calf. In the past, the species formed large aggregations in the middle reaches of the Amazon, but such sights are rare today, with loose groups of only 4-8 individuals generally appearing together in feeding areas.
Manatees make seasonal movements synchronized with the flood regime of the Amazon Basin. They are found in flooded forests and meadows during the flood season, when food is abundant. During the July-August dry season when water levels begin to fall, some populations become restricted to the deep parts of large lakes, where they often remain until the end of the dry season in March. They are thought to fast during this period, their large fat reserves and low metabolic rates – only 36% of the usual placental mammal metabolic rate – allowing them to survive for up to seven months with little or no food. Other populations enter the main water courses during the dry period, where they can find food but must expend energy to hold their position in the current.
Breeding is thought to occur throughout the year, with peaks at different times in different parts of the river system. In the central Amazon, most births occur in February-May, during the period of rising water levels. Females give birth to a single young, after a gestation period of approximately one year. The interbirth interval is around 2-3 years. The lifespan of this species in the wild is unknown, although some individuals have lived more than twelve years in captivity.
The species is listed on Appendix I of CITES and has been protected legally in Brazil since 1973, although subsistence hunting and commercial exploitation both here and in other parts of the species’ range has continued. In Ecuador, the Siona Indians - traditional hunters of the manatee - have a self-imposed ban on taking the species because of its low numbers. However, the species is hunted by other settlers and the military, even within protected areas. There is a national Conservation Action Plan specific to manatees in Colombia, and the species is currently included in a Brazilian Conservation Action Plan for aquatic mammals. However, there is a strong need for a collaborative regional strategy to protect the species. Most research to date has focused on Brazilian populations, with captive studies having been carried out since the 1980s, and long-term studies into manatee movement patterns since 1993. Valuable information on habitat use and migratory routes has been collected, which will be used to develop a management plan for the species in the Amazon Basin. Reintroduction attempts in Brazil and Colombia have had partial success, and some manatees have been born in captivity. Educational programmes are currently underway in Brazil to raise awareness about the importance of conserving this species.
More recently, a ‘National Action Plan for the Conservation of Sirenia’ has been implemented in Brazil. Based on research completed in 2006 by the Special Working Group for Aquatic Mammals and the Brazilian Institute of Environment and Renewable Natural Resources amongst others, this multi-million pound, 5 year conservation plan began in 2010. Coordinated by the National Centre for Research and Conservation of Aquatic Mammals (CMA), this scheme aims to significantly reduce hunting and illegal trade of the Amazonian Manatee whilst also combating pollution and destruction of important mangrove habitat.
Marmontel, M. 2008. Trichechus inunguis. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.4. www.iucnredlist.org. Downloaded on 14 November 2010.
Martins Cantanhede, A., Ferreira Da Silva, V. M., Pires Farias, I. Hrbek, T., Lazzarinis, S. M. and Alves-Gomes, J. 2005. Phylogeography and population genetics of the endangered Amazonian manatee, Trichechus inunguis Natterer, 1883 (Mammalia, Sirenia). Molecular Ecology 14(2): 401-413.
Nowak, R.M. 1999. Walker's Mammals of the World.Sixth edition. The Johns Hopkins University Press, Baltimore and London.
Sirenia Specialist Group 1996. Trichechus inunguis. In: IUCN 2006. 2006 IUCN Red List of Threatened Species. Downloaded on 05 January 2007.
Timm, R. M., Albuja, V. L. and Clauson, B.L. 1986. Ecology, distribution, harvest and conservation of the Amazonian manatee Trichechus inunguis in Ecuador. Biotropica 18(2): 150-156.
Sirenews. October 2005. The newsletter of the IUCN/SSC Sirenia Specialist Group. Number 44.
Sirenews. April 2006. The newsletter of the IUCN/SSC Sirenia Specialist Group. Number 45.
Distribution map based on data provided by the IUCN Global Mammal Assessment.
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