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.
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.
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