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Amazonian manatee “knows” just when to migrate

Léo Ramos Chaves / Photo taken at the São Paulo Aquarium The manatee moves from Lake Mamirauá to Lake Amanã in the dry seasonLéo Ramos Chaves / Photo taken at the São Paulo Aquarium

October through December is the dry season in the region of the middle Solimões River, about 600 kilometers west of Manaus, and the waters in nearby Lake Mamirauá drop so low that one of its most famous inhabitants, the Amazonian manatee (Trichechus inunguis), moves to another lake, called Amanã, to ensure its survival. Although Mamirauá offers the aquatic mammal more food than Amanã, the risky trip – which can last three days and cover as much as 115 kilometers – is fully justified. Lake Mamirauá sits in a várzea, where waters alternately rise and recede, while Lake Amanã lies in an area of perennial flooding. A recent study suggests that the Amazonian manatee possesses biological mechanisms for spatial localization that prompt it to start its forced migration at the best possible moment (Acta Amazonica, January-March 2017). “They seem to have a cognitive map of the region that is updated as the waters drop,” says ecologist Eduardo Moraes Arraut, a remote sensing specialist with Brazil’s National Institute for Space Research (INPE) and author of the paper. According to Arraut, when the Mamirauá starts shrinking, the manatee puts its move off as long as possible so it has more time to feed on local aquatic plants that are not found in Lake Amanã. Yet the animal usually manages to make the trip without getting stuck in shallower stretches along the way, known as migratory bottlenecks. “Maybe he has some kind of sensor to monitor the chemical changes that occur in the water during the dry season,” ventures Miriam Marmontel, oceanographer with the Mamirauá Institute. The study tracked the movements of ten Amazonian manatees for up to four years, in addition to compiling 30 years’ worth of satellite images of the region and 14 years’ worth of information on the discharge of local rivers and lagoons. One worrisome discovery was a new migratory bottleneck along a stretch between Mamirauá and Amanã, which formed in the last 15 years. The water there nearly vanishes during the dry season, hampering the animal’s passage.