Mystery of Amazon manatee migration solved

Jody Bourton, BBC News 23 Dec 09;

The mystery of why Amazonian manatees migrate has been solved.

Only in recent years did scientists find that the secretive aquatic mammal migrates from shallow to deep water.

Now researchers can reveal that the manatees make this perilous journey to avoid being exposed to attack by predators during the low water season.

That means the species maybe at greater risk than previously thought, say scientists, as migration and low water levels make them vulnerable to hunters.

The international team of researchers from Brazil and the UK publish their findings in the Journal of Zoology.

Great escape

The elusive Amazonian manatee (Trichechus inunguis) is a large plant-eating mammal that lives in freshwater.

Due to its peculiar shape it has been described as a cross between a seal and a hippo.

The species is only found in the Amazon River basin from the river mouth to the upper reaches of tributaries of Brazil, Colombia, Ecuador, Guyana, and Peru.

The researchers studied manatees that live within the Mamiraua and Amana Sustainable Development Reserves in the north west of Brazil.

To obtain their results, the researchers asked local inhabitants about the animals' movements, studied the shapes and depths of the local rivers and lakes and then used radio tracking tags to follow the movements of 10 manatees.

During the high water season, between mid May and the end of June, manatees live in quiet lakes called varzeas that form within river flood plains, the scientists found.

Here the manatees consume 8% of their body weight in aquatic plants each day.

Then during the low water season, between October and November, the animals start to migrate as the water level drops.

They journey to deeper water within long narrow lakes called rias, that are submerged river valleys.

They do this because it becomes too dangerous to remain in shallow water, the scientists say.

If the manatees do not move, they become stranded and exposed to hunters such as caimans, jaguars and humans who stalk the water margins.

Lesser of two evils

Moving to the deeper habitat is not easy, as the large mammals must pass through narrow bottlenecks in the aquatic landscape, where human hunters wait for them.

The perilous journey also has another downside; it forces the manatees to fast for several months due to a lack of aquatic plants.

"Amazonian manatees migrate to a habitat that doesn't offer easy living conditions in order to flee from a habitat that becomes inhospitable," says Dr Eduardo Moraes Arraut from the National Institute for Space Research in Sao Paulo, Brazil who undertook the latest study.

By doing so, they choose between the lesser of two evils.

"When you have two options that are not good, you choose the one that is less bad," says Dr Arraut.

Hunters respected

"I was surprised with the difficulty of the conditions the manatee lives in during the low water season," he says.

"I was also badly surprised with the fact that they are probably being killed yearly throughout the Amazon during migration."

Even though hunting manatees is illegal they are prized by local people for their meat and the status a kill bestows on the hunter.

"It is very difficult to kill one and hunters are respected people in their communities," explains Dr Arraut.

"Manatees are in greater danger than previously thought because every year they are probably migrating through narrow channels where they are exposed to hunters," he says.

Dr Arraut hopes to track manatees in other regions of the Amazon to find out if this is occurring elsewhere.