A few years back scientists became aware that Amazonian manatees (Trichechus inunguis) migrate from shallow to deep water each year, starting in October or November when the dry season makes the water level decrease in their favoured habitat. A team of Brazilian and British researchers now claim have found out why these mesmerizing creatures undertake a perilous journey to get to a habitat that doesn’t have much to offer food wise.

By studying manatees living in the Mamiraua and Amana Sustainable Development Reserves in north-western Brazil, the team found out that if the manatees wouldn’t move, the animals would be become stranded in the shallow water and exposed to predators such as caimans and jaguars.

“Amazonian manatees migrate to a habitat that doesn’t offer easy living conditions in order to flee from a habitat that becomes inhospitable,” says team-member Dr Eduardo Moraes Arraut from the National Institute for Space Research in Sao Paulo, Brazil.

A dangerous voyage
In order to reach deeper waters, the manatees must pass through narrow bottlenecks in the landscape where they easily fall prey to predators – including humans. Hunting manatee is illegal in Brazil but the local people have a long tradition of eating the meat.

Once the manatees reach their destination, it isn’t much of a relief either. Due to a lack of aquatic plants in their new home, they are forced to go without food for several months.

“When you have two options that are not good, you choose the one that is less bad,” Dr Arraut explains, adding that the harshness of the low-water conditions surprised him.

Modern technology and ancient knowledge
In order to learn more about the elusive manatees, the international research team asked local inhabitants about the movements of the manatees, and placed modern radio tracking devices on 10 individual manatees. What they found was that the Amazon landscape forces the manatees to migrate through a type of narrow lakes known as rias, a type of submerged river valleys.

Dr Arraut now wishes to track manatees in other parts of the Amazon to find out if they are facing similar obstacles as the populations in Mamiraua and Amana.

The findings have been published in the Journal of Zoology.

http://www.zsl.org/info/publications/

A gentle giant
The Amazon manatee is one of the smallest species of manatee but can nevertheless reach a length of 9.2 ft (2.8 m) and weight 800-1200 lbs (360-540 kg). It is only found in the Amazon River basin, in the countries Brazil, Peru, Colombia, Guyana and Ecuador.

During the high-water season (mid-May to late June), the Amazon manatees live in quiet lakes (so called varzeas) that form within river flood plains in the Amazon. In this hospitable environment, they feast on aquatic plants and typically consume 8% of their weight in greens each day. For a manatee weighing 800 lbs, that means 64 lbs – almost 30 kg!

Picture is depicting a manatee in Florida (not an Amazonian Manatee)