The 4 extant species of Sirenia are divided into the two families; dugong and manatee. There are 3 species of manatee and just one dugong. All 4 species are very large, slow moving marine mammals that feed on vegetation. They are the only marine mammals that are almost true vegetarians. They spend hours each day grazing along the sea bed, either 'hovering' up vegetation with a large mobile upper lip, or by using their front flippers to grasp food and pass it to their mouths. Once the food is in the mouth, it is then crushed between two horny plates, one on the front part of the palate and the other on the lower jaw. It is then ground beneath the teeth before digestion. Their daily food intake is up to one quarter of their body weight and consists of rhizomes (underground stems) of sea grasses plus the odd fish for protein. Due to the large volume of plant matter that has to be digested, sirenians give off a large amount of gas, making them extremely buoyant. To counter this buoyancy, their bones are extremely heavy and dense, allowing them to remain beneath the surface of the water. However, as with all mammals, sirenians need to air for respiration and so have to come up to the surface at least every 20 minutes in order to breathe.
Sirenians are extremely slow breeders, tending to have only one calf every two years. Males will compete for mating rights with females by making low sounds, and pushing each other. The actual mating process is monogamous in dugongs but polygynous in manatees, nut both species are similarly auditory and tactile during the process.
Sirenians can live up to 70 years in the wild, but their numbers are considerably low. They used to be hunted for oil, meat and hide, but now are at greater risk from death or injury caused by boat propellers, fishing nets and the pollution of coastal waters. As a result, despite having no natural enemies and no longer being at risk from large scale hunting, sirenians have fewer numbers than any other mammal order, only 130'000 are estimated to be left.
Length: 2.5-4 m
Weight: 250-900 kg
Social unit: Group
Region: East Africa, West Asia, South Asia, South-East Asia, Australia and Pacific islands.
West Indian Manatee
Length: 2.5-4.5 m
Weight: 200-600 kg
Social unit: Variable
Region: South-East USA to North-East South America and Caribbean
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Author: Emma-Louise Nicholls
Last updated: 20th November 2005
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