The West Indian Manatee (Trichechus manatus)

May 20, 2012 Features / Columnists, Interesting Creatures in Guyana

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The West Indian manatee (Trichechus manatus) is a manatee, and the largest surviving member of the aquatic mammal order Sirenia which also includes the dugong and the extinct Steller’s sea cow. The West Indian manatee is a species distinct from the Amazonian manatee (T. inunguis), and the African manatee (T. senegalensis).

Based on genetic and morphological studies, the West Indian manatee is divided into two subspecies, the Florida manatee (T. m. latirostris) and the Antillean or Caribbean manatee (T. m. manatus). However, recent genetic (mtDNA) research suggests that the West Indian manatee actually falls out into three groups, which are more or less geographically distributed as: (1) Florida and the Greater Antilles; (2) Central and Northern South America; and (3) North eastern South America.
Like other manatees, the West Indian manatee has adapted fully to an aquatic life style, having no hind limbs. Pelage cover is sparsely distributed across the body, which may play a role in reducing the build-up of algae on their thick skin. The average West Indian manatee is approximately 2.7–3.5 m (8.9–11 ft) long and weighs 200–600 kg (440–1,300 lb), with females generally larger than males. The largest individual on record weighed 1,655 kg (3,650 lb) and measured 4.6 m (15 ft) long. This manatee’s colour is grey or brown. Its flippers also have either three or four nails so it can hold its food as it is eating.
As its name implies, the West Indian manatee lives in the West Indies, or Caribbean, generally in shallow coastal areas. However, it is known to withstand large changes in water salinity, and so has also been found in shallow rivers and estuaries. It can live in fresh water, saline water, and even brackish water. It is limited to the tropics and subtropics due to an extremely low metabolic rate and lack of a thick layer of insulating body fat. While this is a regularly occurring species along coastal, southern Florida, during summer this large mammal has even been found as far north as New York City, New York and as far west as Texas.
They are found in fresh water rivers, in estuaries, and in the coastal waters of the Gulf of Mexico and the Atlantic Ocean. Females usually have their first calf when they are about seven or eight years old. Normally they only have one calf every three years because manatees nurse their calf for one or two years, but there are rare occurrences of twins. When a calf is born, they usually weigh between 60 and 70 pounds and are between four and 4.5 feet. The family unit consists of mother and calf, which remain together for up to two years. Males aggregate in mating herds around a female when she is ready to conceive, but contribute no parental care to the calf.
The other subspecies of the West Indian manatee is sometimes referred to as the Antillean manatee (Trichechus manatus manatus). Antillean manatees are sparsely distributed throughout the Caribbean and the Northwestern Atlantic Ocean, from Mexico, east to the Greater Antilles, and south to Brazil. They are found in French Guiana, Suriname, Guyana, Trinidad (however there has been a lack of recent sightings), Venezuela, Colombia, Panama, Costa Rica, Nicaragua, Honduras, Guatemala, Belize, Mexico, Trinidad, Cuba, Haiti, Dominican Republic, Jamaica, and Puerto Rico.
Historically, Antillean manatees were hunted by local natives and sold to European explorers for food. Today, they are threatened by loss of habitat, poaching, entanglement with fishing gear, and increased boating activity.
The West Indian manatee is surprisingly agile in water, and individuals have been seen doing rolls, somersaults, and even swimming upside-down. Manatees are not territorial and do not have complex predator avoidance behaviour, as they have evolved in areas without natural predators. The common predators of marine mammals, such as orcas and large sharks, are rarely (if ever) found in habitats inhabited by this species.
The West Indian manatee is an opportunistic feeder, with large adults consuming 10 to 15 per cent of their body weight in food daily. Manatees feed on about 60 plant species which includes sea grasses as their major food source. They also consume some fish and small invertebrates. Because manatees feed on abrasive plants, their molars are often worn down and are continually replaced throughout life, called “marching molars”.
The West Indian manatee has a high casualty rate due to thermal shock from cold temperatures. During cold weather many die due to their digestive tract shutting down at water temperatures below 68 °F (20 °C). Many manatee deaths are caused by large commercial vessels but are attributed to “recreational watercraft” due to the elimination of that classification.
Although female West Indian manatees are mostly solitary creatures, they form mating herds while in estrus. Most females breed successfully between ages of seven and nine; they are, however, capable of reproduction as early as four years of age. The gestation period (pregnancy) lasts from 12 to 14 months. Normally, one calf is born, although on rare occasions two have been recorded. The young are born with molars, allowing them to consume sea grass within the first three weeks of birth. On average, manatees that survive to adulthood will have between five and seven offspring between the ages of 20 and 26.
The West Indian manatee has been hunted for hundreds of years for meat and hide, and continues to be hunted to this day in Central and South America. Illegal poaching, as well as collisions with speeding motorboats, are a constant source of manatee fatalities.
Due to their low reproductive rates, a decline in manatee population may be hard to overcome. They enjoy protection from the U.S. Endangered Species Act of 1973, and the U.S. Marine Mammal Protection Act of 1972. The West Indian Manatee is also protected by the Florida Manatee Sanctuary Act of 1978, the Manatee Recovery Plan, and the Save the Manatee Club. However, in April 2007, the U.S. Fish and Wildlife Service announced that the West Indian manatee population of Florida had rebounded. It advised that the species be reclassified as threatened rather than endangered.
(Source: Wikipedia – the Free Online Encyclopedia)

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