Manatees

A relative of the elephant, manatees are totally aquatic, herbivorous mammals of the family Trichechidae. This group arose 15–20 million years ago during the Miocene period, a time which also favored the development of a tremendous diversity of aquatic plants along the coast of South America. Manatees are adapted to both marine and freshwater habitats and are divided into three distinct species : the Amazonian manatee (Trichechus inunguis ), restricted to the freshwaters of the Amazon River; the West African manatee (Trichechus senegalensis ), found in the coastal waters from Senegal to Angola; and the West Indian manatee (Trichechus manatus ), ranging from the northern South American coast through the Caribbean to the southeastern coastal waters of the United States. Two other species, the dugong (Dugong dugon ) and Steller's sea cow (Hydrodamalis gigas ), along with the manatees, make up the order Sirenia. Steller's sea cow is now extinct, having been exterminated by man in the mid-1700s for food.

These animals can weigh 1,000–1,500 lb (454–680 kg) and grow to be more than 12 ft (3.7 m) long. Manatees are unique among aquatic mammals because of their herbivorous diet. They are non-ruminants, therefore, unlike cows and sheep, they do not have a chambered stomach. They do have, however, extremely long intestines (up to 150 ft/46m) that contain a paired blind sac where bacterial digestion of cellulose takes place. Other unique traits of the manatee include horizontal replacement of molar teeth and the presence of only six cervical, or neck, vertebrae, instead of seven as in all other mammals. The intestinal sac and tooth replacement are adaptations designed to counteract the defenses evolved by the plants that the manatees eat. Several plant species contain tannins, oxalates, and nitrates , which are toxic, but which may be detoxified in the manatee's intestine. Other plant species contain silica spicules, which, due to their abrasiveness, wear down the manatee's teeth, necessitating the need for tooth replacement. The life span of manatees is long, greater than 30 years, but their reproductive rate is low, with gestation being 13 months and females giving birth to one calf every two years. Because of this the potential for increasing the population is low, thus leaving the population vulnerable to environmental problems.

Competition for food is not a problem. In contrast to terrestrial herbivores, which have a complex division of food resources and competition for the high-energy level land plants, manatees have limited competition from sea turtles . This is minimized by different feeding strategies employed within the two groups. Sea turtles eat blades of seagrasses at greater depths than manatees feed, and manatees tend to eat not only the blades, but also the rhizomes of these plants, which contain more energy for the warm-blooded mammals.

Because manatees are docile creatures and a source of food, they have been exploited by man to the point of extinction . There are currently between 1,500 and 3,000 in the U.S. Also because manatees are slow moving, a more recent threat is taking its toll on these shallow-swimming animals. Power boat propellers have struck hundreds of manatees in recent years, causing 90% of the man-related manatee deaths. This has also resulted in permanent injury or scarring to others. Conservation efforts, such as the Marine Mammals Protection Act of 1972 and the Endangered Species Act of 1973, have helped reduce some of these problems but much more will have to be done to prevent the extirpation of the manatees.

[Eugene C. Beckham ]

RESOURCES

BOOKS

Ridgway, S. H., and R. Harrison, eds. Handbook of Marine Mammals. Vol. 3, The Sirenians and Baleen Whales. London: Academic Press, 1985.

OTHER

Manatees of Florida. [cited May 2002]. <http://www.xtalwind.net/~cfa>.

Save the Manatees Club. [cited May 2002]. <http://www.savethemanatee.org>.