Evolution of Sirenia

Evolution of Sirenia

Many scientists believe that sirenians evolved around fifty-five million years ago from small hoofed animals. The closests living relatives today are elephants and small asian mammals called hyraxes. Although there are vast differences between elephants, hyraxes, and sirenians, fossil evidence shows that all three evolved from a common ancestor. The manatee's physical characteristics are visible remnants of their ancient heritage. The dugong's tusks are another link to elephants. Like elephants, sirenians are enormous animals.

There are five species of Sirenia, but scientists theorize that there were many more in the past. Early forms of manatees are thought to have originated near the Amazon basin in South America. Some remained there to become the Amazonian manatee, while others migrated up through the Caribbean, giving rise to the Antillean and Florida manatees. Another group managed to swim or where carried on currents across the Atlantic and became the West African manatee.

Dugongs thought to have evolved along with mantees once ranged from Europe to Africa, and along the East and West coasts of the Americas. At the present time, they are only found in the Eastern Hemisphere in the Indian and Pacific Oceans. Steller's Sea Cow, another species of Sirenian became extinct by fisherman within 25 to 30 years of its discovery in the 1700's.

Physical Characteristics

An adult manatee has a rounded body, usually light to dark gray or black, that tapers to a horizontally flattened, rounded tail, which provides propulsion and also serves as a rudder. Manatee's range in length from 8 to 14 ft. long, depending on the species and can weigh 440 to 1300lbs. Although males can grow larger than the females. The small head includes a straight snout and a cleft upper lip with bristly hairs. Whiskers can be found on the surface of the lips, each attached seperatly to nerve endings and its own blood supply in the follicle. A manatees only teeth are 24 to 32 molars located in the back of the mouth, in addition to molars, manatees have horny, ridged pads at the front of the upper and lower jaws. The manatees upper lip is cleft with two lobes this isn used for gathering food. There small eyes are located on the sides of the head. Its nostrils, set on the upper surface of the snout are closed tightly by valves when the animal is under water. Their lungs and diagphram run lengthwise along the body, unlike other mammals, in which they run crosswise. This supposedly provides balance in the water. The many muscles surrounding the lungs allow manatees to exspel air more rapidly than other animals, thus making it possible for them to submerge for up to 15 minutes at a time. The paddlike fore limbs are set close to the head; no external limbs exists. The minimal amount of body hair and lack of hind limbs and external ears all reduce friction when the manatee glides threw the water.

Habitat

Manatees inhabit shallow, marshy coastal areas of North, Central and South America and the Caribbean Sea. The Florida manatee primarily

reside in Florida's coastal waters during winter and migrate either as far north as the Carolinas or as far west as Louisiana during the summer months. It can also survive in both fresh and salt water.

Manatees live in small family groups, although they do travel in herds of 15 to 20. Sirenians are the only aquatic mammals that are primarily vegetarians. They are considered herbivores and feed in both fresh and saltwater. They eat over 60 species of plants and forage on seagrass beds in bays, estuaries, and coastal rivers and graze for six to eight hours a day. An adult consumes an amount equal to between 5 and 10 percent of its body weight daily. Manatees have a slower metabolism than other mammals of similar size, which sharply reduces there energy requirements. Manatees ingest snails and other small animals found on and among the seagrasses on which they feed. The shells of these creatures provide calcium and phosphorus which the manatees require. There are also reports of them occasionally feeding on fish. This deviation from their vegetarian diet may be an important source of protein. In captivity, manatees need supplemental minerals to remain healthy.

Conservation of the Manatee

Manatees are not predators themselves, and there are no animals that prey on them except for man and possibly sharks. It is this dependence on vegetation near the shore that makes manatees so vulnerable to human-caused death. These doscile creatures were heavily exploited in the 19th and early 20th century, their hides made for leather, their bones worked like ivory, and their delectable meat eaten.