The West Indian Manatee- A Study in Evolutionary Patterns

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1 The West Indian Manatee- A Study in Evolutionary Patterns Purpose To study three animals as examples of convergent and divergent evolution Name Objective Identify similarities and differences between the manatee, the elephant, and the walrus Introduction The evolution of organisms often takes two distinct routes. Divergent evolution is the path that two closely related groups take when they grow more and more dissimilar through time. Convergent evolution is the process by which two groups that are not closely related grow more and more similar through time. The West Indian Manatee, Trichechus manatus, also known as the sea cow, is a gentle aquatic mammal that once ranged throughout the coastal waters of the southeastern United States. Today, it is primarily restricted to Florida's springs, rivers, and coastal waters. The manatee's numbers have been reduced to less than 1,000 individuals. As a result, it has been placed on the endangered species list. The West Indian Manatee belongs to the order Sirenia. Three other living species that belong to this order are the Amazonian Manatee, the West African Manatee, and the Dugong of the Indopacific area. The order was named for the sirens, or mermaids, of Greek mythology whose haunting songs lured helpless sailors to their deaths. Legend suggests that ancient sailors thought manatees were mermaids, probably because they occasionally lift their heads out of the water. The fossil evidence indicates the manatees belong to a group called subungulates. They appear to have diverged from the other orders in this group in the Eocene Epoch over 60 million years ago. The distribution of the manatee was once much greater than it is today. Their remains are very common in the sediments deposited in shallow coastal areas. The closest living relatives to the manatees are the elephants and hyraxes. Both of these animals are now restricted to Africa and Asia. The largest member of the order Sirenia. was the Stellar's Sea Cow, which was hunted to extinction in the Bering Sea just a few years after its discovery in Like other mammals, the manatee is warm-blooded, breathes air, and nurses its young. Its finely wrinkled skin is covered with sparcely scattered hairs and is sloughed off on a regular basis. This probably is done to control external parasites. The body of the manatee is shaped like a large, gray torpedo. Their forelimbs are flat and paddle-shaped with three external nails. The forelimbs are used for slow maneuvering and walking along the bottom. The hindlimbs are absent and are represented only by fragments of pelvic bones. The manatee swims with powerful movements of its flat tail. The tail allows the manatee a cruising speed of 10 kilometers per hour and a top speed of about 25 kilometers per hour. The manatee is also capable of somersaults, barrel rolls, head and tail stands, and upside-down cruising. The manatee's eyes are small and almost buried in its skin. Although lacking acute vision, manatees investigate objects found in the water and raise their heads to observe things above the surface. The ears of the manatee lack external lobes and its auditory canals are very small. Despite this, its hearing is extremely good. Manatees make a number of squeaks and squeals that other manatees can detect from great distances. The manatee is a herbivore; it eats only plants. The manatee's only teeth are large grinding molars. The upper lip is split and extremely flexible. This allows the manatee to efficiently harvest large quantities of aquatic vegetation. The manatees in Florida are endangered largely due to human activity. In the past, they were hunted extensively for food. Today they are accidentally killed by boats, barges, flood-control structures, and fishing gear. Additionally, their population is harmed by loss of habitat and water pollution. Procedure Using the data sheet, compare the West Indian Manatee with a close relative, the elephant and another marine mammal, the walrus. Look for similarities and differences that illustrate divergent and convergent evolution. Analyses and Conclusions 1. Describe the characteristics that manatees and elephants share in common. 2. Describe the characteristics that manatees and walruses share in common. 3. Explain the possible reasons why manatees more closely resemble walruses than they do elephants. 4. What two evolutionary pathways are illustrated by these three animals?

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