Name_____________________
The West Indian Manatee- A
Study in Evolutionary Patterns
Purpose To study three animals as examples of convergent
and divergent evolution
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
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
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
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?
West Indian Manatee
