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Manatees and How They Live

Copyright 1994 Oceanic Research Group, Inc

All Rights Reserved

In 1493, Columbus entered into his logbook the sighting of a mermaid in the Caribbean. In his log, he mentioned that the mermaid was not as beautiful as sailors had been led to believe. Of course, he had not actually seen a mermaid, but instead, a manatee, and his logbook became the first written historical reference to the now greatly endangered West Indian Manatee. Join us as we explore the underwater world of this fascinating and often misunderstood marine mammal.

The manatee is a marine mammal, like a seal or a whale, but is unique in many ways. Like all mammals, manatees have lungs and breathe air. Therefore, they must hold their breath while diving, and need to surface for air periodically. Manatees have a horizontally flattened tail fluke and a pair of front flippers like whales do. But unlike whales, manatees live in both salt water and fresh water, and move between the two at will.

Manatees are contained within the order Sirenia. The term Sirenia is a reference to the mythical "siren" of Homer's Odyssey, a beautiful mermaid who lured sailers to treacherous waters and reefs. There are only four living species of manatees in the world, all of which are similar in size and appearance, but with minor differences in anatomy. The West Indian Manatee is the only sirenian species found within the United States. The others are found in West Africa, the Amazon and the South Pacific. Until the mid-seventeen-hundreds there were huge manatees called Stellar's Sea Cows in the Bering Sea.

"The Stellars Sea Cow was discovered in 1741 and was hunted so heavily for its meat and hides that within 27 years after its discovery, it was extinct. It was the largest animal in the order Sirenia, the same order which contains the manatees and dugongs we know of today. But unlike those we know today, it lived in cold arctic waters, while all modern manatees need warm tropical waters to survive."
-Maria Rutzmoser, Museum of Comparative Zoology, Harvard University

All manatees are believed to have evolved from a four-footed, plant-eating land mammal more than 60 million years ago. This prehistoric relative may have looked similar to the hyrax, a modern relative of the manatee.

Although manatees look much like whales, blood sampling has shown that they are actually more closely related to elephants than to either whales or seals.

The West Indian Manatee lives throughout the Caribbean and Florida. During the summer, this curious animal roams the coastal ocean as far north as the Carolinas or as far south as Brazil, and throughout the Gulf of Mexico. During the winter, however, low water temperature causes the manatees to seek warmer regions. Manatees cannot tolerate water much below 68 degrees F. During the winter, when the ocean water temperature drops below that, many manatees go inland, traveling up rivers and canals, to gather near sources of warm water.

Natural warm water springs occur throughout Florida in places where pure, clear water, heated deep within the Earth’s crust, flows to the surface.

This is generally the only place where large congregations of manatees can be found, since during the summer they do not normally travel in groups like dolphins or whales. In the warm water springs, the manatees wait out the cold weather, and then return to the coastal ocean in the spring.

The manatee has a body form resembling that of a seal, except that the manatee has a rounded, flattened tail, vaguely similar to a whale's fluke. The manatee swims with slow undulations of the body and tail. Manatees are in fact so slow and leisurely, that many of them have algae or barnacles growing on their backs. For this tropical fish, the algae on a passing manatee is lunch!

The two flippers of the manatee are used to steer and position the manatee while swimming, but they are also used as arms to hold onto objects and manipulate food to the mouth. They can use the flippers to grab people if they are feeling playful. Sometimes the manatee even "walks" on its flippers.

The evolutionary relationship of manatees to land animals is perhaps most obvious in the flippers, which still retain the finger bones of distant ancestors, even though the fingers themselves have long since been lost in order to form a flat paddle shape.

The manatee has small eyes, protected by membranes which can be drawn across the eyeballs. The manatee is capable of seeing in both dim and bright lighting, and can probably see colors, but has somewhat limited depth perception. It produces thick, sticky tears to protect its eyes when they are out of the water. Although the manatee lacks eyelids as we know them, it can still blink to protect its eye.

The ears of the manatee are internal, with no external lobes, but the underwater hearing of the animal is quite good. Manatees produce squeaks and chirps. It is not known for sure why, but it is suspected that they could range from alarm calls, to expressions of happiness. Most scientists believe that the manatee does not use these sounds for echolocation like a dolphin would.

Breathing occurs through a pair of nostrils located on the top of the nose. The manatee can close off its nostrils when diving in order to keep water out. When a breath is taken, 90% of the air in the lungs is refreshed, compared with about 10% in humans. This allows the manatee to make more efficient use of its trips to the surface.

Manatees normally breathe every 2 to 5 minutes, but can stay submerged for as long as 20 minutes if necessary. Like other marine mammals, when diving, a manatee may slow its heart rate to as little as 20% of its normal rate, in order to conserve oxygen.

The mouth of the manatee contains rows of molars, for chewing the tough plants which make up the animal's diet. As these teeth are worn down by the plants, they are replaced with new ones growing in from the back. For the entire life a manatee, it never runs out of new teeth!

"Here we have a manatee skull, and you can see the hard palette in the front (the manatee doesn't have teeth in the front but a hard palette) with rows of grinding molars in the back. There is sand in the plants the manatees eat and it wears down the teeth. The front teeth are more worn, while the ones in the back still have a bit of a point to them, and the teeth go right down inside the jawbone."
-Betsy Dearth, Park Ranger, Homosassa Springs State Wildlife Park

The manatee is a simple and docile grazer. It has evolved no defenses because it has no natural enemies, and eats only aquatic plants. In fact, it is the only completely herbivorous marine mammal in the entire world. Manatees prefer shallow water, between 6 and 7 feet deep, where they can find their favorite foods. Manatees will spend 6 to 8 hours per day feeding, and may consume between 60 and 200 pounds of vegetation, or roughly 10% of their body weight! The nutritious roots of water plants, especially the water hyacinth, are a manatee favorite. Since this proliferating water weed is a nuisance to people, weed chomping manatees are natural weed-control agents! Manatees are, in fact, so effective at eating their way through weed-choked canals, that they have actually been employed in some countries to clear canals for people.

"Manatees are what are called hind-gut animals. Unlike the cow, which does most of the digestion in the stomach, manatees are more like horses, having a long colon for digestion. It takes 148 hours for food to pass through the manatee digestive tract."
-Dr. Jesse R. White, Marine Mammal Veterinarian

The manatee needs to drink water in addition to eating. When in fresh water, this is not a problem, and the animal drinks frequently. However, when the manatee is in the ocean or brackish coastal areas, it cannot get fresh water to drink. In addition, the manatee inadvertently swallows some sea water when it feeds in these areas. The kidneys of the manatee are specially developed to help it rid itself of excess salt. But it is not known how long a manatee can go without fresh water.

When manatees aren't eating, they either sleep, or play. When sleeping, the manatee lies motionless on the bottom, but has to surface for a breath every few minutes. With as little effort as possible, the manatee will surface, breathe, and settle right back to the bottom. This manatee has found herself a nice shady resting spot under a bridge.

When playing, manatees sometimes chase each other, rub each other, or just roll over and over. They tend to be very curious. Manatees will sometimes pop their heads up next to boats to investigate their human company. Although some manatees avoid swimmers and snorkelers, many others swim right up for a closer look. Its quite a sight when a 2,000 pound animal swims over to say hello!

Manatees usually give birth to their young during the winter stay at the warm water springs. At birth, calves are 3 feet long, dark in color, and weigh about 75 pounds. Female manatees form strong bonds with their calves, and are able to recognize them by their sounds and smell. When the calf is young and vulnerable, the mother will protect her offspring by always placing herself between the calf and anything she considers a threat.

The mother has a nipple located under each flipper. The calf stays with its mother for at least a year, nursing from its mother's milk. This large calf is about a year old and 8 feet long. Although it now seeks its own food, it is still nursing from its mother on occasion. Females are generally not sexually mature until age 5 to 9 years old, males until 6 to 9 years. A female gives birth only once every two or three years, usually to a single calf, or, rarely, twins. The gestation period is about 13 months. Orphans are rare among manatees. It is quite common for mature female manatees to adopt orphaned calves and nurse them as their own.

This manatee is about 2 years old. Average mature manatees are 10 feet long and weigh in the neighborhood of 1,200 pounds. However, some individuals may reach 15 feet and over 3,000 pounds! The lifespan of a manatee is thought to be greater than 50 years.

Manatees face many challenges to survive in the wild. They are slow-moving and are frequently hit by boats. In fact, about 85% of the manatees in Florida show horrible scars caused by boat propellers. Many animals are positively identified every year by their tell-tale boat propeller scars. If this isn’t bad enough, approximately 130 manatees die each year in Florida from collision with boats and laceration by the deadly propellers.

"Here we have a manatee rib. Manatees don't make their bone marrow in their ribs, they make it in their backbones, so these ribs are really heavy. Many manatees die each year from being hit by boats and then the ribs are fractured and puncture the lungs. These ribs are really heavy and its hard to imagine the force required to break one of these ribs."
-Betsy Dearth, Park Ranger, Homosassa Springs State Wildlife Park

To reduce this terrible slaughter, many manatee areas in Florida near warm water springs are either off limits to boat traffic, or are posted with a speed limit. Although this does not guarantee the safety of manatees, it at least gives them a chance to get out of the way of oncoming boats.

"Its hard to know exactly how many manatees are out there in the wild. The number we have released (and there is no scientific evidence to support this number) is 1824, which we counted by aerial survey in 1992. Certainly there were manatees that we missed because they were hidden from view, so it is reasonable to estimate that there are probably about 2,000 to 2,500 left in existence [in Florida]."
-Dr. Jesse R. White, Marine Mammal Veterinarian

These low population figures coupled with high death rate and low birth rate all add up to a very questionable future for the Gentle West Indian Manatee. Manatees are dying faster than they can reproduce. It has been estimated that if things do not change soon, there will be less than 500 manatees in Florida by the year 2,000. Unless we act now, the West Indian Manatee is almost guaranteed to go the way of Stellar’s sea cow, and become extinct.

The marine mammal protection act of 1972 and the endangered species act of 1973 were the first steps towards protection of the manatee in the United States. These laws make it illegal to harm, harass or kill any marine mammal, including manatees.

However, these laws are not sufficient to protect the manatee from accidents with boats, loss of habitat and the other hazards they face. In 1978, the Florida Manatee Sanctuary Act was passed, making the entire state of Florida an official manatee sanctuary. In addition, many local environmental organizations are helping the manatee cause by raising funds to provide medical assistance to injured animals and increase public awareness of the plight of the manatee.

Perhaps one of the most impressive of manatee preservation programs has come about almost by accident. Many manatees have begun to accumulate near power plants. These power plants use bay water as coolant. Once released back into the bay, the water is unchanged except for a slightly higher temperature. Manatees use these artificial warm-water sources the same way they use the springs: they accumulate near them for the winter.

In order to help the manatees, the power companies, such as this one in Tampa, have provided for the manatees by giving them a source of fresh water to drink, and prohibiting boat traffic within the manatee zone. In addition, some power companies have even set-up manatee watching platforms for manatee enthusiasts.

Although some critics argue that the manatees are coming to rely upon people too much for their survival, many others see this as just one more way to ensure the survival of this magnificent animal.

There comes a time when people must decide how far they will go in their domination of the environment. We alone now hold the fate of the manatee in our hands. We can save the manatee through simple management of our resources. We need to leave the manatees a portion of the world for themselves. We have to leave them a place to live without boats or houses, pollution or harassment. It is a simple thing, really, to share the world with another species. The time has come to make the decision. Let us hope that we can find it within our hearts to leave some of the Earth to the Gentle West Indian Manatees, so that we may enjoy their company for years to come, and that they are not reduced to skeletons in a museum of what used to be.

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update 6/5/07