Check out this huge M. annularis and accompanying fish at Molasses Reef, Key Largo, Florida.
History points out the foolishness of man. For example, in 1493 Christopher Columbus reported seeing mermaids off the coast of the New World. In actuality these mythological beings that he witnessed were really manatees. Even though Columbus was mistaken, taxonomists perpetuated the cycle by naming the order of mammals that contains both manatees and dugongs after the mermaid-like creatures that plagued lost sailors known as sirens. Despite the case of mistaken identity, the explorer and his crew became the first Europeans to see a manatee in the New World firsthand (O’Shea, 1994). As pointed out by the anecdote above, the first step to understanding manatees is identification.
Manatees are mammals and they belong to the order sirenia with their relatives the dugongs. An easy distinguishing trait between the two sirenians is the tail. Dugongs have a “fluked” tail resembling that of a dolphin’s and manatees have a large, flat, rounded tail (Powell, 2002). To go along with the fluked tail, dugongs are more streamlined than the bulkier manatee and do not have the characteristic three “toe-nails” on their forelimbs. Dugongs would appear to have flippers although both sirenians use their limbs to crawl on the sea floor and hold food (Nowak, 2003).
The two families were believed to share similar territories until manatees “outchewed their relatives,” as Daryl P. Domning, a paleontologist at Howard University put it (O’Shea). As a manatee’s teeth wear down from the high silt content in the vegetation they consume, they are replaced by newer teeth that move in from the back of the jaw. This allowed a more diverse diet than the dugong’s teeth would allow since they continually grow instead of being replaced. Also, a manatee’s upper lip is “more deeply cleft” than a dugong’s, allowing it independent control of the two sides of the “muzzle” (Nowak).
Manatees, or Trichechidae, are broken up into three living species: the West Indian (T. manatus), West African (T. senegalensis), and Amazonian manatees (T. inunguis). The most common to North America, particularly Florida, is the Florida manatee, a subspecies of the West Indian species. In general, manatees are large “fusiform,” or spindle-shaped mammals with a large muzzle that houses their two covered nasal openings (Nowak). Hair is scattered all over their bodies until one reaches the muzzle which is covered in whiskers. On the inside of a manatee’s upper lip are bristles that give the animal traction on the vegetation it pulls into its mouth. They have small eyes and what is believed to be poor eyesight, as well as a small pin hole in the side of the head that leads to their auditory receptors (Powell). Basically, picture an elephant with a much shorter trunk, no floppy ears, and hind legs that have fused into one large paddle.
The West Indian manatee will be the focus here as its two subspecies, the Florida and the Antillean, are found around the Florida Keys and the Carribean. West Indian manatees are typically larger than their Amazonian and West African counterparts and can handle more diverse climates. They have been known to travel as far north as Connecticut and in 1994 an individual was caught in Chesapeake Bay, Maryland. After being returned to Florida the individual was found again in Rhode Island the following year, but returned home on his own. During the winters in Florida it has been recorded that manatees can survive in an average of 13.5° Celsius water, but prefer around 20° Celsius (Nowak). An interesting adaptation the Florida manatee has made is their ability to seek out hot springs and power plants in the winter. Although they are usually solitary animals, manatees some times congregate in groups of hundreds around the warm discharge of water around a power plant. This is an excellent example of how humans can positively influence survival of the endangered manatee (Powell).
Manatees do have natural predators such as sharks, alligators, and killer whales, but none compare to the threat that humans pose to their existence. The human population in Florida has grown from 15,982,378 in 2000 to 17,789,864 in 2005 and this does not include tourists (US Census Bureau, 2006). As the population has grown, so has the number of watercraft. In 2001 twenty five percent of the manatee deaths were due to collisions with boats and the rate continues to rise as more and more humans are taking to the water. The deaths toll does not count the number injured. Boat related injuries are so common that a man by the name of Joseph C. Moore, who worked for the Everglades National Park, found that individuals can be identified by their prop scars (O’Shea). The problem with the injuries is that they eventually lead to a very painful, slow death. A manatee’s lungs are positioned along their back. Deep cuts from propellers can puncture their lungs which in turn leaks oxygen into the body cavity, or pneumothorax. As their bodies fill up with air they find it more and more difficult to submerge and will eventually die of suffocation or infection (Powell).
Being the natural grounds keepers that they are, manatees will eat just about anything lying around. This includes, but is not limited to, fishing line. Not only do they get tangled in the fishing line and drown, but on occasion they ingest hooks that lead to a secondary infection and eventually death. Plastic bags and condoms have also been found in the gut of different manatees during the necropsy which can also wreak havoc on the digestive system. Humans take away from their natural food sources as well by destroying sea grass beds and depleting water reserves. With the population growth comes the need to withdrawal increasingly larger amounts of water from the aquifer for farming, golf courses, and personal use. This can lead to habitat destruction as the grazing grounds of some manatees are dried up (Powell).
Habitat destruction not only affects food sources, but also displaces and kills manatees by other means. Many Florida manatees depend on the warm waters provided by power plants to survive the winters. When the Brevard County power plant shut down in 1977, thirty eight dead manatees were found in the area. The leading cause of death was cold stress syndrome. Although manatees are relatively disease free, cold water can cause a loss of fat, whitish skin (“frostbite”), external sores, and impacted intestines. Also, the shutting down of a power plant or the drying up of a spring displaces manatees which in turn puts them in the path of their number one killer, the watercraft (Powell).
If history does really repeat itself, then both manatees and dugongs alike could be in trouble. A second species of the dugong family existed up until the year 1741. Twenty five years after the discovery of the Steller’s sea cow it was hunted to extinction (O’Shea). The Steller’s sea cow was a much larger Sirenian than exists today and lived in the Bering Sea. It was capable of surviving in much colder water due to its immense amount of blubber. It also differed from surviving Sirenians in that it had no true teeth to speak of (Nowak). Today, there are international regulations governing the hunting of Sirenians.
Hunting is not a real threat to the Florida manatee, but to the other species, including the dugong, it is a problem. Regulations are in place, but enforcement is sketchy at best. Sirenians are hunted using harpoons that are triggered by bait, box traps, “fishing,” large-holed gill nets, and harpooning from platforms that are located next to their regular feeding grounds. In some African countries where manatee and dugong meat is prized, law enforcement agents will often look the other way or indulge in the feast of illegally caught meat themselves. While in Australia, the aboriginal people are allowed to hunt dugong with little restriction (Powell).
There is hope though. The US Geological Survey has started an international program in Belize where they are training local biologists and raising public awareness for the Antillean manatee. Some of their efforts include radio tagging, aerial surveys, and the establishment of a more organized conservation effort. Sirenian International is another organization working on an international scale. Programs have been implemented in Vietnam, West Africa, Colombia, Venezuela, and Cambodia, to name a few. In these countries they work to establish regulatory commissions responsible for the conservation of manatees and dugongs and also to educate the local people in order to raise awareness of the animals endangered status. Sirenian International completes this goal by distributing children’s books, researching the various species in different locations, and checking on the true number of individual specimens in that area (Sirenian International, 2005).
In the US of A there are two nationally recognized statutes that fend for the manatees, as well as other living organisms, and those are the Endangered Species Act and the Marine Mammal Protection Act. Both forms of regulation make it illegal to “take” a manatee, an act that includes killing, harming or harassing them. Statewide in Florida there is the Florida Sanctuary Act of 1978 which separates thirteen “key counties” that must provide some sort of protection on the behalf of the manatee and its environment. As a result of these acts controlled speed zones have been implemented and, according to the US Fish & Wildlife Service, “seven Federal sanctuaries have been established.” More work is underway though as more counties are being supported by the state to develop safer waterways for manatees. Although these actions are not always supported by the public and different private organizations, battles are taking place in the court room due to the lack of a neutral ground in which the manatee enthusiasts and those that oppose new regulation can speak freely. Throughout it all there are still many who work to find manatees in Florida’s waters that have been injured or are sick due to the actions of humans. Zoos and aquariums, with the help of specialists and volunteers, rehabilitate the animals they find in hopes of returning them to the wild and possibly tracking them in the future (Powell).
Manatees have been a part of human culture for hundreds of years. In Mali there is a legend that tells of a young woman wading in a river. Surprised by a group of men approaching, she submerged herself covering her lower half with a fan. She was so embarrassed that she spent the rest of her existence in the water becoming what we know as a manatee. Also in West Africa manatees are thought of as “evil spirits” and only a few know how to kill one without being slain himself or going “mad.” According to O’Shea, along the Amazon, Siona Indian shaman believe an,
ancient god was trapped by a tapir, who cruelly subjected the god to attack by piranhas. The god escaped and in revenge banished one of the tapir’s daughters to live forever in the waters as a manatee (p 68).
Although the tall tales are all equally sexist, it is interesting to discover, not only the depths of human paranoia, but also the roles sirenians have played in superstitious beliefs and legends.
What would the world be like if we lost the manatee altogether? Biologists are still studying manatees in order to understand the evolutionary trail that separated them from their cousins, the elephant and the hyrax. How is it that they are related closer to terrestrial mammals than the dolphins and walruses that share their marine environment? What could this possibly tell us about evolution and natural selection? The answers to these questions are all being pursued. One thing is known, although their numbers are growing in Florida, their mortality rate is climbing as well (Powell). It does not take a scientist to understand the repercussions of such an ill-fated statistic.
Nowak, Ronald M. (2003). Walker’s Marine Mammals of the World. Baltimore, Maryland: The John Hopkins University Press.
O’Shea, Thomas J. (1994, July). Manatees. Scientific American. 66-72.
Powell, James. (2002). Manatees: Natural History & Conservation. Stillwater, Minnesota. Voyageur Press Inc.
Sirenian International. (2005, February 9). Projects Supported by Sirenian International. Retrieved April 15, 2006 from the World Wide Web: www.sirenian.org/aboutSI.html.
US Census Bureau. (2006). State & County Quick Facts: Florida. Retrieved May 25, 2006 from the World Wide Web: http://quickfacts.census.gov/qfd/states/12000.html.
US Fish & Wildlife Service: North Florida Field Office. Manatee Recovery Facts. Retrieved April 15, 2006 from the World Wide Web: www.fws.gov/northflorida/Manatee/manatee-gen-facts.htm.
US Geological Survey. (2006). International Programs: Manatee Conservation in Belize. Retrieved April 15, 2006 from the World Wide Web: http://international.usgs.gov/projects/prjmanatees.htm.
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