Marine Biology Term Paper

The Manatee

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Table of Contents

Section Pages

Introduction 3
Report 4-9
Conclusion 10
Appendix 11-13
Bibliography 14

Introduction Manatees (order Sirenia, genus Trichechus) are the source folklore and possess many unique characteristics. With three recognized species, manatees (along with their relative, the dugong) also possess a unique range, limited mostly to coastal bays and waterways of the warm southern coasts. They are also the world’s only herbivorous marine mammals, and they have a highly-developed mouth with bristles. Although much-loved, the manatee population is currently under threat from man. Boat collisions are among the leading causes of death in manatees; this, as well as their long gestation period (one year), threatens manatee populations. Because of this, West Indian manatees in the United States are protected under federal law by the Marine Mammal Protection Act of 1972, and the Endangered Species Act of 1973.

Report Both manatees and dugongs are thought to be the source of the myth of mermaid stories.[1] The order Sirenia was named after the sirens of Greek mythology, mermaids who lured sailors to their death in shipwrecks by their singing. Perhaps this etymology is due to their unique vocalizations. Florida manatees were first documented to produce underwater vocalizations more than 40 years ago. This was followed by limited analyses of sounds of one or two captive Amazonian manatees and Caribbean manatees. “Hartman (1979) qualitatively described Florida manatee call notes with terms such as squeaks, chirps, and grunts, and described the contexts in which vocalizations were heard. Hartman hypothesized that aspects of manatee sounds may vary with motivational state, and may be individually distinctive and allow individual recognition. There is renewed interest in the study of vocalizations of Florida manatees (Nowacek et al. 2003; Phillips et al. 2004), prompted in part to develop acoustic detection or warning technology to reduce accidental collisions between manatees and boats.”[2]

There are three main species of manatees: the West Indian manatee (Trichechus manatus), the Amazonian manatee (T. inunguis), and the West African manatee (T. senegalensis). All of these species have different ranges (see Figure 1). Manatees are large, gray aquatic mammals with bodies that taper to a flat, paddle-shaped tail. They have two forelimbs, called flippers, with three to four nails on each flipper. Their head and face are wrinkled with whiskers on the snout. The manatee's closest relatives are the elephant and the hyrax (a small, gopher-sized mammal), and they are thought to have evolved from a wading, plant-eating animal. The average adult manatee is about ten feet long and weighs between 800 and 1,200 pounds.[3] Manatees can be found in shallow, slow-moving rivers, estuaries, saltwater bays, canals, and coastal areas. They are particularly found where seagrass beds or freshwater vegetation flourish. Manatees are a migratory species; in the United States, they remain mostly in Florida during the winter. However, in the summer months, they can be found as far west as Texas and as far north as Massachusetts; sightings in Alabama, Georgia and South Carolina are more common during this time of year (see Figure 2). Water temperatures that fall below 21° C (70° F) cause manatees to move into warm water refuge areas. “Unlike many marine mammals that inhabit large open-water areas, manatees often reside in narrow, irregularly shaped bodies of water (man-made canals, rivers, creeks, etc.) that are difficult or impossible to survey using standard methods like line transects (Barlow 1995, Calambokidas and Barlow 2004). Florida manatees aggregate in warm-water outfalls of power plants (see Figure 3) and natural springs in winter, seeking refuge from the colder ambient waters of surrounding bays, rivers, and estuaries. Aerial surveys that intensively cover manatee habitat in small, well-defined areas, like these winter aggregation sites (Packard et al. 1985), have been useful in obtaining minimum population counts.”[4]

West Indian manatees can also be found in the coastal and inland waterways of Central America and along the northern coast of South America, although distribution in these areas may be discontinuous.[5] The freshwater Amazonian Manatee (T. inunguis) inhabits the Amazon River and its tributaries. Amazonian Manatees never venture into salt water. They are found from the mouth of the Amazon River to the upper reaches of the tributaries of Brazil, Colombia, Ecuador, Guyana and Peru.[6] West African Manatees are found in coastal marine and estuarine habitats and in freshwater river systems along the west coast of Africa from the Senegal River south to the Kwanza River in Angola. Areas included in their ranges are Gambia, Liberia, Guinea-Bissau, Guinea, Sierra Leone, Côte d'Ivoire, Ghana, Mali, Nigeria, Cameroon, Gabon, Republic of the Congo, and Democratic Republic of the Congo. They have been found living as high upriver on the Niger River as Gao, Mali.[7] Manatees are gentle and slow-moving animals. Most of their time is spent eating, resting, and traveling. They are also the sea’s only completely herbivorous marine mammal[8]; these herbivores are known for their generalized feeding habits and may ingest 60 species of freshwater and marine plants. They can consume 10-15% of their body weight in vegetation daily. [9] “They [manatees] possess a short muscular snout that is covered by short sinus hairs and modified vibrissae or bristles (see Figure 4). These bristles are short, thick, and robust and are organized in several discrete fields in the perioral region (Reep et al. 1998). Among mammals, manatees are unique in using these perioral bristles in a prehensile manner in conjunction with elaborated facial musculature to bring plants into the mouth (Marshall et al. 1998a, 1998b). Manatees use this vibrissal–muscular complex to manipulate a diverse range of structural forms among aquatic angiosperms.”[10]

Because they are mammals, they must surface to breathe air. They may rest submerged at the bottom or just below the surface of the water, coming up to breathe on an average of every three to five minutes. When manatees are using a great deal of energy, they may surface to breathe as often as every thirty seconds; if resting, manatees have been known to stay submerged for up to twenty minutes. Manatees can swim up to twenty miles per hour in short bursts, but they usually only swim about three to five miles per hour.[11] The reproductive rate for manatees is low. Female manatees are not sexually mature until about five years of age; males are mature at approximately nine years of age, although some males mature earlier. On average, one calf is born every two to five years, and twins are rare. Intervals between births range from two to five years. A two-year interval may occur when a female, or cow, loses a calf soon after birth. The gestation period for female manatees is about a year.[12] Male manatees play no role in raising the calf. Mothers nurse their young for one to two years, so a calf may remain dependent on its mother during that time (see Figure 5). Calves nurse underwater from teats located behind the mother's flippers; they begin to eat plants a few weeks after birth. Newborn calves are capable of swimming to the surface on their own and vocalize at or soon after birth.[13] Manatees have very few predators – alligators, crocodiles, orcas, and sharks are their only natural enemies, and they can live sixty years or more. As with all wild animal populations, a certain percentage of manatee mortality is attributed to natural causes of death such as cold stress, gastrointestinal disease, pneumonia, and other diseases; however, a high number of additional fatalities are from human-related causes. Most human-related manatee fatalities occur from collisions with watercraft. “Between 1993 and 2003, 713 (24%) of 2,940 dead Florida manatees (Trichechus manatus latirostris) recovered from Florida waters and examined were killed by watercraft-induced trauma. It was determined that this mortality was the result of watercraft trauma because the external wound patterns (see Figure 6) and the internal lesions seen during gross necropsy are recognizable and diagnostic.”[14] Other causes of human-related manatee mortality include being crushed and/or drowned in canal locks and flood control structures, ingestion of fish hooks, litter, and monofilament line, and entanglement in crab trap lines. Ultimately, loss of habitat is the most serious threat facing the approximately 3,800 manatees in the United States today. West Indian manatees in the United States are protected under federal law by the Marine Mammal Protection Act of 1972, and the Endangered Species Act of 1973, which make it illegal to harass, hunt, capture, or kill any marine mammal. West Indian manatees are also protected by the Florida Manatee Sanctuary Act of 1978. Violations of these federal or state laws can be met with civil or criminal convictions associated with monetary fines and/or imprisonment. The Florida Manatee Recovery Plan was developed as a result of the Endangered Species Act; this recovery plan is coordinated by the U.S. Fish & Wildlife Service (USFWS) and sets forth a list of tasks geared toward recovering manatees from their current endangered status.[15] Manatees are an important marine mammal, as is any species. Dr. O’Shea provides us with a great justification for their protection and environment impact: “Our nation's attitude toward its own conservation responsibilities is undergoing a period of rapid change. Public values embrace a wide array of species beyond those with a traditional dollar or recreational value. Federal legislation such as the Marine Mammal Protection Act and the Endangered Species Act, new laws of the various states, institution of non-game programs, and growing membership in broad-based conservation groups all signal a widening respect and concern for the diversity of life as a reflection of the quality of life. Permitting the extinction of any species is an irrevocable desecration of this emerging view of the quality of life. Nevertheless, some species are cast more to the forefront of the public eye than others, and in Florida the manatee ranks among them. In some sense perhaps this rank among taxa of concern is not disproportionate. If the future is lost for manatees we lose the only representative of an entire mammalian order in our country, and the only example we have of the marvelous adaptations represented by this level of evolutionary distinctiveness. We will lose our self-respect by eliminating a benign species with no direct conflict with man for resources, a species tough enough and flexible enough to have endured centuries of pressure and environmental change. Florida is undergoing prodigious development, yet remnants of its fauna manage to persist in a remarkable, highly visible interface between wildlife and man. Maintenance of that interface with continued development is a great experiment in how far our society can go while still upholding principles of respect for the diversity of life. Manatees currently have the biological chance and the legal justification to persist, but if the species is lost to future generations it will be a bellwether, a signal that through lack of commitment we can lose the battle to maintain the compatibility of civilization and living diversity.”[16]

Conclusion The manatee is a much-loved marine mammal that is perfectly suited for its environment. Gentle and calm, they have captured the imaginations of people through folklore, as well as science. Though currently threatened by man’s activities, manatees are a protected species, and many people are dedicating much time and money to their survival. Aquariums, zoos, and rescue facilities are educating the public on their plight in hopes of curbing any future problems. Even though manatees may be threatened, they have shown their resilience and continue to live as successfully as they can in their home ranges. We can only hope that, as education increases, so does public awareness of the manatee and its marvelous adaptations so that this amazing creature is around for generations to come.

Appendix
• Figure 1
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The range of the Amazon manatee is marked in red. The green area indicates the range of the West Indian manatee and the orange area is the range of the West African manatee.
Photo taken from: "WWF - Amazon Manatee." WWF - WWF. Web. 16 Apr. 2010. .

• Figure 2
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Range of the West Indian manatee in the United States.
Photo taken from: Save the Manatee Club. Web. 16 Apr. 2010. .

• Figure 3
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Manatees gather at the warm water effluent of Florida Power and Light Company's Riviera Beach power plant.
Photo taken from: Save the Manatee Club. Web. 16 Apr. 2010. .

• Figure 4
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The manatee’s modified vibrissae or bristles.
Photo taken from: http://www.bio.davidson.edu/people/midorcas/animalphysiology/websites/2003/Harshaw/page4.htm

• Figure 5
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A mother manatee (cow) and her calf.
Photo taken from: Save the Manatee Club. Web. 16 Apr. 2010. .

• Figure 6
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Manatee wounds incurred from a boating encounter.

Bibliography
Edwards, Holly H., Bruce B. Ackerman, and John E. Reynolds. "Techniques and Technology Article Estimation of Detection Probability in Manatee Aerial Surveys at a Winter Aggregation Site." The Journal of Wildlife Management 71.6 (2007).

Lightsey, Jessica D., and Sentinel A. Rommel. "Methods Used During Gross Necropsy To Detemine Whatercraft-related Mortality in the Florida Manatee (Trichechus manatus latirostris)." Journal of Zoo and Wildlife Medicine 37.3 (2006): 262-275.

Marshall, Christopher A., Paul S. Kubilis, Glenn D. Huth, and Virginia M. Edmonds. "Food Handling Ability and Feeding-Cycle Lengt of Manatees Feeding on Several Species of Aquatic Plants.” Journal of Mammalogy 81.3 (2000): 649-658.

O'Shea, Thomas J., and Lynn B. Poche. "Aspects of Underwater Sound Communitcation in Florida Manatees (Trichechus manatus latirostris)." Journal of Mammalogy 87.6 (2006).

O'Shea, Thomas J., "The Past, Present, and Future of Manatees in the Southeastern United States: Realities, Misunderstandings, and Enigmas." Proceedings of the Third Southeastern Nongame and Endangered Wildlife Symposium, pp. 184-204.

Save the Manatee Club. Web. 16 Apr. 2010. .

Sverdrup, Keith A., and E. Virginia Armbrust. Introduction to the World’s Oceans. Boston: McGraw-Hill Education, 2009.

“West African manatee.” Encyclopedia Britannica. 2010.

"WWF - Amazon Manatee." WWF - WWF. Web. 16 Apr. 2010. .

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[1] Sverdrup, Keith A., and E. Virginia Armbrust. Introduction to the World’s Oceans. Boston: McGraw-Hill Education, 2009.
[2] O'Shea, Thomas J., and Lynn B. Poche. "Aspects of Underwater Sound Communitcation in Florida Manatees (Trichechus manatus latirostris)." Journal of Mammalogy 87.6 (2006).

[3] Save the Manatee Club. Web. 16 Apr. 2010. .
[4] Edwards, Holly H., Bruce B. Ackerman, and John E. Reynolds. "Techniques and Technology Article Estimation of Detection Probability in Manatee Aerial Surveys at a Winter Aggregation Site." The Journal of Wildlife Management 71.6 (2007).
[5] Ibid.
[6] "WWF - Amazon Manatee." WWF - WWF. Web. 16 Apr. 2010. .
[7] “West African manatee.” Encyclopedia Britannica. 2010.
[8] Sverdrup, Keith A., and E. Virginia Armbrust. Introduction to the World’s Oceans. Boston: McGraw-Hill Education, 2009.
[9] Marshall, Christopher A., Paul S. Kubilis, Glenn D. Huth, and Virginia M. Edmonds. "Food Handling Ability and Feeding-Cycle Lengt of Manatees Feeding on Several Species of Aquatic Plants.” Journal of Mammalogy 81.3 (2000): 649-658.
[10] Ibid.
[11] Save the Manatee Club. Web. 16 Apr. 2010. .
[12] Ibid.
[13] O'Shea, Thomas J., and Lynn B. Poche. "Aspects of Underwater Sound Communitcation in Florida Manatees (Trichechus manatus latirostris)." Journal of Mammalogy 87.6 (2006).
[14] Lightsey, Jessica D., and Sentinel A. Rommel. "Methods Used During Gross Necropsy To Detemine Whatercraft-related Mortality in the Florida Manatee (Trichechus manatus latirostris)." Journal of Zoo and Wildlife Medicine 37.3 (2006): 262-275.

[15] Save the Manatee Club. Web. 16 Apr. 2010. .
[16] O'Shea, Thomas J., "The Past, Present, and Future of Manatees in the Southeastern United States: Realities, Misunderstandings, and Enigmas." Proceedings of the Third Southeastern Nongame and Endangered Wildlife Symposium, pp. 184-204.