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Manatees (Trichechidae)

Manatees

(Trichechidae)

Class Mammalia

Order Sirenia

Family Trichechidae


Thumbnail description
Large, fully aquatic, nearly hairless, herbivorous marine and freshwater mammals

Size
9–13 ft (3–4 m) total length; 1,100–3,300 lb (500–1500 kg) body mass

Number of genera, species
1 genus; 3 species

Habitat
Tropical and subtropical Atlantic; coastal, estuarine and riverine

Conservation status
Vulnerable: 3 species

Distribution
West coast of Africa from Senegal south to Angola; Southeastern United States, throughout the Caribbean to southeastern Brazil; Amazon River basin

Evolution and systematics

The oldest sirenians are from the early Eocene and are related to elephants, hyraxes, and the extinct desmostylids. The trichechids may have arisen from dugongids in the late Eocene or early Oligocene. The subfamily Trichechinae (modern manatees) first appeared in freshwater Miocene deposits in Colombia. It is likely that much of the early developmental history occurred in South America. The spread to North America and Africa was likely in the Pliocene or Pleistocene.

Physical characteristics

Manatees have a standard body length of 9–13 ft (3–4 m) and weigh 1,100–3,300 lb (500–1,500 kg) depending on the species. The body is nearly hairless, robust and oval in cross section. Hind limbs are absent but vestigial pelvic bones remain embedded in the pelvic musculature. The tail is a broad, rounded paddle. Forelimbs are short, flexible, and have tree to four nails except in the Amazonian manatee. Color is gray to brownish and, in the field, may depend on the epiphytes (algae, etc.) that are growing on the skin. The eyes are very small and there is no external ear (pinna). The external ear canal opening is very small and difficult to see. Testes are internal and the male genital opening is anterioventral just posterior to the umbilicus. Mammary glands are paired and there is a single nipple in each axilla. The upper lips are split, covered with stiff vibrissae, and have been described as "prehensile" from the manatee's ability to manipulate vegetation.

Distribution

The manatees are found on both sides of the Atlantic Ocean in tropical and subtropical regions. In West Africa, they range from Senegal southward to Angola. Along the eastern Atlantic seaboard, they range from the southeastern United States (primarily Florida) southward throughout the Caribbean region to southeastern Brazil plus the Amazon River basin.

Habitat

Manatees occur in coastal, estuarine and freshwater/riverine habitats. Since two of the species depend extensively on marine vegetation (seagrasses), they rarely venture into deep waters.

Behavior

Manatees are typically semi-social. The primary social group is the cow-calf pair that may remain intact for two years or more. There is no pair bonding between males and females. Their reproductive behavior has been described as "scramble promiscuity" wherein several males compete for mating rights with a single estrous female. Manatees may undertake local migrations in response to water temperature, water depth, or to the presence/absence of freshwater. Manatees communicate using sound, vision, taste and touch. In turbid waters, acoustic communication is important for maintaining the cow-calf bond. Most manatee sounds are in the 3–5 kHz range but have not been well studied.

Feeding ecology and diet

The manatees are the only marine mammals that are herbivorous. However, they are not obligate herbivores and will consume fish and invertebrates in some areas if they are available. The typical manatee diet consists of a wide variety of marine and freshwater vascular plants and algae as well as terrestrial vegetation that may be accessible on shorelines, overhanging and touching the water or floating such as red mangrove propagules. Manatees may feed on bottom, midwater,

or floating vegetation and they may climb partially out of the water to access shoreline vegetation.

Reproductive biology

Manatees mature at two to 11 years of age. Females may be seasonally polyestrus. Gestation is about 12 months but has not been confirmed. Typically, a single calf is born and is 3.3–4.9 ft (100–150 cm) long. Twins are rare. The mating system has been described as "scramble competition polygamy or polyandry" or "scramble promiscuity." Individual estrous females are pursued by as many as 20 or more males. While males may mature at three to five years of age, they may not be able to secure mating rights until they are physically larger. The typical calving interval is two-and-a-half to three years. There is no pair bonding. Males play no role in care of the young. Calves may be born at any time of the year but there may be seasonal peaks in parts of the range.

Conservation status

The manatees are listed variously as endangered, threatened, or vulnerable under international and national legislation. The Convention on Trade in Endangered Species of

Flora and Fauna (CITES) lists the Amazonian manatee and both subspecies of the West Indian manatee in Appendix I and the West African manatee in Appendix II. The World Conservation Union (IUCN) lists all three species, including the subspecies, as Vulnerable. The United States Endangered Species Act (ESA) of 1973 lists both subspecies of the West Indian manatee and the Amazonian manatee as "endangered" and the West African manatee as "threatened." The African Convention for the Conservation of Nature and Natural Resources lists the West African manatee as "protected" under Class A. The Florida manatee is protected by the Florida Manatee Sanctuary Act in the State of Florida, United States.

Significance to humans

Historically, manatees have provided humans with a meat for food and bones and hides for tools, implements, and leather. Some indigenous peoples may have used manatee parts for medicinal and aphrodisiac purposes. In Guyana and Florida, manatees have been used to clear vegetation-choked canals and waterways. As of 2003, while manatees are protected range-wide, they are still hunted for food in many areas. Ecologically, manatees may benefit humans indirectly by recycling nutrients in seagrass beds, keeping the vegetation in a constantly regenerating state and maintaining habitat for fish and invertebrates used by humans. In some regions, especially in Florida, the manatee is the basis for ecotourism.

Species accounts

List of Species

West Indian manatee
West African manatee
Amazonian manatee

West Indian manatee

Trichechus manatus

taxonomy

Trichechus manatus Linnaeus, 1758, West Indies.

other common names

English: Caribbean manatee, Antillean manatee, Florida manatee; French: Lamantin; German: Seekuh, manati; Spanish: Vaca marina, manatí.

physical characteristics

13 ft (4 m) body length; 3,300 lb (1,500 kg) body mass; color gray; body nearly hairless and slightly compressed dorso-ventrally; no hind limbs; tail broad and spatulate; forelimbs short, flexible and with three to four nails.

distribution

Southeastern United States, Caribbean, to Bahia, Brazil. The Florida manatee occurs primarily in Florida and southeastern Georgia but is known from as far north as Rhode Island and as far west as Texas. A few Florida manatees have apparently crossed the Gulf Stream to the northern Bahamas. The Antillean manatee ranges from Mexico, the Caribbean, to northeastern Brazil. It apparently does not enter the Amazon River.

habitat

Coastal and estuarine areas, freshwater rivers connected to the coast.

behavior

Generally considered solitary or semi-social except for mating herds or winter congregations in warm water refugia. The only long-term social unit is the female-calf pair which may last two or more years. Florida manatees migrate along a north-south axis in response to air and water temperature. In other parts of the range, manatee movements may be dictated by wet and dry seasons.

feeding ecology and diet

The West Indian manatee feeds on submerged, mid-water, floating, overhanging and bank vegetation. Fish and invertebrates are ingested on occasion. Individuals are estimated to consume 10% body weight in vegetation per day.

reproductive biology

Males and females mature at 2.5–6 years. Calving interval is 2.5–3 years. Females are seasonally polyestrous. Both males and females are promiscuous and polygamous. Estrous females may be pursued by 20 or more males for up to a month. There is no pair bonding and males play no role in care of the young. Typically, a single calf is born but twins may account for 1–2% of pregnancies. Calving is broadly seasonal in some areas and both males and females may show seasonal patterns in sexual activity. Some Florida manatees may live for 50 years or more and reproductive senility is not known.

conservation status

Protected throughout the range but laws are difficult to enforce. Unknown numbers are killed illegally each year for food.

In Florida, significant numbers are killed each year from collisions with watercraft, entanglement in fishing gear, ingestion of plastic and recreational fishing gear, or drowning/crushing in canal locks and flood gates. Actual and potential habitat (seagrass) destruction is a significant conservation factor throughout the range. Runoff of anthropogenic chemicals (pesticides, etc) may be a problem range-wide. Natural and artificial warm water refugia in Florida are key habitat components in Florida. The Florida manatee has a minimum population of 3,000–3,500 (as of 2003). Population estimates for the Antillean manatee are not available.

significance to humans

Historically, a source of meat, fat, oil, hide, bone. Some use in clearing waterway vegetation in Florida and Georgetown, Guyana.


West African manatee

Trichechus senegalensis

taxonomy

Trichechus senegalensis Link, 1795, Senegal.

other common names

English: African manatee; French: Lamantin; German: Seekuh, manati.

physical characteristics

Similar in body shape and size to the West Indian manatee but with a blunter snout, protruding eyes, and a slimmer body. Fingernails are present. Good data are lacking.

distribution

West Africa from Senegal southward to Angola.

habitat

Coastal, estuarine, riverine, and lacustrine (lakes).

behavior

Similar to the West Indian manatee but details are lacking. The major social unit is the cow-calf pair.

feeding ecology and diet

Feeds on submerged, mid-water, floating, overhanging, and bank vegetation. Diet may include some invertebrates.

reproductive biology

Details not well known. Thought to be similar to the West Indian manatee.

conservation status

Animals are still hunted throughout their range. Habitat degredation is a potential problem. Range-wide data are lacking and there are no reliable population estimates.

significance to humans

Historically hunted for meat, fat, oil, hides and bones. Culturally significant in some areas and some hunting continues to this day for food and reduce damage to fishing gear and rice fields. In some areas of Cameroon, local people fear manatees and do not hunt them.


Amazonian manatee

Trichechus inunguis

taxonomy

Trichechus inunguis (Natterer, 1883), Rio Madeira, Brazil.

other common names

French: Lamantin de l'amazone; Spanish: Vaca marina amazónica.

physical characteristics

The smallest of the three species of manatee. Maximum body length is 9 ft (3 m) or less and body mass is less than 1,100 lb (500 kg). Color generally gray and most individuals have a white or pink abdominal patch of variable size. Fingernails are absent.

distribution

Throughout the Amazon River basin. No reliable population estimates are available.

habitat

All accessible freshwater rivers and lakes in the Amazon basin. Does not enter saltwater.

behavior

Considered to be semi-social with the cow-calf pair being the dominant social grouping.

feeding ecology and diet

Feeds on a variety of floating, overhanging, and bank vegetation. In the dry season, some Amazonian manatees are thought to fast for several months when water levels in lakes are low and vegetation is not accessible.

reproductive biology

Thought to be similar to the West Indian manatee in general terms, but good details are lacking. Gestation is about one year. Calves are 2.5–3.5 ft (85–105 cm) long and weigh 22–33 lb (10–15 kg) at birth.

conservation status

Still hunted to an unknown extent. Actual and potential habitat destruction and degradation are probably the biggest threats.

significance to humans

Historically hunted for meat, fat, oil, hides and bone. Middle ear bones (stapes) thought to have aphrodisiac powers.


Resources

Books

Dierauf, L. A. and F. M. D. Gulland, eds. CRC Handbook of Marine Mammal Medicine, 2nd ed. Boca Raton, Florida: CRC Press, 2001.

Domning, D. P. "Desmostylia." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig and J. G. M. Thewissen. San Diego: Academic Press, 2002.

——. "Sirenian Evolution." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Hartman, D. S. Ecology and Behavior of the Manatee (Trichechus manatus) in Florida. Lawrence, KS: American Society of Mammalogists, 1979.

Kaiser, H. E. Morphology of the Sirenia. Basel: S. Karger, 1974.

Marmontel, M., D. K. Odell, and J. E. Reynolds III. "Reproductive Biology of South American Manatees." In Reproductive Biology of South American Vertebrates, edited by William C. Hamlett. New York: Springer-Verlag, 1992.

Odell, D. K. "Sirenian Life History." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig and J. G. M. Thewissen. San Diego: Academic Press, 2002.

O'Shea, T. J., B. B. Ackerman, and H. F. Percival. Population Biology of the Florida Manatee. Washington, DC: U. S. Department of the Interior, National Biological Service, 1995.

Reynolds, J. E. III, and D. K. Odell. Manatees and Dugongs. New York: Facts On File, 1991.

Reynolds, J. E. III, and J. A. Powell. "Manatees." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Reynolds, J. E. III, and S. A. Rommel, eds. Biology of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Rice, D. W. Marine Mammals of the World: Systematics and Distribution. Lawrence, KS: The Society For Marine Mammalogy, Special Publication Number 4. 1998.

Rommel, S. A., D. A. Pabst, and W. A. McLellan. "Skull Anatomy." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Rommel, S. A., and J. E. Reynolds III. "Skeletal Anatomy." In Encyclopedia of Marine Mammals, edited by William F. Perrin, Bernd Würsig and J. G. M. Thewissen. San Diego: Academic Press, 2002.

Twiss, J. R., and R. R. Reeves, eds. Conservation and Management of Marine Mammals. Washington, DC: Smithsonian Institution Press, 1999.

Periodicals

Anderson, P. K. "Habitat, niche, and evolution of sirenian mating systems." Journal of Mammalian Evolution 9, nos. 1–2(2003): 55–98.

Deutsch, C. J., J. P. Reid, R. K. Bonde, D. E. Easton, H. I. Kochman, and T. J. O'Shea. "Seasonal movements, migratory behavior, and site fidelity of West Indian manatees along the Atlantic coast of the United States." Wildlife Monographs 151 (2003):1–77.

Domning, D. P. "Bibliography and index of the Sirenia and Desmostylia." Smithsonian Contributions in Paleobiology 80(1996): 1–611.

O'Shea, T. J. "Manatees." Scientific American 271, no. 1 (1994): 50–56.

Organizations

Save the Manatee Club. 500 North Maitland Avenue, Maitland, FL 32751 United States. Phone: (407);539-0990. Fax: (407) 539-0871. E-mail: [email protected] Web site: <http://www.savethemanatee.org>

Sirenian International, Inc.. 200 Stonewall Drive, Fredericksburt, VA 22401 United States. E-mail: [email protected] Web site: <http://www.sirenian.org>

Daniel K. Odell, PhD

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