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Trichechus manatusWest Indian manatee

By Helen Edwards

Geographic Range

The West Indian manatee or sea cow (Trichechus manatus) is commonly found in shallow coastal areas, but can also be found in shallow rivers, estuaries, and canals. In the United States this species is concentrated around Florida in the winter months and migrates as far north as Virginia and as far west as Louisiana in summer months. The range of this species extends through the Caribbean, along the eastern coast of Central America and the northern coast of South America as far as northeastern Brazil. ("The West Indian Manatee in Florida", FPL. 1989; Rathbun, 1990)

Habitat

West Indian manatees are capable of withstanding large changes in salinity and move freely between freshwater and marine habitats. They can be found in shallow rivers, canals, saltwater bays, estuaries and coastal areas. Because of their extremely low metabolic rate and absence of a thick layer of insulating body fat, they are restricted to tropical and subtropical waters.

This species may inhabit clear or muddy waters. Because of their large size, manatees prefer water reaching at least 1 to 2 meters in depth. These animals are most commonly found travelling in waters 3 to 5 meters deep and waters over 6 meters are generally avoided.

If the water is deep enough and the currents are not too strong (under 5 kilometers per hour), manatees are capable of travelling large distances upstream on inland rivers. In St. John's river, manatees live up to 200 km away from the ocean. Manatees found in the Gulf of Mexico are rarely more than a kilometer from the mouth of a river.

(FPL 1989, Rathbun 1990)

  • Aquatic Biomes
  • rivers and streams
  • coastal

Physical Description

The average body length of an adult West Indian manatee is approximately 3 meters but some individuals can reach a length of 4.5 meters including the tail. The average weight of these manatees ranges between 200 and 600 kg, however the largest individuals can weigh up to 1,500 kg. These upper figures are unusual in manatees and females generally reach greater lengths and weights than the males of the species. Newborns measure between 1.2 and 1.4 meters and weigh approximately 30 kg. The adults are grey or brown whereas newborns are darker, a coloration they lose at about one month.

Manatees are somewhat seal-shaped with forelimbs (flippers) adapted for a completely aquatic life and no hind limbs. Lungs extend the length of the animal's body, which is important in controlling position in the water column. Hair is distributed sparsely over the body and the surface layer of skin is continually sloughing off. This is believed to reduce the build-up of algae on their skin.

(FPL 1989, Rathbun 1990)

  • Sexual Dimorphism
  • female larger
  • Range mass
    200 to 1500 kg
    440.53 to 3303.96 lb
  • Average mass
    400 kg
    881.06 lb
  • Range length
    4.5 (high) m
    14.76 (high) ft
  • Average length
    3 m
    9.84 ft
  • Average basal metabolic rate
    70.0056 W
    AnAge

Reproduction

Although individuals of this species are largely solitary, mating herds form when a female is in estrus. These groups are made up of bulls pursuing the sexually receptive female. Courting bulls establish a dominance hierarchy for mating rights while the female attempts to avoid these males during most of her estrus cycle.

Females may attract up to 20 males, which pursue her for one week to one month.

Males reach full reproductive maturity between the ages of 9 and 10, but they are capable of mating as early as 2. Females are capable of reproduction at 4 to 5 years of age. Young females lack the skills necessary to raise calves and are less successful breeders. Most females breed successfully between the ages of 7 and 9. Gestation periods for West Indian manatees range from 12 to 14 months and calves are dependent on their mothers for about 2 years. One calf is produced at a time, however twins have been occasionally recorded. The inter-birth interval is 3 to 5 years, but this period may be shortened in the event of the early death of a calf. Calves nurse underwater from teats near the forelimbs. Calves are born with both molars and premolars and can begin consuming plants soon after birth, usually within the first 3 weeks.

The mother-young pair is the only stable, long-term association within the species. It has been suggested that the mother and her young can recognize each other after weaning and the association continues, to a certain extent, through the subadult years of the young. This long period of parental care might aid in the transfer of information about migration routes and other learned information.

(FPL 1989, Nowak 1999, Rathbun 1990)

  • Key Reproductive Features
  • gonochoric/gonochoristic/dioecious (sexes separate)
  • sexual
  • Average number of offspring
    1
    AnAge
  • Average gestation period
    335 days
    AnAge
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    1278 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    913 days
    AnAge

Lifespan/Longevity

Behavior

The West Indian sea cow has evolved in areas with no natural predators and as a result the members of this species have had no need to develop complex behaviors for predator avoidance. In addition, the areas inhabited by this species have fairly constant temperatures year-round and an abundant food source. Without the need for group foraging techniques or group defense, this species is largely solitary, occasionally forming loose aggregations.

Manatees are not territorial and do not observe any social hierarchy. Most groups are temporary associations, without regard to sex or age. One exception is herds of juvenile males, which are temporary groups that arise from the exclusion of such individuals from reproductive activities. In addition temporary mating herds develop when a female is in estrus.

Manatees use their tail to propel themselves forward and are surprisingly agile in the water. They are capable of complex maneuvering including somersaults, rolls, and swimming upside-down. They are active day and night, resting for several hours at a time near the surface of the water or at the bottom. While resting on the bottom, they rise to the surface to breath every few minutes.

Manatees use various forms of communication in the water. Individuals rub themselves against hard surfaces, possibly secreting a scent to convey information about the reproductive state of the resident females. Manatees also have an acute ability to hear and squeals are often used to keep contact between a mother and calf. Vision seems to be the preferred method of navigation.

(FPL 1989, Nowak 1999, Rathbun 1990)

Communication and Perception

Food Habits

The snout of West Indian manatees is bent further down than other species in this family. This may be related to the food habits of this species. West Indian manatees feed mainly on sea grasses growing on the sea floor and the orientation of the mouth aids in grasping these plants. One of the unique characteristics of manatees is their flexible split upper lip which is used to pass food to the mouth. Manatees are opportunistic feeders, eating the leaves of most plants that can be manipulated by the upper lip. Manatees may also use their flippers to dig up the roots of these plants. This variable diet is most likely necessary to meet their nutritional demands. Some manatees may also eat invertebrates and will eat fish both in captivity and in the wild.

Because of the low nutritional value of the plants consumed, manatees must graze for 6 to 8 hours a day. Each day they consume 5-10 percent of their body weight, which can be over 100 kg in a large individual. This low-nutient diet also has also contributed to the the development of low metabolic rates. Manatees can survive on 25% percent less energy than a typical mammal of similar size.

Manatees feed on abrasive plants and, as a result, their molars are continually replaced throughout life as they wear down. Hind-gut fermentation is another adaptation to the herbivorous diet of the manatee, aiding in breaking down the cellulose of the plants eaten.

(FPL 1989, Nowak 1999, Rathbun 1990)

Economic Importance for Humans: Positive

For centuries West Indian manatees have been hunted for their meat, hide, and bones. This hunting continues in many South and Central American countries. The domestication of manatees has been suggested. Their status as an endangered species makes this an unlikely option. It has also been suggested that manatees be used as a method of aquatic weed control within their range. However, it has been shown that the amount of plant material consumed by these animals is not large enough to make this a viable option.

As the state's official marine mammal, the Florida manatee (a subspecies of Trichechus manatus) is of particular interest to tourists visiting the area.

(FPL 1989)

Economic Importance for Humans: Negative

N/A

Conservation Status

While manatees have no natural predators, their numbers are threatened by human activities. Because of their low reproductive rate, it is difficult for the species to rebound from a decline in numbers. Although the population in Florida has historically been hunted by Native Americans and, later, by the European inhabitants, it was never the victim of commercial hunting. In other parts of their range West Indian manatees have been exploited commercially and, in some cases, this continues. Although protection laws exist in countries such as Costa Rica and Venezuela, illegal poaching still occurs.

One of the main causes of manatee mortality is collisions with motorboats. Manatees are also killed in canal locks and found entangled in fishing nets. They are also threatened by the loss of (or damage to) sea beds due to agricultural and industrial runoff. These same pollutants have been shown to accumulate in the tissues of manatees and some could be toxic to the animals.

Manatee conservation efforts were initiated as early as the eighteenth century, when the English established Florida as a marine sanctuary for the species. In 1893 a state law was established to protect manatees. At the start of the twentieth century fines were established for the killing of a manatee. Manatees are now protected by the U.S. Marine Mammal Act (1972), the U.S. Endangered Species Act (1973), and the Florida Manatee Sanctuary Act (1978).

(Marsh 1994, Oshea 1998, Reynolds 1995, FPL 1989)

Contributors

Helen Edwards (author), University of Michigan-Ann Arbor, Phil Myers (editor), Museum of Zoology, University of Michigan-Ann Arbor.

Glossary

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

References

FPL. 1989. "The West Indian Manatee in Florida" (On-line). Accessed November 18, 1999 at http://www.fpl.com/html/kid_manateebook.html.

Marsh, H. 1994. Sirenian status and conservation efforts. Aquatic Mammals, 20(3): 155-170.

Nowak, R. 1999. Walker's Mammals of the World. 6th edition. Baltimore: John Hopkins University Press.

Oshea, T., V. Correa, M. Ludlow, J. Robinson. 1988. Distribution status and traditional significance of the West Indian Manatee, Trichechus Manatus, in Venezuela. Biological Conservation, 46(4): 281-302.

Rathbun, G. 1990. Manatees. Pp. 525-528 in B Grzimek, ed. Grzimek's Encyclopedia of Mammals, Vol. 4. NJ: McGraw-Hill Publishing Company.

Reynolds, J. 1995. Status and conservation of manatees, Trichechus manatus manatus, in Costa Rica. Biological Conservation, 71(2): 193-196.

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