Trichechus

Diversity

The genus Trichechus , or more commonly known as manatees, includes three currently living species in order Sirenia. The species and their common names are as follows: West Indian manatee ( Trichechus manatus ), Amazonian manatee ( Trichechus inunguis ), and West African manatee ( Trichechus senegalensis ). A fourth extinct species ( Trichechus hesperamazonicus ) was described in 2020. Manatees are slow-moving, gentle giants with large appetites for aquatic vegetation. Legends of mermaids from centuries ago may have actually been manatees seen through the tired, hallucinating eyes of sailors.

Geographic Range

Manatees can be found throughout coastal and freshwater systems of the Amazon Basin ( Trichechus inunguis ), West Africa ( Trichechus senegalensis ), the Gulf of Mexico, and the Caribbean Sea ( Trichechus manatus ). The state of Florida contains the largest population of manatees ( Trichechus manatus latirostris ). However, housing, agriculture, and commercial developments have greatly altered manatee habitats and depleted resources.

• Biogeographic Regions
• nearctic nearctic
• ethiopian ethiopian
• neotropical neotropical

Habitat

Manatees exist in the eastern and western parts of the Atlantic along the tropical coasts. Manatees are mostly found in waters that are shallow, marshy, and slow moving in rivers, bays, canals, estuaries, and coastal areas. They can tolerate varying levels of salinity, but would rather live in habitats with minimal osmotic stress or with freshwater availability. These mammals prefer warm waters (around 24 degrees Celsius) and will migrate and aggregate to warm water areas to survive if water temperatures drop seasonally below a certain point (around 20 degrees Celsius). Manatees usually inhabit areas that contain an abundance of aquatic vegetation such as seagrass beds. Aquatic depth limits range from 0.4 meters to 20 meters. When it comes time for feeding, resting, mating, and calving, manatees utilize secluded canals, creeks, and lagoons near the mouths of coastal rivers.

• Habitat Regions
• saltwater or marine saltwater or marine
• freshwater freshwater

• Aquatic Biomes
• reef reef
• rivers and streams
• coastal coastal
• brackish water brackish water

• Wetlands
• marsh marsh

• Other Habitat Features
• estuarine estuarine

Systematic and Taxonomic History

Manatees are part of the order Sirenia, which includes their relative, the dugongs ( Dugong dugon ). The Steller’s sea cows are also considered part of the order Sirenia, but sea cows went extinct in 1768. The presence of sexually dimorphic, small pelvic bones in manatees, point to their shared ancestry with terrestrial mammals. Originally, manatees were assigned to the order Bruta by Linnaeus in 1758. Linnaeus organized manatees in the same order as elephants, anteaters, pangolins, and sloths. In 1829, manatees, dugongs, and sea cows were assigned to order Sirenia by Fischer. Siren is a latin word that represented underwater human-like creatures that lured sailors into the water with tempting songs, a creature that existed only in folklore. It is believed that sailors most likely saw manatees rather than sirens. The genus Trichechus was named by Linnaeus. The family, Trichechidae includes three existing species of manatees, T. inunguis , T. senegalensis , and T. manatus . The latin word manatus means “having hands” and the Carib word manati means “breast”. T. manatas and T. senegalensis share a more recent common ancestor than with T. inunguis. Two subspecies of T. manatus were proposed in 1934 by Hatt, the Florida manatee T. manatus latirostris and Antillean manatee< T. manatus manatus >. These two subspecies seem to be separated by the seasonal temperatures of the Gulf coast and the strong currents of the Straits of Florida. Trichechus hesperamazonicus was recognized as a new extinct species of manatee in 2020. The species is estimated to be extant in the Brazilian Amazonia during the late Pleistocene age. T. hesperamazonicus' distinct features include a wide space between the posterior lower tooth row and the anterior border of the ramus covering the posterior edge of the tooth row.

Physical Description

Manatee adults average around 10 feet in length and 800 to 1200 pounds in weight. It is known that some manatees can grow to 13 feet and weigh more than 3,000 pounds. Females are usually larger than males. Manatees range from gray to brown in color and have streamlined bodies that are larger in the middle but taper at the ends. They have a large paddle-shaped tail with a pair of front flippers with three to four nails on each flipper. Their faces are wrinkly and contain whiskers. Hair is located all over the body in a sparse pattern. Their skin is wrinkly and the surface layer is constantly replaced by new skin to reduce the build-up of algae growth. Under the skin, is a layer of fat. Manatees have small eyes that can be protected by a nictitating membrane when moved across the eyeballs. The nostrils are located at the top of the snout with valves that close when underwater. They have small ear openings behind the eyes. Despite having lobeless ears and small eyes, manatees have acute hearing and sight abilities. The prehensile lip allows manatees to draw in their food. They rely on grinding cheek teeth with cusps to eat high fiber plants. Manatees have adapted to replace their teeth when they are worn down. Female manatees have a teat against their bodies under each flipper as well as a urinary reproductive opening just in front of the anus. In male manatees, the opening is below the navel. Young manatees can be as small as 3 to 4 feet in length and weight around 60 to 70 pounds. At birth, manatee calves are usually darker in color than the adults.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

Reproduction

Female manatees have a long estrus period. During breeding events, female manatees are followed by a dozen or more males for a week up to a month. Males can establish an order for mating rights. Multiple males will compete against each other by pushing and shoving. Manatees display promiscuous mating behavior, therefore they do not form monogamous bonds. Breeding and birth can happen anytime of the year, but there is a peak in calves born during the spring.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

It is estimated that female manatees do not become sexually mature until they are between 5 to 9 years old. Males are estimated to begin producing sperm at around 2 years old. Copulation is short and involves the male below the female in an abdomen-to-abdomen position. Variations in reproductive behaviors between manatee species have not been identified.

• Key Reproductive Features
• iteroparous iteroparous
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• viviparous viviparous

Calves are usually born one at a time, but there have been recorded instances of twin calves. The range for time between births range from 2-5 years, with the average gestation period being around 13 months. Due to the slow birth rate and long parental investment, manatees have a high adult survival rate. The female manatee has complete responsibility of their calf without the help of the male manatee. A calf may depend on its mother for over two years. Calves nurse from a nipple that is located under their mother’s flipper for three minutes at a time underwater. Manatee milk does not contain lactose, but is rich in fats, proteins, and sodium. After a few weeks of nursing, calves begin to include plants in their diet. Mother manatees and young manatees can recognize each other even after weaning. Some younger manatees may spend their young adulthood living within the range of their mothers. This behavior can be beneficial as young manatees can learn migration routes to winter refuges in this way.

• Parental Investment
• female parental care female parental care
• post-independence association with parents

Lifespan/Longevity

Manatees in the wild are able to live for over 60 years. Their long lifespans could be due to the minimal predation risk. A captive manatee has been able to live for over 45 years. Limits to lifespan can be attributed to common natural causes such as cold stress, pneumonia, or gastrointestinal diseases. Human-related activities have become a threat to manatee longevity. Human-related manatee mortality incidents are largely due to watercraft collisions.

Behavior

Commonly characterized as gentle and slow-moving, manatees spend much of their time eating, resting, and traveling. On average, 6 to 8 hours a day are spent feeding and 2 to 12 hours are spent resting. They rest underwater at the bottom or right below the surface. While resting, they surface to breathe about every 3 to 5 minutes. Manatees are able to hold their breath for up to 20 minutes before needing to surface and breathe. When traveling, they can swim up to 20 miles per hour for short periods of time. However, they usually swim around 3 to 5 miles per hour. Manatees have not been observed to be territorial and they do not rely on a herd for survival. Their lack of requirement for a social structure results in their semi-solitary behavior. Small groups of manatees occur, but these groups are informal and have no dominating leader. Manatees engage in play by body-surfing and barrel rolling with other manatees.

• Key Behaviors
• natatorial natatorial
• motile motile
• nomadic nomadic
• migratory migratory
• solitary solitary

Communication and Perception

Manatees have the ability to communicate with a wide range of sounds. Communication is mostly between mothers and their calves. Mothers have been observed to respond to their calves from almost 200 feet away. Communicating sounds are described as chirps, whistles, and squeaks. Fear, stress, or excitement can be expressed in squeals. Adult manatees communicate during copulation and when playing. It is possible that pitch, volume, and duration of calls convey information. Touch, taste, and smell, may be other forms of communication within manatees. Manatees can also perceive their environment visually. In clear water, visual cues can be noticed from up to 115 feet away. It is understood that manatees can see in color due to the presence of two types of cone cells in their retina.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Young calves drink their mother’s milk, but adult manatees spend much time grazing. Manatees are herbivorous and feed on water grasses, weeds, and algae. In 24 hours, a manatee can eat a tenth of its weight. On average, this is about 100 to 200 pounds of vegetation a day. Manatees are known to eat over 60 species of plants. Some common marine vegetations include manatee grass ( Syringodium filiforme ), turtle grass ( Thalassia testudina ), widgeon grass ( Ruppia maritima ), and shoal grass ( Halodule beaudettei) . Common freshwater vegetation include Florida elodea ( Hydrilla verticillata ), Southern naid ( Najas guadalupensis ), Eurasian watermilfoil ( Myriophyllum spicatum ), tapegrass ( Vallisneria neotropicalis ), coontail ( Ceratophyllum demersum ), water hyacinth ( Eichornia crassipes ), and water lettuce ( Pistia stratiotes ). For a mammal of its size, manatees have very low metabolic rates. This low metabolic rate may explain their sensitivity to cold and slow healing rate. This adaptation most likely allows manatees to stay cool in their warm environments and to live off of their nutrient-poor diets. Unlike their relatives the dugongs, manatees have developed root hypsodonty which is characterized by tall-crowned teeth and enamel that extends below the gum line. This type of dentition is present in mammals that feed on abrasive material. Modern manatees have more teeth that are more resistant to wear than their ancestors. As teeth are worn, new molars move forward from the back of the jaw and push out the older teeth. The rate that this happens is dependent on how abrasive the diet is. The Amazonian manatee has specialized its diet and has thus developed even smaller and more wear-resistant teeth for eating aquatic grasses when compared to the other species. Prehensile lips are adapted to tear and grab plants into a manatee’s mouth. They also utilize their front flippers to guide plants towards their lips or to dig up an entire plant.

• Primary Diet
• herbivore herbivore

Predation

Manatees have no natural predators. However, human activity has drastically reduced the range of their natural habitats by coastal areas.

Ecosystem Roles

The diets of manatees directly affect their ecosystems, which makes them integral parts of their environments. Due to the large quantities of underwater vegetation that they eat, manatees prevent the vegetation from becoming overgrown. They also maintain the diversity of plants by consuming invasive plant species. The excrement of manatees help fertilize sea grasses and many other aquatic plants.

Economic Importance for Humans: Positive

Manatees have been hunted for food and resources since as early as 8500 BCE. Manatee meat was a source of food, while the fat, bones, and hides could be utilized as valuable supplies. Though manatees are not hunted as they were in the past, they continue to benefit modern economies. Ecotourism is a valuable source of income in many coastal areas. Manatees bring millions of dollars to local ecotourism industries by attracting tourists that wish to see the animals in person. Manatees in Florida have been considered for use in invasive aquatic vegetation management, which may be a cost-effective method when compared to chemical usage or nonindigenous species introduction.

• Positive Impacts
• ecotourism ecotourism

Economic Importance for Humans: Negative

There are no known adverse effects of manatees on humans.

Conservation Status

Some natural causes of manatee deaths are exposure to cold weather and red tides. Water temperatures below 68 degrees Fahrenheit leads to increasing metabolic responses. Long-term exposure to cold can lead to malnourishment and hypothermia. Manatees are susceptible to toxin exposure during red tide events. They can accidentally ingest small marine animals that contain toxins from the red tide and inhale brevetoxin from the air. Since manatees have no natural predators, human activities have been largely responsible for the decreasing of manatee populations around the world. Common detrimental human activities to manatee populations have been identified as watercrafts, flood-control structures, fishing gear, poaching, loss of habitat, and pollution. Poaching of manatees is rare, but pollution of estuaries, rivers, and oceans have led to debris ingestion and entanglement. Plastic is the most commonly ingested debris, which can obstruct the digestive tract and kill the animal. Even small amounts can be fatal. Industrial chemicals and pesticides contaminate the water and vegetation that make up the livelihoods of manatees. Entanglements in fishing lines of crab trap lines are also issues. flood-control structures like navigation locks or floodgates can crush manatees that approach too close. As manatees live near coasts, they are prone to interactions with humans. They are vulnerable to waste by the shores and food and water waste left by boats. Watercraft collisions are the most common causes of death among human activities. Blunt trauma from impact is enough to kill manatees. Motors and propellers may not always be deadly to manatees, but these wounds from collisions will possibly result in reproductive or survival difficulties. Evidence of collisions are carried by many manatees as scars or deformities. The use of propeller guards may assist in reducing propeller injuries and deaths, however the slow nature of manatees and their need to breathe at the surface makes it difficult for manatees to avoid fast boats in shallow waters. Regulations to restrict boat speed and limit access to manatee-dense areas have been implemented in some areas in Florida. All three manatee species are listed as vulnerable for extinction on the IUCN Red List. As of 2021, population estimates for each species are 13,000 T. manatus individuals and less than 10,000 T. senegalensis individuals. T. inunguis population counts range from 8,000 to 30,000 individuals. More research is needed for more accurate and updated manatee population estimates. Steps are being taken to preserve these calm mammals. Rescue and rehabilitation programs focus on assisting sick or injured manatees and reintroduction programs around the world have released manatees back to their native habitats. Manatees are protected by the 1973 Endangered Species Act, which makes it illegal for anyone to capture, hunt, harass, or kill them. Many national laws in addition to the Endangered Species Act offer protection to manatees and their habitats. Agencies like the U.S. Fish and Wildlife Service and Amazon Rescue Center are rescuing and rehabilitating injured manatees. With more research about manatees being published, global efforts to protect them can be optimized.

Additional Links

Encyclopedia of Life

Contributors

An-Ping Yu (author), Colorado State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Allen, A., E. Keith. 2015. "Using the West Indian Manatee (Trichechus manatus) as a Mechanism for Invasive Aquatic Plant Management in Florida" (On-line). NSUWorks. Accessed December 08, 2021 at https://nsuworks.nova.edu/cgi/viewcontent.cgi?article=1494&context=occ_facarticles .

Attademo, F., D. Balensiefer, A. Freire, G. Pereira de Sousa, F. Carneiro da Cunha, F. Luna. 2015. Debris ingestion by the Antillean Manatee (Trichechus manatus manatus). Marine Pollution Bulletin , (101)1: 284-287. Accessed September 24, 2021 at https://www.semanticscholar.org/paper/Debris-ingestion-by-the-Antillean-Manatee-manatus-Attademo-Balensiefer/d21988dc8083af2f55f671cc14fbeff14f4e7b67 .

Deutsch, C., C. Self-Sullivan, A. Mignucci-Giannoni. 2008. "Trichechus manatus" (On-line). The IUCN Red List of Threatened Species. Accessed September 24, 2021 at https://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T22103A9356917.en .

Diagne, L. 2015. "Trichechus senegalensis" (On-line). The IUCN Red List of Threatened Species. Accessed September 24, 2021 at https://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T22104A81904980.en .

Domning, D., L. Hayek. 1986. Interspecific and Intraspecific Morphological Variation in Manatees (Sirenia: Trichechus). Marine Mammal Sciences , (2)2: 87-144. Accessed September 24, 2021 at https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1748-7692.1986.tb00034.x .

Fleming, E. 2019. "Coexisting with Florida Manatees" (On-line). Accessed October 22, 2021 at https://defenders.org/blog/2019/11/coexisting-florida-manatees .

Gardner, A., D. Wilson. 2006. "Integrated Taxonomic Information System" (On-line). Accessed September 24, 2021 at https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=180684#null .

Journey North, 2003. "Explore the Eating Habits of Manatees" (On-line). Accessed September 24, 2021 at https://web.archive.org/web/20140429180435/http://www.learner.org/jnorth/tm/manatee/AdaptationsMouth.html .

Larkin, I. 2000. "Reproductive Endocrinology of the Florida Manatee (Trichechus Manatus Latirostris): Estrous Cycles, Seasonal Patterns and Behaviors" (On-line pdf). Accessed September 24, 2021 at http://ufdcimages.uflib.ufl.edu/UF/00/10/06/93/00001/Dissertation.pdf .

Marmontel, M., D. de Souza, S. Kendall. 2016. "Trichechus inunguis" (On-line). The IUCN Red List of Threatened Species. Accessed September 24, 2021 at https://dx.doi.org/10.2305/IUCN.UK.2016-2.RLTS.T22102A43793736.en .

National Geographic, 2021. "Manatees" (On-line). Accessed October 21, 2021 at https://www.nationalgeographic.com/animals/mammals/facts/manatees .

Perini, F., E. Nascimento, M. Cozzuol. 2020. "A new species of Trichechus Linnaeus, 1758 (Sirenia, Trichechidae), from the upper Pleistocene of southwestern Amazonia, and the evolution of Amazonian manatees" (On-line). Taylor & Francis Online. Accessed September 15, 2021 at https://www.tandfonline.com/doi/full/10.1080/02724634.2019.1697882 .

Save the Manatee Club, 1994. "Manatees: An educator’s Guide. Fourth Edition" (On-line pdf). Accessed September 24, 2021 at https://files.eric.ed.gov/fulltext/ED388523.pdf .

Smith, K. 1993. "Manatee Habitat and Human-related Threats to Seagrass in Florida: A Review" (On-line pdf). Accessed September 15, 2021 at https://myfwc.com/media/7270/manatee_habitat_human.pdf .

The Paleobiology Database, 2021. "Trichechus (manatee)" (On-line). Accessed September 24, 2021 at https://paleobiodb.org/classic/checkTaxonInfo?taxon_no=36903&is_real_user=1 .

Van Meter, V. 1989. "The Florida Manatee" (On-line pdf). Accessed September 24, 2021 at http://67.59.130.204/FPLBooklet.pdf .

Winger, J. 2000. "What’s in a Name? Manatees and Dugongs" (On-line). Smithosonian National Zoological Park. Accessed September 24, 2021 at http://nationalzoo.si.edu/Animals/Whats_in_a_name/default.cfm?id=37 .

World Animal Foundation, 2021. "Manatees" (On-line). Accessed September 24, 2021 at https://www.worldanimalfoundation.org/animal_encyclopedia/params/category/173341/item/994814/ .