Manatees (, family Trichechidae, genus Trichechus) are large, fully aquatic, mostly herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species in the order Sirenia: the Amazonian manatee (Trichechus inunguis), the West Indian manatee (Trichechus manatus), and the West African manatee (Trichechus senegalensis). They measure up to long, weigh as much as , and have paddle-like tails.
Manatees are herbivores and eat over 60 different freshwater and saltwater plants. Manatees inhabit the shallow, marshy coastal areas and rivers of the Caribbean Sea, the Gulf of Mexico, the Amazon basin, and West Africa.
The main causes of death for manatees are human-related issues, such as habitat destruction and human objects. Their slow-moving, curious nature has led to violent collisions with propeller-driven boats and ships. Some manatees have been found with over 50 scars on them from propeller blades. Natural causes of death include adverse temperatures, predation by crocodiles on young, and disease.
Etymology
The etymology of the name is unclear, with connections having been made to Latin "hand" and to the term manaty "breast" from the Carib language of native South Americans. The Carib term may refer to the mammary glands of the manatee, which are located on their chests under their armpits. The term sea cow is a reference to the species' slow, peaceful, herbivorous nature, reminiscent of that of bovines. Lamantin (from French ) was commonly used as an alternative name until the 20th century.
Taxonomy
Manatees are three of the four living species in the order Sirenia. The fourth is the Eastern Hemisphere's dugong. A preexisting member of the order known as Steller's sea cow was hunted to extinction in the eighteenth century. The Sirenia are thought to have evolved from four-legged land mammals more than 60 million years ago, with the closest living relatives being the Proboscidea (elephants) and Hyracoidea (hyraxes).
Description
thumb|left|A skeleton of a manatee and calf, the [[Museum of Osteology, Oklahoma City]]
thumb|left|Skull of a West Indian manatee, the Museum of Osteology, Oklahoma City
The life span of manatees is long, as they can live more than thirty years. Manatees weigh , and average in length, sometimes growing to and and females tend to be larger and heavier than males. At birth, baby manatees weigh about each. The female manatee has two teats, one under each flipper, a characteristic that was used to make early links between the manatee and elephants.
The lids of manatees' small, widely spaced eyes close in a circular manner. The manatee has a large, flexible, prehensile upper lip, used to gather food and eat and for social interaction and communication. Manatees have shorter snouts than their fellow sirenians, the dugongs.
Manatee adults have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and premolars. These teeth are repeatedly replaced throughout life, with new teeth growing at the rear as older teeth fall out from farther forward in the mouth, somewhat as elephants' teeth do. a number that may be due to mutations in the homeotic genes. All other mammals have seven cervical vertebrae, other than the two-toed and three-toed sloths.
Like the horse, the manatee has a simple stomach, but a large cecum, in which it can digest tough plant matter. Generally, the intestines are about 45 meters, unusually long for an animal of the manatee's size.
Evolution
Fossil remains of manatee ancestors – also known as sirenians – date back to the Early Eocene. It is thought that they reached the isolated area of the South American continent and became known as Trichechidae. In the Late Miocene, trichechids were likely restricted in South American coastal rivers and they fed on many freshwater plants. Dugongs inhabited the West Atlantic and Caribbean waters and fed on seagrass meadows instead. As the sea grasses began to grow, manatees adapted to the changing environment by growing supernumerary molars. Sea levels lowered and increased erosion and silt runoff was caused by glaciation. This increased the tooth wear of the bottom-feeding manatees.
Behavior
thumb|Endangered Florida manatee (Trichechus manatus)
Apart from mothers with their young, or males following a receptive female, manatees are generally solitary animals. Manatees spend approximately 50% of the day sleeping submerged, surfacing for air regularly at intervals of less than 20 minutes. The remainder of the time is mostly spent grazing in shallow waters at depths of . The Florida subspecies (T. m. latirostris) has been known to live up to 60 years.
Locomotion
Generally, manatees swim at about . However, they have been known to swim at up to in short bursts.
Intelligence and learning
thumb|right|Manatee postures in captivity
Manatees are capable of understanding discrimination tasks and show signs of complex associative learning. They also have good long-term memory. They demonstrate discrimination and task-learning abilities similar to dolphins and pinnipeds in acoustic and visual studies. Social interactions between manatees are highly complex and intricate, which may indicate higher intelligence than previously thought, although they remain poorly understood by science.
Reproduction
Manatees are semi-social. There is no bond between males and females, and males play no role in care of the young. Their reproductive behavior can be described as "scramble promiscuity" wherein several males compete for mating rights with a single estrous female. While males typically mature around age three to five, they may be unable to secure mating rights until they are physically larger. The typical calving interval is two-and-a-half to three years. Manatees typically breed once every two years. Gestation lasts about 12 months and to wean the calf takes a further 12 to 18 months, Females may be seasonally polyestrous. Their ears are large internally but the external openings are small, and they are located four inches behind each eye. Adults communicate to maintain contact and during sexual and play behaviors. Taste and smell, in addition to sight, sound, and touch, may also be forms of communication.
Diet
Manatees are herbivores and eat over 60 different freshwater (e.g., floating hyacinth, pickerel weed, alligator weed, water lettuce, hydrilla, water celery, musk grass, mangrove leaves) and saltwater plants (e.g., sea grasses, shoal grass, manatee grass, turtle grass, widgeon grass, sea clover, and marine algae). Using their divided upper lip, an adult manatee will commonly eat up to 10–15% of their body weight (about ) per day. Consuming such an amount requires the manatee to graze for up to seven hours a day. To be able to cope with the high levels of cellulose in their plant based diet, manatees utilize hindgut fermentation to help with the digestion process. Manatees have been known to eat small numbers of fish from nets.
Feeding behavior
thumbnail|Manatee plate
Manatees use their flippers to "walk" along the bottom whilst they dig for plants and roots in the substrate. When plants are detected, the flippers are used to scoop the vegetation toward the manatee's lips. The manatee has prehensile lips; the upper lip pad is split into left and right sides which can move independently. The lips use seven muscles to manipulate and tear at plants. Manatees use their lips and front flippers to move the plants into the mouth. The manatee does not have front teeth, however, behind the lips, on the roof of the mouth, there are dense, ridged pads. These horny ridges, and the manatee's lower jaw, tear through ingested plant material.
West Indian manatees prefer warmer temperatures and are known to congregate in shallow waters. They frequently migrate through brackish water estuaries to freshwater springs. They cannot survive below 15 °C (60 °F). Their natural source for warmth during winter is warm, spring-fed rivers.
West Indian
The coast of the state of Georgia is usually the northernmost range of the West Indian manatees because their low metabolic rate does not protect them in cold water. Prolonged exposure to water below 20 °C (68 °F) can cause "cold stress syndrome" and death.
West Indian manatees can move freely between fresh water and salt water. However, studies suggest that they are susceptible to dehydration if freshwater is not available for an extended period of time.
Manatees can travel hundreds of miles annually, and have been seen as far north as Cape Cod, and in 1995 and again in 2006, one was seen in New York City and Rhode Island's Narragansett Bay. A manatee was spotted in the Wolf River harbor near the Mississippi River in downtown Memphis in 2006, and was later found dead downriver in McKellar Lake. Another manatee was found dead on a New Jersey beach in February 2020, considered especially unusual given the time of year. At the time of the manatee's discovery, the water temperature in the area was below 6.5 °C (43.7 °F).
The West Indian manatee migrates into Florida rivers—such as the Crystal, the Homosassa, and the Chassahowitzka rivers, whose headsprings are 22 °C (72 °F) all year. Between November and March each year, about 600 West Indian manatees gather in the rivers in Citrus County, Florida such as the Crystal River National Wildlife Refuge.
In winter, manatees often gather near the warm-water outflows of power plants along the Florida coast, instead of migrating south as they once did. Some conservationists are concerned that these manatees have become too reliant on these artificially warmed areas.
Accurate population estimates of the West Indian manatee in Florida are difficult. They have been called scientifically weak because they vary widely from year to year, with most areas showing decreases, and little strong evidence of increases except in two areas. Manatee counts are highly variable without an accurate way to estimate numbers. In Florida in 1996, a winter survey found 2,639 manatees; in 1997, a January survey found 2,229, and a February survey found 1,706.
As of January 2016, the USFWS estimates the range-wide West Indian manatee population to be at least 13,000; as of January 2018, at least 6,100 are estimated to be in Florida.
Population viability studies conducted in 1997 found that decreasing adult survival and eventual extinction were probable future outcomes for Florida manatees unless they received more protection. The U.S. Fish and Wildlife Service proposed downgrading the manatee's status from endangered to threatened in January 2016 after more than 40 years.
There is a small population of the subspecies Antillean manatee (T. m. manatus) found in Mexico's Caribbean coastal area. The best estimate for this population is 200–250. As of 2022, a new manatee habitat was discovered by Klaus Thymann within the cenotes of Sian Ka'an Biosphere Reserve on the Yucatán Peninsula. The explorer and his team documented the discovery with a 12-minute film that is available on the interactive streaming platform WaterBear. The discovery got picked up by the New Scientist in 2024, who featured in a 10-minute short film.
Amazonian
The freshwater Amazonian manatee (T. inunguis) inhabits the Central Amazon Basin in Brazil, eastern Perú, southeastern Colombia, but not Ecuador. It is the only exclusively freshwater manatee, and is also the smallest. Since they are unable to reduce peripheral heat loss, it is found primarily in tropical waters.
West African
They 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 Cuanza River in Angola. They live as far upriver on the Niger River as Koulikoro in Mali, from the coast.
Predation
In relation to the threat posed by humans, predation does not present a significant threat to manatees.
Ship strikes
Their slow-moving, curious nature, coupled with dense coastal development, has led to many violent collisions with propeller-driven boats and ships, leading frequently to maiming, disfigurement, and even death. As a result, a large proportion of manatees exhibit spiral cutting propeller scars on their backs, usually caused by larger vessels that do not have skegs in front of the propellers like the smaller outboard and inboard-outboard recreational boats have. They are now even identified by humans based on their scar patterns. Many manatees have been cut in two by large vessels like ships and tug boats, even in the highly populated lower St. Johns River's narrow channels. Some are concerned that the current situation is inhumane, with upwards of 50 scars and disfigurements from vessel strikes on a single manatee. Often, the lacerations lead to infections, which can prove fatal. Internal injuries stemming from being trapped between hulls and docks and impacts have also been fatal. Testing and studies from the 2000s and 2010s suggested that manatees may be able to hear speed boats and other watercraft approaching, due to the frequency the boat makes. However, a manatee may not be able to hear the approaching boats when they are performing day-to-day activities or distractions. The manatee has a tested frequency range of 8 to 32 kilohertz.
Manatees hear on a higher frequency than would be expected for such large marine mammals. Many large boats emit very low frequencies, which confuse the manatee and explain their lack of awareness around boats. The Lloyd's mirror effect results in low frequency propeller sounds not being discernible near the surface, where most accidents occur. Research indicates that when a boat has a higher frequency the manatees rapidly swim away from danger.
In 2003, a population model was released by the United States Geological Survey that predicted an extremely grave situation confronting the manatee in both the Southwest and Atlantic regions where the vast majority of manatees are found. It states,
thumb|Manatee bearing scars on its back from a boat propeller
According to marine mammal veterinarians: