Manatees: what is a sea cow?

Discover more about slow-moving, seagrass-munching manatees.

What is a manatee?

Manatees are aquatic mammals that belong to a group of animals called Sirenia.

This group also contains dugongs. Dugongs and manatees look quite alike - they're similar in size, colour and shape, and both have flexible flippers for forelimbs. One way to tell them apart is by the shapes of their tails: manatees have a broad, rounded tail, whereas dugongs have fluked tails, a bit like whales' tails.

Manatees are also related to a huge, subarctic sirenian called the Steller's sea cow (Hydrodamalis gigas), which was hunted to extinction in the 1760s.

Manatees are thought to live to the age of about 50 or 60 in the wild, but it is possible they could live longer. In captivity, a male manatee called Snooty lived to the age of 69, but didn't die of natural causes.

Baby manatees, known as calves, are born underwater after a gestation of 12-14 months. When they are born, the calf is guided to the surface by its mother so it can take its first breath. Manatee calves stay close to their mother for up to two years.

Unlike most mammals, manatees only have six bones in their neck - most others, including humans and giraffes, have seven. This short neck allows a manatee to move its head up and down, but not side to side. To look left and right, a manatee must turn its entire body, steering with its flippers. 

Are manatees related to elephants?

Manatees look a bit like walruses or chunky porpoises and are sometimes referred to as sea cows, but they're actually much more closely related to elephants.

Sirenians, proboscideans (which includes elephants, mastodons and woolly mammoths) and embrithopods (an extinct group of animals that looked a bit like rhinos, though they aren't close relatives) are all thought to have descended from a common ancestor. Together these groups belong to another group called Tethytheria.

Relatively little is known about sirenian evolution, but this group likely descended from a four-legged wading mammal. Early forms appeared about 55 million years ago and one of the first true manatees, Potamosiren, lived 13-16 million years ago during the Miocene epoch.

Manatees have pectoral flippers but no hind limbs, only a tail for propulsion. They do have vestigial pelvic bones, however, a leftover from their evolution from a four-legged to a fully aquatic animal. 

Manatees sport some visual similarities to elephants. Both mammals have thick, wrinkled skin and sparse, bristle-like hairs covering their bodies. Manatees' hairs are known as vibrissae and help sense vibrations in the water around them.

Hyraxes may look like rodents such as marmots, but they are actually also close relatives of manatees and elephants. Like manatees, hyraxes have vibrissae, interspersed though their furry coats.

Where do manatees live?

There are three species of manatees: the West Indian manatee (Trichechus manatus), the African manatee (Trichechus senegalensis) and the Amazonian manatee (Trichechus inunguis).

The West Indian manatee reaches about 3.5 metres long and weighs on average around 500 kilograms. It moves between freshwater and saltwater, taking advantage of coastal mangroves and coral reefs, rivers, lakes and inland lagoons.

There are two subspecies of West Indian manatee. The Antillean or Caribbean manatee (Trichechus manatus manatus) is found in waters from the Bahamas to Brazil, whereas the Florida manatee (Trichechus manatus latirostris) is found in US waters, although some individuals have been recorded in the Bahamas.

In winter, the Florida manatee is typically restricted to Florida. When the ambient water temperature drops below 20°C, it takes refuge in naturally and artificially warmed water, such as at the warm-water outfalls from powerplants.

In warmer months, Florida manatees often migrate to neighbouring states such as Georgia, although some travel much further. In 1995, a manatee swam from south-eastern Florida to Rhode Island and back. The manatee, nicknamed Chessie, made a record-breaking round trip of over 4,000 kilometres.

The African manatee is also about 3.5 metres long and found along the west coast of Africa, from Mauritania down to Angola. African manatees also utilise rivers, with the mammals seen in landlocked countries such as Mali and Niger.

The Amazonian manatee is the smallest species, though it is still a big animal. It grows to about 2.5 metres long and 350 kilogrammes. Amazonian manatees favour calm, shallow waters that are above 23°C. This species is only found in freshwater in the Amazon Basin, in Brazil, Colombia, Ecuador and Peru.

Location is another way to tell manatees and dugongs apart, as their ranges don't overlap. Dugongs are only found in the Indo-Pacific, near the coasts of India, East Africa, Malaysia and Australia.

What do manatees eat?

Seagrasses and other aquatic plants make up most of a manatee's diet. Manatees spend about eight hours each day grazing and uprooting plants. They eat up to 15% of their weight in food each day.

African manatees are omnivorous - studies have shown that molluscs and fish make up a small part of their diets. West Indian and Amazonian manatees are both herbivores.

How many teeth do manatees have?

Manatees only have molars, with about 24-32 in their mouth at a time. Due to their abrasive aquatic plant diet, the teeth get worn down and they eventually fall out, so manatees continually grow new teeth that get pushed forward to replace the ones they lose.

Instead of having incisors to grasp their food, manatees use their prehensile lips like a pair of hands to help pull food away from the seafloor and into their mouths. 

How long can manatees stay underwater?

Manatees are fully aquatic, but as mammals, they need to come up to the surface to breathe. When awake, they typically surface every two to four minutes, but they can hold their breath for much longer. The longest recorded dive was 24 minutes by a West Indian manatee in Florida.

Adult manatees sleep underwater for 10-12 hours a day, but they come up for air every 15-20 minutes. Active manatees need to breathe more frequently.

It's thought that manatees use their muscular diaphragm and breathing to adjust their buoyancy. They may use diaphragm contractions to compress and store gas in folds in their large intestine to help them float.

To swim deeper, manatees release the stored gas, using flatulence to their advantage. It has been observed that constipated manatees aren't able to dive and instead get stuck floating at the surface with their distended tail ends higher in the water.

Manatees spend a lot of time at the surface, so sometimes their bodies can become coated in algae, which tends to grow in wet and sunny spots. Some manatees in coastal environments can also sport clusters of barnacles.

How fast can a manatee swim?

Manatees have a much slower metabolism than other mammals of their size. They are typically slow swimmers but can reach a top speed of about 25 kilometres per hour, though only do so in short bursts.

Are manatees endangered?

All manatees are endangered or at a heightened risk of extinction.

The African manatee and Amazonian manatee are both listed as Vulnerable by the International Union for Conservation of Nature (IUCN). 

It is estimated that 140,000 Amazonian manatees were killed between 1935 and 1954 for their meat, fat and skin, with the latter used to make leather. African manatee decline has been tied to incidental capture in fishing nets and hunting. Manatee hunting is now illegal in every country the African species is found in.

The West Indian manatee is also considered Vulnerable as it's estimated there are fewer than 10,000 individuals. But independently, the two subspecies - the Florida and Antillean manatees - are listed by IUCN as Endangered as there are fewer than 2,500 individuals of each. Both are also expected to undergo a decline of 20% over the next 40 years.  

The biggest known cause of death in Florida manatees is boat strikes. The propellors of boats are also a big threat, able to deeply gouge a manatee's thick skin. There are now laws in some parts of Florida to limit boat speeds during winter, which allows slow-moving manatees more time to respond to an imminent threat. 

As with many other marine animals, plastic poses a risk to manatees. A review of almost 1,800 cases of entanglement and plastic consumption among marine mammals in US waters from 2009-2020 found that at least 700 cases involved manatees. Nearly all had swallowed plastic.

According to the Florida Department of State, humans are estimated to cause about half of the known deaths of West Indian manatees.

Dugongs are facing similar threats. Find out how research is unveiling a clearer picture of how these manatee relatives are faring.