Manatees, often called "sea cows," are gentle giants of the aquatic world. If you have traveled along the coast of Florida, you may have seen a manatee grazing and wondered about what they are eating and how they find their food and chew it. These marine mammals, belonging to the family Trichechidae and genus Trichechus, are herbivores, playing a vital role in maintaining healthy aquatic ecosystems. This article explores the diet of manatees, their feeding habits, and the unique adaptations that allow them to thrive on a plant-based diet.
Manatee Species and Their Habitats
Manatees are three of the four living species in the order Sirenia. The fourth is the Eastern Hemisphere's dugong. 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).
The West Indian manatee (Trichechus manatus) inhabits the shallow, marshy coastal areas and rivers of the Caribbean Sea and the Gulf of Mexico. A subspecies, the Antillean manatee (T. m. manatus), is found in Mexico's Caribbean coastal area.
The Amazonian manatee (Trichechus inunguis) is the only exclusively freshwater manatee and is also the smallest. It inhabits the Central Amazon Basin in Brazil, eastern Perú, and southeastern Colombia, but not Ecuador.
The West African manatee (Trichechus senegalensis) is 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.
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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). 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. 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. 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.
The Herbivorous Diet of Manatees
Manatees are herbivores and eat over 60 different freshwater and saltwater plants. Manatees are aquatic herbivores (plant-eaters). Also known as "sea cows," these herbivores usually spend up to eight hours a day grazing on seagrasses and other aquatic plants. Manatees feed almost exclusively on plants that grow in fresh and saltwater environments.
Freshwater Plants
Freshwater plants include:
- Hyacinth
- Pickerelweed
- Alligator weed
- Water lettuce
- Hydrilla
- Water celery
- Musk grass
- Mangrove leaves
Saltwater Plants
Saltwater plants include:
- Sea grasses, such as turtle grass, manatee grass, and shoal grass
- Widgeon grass
- Sea clover
- Marine algae
Manatees extract essential nutrients from these plants, enabling them to maintain their slow metabolism and hefty size.
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Feeding Habits and Consumption
Manatees eat an average of 100-200 pounds of sea grasses and weeds each day. Using their divided upper lip, an adult manatee will commonly eat up to 10%-15% of their body weight (about 50 kg) per day. They large herbivores graze on the grasses and weeds for up to seven hours each day, ultimately consuming about 10 - 15% of their body weight. They graze on over 60 species of plants, consuming them at the surface, mid-water column, and even the sea floor. An average manatee can graze for 6-8 hours a day, contributing significantly to ecosystem balance.
Feeding Behaviors and Adaptations
Manatees use their two front flippers to pull or collect plants toward them. The flippers scoop of vegetation and carry it to their mouths. They also use their flippers to coast along the sandy bottom and dig for roots in the sand. 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.
Prehensile Lips
The manatee has a large, flexible, prehensile upper lip, used to gather food and eat and for social interaction and communication. Manatees have prehensile lips, which means the upper lip is split so that the left and right sides can move almost independently. The lips use seven muscles to manipulate and tear at plants. The manatee can move each side of its lip pads independently. This flexibility allows the manatee to "grab" aquatic plants and draw them into its mouth. The manatee has prehensile lips; the upper lip pad is split into left and right sides which can move independently.
Unique Dental Structure
Manatee adults have no incisor or canine teeth, just a set of cheek teeth, which are not clearly differentiated into molars and premolars. The manatee does not have front teeth, however, behind the lips, on the roof of the mouth, there are dense, ridged pads. Manatees have four rows of teeth. There are 6 to 8 high-crowned, open-rooted molars located along each side of the upper and lower jaw giving a total of 24 to 32 flat, rough-textured teeth.
Continuous Tooth Replacement
Eating gritty vegetation abrades the teeth, particularly the enamel crown; however, research indicates that the enamel structure in manatee molars is weak. To compensate for this, manatee teeth are continually replaced. When anterior molars wear down, they are shed. Posterior molars erupt at the back of the row and slowly move forward to replace these like enamel crowns on a conveyor belt, similarly to elephants. This process continues throughout the manatee's lifetime. The rate at which the teeth migrate forward depends on how quickly the anterior teeth abrade.
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Manatee Anatomy and Physiology
Manatees weigh 400 to 550 kg (880 to 1,210 lb), and average 2.8 to 3.0 m (9 ft 2 in to 9 ft 10 in) in length, sometimes growing to 4.6 m (15 ft 1 in) and 1,775 kg (3,913 lb) and females tend to be larger and heavier than males. At birth, baby manatees weigh about 30 kg (66 lb) each. The lids of manatees' small, widely spaced eyes close in a circular manner. Like the horse, the manatee has a simple stomach, but a large cecum, in which it can digest tough plant matter.
Manatees have two fore limb flippers that they use for steering movements and to hold vegetation while eating. A large, round, flattened paddle-shaped tail is used for swimming. Manatees have finely-wrinkled, leathery looking thick skin that continuously flakes off. Algae often grows on the backs and tails of manatees, which makes their skin color appear green or brown. Barnacles (found mostly on coastal dwelling manatees) often leave round scars from attachment sites; movement from saltwater to freshwater habitats clears the animals of these saltwater hitchhikers. The manatee’s flaky skin adaptation also helps keep the algae and barnacles from building up on the animals. These unique aquatic creatures definitely have some highly specialized features.
Manatees use their tails in an up and down motion to propel themselves forward. Strong swimmers, they are capable of reaching speeds of 15 miles per hour in short bursts. Manatees rest from 2 to 12 hours a day either suspended near the water's surface or lying on the bottom, usually for several hours at a time. While most people tend to see many manatees gathered together at winter warm-water sites, during the rest of the year these animals are semi-social as they travel around the state’s waterways in search of food, mates or places to rest.
Manatees have only six cervical (neck) vertebrae. Most other mammals, including giraffes, have seven. The manatee's rib bones are solid, there is no marrow. The manatee has pelvic bones, but they are not attached to its skeletal frame and are fairly small. They are remnants of a time when manatees lived on land. The bones are found in a cartilage tissue area of the body in the vicinity of the reproductive organs and the urinary bladder. Other remnant bones found in the manatee are the hyoid bones located near the neck region. These bones are similar to the Adam's apple in humans. The bones in a manatee's flipper are similar to a human hand. The jointed "finger-like bones" of the flipper help the manatee move through the water, bring food to its mouth, and hold objects. The manatee's lungs lie along its backbone instead of along its rib cage as is found in most mammals. The lungs are long (1 meter or more in adults), wide (20 cm), and thin (5 cm or less). Manatees are quite agile in the water.
Evolutionary History
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.
Social Behavior and Reproduction
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 1-2 m (3 ft 3 in - 6 ft 7 in). Generally, manatees swim at about 5 to 8 km/h (3 to 5 mph). Manatees are capable of understanding discrimination tasks and show signs of complex associative learning.
Manatees typically breed once every two years; generally only a single calf is born. Manatees reach sexual maturity in 3-5 years (females) and 5-7 years (males) and may live over 65 years in captivity. Gestation is approximately 13 months and usually one calf is born. The calf may stay with its mother (cow) for up to 2 years. Male manatees (bulls) are not part of the family unit. Bulls will leave a cow alone after her breeding period is over. Of the wild manatees that reach adulthood, only about half are expected to survive into their early 20s.
Manatees emit a wide range of sounds used in communication, especially between cows and their calves. 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.
Threats to Manatees
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. 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. 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.
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. The severity of mutilations for some of these individuals can be astounding - including long term survivors with completely severed tails, major tail mutilations, and multiple disfiguring dorsal lacerations. These injuries not only cause gruesome wounds, but may also impact population processes by reducing calf production (and survival) in wounded females - observations also speak to the likely pain and suffering endured.
Another cause of manatee deaths are red tides, a term used for the proliferation, or "blooms", of the microscopic marine algae Karenia brevis.
Conservation Efforts
All three species of manatee are listed by the World Conservation Union as vulnerable to extinction. Fish and Wildlife Service (FWS) does not consider the West Indian manatee to be "endangered" anymore, having downgraded its status to "threatened" as of March 2017. They cite improvements to habitat conditions, population growth and reductions of threats as reasoning for the change. Brazil outlawed manatee hunting in 1973 in an effort to preserve the species.
There are many conservation programs that have been created to help manatees. Save the Manatee Club is a non-profit group and membership organization that works to protect manatees and their aquatic ecosystems. To achieve this, they work to: increase public awareness and education; sponsor manatee research, rescue, rehabilitation and release; and advocate for strong protection measures, such as boat speed zones and sanctuaries.
There are a number of manatee rehabilitation centers in the United States. These include three government-run critical care facilities in Florida at Lowry Park Zoo, Miami Seaquarium, and SeaWorld Orlando. After initial treatment at these facilities, the manatees are transferred to rehabilitation facilities before release. The Columbus Zoo was a founding member of the Manatee Rehabilitation Partnership in 2001.
Manatees in Culture and Folklore
The manatee has been linked to folklore on mermaids. In West African folklore, they were considered sacred and thought to have been once human. Killing one was taboo and required penance. In the cosmogony of the Serer people of Senegal, Gambia, and Mauritania, the cayman and the manatee holds great significance in Serer mythology.