Mammals
West Indian Manatee
Trichechus manatus · also called manatee, sea cow, Florida manatee
The West Indian manatee is a large, slow, blimp-shaped aquatic mammal that spends its days drifting through shallow coastal water eating plants. Despite the “sea cow” nickname, its closest living relatives are elephants, and it shares some of their odder features.
Overview
West Indian manatees live in warm, shallow water — coastal lagoons, seagrass flats, estuaries, rivers, canals, and springs — around the tropical and subtropical western Atlantic. The species has two subspecies: the Florida manatee, in the southeastern United States, and the Antillean manatee, from Mexico and the Caribbean to Brazil. They move freely between salt and fresh water, and Florida animals migrate seasonally in search of warmth and food.
An aquatic herbivore built like a barge
Manatees are the only large mammals that live entirely in water and eat only plants. The body is adapted for slow, efficient grazing: paddle-like flippers with vestigial toenails, a horizontal tail fluke for gentle propulsion, body-length lungs used as buoyancy trim, and a divided, prehensile upper lip that gathers vegetation. Like elephants, manatees have “marching molars” — teeth that form at the back of the jaw and migrate forward, falling out at the front, so a manatee never runs out of grinding surface no matter how abrasive its diet.
Life at low speed
A manatee’s metabolism is remarkably slow, and it carries little insulating fat, so cold water is dangerous. When temperatures drop, Florida manatees congregate at natural springs and the warm outflows of power plants, sometimes hundreds together, waiting out the winter. Otherwise they lead unhurried, largely solitary lives, resting and feeding, coming up to breathe every few minutes.
Reproduction
Manatees breed slowly. A female first calves at around five years old and then produces a single calf roughly every two to five years after a year-long pregnancy. The calf nurses for one to two years and learns migration routes and warm-water refuges from its mother.
Threats and conservation
The West Indian manatee is Vulnerable. The Florida population recovered strongly from a few hundred animals in the 1970s to several thousand under legal protection, but it remains at risk from boat strikes (the top cause of death), cold stress, loss of warm-water refuges as power plants retire, harmful algal blooms (red tide), entanglement and entrapment in gates and locks, and — acutely in recent years — starvation caused by seagrass die-offs from water pollution. The Antillean subspecies is scarcer and additionally threatened by hunting in parts of its range.
West Indian Manatee: common questions
What is a manatee related to?
Elephants, most closely — along with the small rock-dwelling hyraxes. Manatees and their Pacific relative the dugong form the order Sirenia, the only fully aquatic herbivorous mammals. They share with elephants a set of unusual traits — replacement molars that move forward through the jaw as older ones wear out and drop off, toenails on the flippers (a relic of hoofed ancestors), and, in males, internal testes. Sirenians returned to the water tens of millions of years ago, entirely independently of whales and seals.
Why do manatees gather at Florida's springs and power plants in winter?
They cannot survive prolonged cold. A manatee has a very low metabolic rate for its size and only a thin layer of fat, so water below about 20 °C causes "cold stress syndrome," which damages the skin and gut and can be fatal. When Florida's coastal waters cool, manatees crowd into places that stay warm year-round — natural springs that flow at a constant ~22 °C, and the heated outflows of power stations. Hundreds may pack into a single spring run. This has created a conservation dilemma, since manatees now rely partly on power-plant discharges that will eventually be shut down.
How much does a manatee eat?
A great deal of low-quality food. Manatees graze aquatic plants for six to eight hours a day and consume roughly 10% of their body weight — 50 to 70 kilograms of vegetation — every day. Their split, muscular upper lip works almost like a pair of hands to gather plants and stuff them into the mouth. All that grazing shapes seagrass beds and, in some rivers, helps keep waterways clear of invasive water hyacinth.
Why are manatees hit by boats so often?
Manatees are slow, they spend much of their time just below the surface in the shallow, murky coastal waters where boats also travel, and they cannot move quickly out of the way. Collisions with boat hulls and propellers are the leading direct human cause of manatee death and injury, and most adult Florida manatees carry scars from past strikes, used by researchers to identify individuals. Speed zones in manatee areas are the main countermeasure.