Why Are Manatees Called Sea Cows? Grazers of the Sea

Grazers of underwater pastures

The West Indian manatee eats more than 60 kinds of plants, but its staple food is seagrass. Seagrasses are true flowering plants that grow in shallow, sunlit seawater and form dense meadows on sandy and muddy bottoms, so a seagrass bed really is a kind of pasture under the sea. One common Caribbean species is even called manatee grass.

Like cattle, manatees spend a large part of the day eating. They graze for about five to eight hours and take in roughly 4–10% of their body weight in wet plants daily, which means about 18–45 kg (40–100 lb) for a 450 kg (1,000 lb) adult. In rivers and springs they crop floating and rooted freshwater plants such as water hyacinth, hydrilla and eelgrass, and they will reach up to pull grasses and mangrove leaves from the water’s edge.

Their pace adds to the cow-like image. Manatees usually cruise at only 5–8 km/h (3–5 mph), drift calmly between meals and rest for hours on the bottom. They are docile, and in winter they gather in loose “herds” at warm springs.

Cows, bulls and calves

The farmyard vocabulary also helps. A female manatee is called a cow, a male a bull and a young one a calf. These words are used for many large mammals, including elephants, whales and seals, so they do not make manatees cattle, but they strengthen the nickname.

Many languages use the same idea. In German a manatee is a Seekuh and in Dutch a zeekoe, both meaning “sea cow”, and in Spanish it is often called vaca marina. In Brazilian Portuguese the manatee is a peixe-boi, literally an “ox-fish”. The dugong, the manatee’s Indo-Pacific cousin, is also widely called a sea cow, and scientists often use “sea cows” as an informal name for the whole order Sirenia.

Do manatees digest food like cows?

No. Cows are ruminants. They have a stomach with four chambers, swallow grass, bring it back up as cud to chew again, and let microbes ferment it at the front of the gut.

Manatees are hindgut fermenters, like horses and elephants. They have a simple stomach and an extremely long intestine, and microbes in the large intestine break down the tough cellulose in plant cell walls. Food moves very slowly, taking about a week to pass through, which helps manatees squeeze as much energy as possible out of a low-quality diet. Combined with an unusually low metabolic rate, this explains why manatees can live on seagrass and why they move so slowly.

Marching molars: teeth built for gritty grass

Seagrass and river plants are coated with sand and silt that wear teeth down fast. Adult manatees have no front teeth. Instead, rough, horny pads at the front of the jaws grip and tear plants, a little like the hard dental pad that cattle have in place of upper front teeth.

At the back, a manatee has six to eight cheek teeth in each half of each jaw. New molars form at the rear and slowly move forward, about 1–2 mm a month, while worn teeth drop out at the front. This conveyor belt, often called “marching molars”, keeps going throughout life. Elephants also replace their molars from back to front, but they have only six sets, which is one of many clues that the two animals are related.

The dugong has a different system. Its molars are simple pegs that are not replaced in the same way, and males grow a pair of short tusks.

Are manatees related to cows?

Manatees and cows are both mammals, but they sit on very different branches of the family tree. Sirenians belong to a group called Afrotheria, which arose in Africa. Within that group, their closest living relatives are elephants and hyraxes, small furry animals that look a bit like large guinea pigs. Aardvarks, tenrecs and elephant shrews are more distant cousins.

Cows are even-toed hoofed mammals. Their closest relatives include pigs, deer, hippos and, surprisingly, whales and dolphins, as explained in Are hippos related to whales? In other words, a cow is more closely related to a dolphin than to a manatee.

The ancestors of manatees walked on land. One of the earliest known sirenians, Pezosiren portelli, lived in what is now Jamaica about 50 million years ago. It had four sturdy legs and probably lived much like a small hippo, wading and feeding in shallow water. Shared features such as nails on the flippers and molar replacement still link today’s manatees with their elephant cousins, like the African bush elephant.

Steller’s sea cow: the giant that carried the name

The one animal officially named a sea cow was Steller’s sea cow (Hydrodamalis gigas), a huge relative of the dugong. It was described by the German naturalist Georg Wilhelm Steller in 1741, after the ship of explorer Vitus Bering was wrecked on Bering Island in the North Pacific.

Steller’s sea cow could reach about 8–9 m (26–30 ft) and probably weighed 8–10 tonnes, far more than any living sirenian. It lived in cold water, fed on kelp and other seaweeds, and had no teeth at all, only tough horny plates for crushing food. By 1741 only a small population survived around the Commander Islands. Fur hunters and sailors killed the slow, tame animals for meat, fat and hides, and the species was extinct by about 1768, just 27 years after its discovery.

How many kinds of sea cows are there?

Four sea cows survive today, all of them plant-eaters that live in warm water:

All four are listed as Vulnerable on the IUCN Red List. The easiest way to tell a dugong from a manatee is the tail, which is fluked like a dolphin’s in the dugong and rounded like a paddle in manatees. More differences are explained in manatee vs dugong.