Steller's sea cow (Hydrodamalis gigas) was an extinct sea animal first discovered by Georg Wilhelm Steller in 1741. At that time, it lived only near the Commander Islands in the Bering Sea, between Alaska and Russia. Earlier, during the Pleistocene epoch, its range covered much of the North Pacific, but it likely became restricted to a smaller area due to changes in ice ages. Indigenous people may have seen the animal before Europeans arrived. Steller first saw it during Vitus Bering's Great Northern Expedition when the crew was stranded on Bering Island. Most knowledge about its behavior comes from Steller's observations, recorded in his book On the Beasts of the Sea, published after his death. Within 27 years of Europeans discovering it, the slow-moving animal was hunted to extinction for its meat, fat, and skin.
Adults in the 18th century weighed 8–10 tons (8.8–11.0 short tons) and grew up to 9 meters (30 feet) long. It belonged to the Dugongidae family, which today includes only the dugong (Dugong dugon), a 3-meter (9.8-foot) long animal. Steller's sea cow had a thicker fat layer than other sea mammals, helping it survive cold waters. Its tail was split, like those of whales and dugongs. It lacked true teeth but had white bristles on its upper lip and two hard plates in its mouth for chewing. It mainly ate kelp and made sounds like sighs and snorts to communicate. Steller believed it lived in small family groups, mated with one partner, and raised its young, similar to modern sea mammals.
Description
Steller's sea cows grew to be 8 to 9 meters (26 to 30 feet) long as adults, which is much larger than living sirenians. In 1987, a nearly complete skeleton was found on Bering Island that measured 3 meters (9.8 feet). In 2017, another skeleton was discovered on the same island, measuring 5.2 meters (17 feet), and likely reached about 6 meters (20 feet) in life. Georg Steller wrote two different estimates for the sea cow's weight: 4 and 24.3 metric tons (4.4 and 26.8 short tons). Scientists believe the true weight was probably between 8 and 10 metric tons (8.8 to 11.0 short tons). This large size made the sea cow one of the biggest mammals of the Holocene epoch, along with baleen whales and some toothed whales. This size may have helped the sea cow keep its body warm by reducing the amount of surface area compared to its body volume.
Unlike other sirenians, Steller's sea cow was positively buoyant, meaning it could not fully submerge underwater. It had thick skin, 2.5 centimeters (1 inch) thick, to protect itself from sharp rocks and ice and possibly to keep its skin from drying out when not submerged. Its blubber was 8 to 10 centimeters (3 to 4 inches) thick, an adaptation to the cold Bering Sea. Its skin was brownish-black, with white patches on some individuals. The skin was smooth on its back and rough on its sides, with crater-like depressions likely caused by parasites. This rough texture led to the nickname "bark animal." Hair on its body was sparse, but the insides of its flippers had bristles. Its forelimbs were about 67 centimeters (26 inches) long, and its tail fluke was forked.
The sea cow's head was small compared to its large body. Its upper lip was large and broad, extending far beyond the lower jaw, making the mouth appear to be located under the skull. Unlike other sirenians, Steller's sea cow had no teeth. Instead, it had a dense arrangement of white bristles on its upper lip. These bristles were about 3.8 centimeters (1.5 inches) long and were used to tear seaweed and hold food. The sea cow also had two keratinous plates, called ceratodontes, on its palate and jaw, used for chewing. According to Steller, these plates were held together by interdental papillae, part of the gums, and had small holes containing nerves and arteries.
Like all sirenians, the sea cow's snout pointed downward, helping it grasp kelp. Its nostrils were about 5 centimeters (2 inches) long and wide. In addition to bristles inside its mouth, the sea cow had stiff bristles 10 to 12.7 centimeters (3.9 to 5.0 inches) long on its muzzle. Its eyes were small, located halfway between its nostrils and ears, with black irises, livid eyeballs, and canthi not visible externally. It had no eyelashes but a nictitating membrane, which covered its eyes to prevent injury while feeding. Its tongue was small and stayed in the back of the mouth, unable to reach the chewing pads.
The sea cow's spine likely had seven cervical (neck), 17 thoracic, three lumbar, and 34 caudal (tail) vertebrae. Its ribs were large, with five of 17 pairs connected to the sternum; it had no clavicles. Like all sirenians, its scapula was fan-shaped, larger on the back side and narrower near the neck. The anterior border of the scapula was nearly straight, unlike the curved scapulae of modern sirenians. The bones of Steller's sea cow were pachyosteosclerotic, meaning they were both bulky and dense. In all collected skeletons, the manus (hand) was missing. Since Dusisiren, the sister taxon of Hydrodamalis, had reduced phalanges, Steller's sea cow may not have had a manus at all. The only specimen with preserved bones from the wrist down is a specimen studied in Komae, Tokyo, Japan, which shows a fragment of a metacarpal bone.
The sea cow's heart weighed 16 kilograms (35 pounds). Its stomach was 1.8 meters (6 feet) long and 1.5 meters (5 feet) wide. Its intestinal tract was about 151 meters (500 feet) long, more than 20 times the animal's length. It had no gallbladder but a wide common bile duct. Its anus was 10 centimeters (0.33 feet) wide, with feces similar to those of horses. The male's penis was 80 centimeters (2.6 feet) long. Genetic evidence shows that Steller's sea cow evolved similar traits in genes related to metabolism and immunity as other marine mammals, including leptin, which is involved in energy balance and reproduction.
Ecology and behavior
Steller's sea cow may have had natural predators, but it is not known for sure. It might have been hunted by killer whales and sharks. However, its buoyancy could have made it hard for killer whales to drown it, and the rocky kelp forests where the sea cow lived might have kept sharks away. Steller noted that adult sea cows protected their young from predators.
Steller described a parasite on the sea cows that looked like a whale louse (Cyamus ovalis). However, the parasite has not been identified because the sea cow is extinct, and the original samples collected by Steller are no longer available. This parasite was first named Sirenocyamus rhytinae in 1846 by Johann Friedrich von Brandt. Later, it was reclassified as Cyamus rhytinae. It was the only known species of cyamid amphipod found on a sirenian. Steller also observed an endoparasite in the sea cows, which was likely an ascarid nematode.
Like other sirenians, Steller's sea cow was an obligate herbivore, meaning it ate only plants. It spent most of the day feeding, lifting its head every 4–5 minutes to breathe. Kelp was its main food source, making it an algivore. Scientists have identified several kelp species the sea cow likely ate, including Agarum spp., Alaria praelonga, Halosaccion glandiforme, Laminaria saccharina, Nereocyctis luetkeana, and Thalassiophyllum clathrus. The sea cow only ate the soft parts of the kelp, leaving the tougher stems and holdfasts to wash up on shore. It may have also eaten seagrass, but seagrass was not common enough to support a large population. The seagrass in its habitat, such as Phyllospadix spp. and Zostera marina, may have grown too deep or been too tough for the sea cow to eat. Since the sea cow floated, it likely fed on canopy kelp, which grows near the surface. Kelp releases a chemical to protect itself from grazing, but canopy kelp releases less of this chemical, allowing the sea cow to eat it safely. Steller noted that the sea cow grew thin during cold winters, suggesting it fasted when kelp growth was low. Fossils of Pleistocene sea cows from the Aleutian Islands were larger than those from the Commander Islands, indicating that the smaller size of Commander Island sea cows may have been due to less food and a less favorable habitat.
Steller described the sea cow as highly social, living in small family groups. It helped injured members and was likely monogamous. The young were kept near the front of the herd for protection. Steller observed that when a female was captured, other sea cows attacked the hunting boat by ramming it. After the hunt, her mate followed the boat to shore even after the captured animal had died. Mating likely occurred in early spring, and the gestation period was a little over a year, with calves born in autumn. Steller noticed more calves in autumn than in other seasons. Female sea cows had only one set of mammary glands, so they likely gave birth to one calf at a time.
The sea cow used its forelimbs for swimming, feeding, walking in shallow water, defending itself, and holding on to its partner during mating. Steller noted that the forelimbs also helped the sea cow stay anchored to avoid being swept away by strong waves. While feeding, the sea cow moved slowly by swaying its tail from side to side. For faster movement, it beat its tail vertically. The sea cow often slept on its back after eating. Steller reported that the sea cow made little noise, only heavy breathing sounds, raspy snorts like a horse, and sighs.
Although Steller's sea cow was large, it may have been preyed upon by transient orcas (Orcinus orca). Steller observed that foraging sea cows with calves kept their young between themselves and the shore, a behavior that may have been a response to orcas. Early indigenous peoples of the North Pacific may have relied on sea cows for food, which could have led to their extinction in parts of the North Pacific outside the Commander Islands. Steller's sea cows may have had a mutualistic (or possibly parasitic) relationship with seabirds. Steller saw birds perching on the sea cows to eat the parasite Cyamus rhytinae. This relationship, which disappeared with the sea cow, may have provided food for many birds, similar to how oxpeckers feed on African megafauna.
Taxonomy
Steller's sea cow was part of the genus Hydrodamalis, a group of large sirenians. Its closest relative was Dusisiren. Both Hydrodamalis and Dusisiren are classified in the subfamily Hydrodamalinae, which separated from other sirenians about 4 to 8 million years ago. Steller's sea cow belongs to the family Dugongidae. The only living member of this family is the dugong (Dugong dugon), making it Steller's sea cow's closest living relative.
Steller's sea cow descended from the Cuesta sea cow (H. cuestae), an extinct tropical sea cow that lived off the coast of western North America, especially California. The Cuesta sea cow likely died out because of the cooling of the oceans during the Quaternary glaciation. While many populations disappeared, the lineage of Steller's sea cow adapted to colder climates. Some scientists believe the Takikawa sea cow (H. spissa) of Japan is the same species as the Cuesta sea cow. However, comparisons of brain cases suggest the Takikawa and Steller's sea cows evolved further than the Cuesta sea cow, leading some researchers to think the Takikawa sea cow is a separate species. Over time, the genus Hydrodamalis grew larger and lost teeth and phalanges as a response to the Quaternary glaciation.
According to a 2015 study by Mark Springer, the following species are related to Hydrodamalis:
† Anomotherium langewieschei
Trichechus senegalensis
† Eotheroides aegyptiacum
† Halitherium schinzii
† Priscosiren atlantica
† Metaxytherium krahuletzi
† Metaxytherium serresii
† Metaxytherium medium
† Metaxytherium floridanum
† Metaxytherium crataegense
† Metaxytherium arctodites
† Hydrodamalis cuestae
A 2004 study by Hitoshi Furusawa included the following species:
† Dusisiren reinharti
† Dusisiren takasatensis
† Hydrodamalis cuestae
† Hydrodamalis spissa
Steller's sea cow was discovered in 1741 by Georg Wilhelm Steller, who was shipwrecked on Bering Island. He studied the island's wildlife, including sea cows, sea otters, Steller sea lions, and northern fur seals. Steller described these animals in detail, and his account was published in 1751 as The Beasts of the Sea. Zoologist Eberhard von Zimmermann later named Steller's sea cow Manati gigas in 1780. Biologist Anders Jahan Retzius placed it in the genus Hydrodamalis in 1794, honoring Steller. In 1811, naturalist Johann Karl Wilhelm Illiger moved it to the genus Rytina, a name many used. The correct scientific name, Hydrodamalis gigas, was first used in 1895 by Theodore Sherman Palmer.
For many years after its discovery, no bones of Steller's sea cow were found. This may have been because rising and falling sea levels during the Quaternary period buried many bones. The first bones were discovered in about 1840, over 70 years after the species was thought extinct. The first partial skull was found in 1844, and the first full skeleton in 1855. These remains were sent to Saint Petersburg and Moscow in the 1850s and 1860s. Most full skeletons were found between 1878 and 1883, with 12 of 22 known skeletons discovered during this time. Additional bones were found in 1983 and 1991. Most museum displays use bones from multiple individuals. Adolf Erik Nordenskiöld, Benedykt Dybowski, and Leonhard Hess Stejneger collected many remains in the late 1800s, which were used to create composite skeletons. As of 2006, 27 nearly complete skeletons and 62 complete skulls had been found, though most were made from bones of two to 16 different individuals.
The Pallas Picture is the only known drawing of Steller's sea cow believed to come from a complete specimen. It was published in 1840 by Peter Simon Pallas in Icones ad Zoographia Rosso-Asiatica. Pallas did not name the source, but it may have been based on a drawing by Friedrich Plenisner, a painter in Vitus Bering's crew. Most of Plenisner's drawings were lost during transport.
A similar drawing appeared on a 1744 map by Sven Waxell and Sofron Chitrow, published in 1893 by Pekarski. The map showed Vitus Bering's route and included images of Steller's sea cow and Steller's sea lion. This drawing included inaccuracies, like eyelids and fingers, suggesting it was not based on a real specimen.
In 1846, Johann Friedrich von Brandt created the "Ideal Image" based on the Pallas Picture. In 1868, he made the "Ideal Picture" using collected skeletons. Two other drawings were found in Waxell's diary in 1891, showing a sea cow with large eyes, a big head, and a tail fluke. This image may have been a distorted view of a juvenile. Another drawing, found by Alexander von Middendorff in 1867, is likely a copy of the Tsarskoye Selo Picture.
Range
The area where Steller's sea cow lived when it was first discovered seems to have been limited to the shallow waters near the Commander Islands, which include Bering and Copper Islands. These islands were not inhabited until 1825, when the Russian-American Company moved Aleuts from Attu Island and Atka Island there.
The first fossils found outside the Commander Islands were discovered in layers of soil from a warm period during the Pleistocene era in Amchitka. Additional fossils from the late Pleistocene were found in Monterey Bay, California, and Honshu, Japan. This suggests that the sea cow had a much larger range in ancient times, but it is possible these fossils belong to other species of Hydrodamalis. The southernmost discovery is a rib bone from the Middle Pleistocene found on the Bōsō Peninsula in Japan. Three sea cow remains were found preserved in the South Bight Formation on Amchitka. Because late Pleistocene warm periods are rare in the Aleutians, this discovery indicates that sea cows were common during that time. Steller described the sea cow as living in shallow, sandy areas and near the mouths of freshwater rivers. Genetic studies show that Steller's sea cow and the modern dugong experienced a population bottleneck, a sharp drop in numbers, around 400,000 years ago.
Bone pieces and reports from Aleut people suggest that sea cows once lived in the Near Islands, possibly with groups that interacted with humans in the western Aleutian Islands before Steller’s discovery in 1741. A sea cow rib found on Kiska Island in 1998 was dated to about 1,000 years old and is now in the Burke Museum in Seattle. However, the dating might be incorrect due to the marine reservoir effect, which makes marine fossils appear older than they are. A 2004 study found sea cow bones on Adak Island that were about 1,700 years old and bones on Buldir Island that were about 1,600 years old. These bones might have belonged to whales and been misidentified. Steller’s sea cow rib bones were also found on St. Lawrence Island, from a specimen that lived between 800 and 920 CE.
Interactions with humans
Genetic evidence shows that Steller's sea cows near the Commander Islands were the last of a much larger group that once lived along the North Pacific coasts. These sea cows had the same level of genetic variety as the last, closely related woolly mammoths on Wrangel Island. During cold periods when sea levels and temperatures dropped, their habitat shrank, splitting the population into smaller groups. By about 5,000 years ago, when sea levels became stable, their numbers had already dropped sharply. These facts suggest that even without humans, the Steller's sea cow would have likely gone extinct due to natural changes in climate and sea levels, with only a small group remaining at risk from genetic problems.
Steller's sea cows may have lived in the Aleutian Islands, possibly causing the Aleut people to move westward to hunt them. This might have led to their local extinction there, though evidence is unclear. On St. Lawrence Island, the Siberian Yupik people, who lived there for 2,000 years, may have hunted the sea cows to extinction because their diet relied heavily on marine mammals. The Medieval Warm Period, which reduced kelp growth, might also have caused their local extinction there. Some scientists think the decline of sea cows could have been indirectly caused by aboriginal people hunting sea otters, which led to more sea urchins and less kelp. However, by the time Europeans arrived, sea cows were already limited to uninhabited islands and were in danger of extinction.
When Europeans discovered the sea cows, there may have been about 2,000 left. This small group was quickly hunted by fur traders, seal hunters, and others who followed Vitus Bering’s route to Alaska. They were also hunted for their valuable fat. Early attempts to hunt them failed because of their strong bodies and thick skin. After a successful hunt, the animals were easier to catch, and their meat became a prize for fur traders.
Steller's sea cows were not the most important species in their ecosystem, but they likely helped kelp forests grow by allowing more light to reach the plants and spreading kelp to new areas. Their absence may have disrupted the balance of marine life, possibly affecting species like the black leather chiton. Today, kelp forests depend on seasonal storms and currents for nutrient flow, but sea cows may have kept this process active year-round.
Some reports claim Steller's sea cows survived after 1768, when they were officially declared extinct. For example, people on Attu Island said they saw them into the 1800s. In 1963, a ship reported seeing six large animals near Kamchatka, and fishermen in other areas also claimed sightings.
Steller described the sea cow’s meat as tasting like corned beef but tougher and redder. The meat lasted a long time, possibly because the animals ate a lot of salt. Their fat was used for cooking and as lamp oil, and their milk could be made into butter. Their thick hides were used for clothing and boats.
In the late 1800s, bones and fossils of Steller's sea cows were valuable and sold to museums. After 1900, trade slowed, but some bones are still sold today for items like knife handles. Because the species is extinct, products made from their bones are legal in the U.S. and not restricted by laws like the Marine Mammal Protection Act or CITES. However, selling unfossilized bones is usually not allowed, as some may not be real.
In 1961, a researcher named Elizabeth Porfirevna Orlova studied the Aleuts of Commander Island and recorded notes about their culture and history.