Steller's sea cow (Hydrodamalis gigas) was an extinct sea mammal first described by Georg Wilhelm Steller in 1741. It lived near the Commander Islands in the Bering Sea, between Alaska and Russia. During the Pleistocene epoch, its range covered much of the North Pacific, but it likely became limited to the Commander Islands due to changes in ice ages. Indigenous people may have encountered 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 notes, written after his death in a book called On the Beasts of the Sea. Within 27 years of Europeans discovering it, the animal was hunted to extinction because it was slow, easy to catch, and valued 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 more blubber than other sea mammals, helping it survive cold waters. Its tail was split, like those of whales and dugongs. It did not have teeth but had white bristles on its upper lip and two hard plates in its mouth to chew food. It mainly ate kelp and made sounds like sighs and snorts to communicate. Steller believed it lived in small family groups, was monogamous, and cared for 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, much larger than living sirenians. In 1987, a nearly complete skeleton was discovered on Bering Island measuring 3 meters (9.8 feet). In 2017, another skeleton was found on Bering Island measuring 5.2 meters (17 feet), and likely reached about 6 meters (20 feet) in life. Georg Steller's writings include two conflicting weight estimates: 4 and 24.3 tons (4.4 and 26.8 short tons). Scientists believe the true weight was between these values, around 8 to 10 tons (8.8 to 11.0 short tons). This size made the sea cow one of the largest mammals of the Holocene epoch, along with baleen whales and a few toothed whales. It likely helped the animal conserve heat by reducing its surface-area to volume ratio.
Unlike other sirenians, Steller's sea cow was positively buoyant, meaning it could not fully submerge underwater. It had thick outer skin, 2.5 centimeters (1 inch) thick, to protect against sharp rocks and ice and possibly prevent 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. The sea cow's skin was brownish-black, with white patches on some individuals. Its back was smooth, while its sides had rough, crater-like depressions, likely caused by parasites. This texture earned it the nickname "bark animal." Its body had sparse hair, but the insides of its flippers were covered in bristles. The 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, its upper lip had a dense array of interlacing white bristles, about 3.8 centimeters (1.5 inches) long, used to tear seaweed and hold food. It also had two keratinous plates, called ceratodontes, on its palate and mandible, used for chewing. These plates (or "masticatory pads") were held together by interdental papillae, a 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 protruding from its muzzle. Its eyes were small, located halfway between its nostrils and ears, with black irises and livid eyeballs. The canthi (outer corners of the eyes) were not visible externally. It had no eyelashes but possessed a nictitating membrane, a protective layer that covered its eyes during feeding. Its tongue was small and remained in the back of the mouth, unable to reach the chewing pads.
The sea cow's spine likely had seven cervical (neck), 17 thoracic (chest), three lumbar (lower back), and 34 caudal (tail) vertebrae. Its ribs were large, with five of 17 pairs connected to the sternum. It had no clavicles (collar bones). Like all sirenians, its scapula (shoulder blade) was fan-shaped, larger on the posterior 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, a relative of Hydrodamalis, had reduced finger bones, 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 in Komae, Tokyo, Japan, which preserves a fragment of a metacarpal bone.
The sea cow's heart weighed 16 kilograms (35 pounds). Its stomach measured 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 had a wide common bile duct. Its anus was 10 centimeters (0.33 feet) wide, with feces resembling those of horses. The male's penis was 80 centimeters (2.6 feet) long. Genetic evidence shows convergent evolution with other marine mammals in genes related to metabolic and immune function, including leptin, which is linked to energy balance and reproduction.
Ecology and behavior
Whether Steller's sea cow had natural predators is unknown. It may have been hunted by killer whales and sharks, though its ability to float easily might have made it hard for killer whales to drown it. The rocky kelp forests where the sea cow lived might have kept sharks away. According to Steller, adult sea cows protected their young from predators.
Steller described a parasite on the sea cows that looked like a whale louse (Cyamus ovalis), but the parasite was never fully identified because the sea cow went extinct and the original samples collected by Steller were lost. This parasite was first named Sirenocyamus rhytinae in 1846 by Johann Friedrich von Brandt, but it was later placed in the genus Cyamus as Cyamus rhytinae. It was the only known species of cyamid amphipod to live on a sirenian. Steller also noted an endoparasite in the sea cows, likely an ascarid nematode.
Like other sirenians, Steller's sea cow only ate plants and spent most of the day feeding, lifting its head every 4–5 minutes to breathe. Kelp was its main food, making it an algivore. The sea cow likely ate several types of kelp, including Agarum spp., Alaria praelonga, Halosaccion glandiforme, Laminaria saccharina, Nereocyctis luetkeana, and Thalassiophyllum clathrus. It only ate the soft parts of the kelp, leaving the tougher stems and holdfasts to wash up on shore in large piles. The sea cow may have also eaten seagrass, but seagrass was not common enough to support its population. The seagrass species in its habitat (Phyllospadix spp. and Zostera marina) may have grown too deep underwater or been too tough for the sea cow to eat. Since the sea cow floated, it likely fed on canopy kelp, as it could only access food no deeper than 1 meter (3.3 feet) below the tide. Kelp releases a chemical to protect itself from grazing, but canopy kelp releases less of this chemical, allowing the sea cow to feed safely. Steller noted that the sea cow grew thin in winter, suggesting it fasted during periods of low kelp growth. Fossils of Pleistocene Aleutian Island sea cows were larger than those from the Commander Islands, indicating that Commander Island sea cows may have grown slower due to less food and a less favorable habitat compared to the warmer Aleutian Islands.
Steller described the sea cow as highly social (gregarious), living in small family groups and helping injured members. It was also monogamous. Steller's sea cow may have cared for its young, with calves kept at the front of the herd for protection. Steller reported that when a female was captured, other sea cows attacked the hunting boat by ramming and rocking it. After the hunt, the female's mate followed the boat to shore, even after the captured animal had died. Mating likely occurred in early spring, with a gestation period of over a year, and calves were likely born in autumn, as Steller observed more calves in autumn than in other seasons. Female sea cows had only one set of mammary glands, suggesting they 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 onto its partner during mating. According to Steller, the forelimbs also helped the sea cow stay anchored to avoid being swept away by strong nearshore waves. While grazing, the sea cow moved slowly by swaying its tail (fluke) from side to side; faster movement was achieved by strong vertical tail beats. They often slept on their backs after feeding. Steller noted that the sea cow made little noise, only heavy breathing sounds, raspy snorts similar to a horse, and sighs.
Despite their large size, Steller's sea cows may have been prey for local killer whales (Orcinus orca). Steller observed that foraging sea cows with calves always kept their young between themselves and the shore, a behavior likely caused by killer whales. Early indigenous peoples of the North Pacific may have relied on the sea cow for food, which could have led to its 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 local seabirds. Steller often saw birds perching on the sea cows to eat the parasitic Cyamus rhytinae. This relationship, which disappeared with the sea cows, may have been a food source for many birds. It is similar to the interactions between oxpeckers (Buphagus) and living African megafauna.
Taxonomy
Steller's sea cow was part of the genus Hydrodamalis, a group of large sirenians. Its closest relative was Dusisiren. Both groups' ancestors lived in tropical mangroves before moving to the cold North Pacific. Hydrodamalis and Dusisiren are grouped in the subfamily Hydrodamalinae, which split from other sirenians about 4 to 8 million years ago. Steller's sea cow belongs to the family Dugongidae, which includes the living dugong (Dugong dugon). The dugong is Steller's sea cow's closest living relative.
Steller's sea cow descended from the Cuesta sea cow (H. cuestae), an extinct tropical species that lived near California. The Cuesta sea cow likely died out because of climate changes during the Quaternary glaciation. While many populations disappeared, the lineage of Steller's sea cow adapted to colder climates. Some scientists think the Takikawa sea cow (H. spissa) of Japan is the same species as the Cuesta sea cow. However, comparisons of braincase structures suggest the Takikawa and Steller's sea cows evolved more than the Cuesta sea cow, leading some to believe the Takikawa sea cow is a separate species. The genus Hydrodamalis evolved larger body size and lost teeth and finger bones as a response to colder climates.
Based on a 2015 study by Mark Springer:
† Anomotherium langewieschei
Trichechus senegalensis
† Eotheroides aegyptiacum
† Halitherium schinzii
† Priscosiren atlantica
† Metaxytherium krahuletzi
† Metaxytherium serresii
† Metaxytherium medium
† Metaxytherium floridanum
† Metaxytherium crataegense
† Metaxytherium arctodites
† Hydrodamalis cuestae
Based on a 2004 study by Hitoshi Furusawa:
† Dusisiren reinharti
† Dusisiren takasatensis
† Hydrodamalis cuestae
† Hydrodamalis spissa
Steller's sea cow was discovered in 1741 by Georg Wilhelm Steller, who was named after it. Steller studied wildlife on Bering Island during a shipwreck. He described sea cows, sea otters, Steller sea lions, and northern fur seals. His writings were published in 1751 as The Beasts of the Sea. Zoologist Eberhard von Zimmermann named it Manati gigas in 1780. Biologist Anders Jahan Retzius placed it in the genus Hydrodamalis in 1794. Naturalist Johann Karl Wilhelm Illiger later moved it to the genus Rytina. The correct name Hydrodamalis gigas was first used in 1895.
For many years after its discovery, no bones of Steller's sea cow were found. This may have been because sea level changes during the Quaternary period buried bones. The first bones were found in 1840, over 70 years after the species was thought extinct. A skull was discovered in 1844, and a full skeleton in 1855. These were sent to Saint Petersburg and Moscow. Most skeletons were found between 1878 and 1883, with 12 of 22 dated skeletons discovered during this time. A skeleton was found in 1983, and 90 bones were collected in 1991. Only two to four museum displays use bones from a single individual. Scientists like Adolf Erik Nordenskiöld and Leonhard Hess Stejneger collected bones in the 1800s to create composite skeletons. As of 2006, 27 nearly complete skeletons and 62 skulls were found, though most are made from bones of multiple individuals.
The Pallas Picture is the only known drawing of Steller's sea cow from a complete specimen. It appeared in Peter Simon Pallas's 1840 work Icones ad Zoographia Rosso-Asiatica. Pallas did not name the source. Stejneger suggested it might be based on drawings by Friedrich Plenisner, an artist on Vitus Bering's crew. Most of Plenisner's work was lost.
A similar drawing appeared on a 1744 map by Sven Waxell and Sofron Chitrow, published in 1893. The map showed Vitus Bering's expedition and included images of Steller's sea cow and sea lion. The drawing had 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. One showed a sea cow with large eyes, a big head, and a horizontal tail. Another image, found in 1867, was likely a copy of the Tsarskoye Selo Picture.
Range
The range of Steller's sea cow when it was first discovered was limited to the shallow seas 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 to live there.
Fossils found outside the Commander Islands were discovered in layers of soil from the interglacial Pleistocene period in Amchitka. Additional fossils from the late Pleistocene were found in Monterey Bay, California, and Honshu, Japan. This suggests that Steller's sea cow had a much larger range in ancient times. However, it is possible that some of these fossils belong to other species in the Hydrodamalis group. The southernmost fossil is a rib bone from the Middle Pleistocene found on the Bōsō Peninsula of Japan. Three sea cow remains were discovered in the South Bight Formation of Amchitka. Because interglacial deposits are rare in the Aleutians, this finding indicates that sea cows were common during that time. According to Steller, the sea cow lived in shallow, sandy shorelines and near the mouths of freshwater rivers. Genetic studies show that Steller's sea cow, along with the modern dugong, experienced a population bottleneck, a sharp drop in numbers, around 400,000 years ago.
Bone fragments and reports from Aleut people suggest that sea cows once lived in the Near Islands. These populations may have been in contact with humans in the western Aleutian Islands before Steller discovered the species in 1741. A sea cow rib found on Kiska Island in 1998 was dated to about 1,000 years old. This bone is now in the Burke Museum in Seattle. However, the dating might be incorrect due to the marine reservoir effect, which makes marine samples appear older than they are. This effect occurs because the ocean holds a large amount of carbon, and the animal may have died between 1710 and 1785. 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. It is possible these bones belonged to whales and were misclassified. Rib bones of Steller's sea cow were also found on St. Lawrence Island, from an individual 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 coasts of the North Pacific. These sea cows had the same level of genetic diversity as the last, less diverse group of woolly mammoths on Wrangel Island. During cold periods when sea levels and temperatures dropped, the area where these animals lived became smaller, splitting their population. By about 5,000 years ago, when sea levels stabilized, their numbers had dropped sharply. These facts suggest that even without humans, Steller's sea cows would have faced a high risk of extinction due to natural changes in climate and sea levels, with only a small group left and facing serious genetic challenges.
Steller's sea cows may have influenced the movement of the Aleut people, who lived in the Aleutian Islands. If the Aleuts moved westward to hunt the sea cows, this might have led to their local extinction, though evidence is not certain. 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 culture relied heavily on marine mammals. The Medieval Warm Period, which made kelp less available, might have also caused the sea cows to disappear from that area. Some scientists think the decline of sea otters, hunted by local people, led to more sea urchins, which damaged kelp forests that sea cows depended on. Even without hunting otters, sea cows may have been easy targets for hunters, leading to their extinction in some areas. By the time Europeans arrived, sea cows lived only near uninhabited islands and were already in danger.
When Europeans discovered Steller's sea cows, there may have been only about 2,000 left. These animals were quickly hunted by fur traders, seal hunters, and others traveling along 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. Later, hunters used harpoons and bayonets to kill them, dragging them onto shore when the tide was low. This made hunting easier, and fur traders began visiting the Commander Islands to restock supplies.
Steller's sea cows were not a keystone species, but they likely helped kelp forests grow by allowing more sunlight to reach the plants and spreading kelp seeds. Their presence may have helped sea otters and other marine animals live together. Without them, kelp forests may have become less productive, and ecosystems may have been harmed before modern issues like overharvesting and climate change.
Some people claimed to see Steller's sea cows after they were thought to be extinct. In the 1800s, natives on Attu Island said they hunted them. In 1963, a ship near Kamchatka reported seeing six large animals that looked like sea cows. Fishermen in other areas also claimed to see them, but these sightings are not confirmed.
Steller described the meat of sea cows as tasting like corned beef but tougher and redder. It lasted a long time because of the salt in their diet. Their fat was used for cooking and as oil. Female sea cows had thick, sweet milk that could be used for food or butter. Their hides were strong and used to make clothing and boats.
In the late 1800s, bones and fossils of sea cows were valuable and sold to museums. Most were collected then, and trade slowed after 1900. Today, some bones are still sold, but laws limit the trade of unfossilized bones because they may not be real. Items made from sea cow bones are legal in the U.S. because the species is extinct and not protected by certain laws.
An ethnographer named Elizabeth Porfirevna Orlova studied the Aleuts of the Commander Islands in 1961. Her research included notes about their culture and history.