Steller's sea cow (Hydrodamalis gigas) is an extinct sirenian described by Georg Wilhelm Steller in 1741. At that time, it was found only around the Commander Islands in the Bering Sea between Alaska and Russia; its range extended across the North Pacific during the Pleistocene epoch, and likely contracted to such an extreme degree due to the glacial cycle. It is possible that indigenous populations interacted with the animal before Europeans. Steller first encountered it on Vitus Bering's Great Northern Expedition when the crew became shipwrecked on Bering Island. Much of what is known about its behavior comes from Steller's observations on the island, documented in his posthumous publication On the Beasts of the Sea. Within 27 years of its discovery by Europeans, the slow-moving and easily caught mammal was hunted into extinction for its meat, fat, and hide.
Some 18th-century adults would have reached weights of and lengths up to . It was a member of the family Dugongidae, of which the long dugong (Dugong dugon) is the sole living member. It had a thicker layer of blubber than other members of the order, an adaptation to the cold waters of its environment. Its tail was forked, like that of whales or dugongs. Lacking true teeth, it had an array of white bristles on its upper lip and two keratinous plates within its mouth for chewing. It fed mainly on kelp, and communicated with sighs and snorting sounds. Steller believed it was a monogamous and social animal living in small family groups and raising its young, similar to modern sirenians.
Description
Steller's sea cows are reported to have grown to long as adults, much larger than extant sirenians. In 1987, a rather complete skeleton was found on Bering Island measuring . In 2017, another such skeleton was found on Bering Island measuring , and in life probably about . Georg Steller's writings contain two contradictory estimates of weight: . The true value is estimated to fall between these figures, at about . This size made the sea cow one of the largest mammals of the Holocene epoch, along with baleen whales and some few toothed whales, and was likely an adaptation to reduce its surface-area to volume ratio and conserve heat.
Unlike other sirenians, Steller's sea cow was positively buoyant, meaning that it was unable to submerge completely. It had a very thick outer skin, , to prevent injury from sharp rocks and ice and possibly to prevent unsubmerged skin from drying out. The sea cow's blubber was thick, another adaptation to the frigid climate of the Bering Sea. Its skin was brownish-black, with white patches on some individuals. It was smooth along its back and rough on its sides, with crater-like depressions most likely caused by parasites. This rough texture led to the animal being nicknamed the "bark animal". Hair on its body was sparse, but the insides of the sea cow's flippers were covered in bristles. The fore limbs were roughly long, and the tail fluke was forked.
The sea cow's head was small and short in comparison to its huge body. The animal's upper lip was large and broad, extending so far beyond the lower jaw that the mouth appeared to be located underneath the skull. Unlike other sirenians, Steller's sea cow was toothless and instead had a dense array of interlacing white bristles on its upper lip. The bristles were about in length and were used to tear seaweed stalks and hold food. The sea cow also had two keratinous plates, called ceratodontes, located on its palate and mandible, used for chewing. According to Steller, these plates (or "masticatory pads") were held together by interdental papillae, a part of the gums, and had many small holes containing nerves and arteries.
As with all sirenians, the sea cow's snout pointed downwards, which allowed it to better grasp kelp. The sea cow's nostrils were roughly long and wide. In addition to those within its mouth, the sea cow also had stiff bristles long protruding from its muzzle. Steller's sea cow had small eyes located halfway between its nostrils and ears with black irises, livid eyeballs, and canthi which were not externally visible. The animal had no eyelashes, but like other diving creatures such as sea otters, Steller's sea cow had a nictitating membrane, which covered its eyes to prevent injury while feeding. The tongue was small and remained in the back of the mouth, unable to reach the masticatory (chewing) pads.
The sea cow's spine is believed to have had seven cervical (neck), 17 thoracic, three lumbar, and 34 caudal (tail) vertebrae. Its ribs were large, with five of 17 pairs making contact with the sternum; it had no clavicles. As in all sirenians, the scapula of Steller's sea cow was fan-shaped, being larger on the posterior side and narrower towards the neck. The anterior border of the scapula was nearly straight, whereas those of modern sirenians are curved. Like other sirenians, the bones of Steller's sea cow were pachyosteosclerotic, meaning they were both bulky (pachyostotic) and dense (osteosclerotic). In all collected skeletons of the sea cow, the manus is missing; since Dusisiren—the sister taxon of Hydrodamalis—had reduced phalanges (finger bones), Steller's sea cow possibly did not have a manus at all. The only specimen with preserved bones from the wrist down is a specimen under study found in Komae, Tokyo, Japan, which preserve a fragment of metacarpal bone.
The sea cow's heart was in weight; its stomach measured long and wide. The full length of its intestinal tract was about , equaling more than 20 times the animal's length. The sea cow had no gallbladder, but did have a wide common bile duct. Its anus was in width, with its feces resembling those of horses. The male's penis was long. Genetic evidence indicates convergent evolution with other marine mammals of genes related to metabolic and immune function, including leptin associated with energy homeostasis and reproductive regulation.
Ecology and behavior
Whether Steller's sea cow had any natural predators is unknown. It may have been hunted by killer whales and sharks, though its buoyancy may have made it difficult for killer whales to drown, and the rocky kelp forests in which the sea cow lived may have deterred sharks. According to Steller, the adults guarded the young from predators.
Steller described an ectoparasite on the sea cows that was similar to the whale louse (Cyamus ovalis), but the parasite remains unidentified due to the host's extinction and loss of all original specimens collected by Steller. It was first formally described as Sirenocyamus rhytinae in 1846 by Johann Friedrich von Brandt, although it has since been placed into the genus Cyamus as Cyamus rhytinae. It was the only species of cyamid amphipod to be reported inhabiting a sirenian. Steller also identified an endoparasite in the sea cows, which was likely an ascarid nematode.
Like other sirenians, Steller's sea cow was an obligate herbivore and spent most of the day feeding, only lifting its head every 4–5 minutes for breathing. Kelp was its main food source, making it an algivore. The sea cow likely fed on several species of kelp, which have been identified as Agarum spp., Alaria praelonga, Halosaccion glandiforme, Laminaria saccharina, Nereocyctis luetkeana, and Thalassiophyllum clathrus. Steller's sea cow only fed directly on the soft parts of the kelp, which caused the tougher stem and holdfast to wash up on the shore in heaps. The sea cow may have also fed on seagrass, but the plant was not common enough to support a viable population and could not have been the sea cow's primary food source. Further, the available seagrasses in the sea cow's range (Phyllospadix spp. and Zostera marina) may have grown too deep underwater or been too tough for the animal to consume. Since the sea cow floated, it likely fed on canopy kelp, as it is believed to have only had access to food no deeper than below the tide. Kelp releases a chemical deterrent to protect it from grazing, but canopy kelp releases a lower concentration of the chemical, allowing the sea cow to graze safely. Steller noted that the sea cow grew thin during the frigid winters, indicating a period of fasting due to low kelp growth. Fossils of Pleistocene sea cow populations from the Aleutian Islands were larger than those from the Commander Islands, indicating that the growth of Commander Island sea cows may have been stunted due to a less favorable habitat and less food than the warmer Aleutian Islands.
Steller described the sea cow as being highly social (gregarious). It lived in small family groups and helped injured members, and was also apparently monogamous. Steller's sea cow may have exhibited parental care, and the young were kept at the front of the herd for protection against predators. Steller reported that as a female was being captured, a group of other sea cows attacked the hunting boat by ramming and rocking it, and after the hunt, her mate followed the boat to shore, even after the captured animal had died. Mating season occurred in early spring and gestation took a little over a year, with calves likely delivered in autumn, as Steller observed a greater number of calves in autumn than at any other time of the year. Since female sea cows had only one set of mammary glands, they likely had one calf at a time.
The sea cow used its fore limbs for swimming, feeding, walking in shallow water, defending itself, and holding on to its partner during copulation. According to Steller, the fore limbs were also used to anchor the sea cow down to prevent it from being swept away by the strong nearshore waves. While grazing, the sea cow progressed slowly by moving its tail (fluke) from side to side; more rapid movement was achieved by strong vertical beating of the tail. They often slept on their backs after feeding. According to Steller, the sea cow was nearly mute and made only heavy breathing sounds, raspy snorting similar to a horse, and sighs.
Despite their large size, as with many other marine megafauna in the region, Steller's sea cows may have been prey for the local transient orcas (Orcinus orca); it is likely that they experienced predation, as Steller observed that foraging sea cows with calves would always keep their calves between themselves and the shore, and orcas would have been the most likely candidate for causing this behavior. In addition, early indigenous peoples of the North Pacific may have depended on the sea cow for food, and it is possible that this dependency may have extirpated the sea cow from portions of the North Pacific aside from the Commander Islands. Steller's sea cows may have also had a mutualistic (or possibly even parasitic) relationship with local seabird species; Steller often observed birds perching on the exposed backs of the sea cows, feeding on the parasitic Cyamus rhytinae; this unique relationship that disappeared with the sea cows may have been a food source for many birds, and is similar to the recorded interactions between oxpeckers (Buphagus) and extant African megafauna.
Taxonomy Phylogeny
Steller's sea cow was a member of the genus Hydrodamalis, a group of large sirenians, whose sister taxon was Dusisiren. Like those of Steller's sea cow, the ancestors of Dusisiren lived in tropical mangroves before adapting to the cold climates of the North Pacific. Hydrodamalis and Dusisiren are classified together in the subfamily Hydrodamalinae, which diverged from other sirenians around 4 to 8 mya. Steller's sea cow is a member of the family Dugongidae, the sole surviving member of which, and thus Steller's sea cow's closest living relative is the dugong (Dugong dugon).