Steller's sea cow
|Steller's sea cow|
|Drawing by Steller in 1743 of a dead female, thought to be the only first-hand illustration|
The Steller's sea cow, Hydrodamalis gigas, is an extinct herbivorous marine mammal of the North Pacific Ocean. It was the largest member of the order Sirenia, which includes its closest living relative, the dugong (Dugong dugon), and the manatees (Trichechus spp.). The Steller's sea cow reached up to 9 metres (30 ft) in length, making it among the largest mammals other than whales to have existed in the holocene epoch. Steller's sea cow was first described by Georg Wilhelm Steller. Although the Steller's sea cow had formerly been abundant throughout the North Pacific, by the mid 1700s, its range had been limited to a single, isolated population surrounding the uninhabited Commander Islands. It was hunted for its meat, skin, and fat by fur traders, and was also hunted by aboriginals of the North Pacific coast. Within 27 years of discovery by Europeans, the slow-moving and easily captured Steller's sea cow was hunted to extinction.
Description and ecology
The sea cow grew to at least 8 to 9 m (26 to 30 ft) in length as an adult, much larger than the manatee or dugong; however, concerning their weight, Steller's work contains two contradictory estimates: 4 and 24.3 metric tons (4.4 and 26.8 short tons). The true value is estimated to lie between these figures, at about 8–10 metric tons (8.8–11.0 short tons). Their large size was probably to reduce their surface-area-to-volume ratio and conserve heat. The forelimbs, according to Steller, were used as sort of a holdfast to anchor themselves down to prevent being swept away by the strong nearshore waves around their habitat. Unlike other sirenians, the Steller's sea cow was positively buoyant, meaning they could not completely submerge. They had a thick epidermis to prevent injury from abrasions on sharp rocks and ice, and possibly to prevent the skin that was not submerged from drying out.
Its head was small and short compared to the huge body. The upper lip was large and broad, and extended so far beyond the mandible, that the mouth appeared to be located underneath the skull. Instead of teeth, Steller's sea cow had a dense array of white bristles, 1.5 inches (3.8 cm) long, which were used to pull out seaweed and hold food, and used keratinous plates for chewing. According to Steller, these plates, or "masticatory bones", were held together by papillae and had many small holes for nerves and arteries. The rostrum was pointed downwards, as in all sirenians, to better grasp kelp. Like other sirenians, the Steller's sea cow was an obligate herbivore, and kelp was most likely their main food source. They may have also fed on seagrasses, but this could not have been a main food source for supporting a viable population, because grasses did not occur in sufficient quantity. Since this animal floated, it most likely fed on canopy kelp. Kelp releases a chemical deterrent to prevent grazing, but canopy kelp release a lower concentration, allowing the sea cows to graze without developing resistance.
Whether or not Steller's sea cows had any predators is unknown. They may have been hunted by killer whales and sharks, but their buoyancy may have made it difficult for killer whales to drown them, and the rocky kelp forests may have protected them from sharks. According to Steller, the young were guarded by the adults from predators.
Taxonomy and range
|Relations within Sirenia|
|Based on a 2004 study by Hitoshi Furusawa|
The Steller's sea cow was a direct descendant of the Cuesta sea cow, an extinct tropical sea cow of California. It most likely went extinct due to the onset of the Ice Ages and the subsequent cooling of the oceans; lineages which could not adapt died out, and those that could started the lineage of the Steller's sea cow. The Steller's sea cow was a member of the genus Hydrodamalis, a group of large sirenians, whose sister taxon was Dusisiren. Much like the Steller's sea cow, the ancestors of the Dusisiren were associated with tropical mangroves, and adapted to the cold climates of the North Pacific and to consuming kelp.
Steller's sea cow was discovered in the mid-18th century by Georg Wilhelm Steller, and subsequently named after him; Steller studied a relict population near Bering Island while he was shipwrecked there. His account was written in his posthumous publication De bestiis marinis, or "The Beasts of the Sea". In 1811, naturalist Johann Karl Wilhelm Illiger placed the Steller's sea cow under the genus Rhytina, which many writers at the time adopted. However, the animal had already been classified long before this. Zoologist Eberhard August Wilhelm von Zimmermann had described its specific name as gigas in 1780, and biologist Anders Jahan Retzius, 17 years before Illiger, had described the sea cow as Rhytina and placed it under the genus Hydrodamalis. He, however, described its specific name as stelleri, as Steller was the first person to describe it. It was not until the 1900s that Hydrodamalis gigas was used.
Their range at the time of their discovery was apparently restricted to the Commander Islands, though fossils dating to the late Pleistocene were found in Monterey Bay, California. The first fossils discovered outside the Commander Islands were interglacial Pleistocene deposits in Amchitka. There is evidence that sea cows also inhabited the Near Islands during historic times.
The Steller's sea cow was quickly wiped out by the sailors, seal hunters, and fur traders who followed Vitus Bering's route past the islands to Alaska, who hunted it for its meat before sailing to nearby islands in search of sea otter pelts. It was also hunted for its valuable subcutaneous fat, which was not only used for food (usually as a butter substitute), but also for oil lamps because it did not give off any smoke or odor and could be kept for a long time in warm weather without spoiling. By 1768, 27 years after it had been discovered by Europeans, Steller's sea cow was extinct. It has been argued that the Steller's sea cow's decline may have also been an indirect response to the harvest of sea otters by aboriginal people from the inland areas. With the otters reduced, the population of sea urchins would have increased and reduced availability of kelp, the sea cow's primary source of food. Thus, aboriginal hunting of both species may have contributed to the sea cow's disappearance from continental shorelines. In historic times, though, aboriginal hunting had depleted sea otter populations only in localized areas. The sea cow would have been easy prey for aboriginal hunters, who would likely have exterminated accessible populations with or without simultaneous otter hunting. In any event, the sea cow was limited to coastal areas off islands without a human population by the time Bering arrived, and was already endangered. It is possible that the extinction of these remaining endangered populations of sea cow could have been affected solely by the hunting of the sea cow for meat by fur-trading mariners of the time, and no other factors (such as overpopulation of sea urchins) need contributed. Zoologist Leonhard Hess Stejneger estimated in 1887 that there had been fewer than 1,500 individuals remaining at the time of their discovery, and thus had been in immediate danger of extinction.
In 1963 the Russian magazine Priroda (Nature), official journal of the Academy of Sciences of the USSR, published an article reporting a possible sighting. In 1962 the whaling ship Buran had reported a group of large marine mammals in shallow water off Kamchatka, grazing on seaweed. These were considered possibly a surviving sea cow population, but there were no further known reports.
Portrayals in media
Tales of a Sea Cow is a 2012 film by Icelandic-French artist Etienne de France "documenting" a fictional 2006 re-discovery by scientists of a population of Steller's sea cows off the coast of Greenland via sound recordings or their calls. This film has been exhibited in public institutions such as art museums and universities in Europe. Art critic Annick Bureaud found the film a "tongue in cheek and joyous but unsettling fable".
- List of extinct animals
- List of extinct animals of North America
- List of recently extinct mammals
- Evolution of sirenians
- Holocene extinction
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