The last population was discovered when a Russian explorer was stranded on Bering Island in 1741, at that time it is thought that 1000-2000 individuals remained. The crew slaughter the animals for meat and for skin to use as leather. Steller’s sea cow is thought to have become extinct by 1768 although there were later unconfirmed reports of its presence in various areas. This species is classified as extinct by the IUCN.
In the late Pleistocene this genus evidently occurred all around the rim of the North Pacific from Japan to California. Hunting by primitive people and climate change restricted its range to the Bering Sea. Pleistocene remains of this species have been found on Amchitka Island in the Aleutians and in Monterey Bay off California.
The giant sea cow had dark brown or grey, hairless skin with a very thick rough outer epidermis similar in appearance to tree bark, which was parasitized by many small crustaceans. Its head was small in proportion to the body, and the forelimbs were also relatively small and flipper-like. The tail had two pointed lobes, as in the living dugong. There was no trace of any hind limbs. The species had no functional teeth, but instead had pads of keratin with corresponding v-shaped ridges in the upper and lower jaws, which it used to grind its food.
Although Steller provided no numerical population or life-history data on the sea cows he observed, a 1741 population of ?1,500 animals was suggested by Leonhard Stejneger, who visited Bering Island from 1882-1883. More recent population estimates have suggested that this population may have consisted of 2,000 animals. Steller's observations suggest that sea cow pairs were monogamous and family groups remained together. He also recorded that sea cows were unable to dive, and were dependent upon shallow inshore kelp beds for food, browsing slowly like cows. The species was apparently fearless of humans, and attempted to aid individuals being attacked by hunters.
However, the sea cow's narrow relictual distribution was not readily apparent to early naturalists. The possibility of the continued survival of the species in other remote parts of the North Pacific, a region well away from familiar Western European trading or whaling routes, was suggested by several scientists during the nineteenth century, notably the Finnish scientist Adolf Nordenskiöld. Leonhard Stejneger’s eighteen-month stay on Bering Island from 1882-1883 partly served to assess Nordenskiöld’s claims for the sea cow's continued survival, and concluded that a supposed sea-cow reported from Bering Island in 1854 was in fact a stray female narwhal.
Several scientists have recently proposed that hunting alone was insufficient to wipe out sea cows, and that this extinction event was actually driven largely by the removal of their food source due to a sea urchin population explosion, triggered by the contemporaneous extirpation of local sea otter populations by Russian hunters. Removing sea otters from nearshore marine environments generates an alternate stable community dominated by sea urchins, which largely eliminate shallow-water kelps on which sea cows would have fed. However, hunting records from eighteenth century Russian expeditions to the Commander Islands have allowed historical levels of sea cow hunting to be modelled. This indicates that sea cows were indeed massively and wastefully overexploited, being hunted at over seven times the sustainable limit; a mean of 123.3 individuals/year were killed between 1743 and 1763. Environmental changes caused by sea otter declines are therefore extremely unlikely to have contributed to this extinction event.
Prehistoric human hunting elsewhere across its North Pacific range is believed to have restricted Steller's sea cow to the uninhabited Commander Islands by the late Holocene. It is generally thought that the last relict population of sea cows around the Commander Islands was rapidly exterminated by direct overharvesting by Russian fur hunters, who used the islands as regular stopping-off points to stock up on supplies following their discovery in 1741. The species was a tempting target for hunters due to its large size, slow movements and apparent fearlessness. Sea cow meat was easily prepared and extremely tasty, blubber was used for cooking and lamp oil, milk from females was drunk or used for butter, and the skin was used as leather for shoes, belts and boats. One early report states that one sea cow could feed 33 men for a month, and it was customary to store sea cow meat to provision ongoing voyages of at least 12 months. Such provisioning also helped eradicate other insular vertebrates, notably the dodo and several giant tortoise species. These records also suggest that five times as many animals were killed as were actually utilized, through wasteful hunting methods; speared animals were typically allowed to swim away in the hope they would later die and float to shore. The mining engineer Petr Yakovlev, who overwintered on Bering Island in 1754-1755, displayed unusual awareness of extinction by anticipating the sea cow’s disappearance through wasteful overexploitation, and unsuccessfully requesting an ukase [Russian edict or decree] from the Bol’sheretsk Chancellery prohibiting further hunting. However, hunting continued and the last sea cow was killed around 1768 on Bering Island. Another species found only on the Commander Islands, the flightless spectacled cormorant (Phalacrocorax perspicillatus), was also hunted to extinction shortly afterwards (Steller wrote that 'one single bird was sufficient for three starving men').
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