Syrena morska

Wstęp

Steller's sea cow (Hydrodamalis gigas) is an extinct sirenian discovered by Europeans in 1741. At that time it was found only around the Commander Islands in the Bering Sea between Alaska and Russia; its range was more extensive during the Pleistocene epoch, and it is possible that the animal and humans previously interacted. Eighteenth century adults would reach weights of 8–10 tona metryczna (8,8–11,0 tony amerykańskiej) and lengths up to 9 metrów (30 ft).

It was a part of the order Sirenia, and member of the family Dugongidae, of which its closest living relative, the 3 metry (9,8 ft) long dugong (Dugong dugon), is the sole surviving 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 cetaceans. 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. Evidence suggests it was a monogamous and social animal, living in small family groups and raising its young, similar to extant sirenians.

The sea cow was named after Georg Wilhelm Steller, a naturalist who discovered the species in 1741, 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 twenty-seven years of discovery by Europeans, the slow-moving and easily caught mammal was hunted into extinction for its meat, fat, and hide.

Description

The skull of a Steller's sea cow, Natural History Museum of London

Steller's sea cows grew to be 8 do 9 m (26 do 30 ft) long as adults, much larger than extant sirenians. Georg Steller's writings contain two contradictory estimates of weight: 4 i 24,3 tona metryczna (4,4 i 26,8 tony amerykańskiej). The true value is estimated to fall between these figures, at about 8–10 tona metryczna (8,8–11,0 tony amerykańskiej). This size made the sea cow one of the largest mammals of the Holocene epoch, aside from whales. The sea cow's large size 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 completely submerge. It had a very thick outer skin, 2,5 centymetra (1 in), to prevent injury from sharp rocks and ice, and possibly to prevent unsubmerged skin from drying out. The sea cow's blubber was 8–10 centymetrów (3–4 in) thick, another adaptation to the frigid climate of the Bering Sea where it lived. 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 forelimbs were roughly 67 centymetrów (26 in) 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 approximately 3,8 centymetra (1,5 in) in length and were used to tear seaweed stalks and hold food. The sea cow also had two keratinous plates 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.

Model in the Natural History Museum of London

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 5 centymetrów (2 in) long and wide. In addition to those within its mouth, the sea cow also had stiff, 10–12,7 centymetra (3,9–5,0 in) long bristles 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 7 cervical (neck), 17 thoracic, 3 lumbar, and 34 caudal (tail) vertebrae. Its ribs were large, with 5 of 17 pairs making contact with the sternum; the sea cow 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), it is possible that Steller's sea cow did not have a manus at all.

The sea cow's heart was 16 kilogramów (35 lb) in weight; its stomach measured 1,8 metra (6 ft) long and 1,5 metra (5 ft) wide. The full length of its intestinal tract was about 151 metrów (500 ft) long, 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 10 centymetrów (0,33 ft) in width, with its feces resembling that of horses. The male's penis was 80 centymetrów (2,6 ft) long.

Ecology and behavior

Illustrations of the dentition of Steller's sea cow by Johann Christian Daniel von Schreber

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. It was the only species of cyamid amphipod to be reported inhabiting a sirenian. Steller also identified an endoparasite on 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 to 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 1 metr (3,3 ft) 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 Aleutian Island sea cow populations 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). The sea cow 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, it is likely that they had one calf at a time.

The sea cow used its forelimbs for swimming, feeding, walking in shallow water, defending itself, and for holding on to its partner during copulation. According to Steller, the forelimbs 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.

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, whose sole surviving member, and thus Steller's sea cow's closest living relative, is the dugong (Dugong dugon).

Steller's sea cow was a direct descendant of the Cuesta sea cow (H. cuestae), an extinct tropical sea cow that lived off the coast of western North America, particularly California. The Cuesta sea cow is thought to have gone extinct due to the onset of the Quaternary glaciation and the subsequent cooling of the oceans. Many populations died out, but the lineage of Steller's sea cow was able to adapt to the colder temperatures. The Takikawa sea cow (H. spissa) of Japan is thought of by some researchers to be a taxonomic synonym of the Cuesta sea cow, but based on a comparison of endocasts, the Takikawa and Steller's sea cows are more derived than the Cuesta sea cow. This has led some to believe that the Takikawa sea cow is its own species. The evolution of the Hydrodamalis genus was characterized by increased size and a loss of teeth and phalanges as a response to the onset of the Quaternary glaciation.

Research history

Steller's sea cow was discovered in the mid-18th century (1741) by Georg Wilhelm Steller, and was named after him. Steller researched the wildlife of Bering Island while he was shipwrecked there for about a year; the animals on the island included relict populations of sea cows, sea otters, Steller sea lions, and northern fur seals. Steller's account was included in his posthumous publication De bestiis marinis, or The Beasts of the Sea, which was published in 1751 by the Russian Academy of Sciences in Saint Petersburg. Zoologist Eberhard von Zimmermann formally described Stellar's sea cow in 1780 as Manati gigas. Biologist Anders Jahan Retzius in 1794 put the sea cow in the new genus Hydrodamalis, with the specific name of stelleri, in honor of Steller. In 1811, naturalist Johann Karl Wilhelm Illiger reclassified Steller's sea cow into the genus Rytina, which many writers at the time adopted. The name Hydrodamalis gigas, the correct combinatio nova if a separate genus is recognised, was first used in 1895 by Theodore Sherman Palmer.

Stejneger's 1925 reconstruction of Steller measuring a sea cow in 1742

For decades after its discovery, no skeletal remains of a Steller's sea cow were discovered. This may have been due to rising and falling sea levels over the course of the Quaternary period, which could have left many sea cow bones hidden. The first bones of a Steller's sea cow were unearthed in about 1840, over 70 years after it was presumed extinct. The first partial sea cow skull was discovered in 1844 by Ilya Voznesensky while on the Commander Islands, and the first skeleton was discovered in 1855 on northern Bering Island. These specimens were sent to Saint Petersburg in 1857, and another nearly complete skeleton arrived in Moscow around 1860. Most of the skeletal remains were unearthed in the late 1800s: between 1878 and 1883, 12 of the known 27 skeletons were discovered. Adolf Erik Nordenskiöld, Benedykt Dybowski, and Leonhard Hess Stejneger each unearthed many bones from different individuals during this period, from which composite skeletons were assembled. As of 2006, 27 nearly complete skeletons and 62 complete skulls have been found.

Illustrations

The Pallas Picture is the only known drawing of Steller's sea cow believed to be from an actual specimen. It was published by Peter Simon Pallas in his 1840 work Icones ad Zoographia Rosso-Asiatica. Pallas did not specify a source; Stejneger suggested it may have been one of the original illustrations produced by Friedrich Plenisner, a member of Vitus Bering's crew as a painter and surveyor who drew a figure of a female sea cow on Steller's request. Most of Plenisner's depictions were lost during transit from Siberia to Saint Petersburg.

Another drawing of Steller's sea cow similar to the Pallas Picture appeared on a 1744 map drawn by Sven Waxell and Sofron Chitrow. The picture may have also been based upon a specimen, and was published in 1893 by Pekarski. The map depicted Vitus Bering's route during the Great Northern Expedition, and featured illustrations of Steller's sea cow and Steller's sea lion in the upper-left corner. The drawing contains some inaccurate features such as the inclusion of eyelids and fingers, leading to doubt that it was drawn from a specimen.

Johann Friedrich von Brandt, director of the Russian Academy of Sciences, had the "Ideal Image" drawn in 1846 based upon the Pallas Picture, and then the "Ideal Picture" in 1868 based upon collected skeletons. Two other possible drawings of Steller's sea cow were found in 1891 in Waxell's manuscript diary. There was a map depicting a sea cow, as well as a Steller sea lion and a northern fur seal. The sea cow was depicted with large eyes, a large head, claw-like hands, exaggerated folds on the body, and a tail fluke in perspective lying horizontally rather than vertically. The drawing may have been a distorted depiction of a juvenile, as the figure bears a resemblance to a manatee calf. Another similar image was found by Alexander von Middendorff in 1867 in the library of the Russian Academy of Sciences, and is probably a copy of the Tsarskoye Selo Picture.

Range

Szablon:OSM Location map The range of Steller's sea cow at the time of its discovery was apparently restricted to the shallow seas around the Commander Islands, which include Bering and Copper Islands. The Commander Islands remained uninhabited until 1825 when the Russian-American Company relocated Aleuts from Attu Island and Atka Island there. The first fossils discovered outside the Commander Islands were found in interglacial Pleistocene deposits in Amchitka, and further fossils dating to the late Pleistocene were found in Monterey Bay, California, and Honshu, Japan. This suggests that the sea cow had a far more extensive range in prehistoric times. It cannot be excluded that these fossils belong to other Hydrodamalis species. The remains of three individuals were found preserved in the South Bight Formation of Amchitka; as late Pleistocene interglacial deposits are rare in the Aleutians, the discovery suggests that sea cows were abundant in that era. According to Steller, the sea cow often resided in the shallow, sandy shorelines and in the mouths of freshwater rivers.

Bone fragments and accounts by native Aleut people suggest that sea cows also historically inhabited the Near Islands, potentially with viable populations that were in contact with humans in the western Aleutian Islands prior to Steller's discovery in 1741. A sea cow rib discovered in 1998 on Kiska Island was dated to around 1,000 years old, and is now in the possession of the Burke Museum in Seattle. The dating may be skewed due to the marine reservoir effect which causes radiocarbon-dated marine specimens to appear several hundred years older than they are. Marine reservoir effect is caused by the large reserves of C¹⁴ in the ocean, and it is more likely that the animal died between 1710 and 1785.

A 2004 study reported that sea cow bones discovered on Adak Island were around 1,700 years old, and sea cow bones discovered on Buldir Island were found to be around 1,600 years old. It is possible the bones were from cetaceans and were misclassified. Rib bones of a Steller's sea cow have also been found on St. Lawrence Island, and the specimen is thought to have lived between 800 and 920 CE.

Interactions with humans

Extinction

Interactions with Europeans

1966 Soviet postage stamp depicting Bering's second voyage and the discovery of the Commander Islands

Steller's sea cow was quickly wiped out by fur traders, seal hunters, and others who followed Vitus Bering's route past its habitat to Alaska. It was also hunted to collect its valuable subcutaneous fat. By 1768, just twenty-seven years after it had been discovered by Europeans, the species was extinct. In 1887 Stejneger estimated that there had been fewer than 1,500 individuals remaining at the time of Steller's discovery, and argued there was already an immediate danger of the sea cow's extinction.

The first attempt to hunt the animal by Steller and the other crew members was unsuccessful due to its strength and thick hide. They had attempted to impale It and haul it to shore using a large hook and heavy cable, but the crew could not pierce its skin. In a second attempt a month later a harpooner speared an animal, and men on shore hauled it in while others repeatedly stabbed it with bayonets. It was dragged into shallow waters, and the crew waited until the tide receded and it was beached to butcher it. After this, they were hunted with relative ease, the challenge being in hauling the animal back to shore. This bounty inspired maritime fur traders to detour to the Commander Islands and restock their food supplies during North Pacific expeditions.

Interaction with aboriginals

Sea otters are keystone species and keep sea urchin populations in check. Its depopulation in the Aleutian Islands may have led to the decline of kelp and subsequently of sea cows.

The presence of Steller's sea cows in the Aleutian Islands may have caused the Aleut people to migrate westward to hunt them. This possibly led to the sea cow's extinction in that area, assuming the animals survived in that region into the Holocene epoch, but there is no archaeological evidence.

It has also been argued that the decline of Steller's sea cow may have been an indirect effect of the harvesting of sea otters by the area's aboriginal people. With the otter population reduced, sea urchin population would have increased, in turn reducing the stock of kelp, it's principal food. In historic times, though, aboriginal hunting had depleted sea otter populations only in localized areas, and as the sea cow would have been easy prey for aboriginal hunters, accessible populations may have been exterminated with or without simultaneous otter hunting. In any event, the range of the sea cow was limited to coastal areas off uninhabited islands by the time Bering arrived, and the animal was already endangered.

One factor potentially leading to extinction of Steller's sea cow, specifically off the coast of St. Lawrence Island, was the Siberian Yupik people who have inhabited St. Lawrence island for 2,000 years. They may have hunted the sea cows into extinction, as the natives have a dietary culture heavily dependent upon marine mammals. The onset of the Medieval Warm Period which reduced the availability of kelp may have also been the cause for their extirpation in that area.

Later reported sightings

Sea cow sightings have been reported after the official 1768 date of extinction. According to Lucien Turner, an American ethnologist and naturalist, the natives of Attu Island reported that the sea cows survived into the 1800s, and were sometimes hunted. A sighting report was made by a passenger on the KruzenshternSzablon:'s world voyage between 1803 and 1806, near the Nordenskiöld Archipelago.

In 1963 the official journal of the USSR's Academy of Sciences published an article announcing a possible sighting. The previous year the whaling ship Buran had reported a group of large marine mammals grazing on seaweed in shallow water off Kamchatka, in the Gulf of Anadyr. The crew reported seeing six of these animals ranging from 6 do 8 metrów (20 do 26 ft), with trunks and split lips. There have also been alleged sightings by local fishermen in the northern Kuril Islands, and around the Kamchatka and Chukchi peninsulas. These sightings may have been mistaken identifications of extant arctic marine mammals such as the narwhal (Monodon monoceros) and the northern elephant seal (Mirounga angustirostris).

Commercial value

Steller's sea cow was described as being "tasty" by Steller; the meat was said to have a taste similar to corned beef, though it was tougher, redder, and needed to be cooked longer. The meat was abundant on the animal, and slow to spoil, perhaps due the high amount of salt in the animal's diet effectively curing it. The fat could be used for cooking and as an odorless lamp oil. The thick, sweet milk of female sea cows could be drunk or made into butter, and the thick, leathery hide could be used to make clothing such as shoes and belts and large skin boats sometimes called baidarkas or umiaks.

Towards the end of the 19th century, bones and fossils from the by then extinct animal were valuable and often sold to museums for high prices. Most were collected during this time, limititing trade after 1900. Some are still sold commercially, as the highly dense cortical bones are well-suited for making items such as knife handles and decorative carvings. Because the sea cow is extinct, native artisan products made in Alaska from this "mermaid ivory" are legal to sell in the United States and do not fall under the jurisdiction of the Marine Mammal Protection Act (MMPA) or the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), which restrict the trade of marine mammal products. Although the distribution is legal, the sale of unfossilized bones is generally prohibited and trade in products made of the bones is regulated because some of the material is unlikely to be authentic and probably comes from other arctic cetaceans.

Portrayals in media

Kotick the white seal talking to sea cows in Rudyard Kipling's The Jungle Book (1895)

In the story The White Seal from The Jungle Book by Rudyard Kipling, which takes place in the Bering Sea, "Kotick" the rare white seal consults Sea Cow during his journey to find a new home.

Tales of a Sea Cow is a 2012 film by Icelandic-French artist Etienne de France "documenting" a fictional 2006 re-discovery of a population of Steller's sea cows off the coast of Greenland. The film has been exhibited in art museums and universities in Europe.

Steller's sea cows appear in two books of poetry: Nach der Natur (1995) by Winfried Georg Sebald, and Species Evanescens (2009) by Russian poet Andrei Bronnikov. Bronnikov's book depicts the events of the Great Northern Expedition through the eyes of Steller; Sebald's book looks at the conflict between man and nature, including the extinction of Steller's sea cow.