North Pacific Ocean: The range of Hydrodamalis gigas in historic times appears to have been limited to the coastal waters of the Komandorskiye and Blizhnie Islands in the Bering Sea. Accounts of sightings from other islands in the Bering Sea, along the northwest coast of North America and the northeast coast of Asia, in the Arctic Ocean and Greenland are difficult or impossible to confirm and generally discounted. Fossil evidence indicates that the past distribution of genus Hydrodamalis was much wider, including the coasts of Japan and North America. Fossil remains of Hydrodamalis cuestae are known from as far south as the southern coast of California.

Biogeographic Regions: arctic ocean (Native ); pacific ocean (Native )

The few first-hand accounts of Hydrodamalis that are available note several distinctive features. The animal was considerably larger than any other extant sirenian. Steller (1751) gives a length of 296 inches (7.5 meters) for a female specimen that he examined. Larger sizes have been suggested, but after examination of available skeletal material Domning (1978) estimated an upper size limit of about 7.9 meters. However, Domning (1978) also notes that the Bering Sea population appears to have occupied a sub-optimal habitat for the species that may have prevented individuals from reaching their maximum possible size. Published mass estimates range from 5400 to 11,196 kilograms. It has been reported that Hydrodamalis displayed sexual size dimorphism, but Domning (1978) could find no evidence to support this assertion.

Steller (1751) describes the head and neck as being short and weakly delimited from the rest of the body. Pinnae were absent, the nostrils were paired and located near the tip of the snout, and the eyes were relatively small. Many large, vibrissae-like bristles surrounded the mouth. Teeth were absent in adults, but the keratinous rostral pads found in other sirenians were retained in Hydrodamalis. The neck appears to have been more flexible than in other living sirenians and may have helped the animal feed over a wider area with less movement of the large body. The body tapered cranially and caudally, but the belly and sides were rounded and swollen looking. When healthy, the back was slightly convex. Hydrodamalis resembled other dugongids in having a whale-like fluke at the end of its tail. The skin of the animal was unusual in having a black, thickened, bark-like epidermal layer that may have protected it from abrasion against rocks in the shallows where it fed.

The forelimbs of Hydrodamalis are highly derived relative to the flipper-like limbs of most other sirenians. They lack phalanges and show several specializations for a distinctive style of locomotion. Steller (1751) described the forelimbs as being relatively short with a distinct hook-like shape. Several recent artistic reconstructions portray Hydrodamalis as having flipper-like limbs, but drawings made by observers who had seen live individuals support Steller's (1751) account. The epidermal layer was very thick on the limbs, and Steller (1751) describes Hydrodamalis using the limbs to pull itself along while feeding in shallow water.

Domning (1978) examined the skeletal structure of the forelimbs and pectoral girdle of Hydrodamalis. Almost all of the bones show extensive modification and changes in muscle originations and insertions that reflect a greater emphasis on parasagittal movements of the limb. Domning compares these specializations to those of graviportal mammals and tree sloths, animals that also have relatively straight limbs that move in a parasagittal plane. Hydrodamalis appears to have had a narrower, deeper chest in the area of the pectoral girdle than most other sirenians, bringing the limbs closer to the midline of the body and allowing greater fore-aft mobility of the limbs. This reconstruction strongly suggests that Hydrodamalis was specialized to "walk" along in the shallows while feeding, as described by Steller (1751).

Steller (1751) and other first-hand observers also describe Hydrodamalis as being unable to dive or even completely submerge its body. Sirenians generally have precise control of their buoyancy as a result of specializations of their skeleton, diaphragm and lungs (Domning and de Buffrénil, 1991). Domning (1978) speculated that increased buoyancy may have been indirectly selected for as a consequence of large body size because of corresponding increases in lung volume, intestinal volume and thickness of blubber. There may also have been a direct selective advantage to increased buoyancy because it would have reduced the area accessible to parasites, reduced drag when swimming, reduced heat loss to the water via conduction, and allowed Hydrodamalis to enter shallower waters to feed and escape predators. However, Domning (1978) disputes Steller's (1751) claim that Hydrodamalis could not dive, even if it spent most of its time floating.

Range mass: 5400 to 11196 kg.

Average mass: 8000 kg.

• Marine

Hydrodamalis is known to have occurred in cold, shallow, coastal marine waters rich in algae and sea grass. Herds were frequently found near the mouths of streams or rivers. Its range was restricted to islands in the Bering Sea during historic times, but extended to California and Japan during prehistoric times.

Habitat Regions: temperate ; polar ; saltwater or marine

Aquatic Biomes: coastal

Other Habitat Features: intertidal or littoral

First-hand accounts of the feeding habits of Hydrodamalis are often vague and contradictory. Based on Steller's (1751) descriptions of plants he saw Hydrodamalis eating, it appears that brown and red algae were its primary food sources, with sea grass a minor component of the diet (Domning, 1976; 1978; Anderson, 1995). Living sirenians are known to ingest brown algae in times of food shortage, but this does not appear to be a preferred food. Steller (1751) describes Hydrodamalis as feeding on parts of algae and sea grass growing near the surface or on rocks in the shallows. Seasonal food availability may have been a problem for the Bering Sea population, as Steller (1751) describes individuals losing enough weight during the winter months to cause their ribs and vertebrae to be visible under the skin.

 Many features of the feeding system of Hydrodamalis indicate that it had adapted to its unusual diet of soft kelps and algae. Perhaps the most obvious modification is the absence of teeth. Hydrodamalis retained the keratinous rostral pads found in other sea cows, and the presence of interlocking ridges and grooves on these pads as well as reinforcement of the rostrum may be evidence that the animal used these pads to masticate its food. There is also skeletal and myological evidence that suggests the cropping and mashing motions of the front of the mouth were emphasized at the expense of more traditional chewing movements in Hydrodamalis. In his account, Steller (1751) also describes the animal as masticating its food with the keratinous plates. Living dugongids finely chew sea grasses, but tend to swallow ingested algae relatively intact. Based on this observation, Domning (1978) hypothesized that Hydrodamalis may have simply ripped off pieces of kelp and swallowed them with little or no processing in the mouth. The great enlargement of the gut reported by Steller (1751) probably reflects the need for more thorough chemical digestion of food due to the lack of thorough mastication. The amount of rostral deflection seen in Hydrodamalis is consistent with an emphasis on surface feeding habits, and the highly mobile lips were used in gathering and cropping food, as in other extant sea cows. The claw-like forelimbs may also have been used to dislodge plant matter from rocks.

Few details are known of the mating system of Hydrodamalis. Steller (1751) describes them as monogamous, and mating activities appear to have been concentrated in the early spring. Offspring were observed to be born at anytime of year, but most births took place in early autumn. Females produced only one calf per breeding attempt. Steller (1751) inferred the length of gestation to be over one year.

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); viviparous

It is not known exactly when the last individual of Hydrodamalis died, but it appears likely that the species was extinct by 1768. Yakolev, a first-hand observer of Hydrodamalis, claims that an order was given to the headquarters of the outpost on the Komandorskiye Islands on November 27, 1755, prohibiting hunting of the sea cows (translated in Domning, 1978). However, he also notes that by this time Hydrodamalis was extremely rare.

Much has been written about the extinction of Hydrodamalis at the hands of humans. The hunting practices described in first-hand accounts are extremely wasteful. Often, hunters would simply wade out to an individual, spear it, and then allow the animal to swim off, hoping that it would later die and drift to shore. No sustained yield practices were used, and the low reproductive rate of the population, combined with its probable existence in a sub-optimal environment likely hastened the species' decline. Anderson (1995) has also noted that the intense hunting of sea otters on the Bering Sea islands may have contributed to the final extinction of Hydrodamalis. It is known that sea urchin populations can severely deplete sea grass and algae communities when otters are removed, and as this happened on the Bering Sea islands, the sea cows would have faced a new competitor for food. A similar course of events may have occurred 12,000-14,000 years earlier along the coast of Asia and North America as aboriginal peoples colonized the areas and began hunting otters and sea cows (Anderson, 1995).

IUCN Red List of Threatened Species: extinct

Hydrodamalis had no negative economic effects on humans.

Hydrodamalis was hunted primarily as a source of food. Steller (1751) describes the meat as being easily prepared and similar to beef in taste and texture. The blubber was useful for cooking and was also a source of lamp oil. The milk of harvested cows was consumed directly or made into butter. The thick, tough hide was used for shoes, belts and to make skin-covered boats.