The earliest records revealing the presence of sirenians in the northern Pacific are from California and Baja California. It is believed that the existence of a Central American seaway during the Miocene provided a means for species which had previously occurred in the tropical waters of the Caribbean to eventually make their way to the Pacific.
In Our Region
Hydrodamalis cuestae is particularly interesting to study, because we have an example of another species in the same genus that only very recently became extinct, for comparison. Hydrodamalis gigas, the Steller's Sea Cow, is also now extinct, but was observed in 1741 and 1742 by the biologist Georg Wilhelm Steller, the only scientist to see it before it was hunted to extinction around 1768. The ship's naturalist and physician on board during Vitus Bering's second expedition to North America, Steller observed the animal while he was shipwrecked on Bering Island and described it in great detail.
Although Steller was unaware of it at the time, the presence of Hydrodamalis gigas off Bering Island was only as an evolutionary relict, a small population confined to a very restricted area of cold waters near the Kamchatkan peninsula.
This area was probably not the optimal habitat for the species. There is evidence that the sea cows were often injured or suffocated by ice floes and were not able to find enough food to stay healthy. This cold-water adapted species had in fact evolved from sirenian ancestors that had formerly occupied tropical warm waters along the California coast with its warm, shallow bays some 35 million years ago.
During the Miocene when Hydrodamalis cuestae was living off California, scientists believe that the profile of our coast was very different from today. The coast of California was fragmented into many shallow bays, estuaries and inland seas, which provided ideal habitat for the sirenians, unlike today's rocky exposed shore line.
Sirenian evolution over a period of approximately four million years allowed for their gradual adaptation to colder waters and a more northward range extension, accompanied by an increase in body size. Food items, too, probably changed from marine plants growing in sheltered waters, such as seagrasses, to kelps and other cold-tolerant algae growing in colder, less protected waters.
Because we have historical information about H. gigas, we know a good bit about the ecology of this animal. Although we cannot be sure, we can surmise that H. cuestae may have had similar habits. Hydrodamalis gigas lived on the surface, in shallow bays, constantly feeding on marine plants in the kelp beds. It was a slow-moving, massive animal, completely unafraid of humans.
Steller observed that the sea cows fed on algae near or on the surface, and may not in fact have been able to submerge the entire body, although they were apparently able to keep their heads under water for as long as five minutes.
Using their claw-like short front appendages, they were able to dig and tear algae off rocks, and to help propel themselves along in shallow water, in a motion that combined elements of swimming and walking.
Steller's sea cows were gregarious, and young ones were kept in the middle of the herd for protection. Babies were born year-round but most often in the fall. Normally one calf was produced, and the mother cow had only two mammae or milk glands.
These animals were extremely vulnerable to hunters, and their extinction was due to human predation. They were very large and slow-moving, not at all aggressive, had no hiding places, were found close to land, and were easy to kill with a simple spear that was stout enough to penetrate the hide.
People could often just wade out to them and kill them at will. The meat and fat made for good eating, and the unfortunate result was that the population of H. gigas was decimated in a period of only 27 years, with the slaughter of over 2000 animals.
Text: Margaret Dykens and Lynett Gillette