Steller's Sea Cow Was Already on the Road to Extinction Before Humans Arrived

The extinction of Steller's sea cow (Hydrodamalis gigas) has often been cited as an example of a species driven to extinction by human hunting. In 1741, Vitus Bering's Great Northern Expedition reached the Commander Islands and encountered this enormous marine mammal around Bering Island and Medny Island. Within less than 30 years, sailors and fur traders had hunted the animals extensively for their meat and fat, and the last known population disappeared during the second half of the eighteenth century.

Reconstruction model of Steller's sea cow (Hydrodamalis gigas)（Image source：Emőke Dénes， CC BY-SA 4.0 ）

Skeleton of Steller's sea cow（Image source：Daderot，CC0 1.0 ）

Illustration of Vitus Bering's Great Northern Expedition hunting Steller's sea cows on the Commander Islands（Image source：Sharko FS et al. (2021)， CC BY 4.0 ）

Steller's sea cow belonged to the order Sirenia, and its closest living relative is the dugong (Dugong dugon). The species once inhabited the coastal waters of the North Pacific during the Pleistocene and Holocene, with a range far greater than the one it occupied in the eighteenth century. During the glacial periods of the Late Pleistocene, sea level in the Bering Sea dropped substantially, creating extensive new coastal habitats. Because Steller's sea cows lived in shallow waters and depended on nearshore plant resources, these conditions provided them with abundant suitable habitat. Fossil evidence indicates that the species once ranged widely across the North Pacific, from Japan through the Aleutian Islands and as far as the coast of California in North America.

Distribution range of Steller's sea cow in the North Pacific during the Late Pleistocene（Image source：Sharko FS et al. (2021)， CC BY 4.0 ）

This period of prosperity did not last indefinitely. During the transition from the Late Pleistocene to the Early Holocene, global temperatures increased and sea levels rose rapidly as glaciers retreated. Large, continuous stretches of coastal habitat began to shrink and fragment, and the ranges of sirenians became increasingly divided. Steller's sea cow was a stenobiontic herbivorous marine mammal with relatively narrow habitat requirements. It fed primarily on brown algae, particularly kelp, and was adapted to the cold coastal waters of the Bering Sea region. Its anatomy also limited its ability to dive deeply. As suitable shallow-water feeding grounds declined, the animals had relatively little flexibility to relocate into other marine environments.

Researchers recovered DNA from the petrous bone of a Steller's sea cow specimen excavated on Bering Island and housed at the Museum of the World Ocean in Kaliningrad, Russia. The bone contained a relatively high proportion of endogenous DNA, allowing the researchers to obtain nuclear genomic data at approximately 25.4-fold coverage. Genomic analysis also showed that the specimen was female. Using this DNA, the researchers attempted to reconstruct population changes extending far beyond the lifetime of the individual itself.

Petrous bone of Steller's sea cow（Image source：Sharko FS et al. (2021)， CC BY 4.0 ）

Mammals are diploid, carrying two sets of chromosomes inherited from their parents. The paternal and maternal copies of the same genomic region can differ in sequence, and these differences contribute to what is known as heterozygosity. The degree of heterozygosity across the genome can therefore be used as an indicator of genetic diversity. When the researchers compared the two chromosome copies in this Steller's sea cow, they found that its genetic diversity was already quite low. The average autosomal heterozygosity was approximately 1.19 heterozygous sites per 1,000 nucleotides. This value was close to that seen in woolly mammoths (Mammuthus primigenius) that had undergone severe population decline. The last genetically inbred woolly mammoth population on Wrangel Island had a value of about 1.00, while a Late Pleistocene woolly mammoth from Oymyakon in Siberia had a value of approximately 1.25. The modern dugong, by comparison, has a substantially higher value of about 2.19. These results indicate that the last Steller's sea cow population on the Commander Islands retained relatively little genetic variation and may already have undergone a prolonged demographic decline before its final extinction.

Autosomal heterozygosity in different mammalian species: dugong (Dugong dugon); Oymyakon woolly mammoth (Mammuthus primigenius); Steller's sea cow (Hydrodamalis gigas); Wrangel Island woolly mammoth (Mammuthus primigenius); beluga whale (Delphinapterus leucas); walrus (Odobenus rosmarus); narwhal (Monodon monoceros); and polar bear (Ursus maritimus)（Image source：Sharko FS et al. (2021)， CC BY 4.0 ）

The genomic data also suggest that both Steller's sea cow and the modern dugong experienced a major population decline, with their effective population sizes reaching a low point approximately 400,000 years ago. Woolly mammoths and the Pleistocene Lena horse (Equus lenensis), in contrast, experienced at least two major population bottlenecks over a comparable timescale. However, methods that reconstruct demographic change from the genome of a single individual are better suited to identifying broad trends over tens or hundreds of thousands of years and cannot accurately resolve population fluctuations during the final few thousand years before extinction. The analysis therefore cannot determine exactly how many episodes of decline Steller's sea cow experienced shortly before it disappeared. What it does indicate is that by the time Paleolithic humans arrived in the Beringia region around 25,000–30,000 years ago, the species had already been undergoing a long-term population decline.

Paleolithic humans may still have placed some hunting pressure on Steller's sea cows, although paleontological and archaeological evidence for this remains limited. Among dozens of Holocene coastal archaeological sites in the Bering Sea region, Steller's sea cow remains have been reported from only a small number of localities. The researchers therefore argue that humans were unlikely to have initiated the species' population decline, which had begun hundreds of thousands of years earlier. By the eighteenth century, however, the situation had changed dramatically. When European sailors and fur traders began systematically hunting the surviving population on the Commander Islands, they encountered a remnant population whose range had already become severely restricted, whose genetic diversity was low, and whose survival had long been constrained by environmental change. Although the population may still have numbered around 2,000 animals, it may already have been close to the brink of extinction. Intensive hunting became the final blow.

Changes in Pleistocene coastlines, the loss of feeding habitat, fragmentation of the species' geographic range, and declining genetic diversity had all placed increasing pressure on Steller's sea cow long before Europeans encountered it. By that time, the species may already have lost much of the habitat and population resilience it once possessed. Human hunters ultimately delivered the last irreversible blow to a population that had already become exceptionally vulnerable.

Author: Shui-Ye You

Reference:

Sharko FS et al. (2021). Steller's sea cow genome suggests this species began going extinct before the arrival of Paleolithic humans. Nature Communications.