The Steller’s Sea Cow, Hydrodamalis gigas, was first described by German naturalist and explorer Georg Wilhelm Steller in 1741, and became extinct 27 years later, presumably as a result of Human activities. As a Sirenian reaching over 10 m in length and weighed in excess of 10 metric tons and living in the subarctic waters around the Commander Islands, of the Russian Far East, Steller’s Sea Cow was a very unusual Animal (all other known Sirenian species are tropical or subtropical in distribution, and the largest reach about 4 m in length and weigh at most 1.5 tons). As with many other extinct Animals, understanding the biology of the Steller’s Sea Cow, has been difficult.
In a paper published in the journal Science Advances on 4 February 2022, a team of scientists led by Diana Le Duc of the Institute of Human Genetics at the University Medical Center Leipzig, and the Department of Evolutionary Genetics at the Max Planck Institute for Evolutionary Anthropology, Akhil Velluva, also of the the Department of Evolutionary Genetics at the Max Planck Institute for Evolutionary Anthropology, and of the Rudolf Schönheimer Institute of Biochemistry at the University of Leipzig, and Molly Cassatt-Johnstone of the Department of Ecology and Evolutionary Biology at the University of California, Santa Cruz, present the results of a study which attempts to understand the biology of the Steller’s Sea Cow by analysing genetic material from its bones.
Le Duc et al. extracted DNA from the bones of 12 Sea Cows from Bering Island, as well as two well preserved museum specimens (SNMB N51667 from the Braunschweig Natural History Museum and SC16.JK045 from the collection of the Kamchatka Branch of the Pacific Geographical Institute). Analysis of the mitochondrial DNA (mDNA) of the Bering Island bones confirmed that these were from different individuals, radiocarbon dated to 2205 to 1155 before the present. They were able to generate a comparative dataset consisting of 4877 orthologous genes, which was compared to previously published genomes of the Dugong, Dugong dugon, and Florida Manatee, Trichechus manatus latirostris.
It was found that two of the genes which code for arachidonate lipoxygenases were inactivated via premature stop codons in all the Steller’s Sea Cow individuals. In Humans, inactivation of either of these genes leads to a disease called congenital ichthyosis, in which the patient develops hyperkeratotic, dry, thickened, scaling skin. This is interesting, as Steller’s Sea Cows are described as having had skin resembling 'the bark of an old oak tree'. Since these genes were inactivated in all of the individuals examined, Le Duc et al. calculate that this trait was either fixed, or close to being fixed in the population, suggesting that it conveyed an evolutionary advantage to the Steller’s Sea Cow. Examination of the same genes in other Sirenians, Seals, Sea Otters, and Polar Bears found that they were still active, although they were inactive in Cetaceans (Whales and Dolphins). Cetaceans do not show scaly or bark-like skin, but they do shed their skin at a very high rate compared to other Mammals, which may make these genes irrelevant to them. The shedding of skin by Cetaceans is facilitated by the loss of functional genes for desmosomes (specialised adhesive protein complexes) in the upper skin layers. These genes were found to be functional in the Steller’s Sea Cows, which would account for their 'bark-like' skin.
The loss of arachidonate lipoxygenase genes in Cetaceans has been suggested as an adaptation to a life spent completely in a marine environment, along with the hair-loss seen in Cetaceans and Sirenians. However, the functioning arachidonate lipoxygenase genes seen in other, extant, Sirenians implies that moving to a marine environment is not enough, in itself, to make the loss of these genes advantageous, while the convergent loss of these genes in Cetaceans and Steller’s Sea Cow suggests that is does convey an advantage of some sort, and presumably one which applies to both Cetaceans and the Steller’s Sea Cow, but not to other Sirenian species.
The one notable difference in the habitat of the Steller’s Sea Cow and other Sirenians is the much colder waters that this species lived in, and while Cetaceans are found in all marine environments, many species do spend at least part of their lives in cold waters, and it is not unreasonable to postulate that the last common ancestor of all modern species might have lived in a cold climate.
Another gene that was found to be inactivated in both the Steller’s Sea Cow and Cetaceans, but not extant Sirenians or other Marine Mammals was NPFFR2, which encodes for neuropeptide FF receptor-2, which is linked to thermogenesis. Mice in which this gene is inactivated are known to show increased fat accumulation, and lose the ability to generate heat through the activation of brown adipose tissue. It is thought that brown adipose tissue thermogenesis is less important in larger Animals, which conserve heat better due to their lower body volume to surface area ratio. Steller’s Sea Cow and Cetaceans also show inactivation of the UCP1 gene, which has been linked to the regulation of thermogenesis by brown adipose tissue. In combination, this strongly suggests that both the Steller’s Sea Cow and Cetaceans have sacrificed brown adipose tissue thermogenesis in return for larger size and a greater development of fatty tissues, as an adaptation to living in cold climates.
A larger size and greater fat accumulation capacity also enables Animals to endure longer periods of fasting, which may also have been important for the Steller’s Sea Cow. The species is thought to have subsisted almost entirely upon Dragon Kelp, Eularia fistulosa, a species found throughout the North Pacific from May to September, but absent for the rest of the year. The Pleistocene fossil record of Steller’s Sea Cows suggests that they were once found from Japan to the Baja Peninsula in Mexico, a distribution which suggests the ability to disperse over long distances, and in particular making crossings between oceanic islands. Such crossings would involve being away from the species food source, and could conceivably have been made outside of the Kelp season by non-feeding Sea Cows.
Georg Wilhelm Steller described a population of about 1000 Sea Cows living around Bering Island during the winter of 1741–1742. If this is a typical population size, and the species once inhabited other Kelp forests around the North Pacific, then there may once have been as many as 200 000 individuals living across the west coast of North America, Kamchatka, and the Kurile Islands.
Le Duc et al. used the pairwise sequentially Markovian coalescent (a method which uses the genome sequence of a single individual to estimate demographic history covering a time span of thousands of generations) to estimate long-term population trends in the Steller’s Sea Cow. From this they estimate that the species had been in decline for around half a million years, possibly with some population stability during the Eemian Interglacial, between 130 000 and 80 000 years ago. This would suggest that, while Humans were probably the final push that drove the species to extinction, the population encountered by early European explorers in the Commander Islands was already a relict of a fast-declining species.
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