Steller's Sea Cow-Hydrodamalis gigas

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Steller's Sea Cow-Hydrodamalis gigas


An illustration of Hydrodamalis gigas being hunted by humans (Homo sapiens). Artist unknown.

Temporal range: Holocene (until 1768 CE).

Scientific classification:

Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Superphylum: Deuterostomia
Phylum: Chordata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Clade: Theriodontia
Clade: Eutheriodonta
Suborder: Cynodontia
Clade: Epicynodontia
Infraorder: Eucynodontia
Parvorder: Probainognathia
Superfamily: Chiniquodontoidea
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Clade: Eutheria
Infraclass: Placentalia
Magnorder: Atlantogenata
Superorder: Afrotheria
Clade: Paenungulata
Clade: Tethytheria
Order: Sirenia
Family: Dugongidae
Subfamily: †Hydrodamalinae
Genus: †Hydrodamalis
Species: †H. gigas

Steller's sea cow (Hydrodamalis gigas) was the largest sirenian that ever lived.

Description:
H. gigas was ~9 meters in length[1] and weighed ~8-10 metric tons.[2] Its forelimbs were used for swimming, locomotion in more shallow areas, etc. The animal's skin was thick and tough to the point of being described by Georg Wilhelm Steller as "more like unto the bark of an ancient oak than unto the skin of an animal". Furthermore, it was also described as "black, mangy, wrinkled, rough, hard, and tough; it is void of hairs, and almost impervious to an ax or to the point of a hook".[3] Steller's sea cow could not swim fast nor could it submerge.[4]

Feeding:
The jaws had no teeth; instead there were bristles that pulled out seaweed and held food. Two pieces of bone were used to chew.[3] Kelp was a food source for the sea cow.[4]

Extinction:
<1,500 Steller's sea cows were left when Steller described them in 1741.[5] These particular sea cows lived in the Commander Islands. Human hunting probably caused it to become extinct elsewhere in previous times; its survival in the Commander Islands is implied to be due to the place being uninhabited by humans. Aboriginal hunters likely not only hunted sea cows, but also overexploited sea otters that fed on urchins: without otters keeping urchin numbers in check, increasing urchin numbers would have deprived sea cows of food.[6] When news of the Steller's sea cow spread in 1742, it was hunted once again. Apparently, "new hunting expeditions were formed almost every year".[7] Likewise, sea otters were also hunted extensively, increasing the population of sea urchins and thus decreasing the abundance of kelp, which formed the staple diet of Steller's sea cow.[6] An assessment of whether or not hunting by itself could have caused the extinction of Steller's sea cow was made; judging from how quickly the animal went extinct on Bering Island it was indeed more than enough to cause its extinction, let alone with the aid of other factors.[8] Overall, the Steller's sea cow's extinction was a result of human hunting and was facilitated by the reduced abundance of food supply (which was also-albeit indirectly-anthropogenic). Steller's sea cow was extinct by 1768, 27 years after it was first described.[8]


Mounted skeleton of H. gigas.

References:

[1] Helene Marsh, Thomas J. O'Shea, & John. E. Reynolds (2012). Ecology and Conservation of the Sirenia: Dugongs and Manatees.
[2] "The Weight of the Steller Sea Cow" (Scheffer, 1972).
[3] Georg Wilhelm Steller (1751). De Bestiis Marinis, or, The Beasts of the Sea.
[4] Richard Ellis (2004). No Turning Back: The Life and Death of Animal Species.
[5] Caryn Self-Sullivan (2007). "Evolution of the Sirenia".
[6] "Competition, Predation, and the Evolution and Extinction of Steller's Sea Cow, Hydrodamalis gigas" (Anderson, 1995).
[7] www.sirenian.org/stellers.html
[8] www.iucnredlist.org/details/10303/0

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Steller’s sea cow genome suggests this species began going extinct before the arrival of Paleolithic humans

Anthropogenic activity is the top factor directly related to the extinction of several animal species. The last Steller’s sea cow (Hydrodamalis gigas) population on the Commander Islands (Russia) was wiped out in the second half of the 18th century due to sailors and fur traders hunting it for the meat and fat. However, new data suggests that the extinction process of this species began much earlier. Here, we present a nuclear de novo assembled genome of H. gigas with a 25.4× depth coverage. Our results demonstrate that the heterozygosity of the last population of this animal is low and comparable to the last woolly mammoth population that inhabited Wrangel Island 4000 years ago. Besides, as a matter of consideration, our findings also demonstrate that the extinction of this marine mammal starts along the North Pacific coastal line much earlier than the first Paleolithic humans arrived in the Bering sea region.

This paper also says something interesting about the size of this animal. Although the species could weigh up to 10 metric tons (or 11 short tons), the ones found on the Commander Islands were much smaller.

H. gigas was up to 11 tons in mass and up to 10 meters in length; however, the adult animals from the last population that was discovered on the Commander Islands were significantly smaller. Biologist Georg Wilhelm Steller (1709–1746), who participated in the Vitus Bering’s Expedition, stated that these sirenian species weighted around 4500–5900 kg and had a body length of ~750 cm.

www.nature.com/articles/s41467-021-22567-5

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Here's a bit more information about the feeding apparatus of the sea cow. The "two pieces of bone" mentioned in the OP actually refers to two keratinous pads on the rostrum and mandible. These horny pads were outfitted with diagonal ridges that likely helped mash kelp. You can kind of see them reconstructed on this skull.


Image source->

Here's another picture (with only the rostral pad).


Image source->

And a more detailed illustration.




Arguably even weirder are the forelimbs of this creature. According to Steller, no traces of phalanges could be found, and the forelimbs ended with the carpals and metacarpals. The fact that we have never recovered any manus elements from H. gigas (Mattioli & Domning, 2006), and the fact that its close relative Dusisiren had vestigial phalanges (Takahashi et al., 2006), supports this claim and suggests H. gigas could have lost them entirely. They were reported to have thickened coverings and stiff bristles, with the metacarpals forming a "claw-like" "hand". These forelimbs apparently had a variety of functions: "walking" in the shallows, serving as holdfasts on the rocky nearshore areas it inhabited (which were subject to strong waves), digging and pulling out seaweed from the rocks, fighting*, and for clasping mates (Marsh et al., 2012).

*I don't know how these forelimbs could be used for fighting. Steller mentions that when taken by a hook and dragged from the water, it would use its forelimbs to resist. So much so, in fact, that the cuticle on its arms would get torn off. It's possible Steller was just referring to that.


Lastly, despite the presence of pachyosteosclerotic bones (i.e. combination of thickened and denser bones), the positive buoyancy of this sirenian was likely a result of its specific gravity (i.e. relative density) being less than that of a neutrally buoyant sirenian. Not only was it huge (mass increases by the cube, the mass of the bones hardly increases more than the square), but it also had a massive belly with large intestines, as well as large lungs (Marsh et al., 2012).


This was no mere scaled up dugong or manatee. Even without its size this was a highly unusual creature in its own right.

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Genomic basis for skin phenotype and cold adaptation in the extinct Steller’s sea cow

Steller’s sea cow, an extinct sirenian and one of the largest Quaternary mammals, was described by Georg Steller in 1741 and eradicated by humans within 27 years. Here, we complement Steller’s descriptions with paleogenomic data from 12 individuals. We identified convergent evolution between Steller’s sea cow and cetaceans but not extant sirenians, suggesting a role of several genes in adaptation to cold aquatic (or marine) environments. Among these are inactivations of lipoxygenase genes, which in humans and mouse models cause ichthyosis, a skin disease characterized by a thick, hyperkeratotic epidermis that recapitulates Steller’s sea cows’ reportedly bark-like skin. We also found that Steller’s sea cows’ abundance was continuously declining for tens of thousands of years before their description, implying that environmental changes also contributed to their extinction.

www.science.org/doi/full/10.1126/sciadv.abl6496

[Exalt]

A question: I'm by no means a physicist, but if the animal had "positive buoyancy" doesn't that mean that it would have to go to a lot of effort to stay in one spot in order to feed?

Additionally, this isn't so scientific but if it had similar behavior to it's cousins, could this be the truest case of a "gentle giant" in the animal kingdom?

I ended up reading a bit into Sirenians and how they are predated on so rarely: it's very sad how this animal may have had no ability to defend itself from humans, in spite of it's size.

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A question: I'm by no means a physicist, but if the animal had "positive buoyancy" doesn't that mean that it would have to go to a lot of effort to stay in one spot in order to feed?

It would have stayed near the surface of the water where giant kelp canopies were. That way it could just browse on kelp without having to put in any real effort to be submerged.

[Exalt]

Maybe I should have thought to look into how ocean plants and shallow sea ecosystems work first. Thank you for answering.

[Exalt]

One more thing that I meant to ask: is the sea cow the largest known aquatic herbivore, or does another animal earn that title?

[Exalt]

Steller apparently once observed that a Sea Cow rammed a boat carrying his mate, and it stayed on the beach that the boat shored at for three days, so perhaps if it's size wasn't enough to deter antagonistic animals, it could do this.

Both Steller and modern scientists apparently suggest that the SSC was on the decline even before Europeans began hunting it, and much of it's range has already shrunk. In it's heyday, what might it have shared ecosystems with? I can't say I know much about cenozoic seas beyond whale evolution, episode 2 of walking with beasts, and some species that I've learned about from the Pisco formation.

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Aug 20, 2023 22:41:26 GMT 5 Exalt said:

Steller apparently once observed that a Sea Cow rammed a boat carrying his mate, and it stayed on the beach that the boat shored at for three days, so perhaps if it's size wasn't enough to deter antagonistic animals, it could do this.

Both Steller and modern scientists apparently suggest that the SSC was on the decline even before Europeans began hunting it, and much of it's range has already shrunk. In it's heyday, what might it have shared ecosystems with? I can't say I know much about cenozoic seas beyond whale evolution, episode 2 of walking with beasts, and some species that I've learned about from the Pisco formation.

Well, when Bering Island was discovered by Europeans, the island not only had sea cows, but also what were apparently relict populations of sea lions, fur seals, and sea otters. The sea otters being there is consistent with something I read previously where the overhunting of sea otters may have led to the proliferation of sea urchins that ate the kelp the sea cows ate (exacerbating its extinction).

[Exalt]

Yeah, I saw some of that, and I watched a video discussing keystone species, and Otters seem to still be an important species in their ecosystems today.

Also, I watched a video just a bit ago of a Polar Bear attempting to hunt Walruses, and I can see now why the Sea Cows might not have had trouble with species that aren't weird apes that have access to metal.

As for the Sea Cow's range, it apparently did once occupy parts of the ocean west of North America, but I'm not sure what else.