Steller's sea cows were larger than many modern whales

No verified soft tissue of the Steller's sea cow survives making reconstructions of the animals, such as this one from 1910, difficult © Florilegius/NHM Images

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Steller's sea cow: the first historical extinction of a marine mammal at human hands

Within just 27 years of being formally described, humans had completely eradicated a marine mammal unlike anything seen today.

Steller's sea cows were extraordinary creatures.

Their closest living relatives are the dugong and manatees, known collectively as the sirenians. But while all four surviving species of sirenian live in warm tropical waters, Steller's sea cow had become highly specialised to the sub-Arctic waters of the northern Pacific Ocean.

This specialisation included growing to incredible sizes: adults could reach up to 10 metres in length while weighing up to 11 tonnes, bigger than many modern whales. To put this into perspective, an adult male killer whale can come in at eight metres long and weigh up to six tonnes.

Richard Sabin, Principal Curator of Mammals at the Museum, says, 'Steller's sea cow, Hydrodamalis gigas, is unusual for a modern mammal in as much as we know little of it from a true natural history perspective.

The sea cows skull in comparison with a dugong skull

Comparing a Steller's sea cow skull (right) with that of a modern dugong (left) gives a good idea of just how big they were © The Trustees of the Natural History Museum, London

'They were subject to very little detailed scientific investigation, so we can only really guess as to their biology and behaviour. We can use the dugong as a living example of a closely related species, to make assumptions as to how these animals were fitting in or contributing to their ecosystems, but there were limited methodological observations of the sea cows and so we have little solid data.'

Now, our chances to study them alive are long gone, as they were hunted into extinction by 1768. 

Fashion victim

It was Europeans' insatiable desire for beautiful fur hats and coats that led to Steller's sea cow becoming an incidental victim of the international fur trade.   

'It was like the gold rush,' explains Richard. 'The demand for fur increased because Europeans had colonised areas in North America that were then found to have great reserves of fur-bearing animals, ripe for exploitation.'

Even though the Steller's sea cow was not the primary target, what sealed their fate was the discovery by Russian fur traders of huge numbers of sea otters living around the islands scattered from Japan across what is now the Bering Sea and down into North America. This was the exact same habitat of the sea cows.

Steller's sea cow lived around the Comander Islands in the Bering Sea

The sea cows were only found living around the Commander Islands © The Trustees of the Natural History Museum, London

'The shallow waters around the Commander and Aleutian islands were primarily the feeding areas for the Steller's sea cow,' says Richard. 'It was unfortunate that the sea cows lay in an area along a route that was necessarily used by the hunters moving between North America and Russia.

'It's a long, cold journey and if you suddenly find that you are able to get your hands on fresh meat, you're going to take that opportunity.'

So as the traders made their way from Russia and China across to North America to hunt the otters, they would stock up on the meat of the Steller's sea cow to tide them over. 

The perfect storm

It was on one of these Russian expeditions, captained by Vitus Bering - who would eventually give this stretch of water his name - that German zoologist Georg Wilhelm Steller first came across the marine mammals.

He would become the first and only scientist recorded to have seen the animals alive, having formally 'discovered' the sea cows in 1741 when the ship on which he was travelling became marooned on what is now Bering Island. It is after Georg that the sea cows are now named.

The Steller's sea cow was hunted into extinction by fur traders

At 10 metres long and weighing up to 11 tonnes, it was reported that a single sea cow could feed 33 men for an entire month © Leonhard Stejneger (1851 – 1943)/Wikimedia Commons

Almost everything we know about the animals' behaviour and ecology stems from his brief observations of them, more frequently than not as they were being slaughtered and dragged up the beaches to be butchered.

Steller noted how the animals were highly gregarious, gathering in large groups as they browsed on kelp in the shallow waters surrounding the islands. 

Unfortunately, this social behaviour likely worked against them. Steller recounted that when sailors targeted a female sea cow, a male swam after the boat, trying to ram it. The male then followed the boat all the way to shore, even once the female had died. 

Their only protection from the hail of harpoons was also one of their main adaptations to their lives in cold water.

Steller noted that the sea cows' blubber was astonishingly thick, reaching up to 10 centimetres in places. He also wrote about how delicious it tasted, having something of a hint of almond to it.

The vertebra of a Steller's sea cow showing cut marks

The cut marks on this vertebra are a poignant reminder of the sea cow's demise © The Trustees of the Natural History Museum, London

It seems that almost every aspect about these animals contributed to their decline. From their diet of kelp that forced them into shallow water, their social behaviour that put surviving sea cows in further danger, or the thick blubber that not only meant that buoyancy was always an issue but also made them apparently just so delectable.  

By 1768, less than three decades after they were first described, the Steller's sea cow was extinct.

Learning from extinction

The speed at which the mammals were driven to extinction suggests that, despite what fur traders may have thought at the time, the sea cows were probably never that numerous to begin with.

There is evidence to suggest that in the past the animals may have had a slightly wider distribution, possibly stretching down the western coast of North America. Those found in 1741 may simply have been a remnant population that, without the intervention of humans, had managed to survive. 

The rib of a Steller's sea cow, driven to extinction by humans in 1786

Many Steller's sea cow specimens made their way back to Russia, such as this rib gifted by the Academy of St Petersburg © The Trustees of the Natural History Museum, London

The sea cows had become incredibly specialised to their sub-Arctic environment.

They had lost their teeth, to be replaced by large horny pads perfect for eating kelp and not much else. Their bones had become astonishingly dense to help counteract the problems they faced with buoyancy, due to their huge amount of blubber. They even had a transparent third eyelid (a nictitating membrane) to help protect their eyes from damage underwater.

This all means they were a highly restricted species and likely only had a small population.

The fur traders probably never understood that the sea cow could be hunted out of existence, but it marks an important point in humanity's relationship with extinction.

Richard adds, 'What fascinates me most about the development of our awareness of extinctions caused directly by human actions, is at what point in our recent history did we realise - from a compassionate perspective and not an economic one - that numbers were decreasing and there were problems on the horizon?'

Unfortunately for Steller's sea cow, this realisation came far too late.

Richard says, 'They are held up as an example of the first sea mammal in modern times made extinct by human ignorance and greed.'