Sea Otters, Hunters, and Steller’s Sea Cows – Replaying a Recent Extinction

The nearly complete skeleton of a Steller’s sea cow (Hydrodamalis gigas) – it is missing bones from the wrist and hand. From Woodward, 1885.

It did not take long for the last remaining population of Steller’s sea cow to be driven into extinction. Discovered by the German naturalist Georg Steller around the Bering Sea’s Commander Islands in 1741, this enormous and peculiar sirenian became an easy target for Russian hunters. By 1768, it was gone. (The marine mammal would not be scientifically described until 1780, and today it is formally known as Hydrodamalis gigas.) Yet, despite the clear role of hunting in the disappearance of this species, some scientists have suggested that an ecological cascade triggered by the hunting of another species also helped tip the massive dugongs into extinction, and so scientists S.T. Turvey and C.L. Risley have dug back into earlier research about the sea cows to find out which factors were most important in their demise.

Figuring out why a species became extinct is often a tricky business, especially in distinguishing direct and indirect causes. In the case of Steller’s sea cow, it is hypothesized that they were easy targets for prehistoric hunters and extirpated from most of their range before Russian sailors began to lay into them. Hence hunting by humans obviously played an important, direct role in the extinction of Steller’s sea cow, but decreases among sea otter populations may have played an indirect role. Sea otters have long been highly prized for their thick pelts, and as they were killed (by both aboriginal people and Russian fur hunters) populations of sea urchins – among their most regular prey – exploded. The inflated sea urchin populations then fed on shallow-water kelp unhindered, and since Steller’s sea cow relied upon kelp for sustenance their decline may have been at least partly caused by these changes. The extinction of Steller’s sea cow would have been caused by a one-two punch of ecological destabilization and overhunting, both of which were attributable to human activities.

It is extremely difficult to test whether it was hunting alone or hunting plus ecological disturbance which led to the extinction of Steller’s sea cow. Most of what we know about the natural history of the animal comes from the notes of Georg Steller, and he did not keep exact counts of how many sea cows there were when he discovered them. Likewise, hunters did not keep precise details of how many sea cows they killed – we only have rough estimates of how much meat a sea cow yielded and how long that might last a crew. On the basis of these notes the 19th century naturalist Leonhard Stejneger tried to come up with rough estimates of how many sea cows were killed by each crew each year between 1743 and 1762, inflating the number to account for those which must have been killed but not eaten, and he presumed that the starting number of the population was less than 1,500 individual animals.

Although Stejneger’s back-of-the-envelope calculations were made over a century ago and have not been verified, Turvey and Risley combined them with what is known about the life history of living dugongs in an attempt to find out if hunting alone could account for the extinction of the sea cows. They estimated how sea cow populations would have been affected by different hunting models (i.e. non-wasteful hunting or hunting for immediate consumption only) and tried to figure out what a sustainable take of Steller’s sea cows would have been. As it turned out, the sea cows could have only sustained a very light level of hunting. Assuming Stejneger’s hypothesized starting point of 1,500 individuals, a sustainable harvest of the slow-reproducing mammals would have been only 17 individuals a year. This is far, far below the estimated yearly average take – about 123 individuals – and so it is little wonder that Steller’s sea cow was wiped out so quickly.

Additionally, the authors found that Stejneger’s estimate for the starting sea cow population was probably too low. If hunters killed sea cows at the rate he proposed, they probably would have only lasted until 1756. Given that we know hunters stuffed their holds with sea cow meat and probably killed far more than they actually needed, it is more likely that the starting population was around 2,900 animals. This was still a small number – the remnants of a once more widespread species – but it attests to the extreme amount of animals killed but left to rot in the cold sea.

Estimated Steller’s sea cow kills between 1743 and 1762 as proposed by Stejneger. In each bar, the white sections with slashes represent animals killed and consumed immediately, grey sections represent animals killed for immediately consumption but wasted, white sections represent animals killed for provisions, and the black sections represent animals killed for provisions but wasted. From Turvey and Risley, 2010.

Whether these estimates accurately represent what occurred and the broader implications that might have is another matter. The new estimates are directly based off of old estimates made by Stejneger which were based upon the limited information he was able to gather about the crews working around the Commander Islands. There is no doubt that there were more crews there than he could account for – he recognized himself that the records he had to work with were “very defective” in this regard – and this makes it all the more difficult to come up with an accurate understanding of the last days of the sea cows. Turvey and Risley’s new estimates are certainly interesting, but as estimates based upon estimates without any way to verify that these numbers are correct, it is hard to know if they were right. Indeed, tracking the decline of the sea cows depends upon knowing how many there were to start with and the rate at which they were being killed, both of which are based upon assumptions which can’t be verified.

Despite my reservations about the data used to come up with the new estimates, however, there is no reason to doubt that hunting played a major role in the extinction of Steller’s sea cow. From studies of their living relatives and other large mammals we can safely hypothesize that they would have reproduced slowly, and if hunters were killing numerous individuals to provision their ships (with animals which were killed but never eaten added to this number) it is reasonable that hunting was the chief culprit behind the extinction. As the authors state, the proposed ecological effects of the sea otter decline/sea urchin rise may have still occurred, but its influence on populations of sea cows between 1741 and 1768 appears to have been minimal compared to hunting. It may be more profitable to explore this hypothesis for the time period before the 1740’s, during the time when sea cow populations were disappearing, but such details are not considered in the new paper.

The rapid extermination of such large mammals by small groups of humans using unsophisticated weapons (i.e. harpoons) leads Turvey and Risley to suggest that the human-caused extinction of the sea cow throws credence to the controversial “overkill hypothesis” for the extinction of the Pleistocene megafauna. If, like the sea cows, large Pleistocene mammals became restricted to small populations then they could be quickly wiped out by small bands of people. This is certainly plausible, but the picture may be too narrow. Even if humans delivered the ultimate coup de grace to some megafaunal species, we must ask why those species became restricted to refugia or scattered in smaller populations in the first place. Ecological changes driven by climate change may have made some species more extinction prone, and the ecological consequences of the removal of large species – which often play a role in shaping local ecology – are just beginning to be understood. The caricature of “humans show up, large animals go extinct” does not work and masks more complex interactions which we do not yet fully understand (such as the extinction of species which show no sign of being hunted by humans and the changes in populations of small mammals). Arguing over single causes to explain the whole of the last extinction may be fertile fodder for pop sci articles and documentaries, but, in the case of the end-Pleistocene mass extinction, it seems that there were an intertwining array of factors which varied from continent to continent, from the dispersal of humans to the development of a warmer, wetter global climate. Perhaps, as some of the intense rhetoric surrounding the issue dies down, more nuanced approaches will help us better understand why some of the last great mammals disappeared..

Turvey, S., & Risley, C. (2006). Modelling the extinction of Steller’s sea cow Biology Letters, 2 (1), 94-97 DOI: 10.1098/rsbl.2005.0415