“But how could they have killed them all with just pointy sticks?”

This question, or a variation thereof, has been asked of me, seemingly whenever I bring up the concept of overkill as the likely cause of Pleistocene megafaunal extinction. As long as people have known about mammoths, mastodons, and Megatherium, they have wondered why they are no longer around. For a period it was fashionable to blame extinction on climate change: firstly the Noachian flood, and then later the meandering cycles between glacial and interglacial conditions that the science of geology had discovered. However, as knowledge of the oscillations of the climate improved, it soon became obvious that the megafauna had survived numerous switches between warm and fruitful, and cold and frigid, each time passing through unscathed. What was different about the end of the Pleistocene that caused so many to disappear? Even early on, a few brave souls, knowing how humans treated the dodo, the solitaire and the great auk, put forth humanity as the despoiler of a Pleistocene paradise. In the late 20^th century these ideas were eloquently condensed and elaborated upon by the great Paul Martin, who published his ideas on overkill: the idea that the megafauna were wiped out by a unique species of mammal. Us.

On the face of it, the evidence is overwhelming, the pattern is clear. Modern humans appear in a pristine environment; then all the largest mammals disappear. It happens at 60,000 years ago in Australia, it happens at 30,000 years ago in Europe, it happens at 10,000 years ago in the Americas. And there is more. Aside from the continental extinctions, megafaunal island extinctions in the prehistoric point the finger at us too. Continental mammoths go extinct everywhere by about 10,000 years ago, except on the isolated Arctic islands of Wrangel and St Paul. There, they hung on for a few thousand more years (until 1600BC on Wrangel) before going extinct at exactly the time when the first human inhabitants are known to have arrived. Similarly, the Caribbean was home to a number of ground sloth species, close relatives of the giants found in North and South America. While the giant Mylodon and Megatherium went extinct about 10,000 years ago, the island sloths survived well into the Holocene before finally joining the choir invisible, you guessed it, right after the first humans sailed there.

To be sure, it seems at first ridiculous, that animals with the size and power of our modern elephant or rhino, could be easily wiped out by humans with stone-age technology. People have been in Africa since the dawn of humankind, hunting elephants with lithics- why are they not extinct? This, to me, is the very crux of the question. In Africa, and south Asia, where humans have lived the longest, we still have megafauna. Those animals have evolved in step with us. As our abilities to hunt improved, so did their ability to avoid being hunted. Run or hide. Fight or flight. In essence, Africa and Asia were humanity’s proving ground, the battlefield for an arms race between megafauna and man. Outside of these regions, animals were naïve, unable to recognise us for the threat we were until it was too late.

Humans have so utterly conquered the globe that the idea that animals once had no experience of our ways, and were likely to greet us with curiosity or indifference is seen as strange. I can only think of perhaps two ecosystems where most animals remain naïve- the Galapagos archipelago and the Antarctic. Actually, there is one other ecosystem where megafauna are mostly naïve- interactions between people and cetaceans in the ocean show that despite our recent efforts at harvesting the majority of wild whales, they still regard us as insignificant and harmless. The history of whaling in the 20^th century has shown how fatal this naivety can be to megafaunal species. Whales had the entire ocean to escape into, yet a few species came within a hair’s breadth of extinction. Where a species habitat niche is more constrained the effects are necessarily more severe. This lesson has been learnt before.

Map of Kamchatka with Bering and Medny islands from Wikimedia Commons

Map of islands of the Bering strait mentioned in the text. Image from Crerar et al.

Georg Wilhelm Steller was a German naturalist with a need to prove himself. Like many scientists in the 18^th century, the ship-borne age of discovery also had a need for him, and he was accepted aboard an expedition to remote Kamchatka with Vitus Bering (after whom the strait is named). Scurvy and shipwreck eventually took most of the ship’s crew and they were stranded on the uninhabited Commander islands (Bering and Medny[Copper] islands) about 100 miles east of Kamchatka. Despite his parlous situation, Georg managed to survey the biology of the islands. His most spectacular (and probably life-saving) discovery was a previously unknown species of giant sirenian (the family to which sea-cows, manatees, and dugongs belong). The eight metre long Steller’s sea cow (Hydrodamalis gigas) was truly a gentle giant, it ate only kelp and harmed no-one.

The cows lived in herds in shallow water and exhibited no reaction to the humans who would walk up to them, pierce them with a billhook and drag them onto shore to be butchered. Steller observed that herd members came to the assistance of injured herd-mates, and that the survivors of monogamous pairings would lie close to the flensed remains of their partner. To the starving survivors of Bering’s expedition, the animals were manna from Heaven: Steller himself likened the sea-cow meat to beef and the blubber to “the best Holland butter”. There’s no doubt that the nutritious sea-cow kept the expedition survivors alive during the bitter winter. Bering died on the island, but Steller and some other crew managed to return to Russia and stories of their adventure reached far and wide. The news of the epicurean delights to be found in the Commander islands led directly to the overharvesting of Hydrodamalis by fur-hunting expeditions. The speed and rapacity with which this was achieved is frankly shocking. The Medny island population was extinct within nine years. The Bering island population within twenty-seven years.

Skeleton of Hydrodamalis gigas from Bering island in the Helsinki museum. Note the lack of carpal bones, which had been lost in this species. Image via Wikimedia Commons

Reconstruction of Hydrodamalis gigas based on Steller’s account of the living animal. Image via Wikimedia Commons

And so, a Pleistocene giant, that once ranged from California to Japan, had a last stand on the frigid islands of the north Pacific in the second half of the eighteenth century. The animal was huge, had a very specific habitat requirement, was naïve (“they are not afraid of man in the least” according to Steller), slow-moving, and more importantly slow-reproducing. It’s hard not to see the parallels between the story of Steller’s sea cow and the mammoth- both survived on inhospitable islands, while the rest of their species had long perished, before finally succumbing to human exploitation.

An interesting thought experiment is prompted by a recent paper that reports a previously unrecognised population of Hydrodamalis from St. Lawrence island in the Bering strait. A combination of radiocarbon dating, ancient DNA, and stable isotope analysis showed that these animals went extinct around the 11^th century, when Yupik groups colonised St.Lawrence. If the Commander island sea-cows had not been recorded by Steller, we would have an extinction pattern exactly analogous to the mammoth or ground sloths: continental extinction followed by prehistoric island extinction. Would we instead be arguing that Hydrodamalis gigas succumbed to the effects of climate change?

Hydrodamalis gigas rib collected on Bering island and showing evidence of pathology. Likely the result of piercing with a 3-pronged harpoon. The individual survived for some time after. Image from Burdin 2012

Postscript

The story of the Steller’s sea cow is a cautionary tale about exploitation of natural resources. In the 27 years that they were known to science, it is estimated that around seven hundred trappers and sailors passed through the Commander islands and killed the cows for food. Their hunting methods were so wasteful that despite each cow containing enough meat for 30-40 people, the entire population of about 2500 animals was slaughtered. Now, even in the death of extinction, their very bones are prized. Carvers and whittlers seek out the thick, dense, ribs of the sea-cow for their work. The bones have found a special niche in the crafting of expensive knife handles. It is through exploitation of this fossil resource that scientists first came to discover that a population had once lived on St Lawrence. Dr Lorelei Crerar noticed the sea cow ribs on display at a craft convention in Atlanta, Goergia and was intrigued by their origin.

Written by Ross Barnett (@DeepFriedDNA)

Further Reading:

Burdin, A. “Climate Change: A view through the prism of Steller’s sea cow extinction.” NPS Reports (2012) [FullText]

Crerar, L. D., A. P. Crerar, D. P. Domning, and E. C. M. Parsons. “Rewriting the History of an Extinction- Was a Population of Steller’s Sea Cows (Hydrodamalis Gigas) at St Lawrence Island Also Driven to Extinction?”. Biology Letters (2015).[FullText]

Domning, D. P., J. Thomason, and G. B. Corbett. “Steller’s Sea Cow in the Aleutian Islands.” Marine Mammal Science 23, no. 4 (2007): 976-83.[Abstract]

Forsten, A., and P. M. Youngman. “Hydrodamalis Gigas.” Mammalian Species, no. 165 (1982): 103.[FullText]

Guthrie, R. D. “Radiocarbon Evidence of Mid-Holocene Mammoths Stranded on an Alaskan Bering Sea Island.” Nature 429 (2004): 746-49.[Abstract]

Martin, P. S. “Prehistoric Overkill: The Global Model.” In Quaternary Extinctions: A Pregistoric Revolution, edited by P. S. Martin and R. G. Klein, 354-403. Tucson: University of Arizona Press, 1984.[Book]

Steadman, D. W. “Asynchronous Extinction of Late Quaternary Sloths on Continents and Islands.” Proceedings of the National Academy of Sciences of the U.S.A. 102, no. 33 (2005): 11763-68.[FullText]

Stejneger, L. “How the Great Northern Sea-Cow (Rytina) Became Exterminated.” The American Naturalist 21, no. 12 (1887): 1047-54.[FullText]

Steller, G. H. De Bestiis Marinis (1751) [Book]

Vartanyan, S. L., K. A. Arslanov, J. A. Karhu, G. Possnert, and L. D. Sulerzhitsky. “Collection of Radiocarbon Dates on the Mammoths (Mammuthus Primigenius) and Other Genera of Wrangel Island, Northeast Siberia, Russia.” Quaternary Research (2008).[FullText]