What really happened during the final days of the woolly mammoths?

The mysterious extinction of an isolated population of mammoths on an Alaskan island reveals the impact of rising sea levels.

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A barren island perched between Russia and the United States may not be the first place that comes to mind when considering the fateful battlegrounds where mammals have confronted extinction throughout history. However, for a team of geoscientists, anthropologists and evolutionary biologists, Alaska’s Saint Paul Island – situated in the volcanic Pribilof archipelago of the Bering Sea  has become the key to understanding the final days of the woolly mammoth.

The Ice Age giants roamed North America, Europe and Asia until around 10,500 years ago, inspired the cave paintings of Neolithic period Homo sapiens, and now, according to new research, had a population that managed to survive well beyond the rise of global temperatures that brought an end to the last glacial period.

Saint Paul Island, once part of a land bridge connecting Russians and Americans, found itself distanced from the two countries by rising sea levels, diminishing in size until around 6,000 years ago. For the scientists, there were two unanswered questions hanging over the Alaskan mammoths trapped on the island: how did they survive for millennia on an isolated bit of land? And what brought about their eventual demise?

The prolonged survival of the mammoths is a mystery yet to be solved, but what has been discovered is that they were desperately running out of drinking water; engulfed by rising sea levels, they were dying of thirst.

The study published in the Proceedings of the National Academy of Sciences was led by Pennsylvania State University’s Russell Graham. By analysing newly collected mammoth remains from a cave on the island, the survival of the mammals until around 5,600 years ago was confirmed, separating the causes of survival and extinction of this particular population of woolly mammoth from their mainland counterparts, who died out long before.

To determine the factors that abruptly ended the woolly mammoths’ survival at that point 5,600 years ago, a number of indicators of extinction were looked at – none of which, according to the researchers, were found to be key extinction drivers on the island.

In 2013, the researchers were drawn to a freshwater lake around the centre of Saint Paul – one which held sediment cores filled with information (such as ancient DNA, fungal spores, pollen and plant remains) correlating to the presence of the megafaunal species. These cores coupled with geographical and environmental data indicated that during the mammoths’ time on Saint Paul Island, vegetation remained stable, volcanic activity minimal. There was no fight for food or fiery volcanic eruption contributing to the extinction.

Graham’s scientists also found that the absence of humans on the island prior to the settling of the Aleut people and the arrival of Russian fur traders and whalers in 1788 meant human presence wasn’t a factor, though it did make it a perfect location to explore what drove survival and extinction of a species in the absence of humans. For the team, the mystery thickened.

It became increasingly clear that what ultimately caused the extinction was not the evasion of the effect of climate change at the end of the Ice Age, but the delayed onset of it. The researchers found that reduced availability of freshwater, rising sea levels and increased competition as a result of shrinking land mass caused the dwindling of the population and set extinction in motion.

According to the research, when sea levels rise there is a posed threat of freshwater becoming scarce; as noted by the paper, “a warming climate causes lakes to become shallower, leaving the animals unable to quench their thirst”. Salt water overwhelmed many lakes and reservoirs on the island which were already relatively shallow to begin with, cutting the mammoths off further and further from drinkable water.

Speaking to the BBC, Graham said:

“As the other lakes dried up, the animals congregated around the water holes. They were milling around, which would destroy the vegetation – we see this with modern elephants. And this allows for the erosion of sediments to go into the lake, which is creating less and less fresh water. The mammoths were contributing to their own demise.”

Although the last known woolly mammoths on the planet lived circa 4,000 years ago on Wrangel Island, the end of the Saint Paul group highlights just how crucial freshwater availability  or lack thereof  is to the perseverance of mammalian species.

For Graham and his team, the findings carry a stark warning about global warming. Rising sea levels mean “small island populations” of humans and animals could face difficulties similar to the ones faced by the Saint Paul Island woolly mammoths; there is a cruel irony to fearing shortages of freshwater when swamped by the salty waters of the sea.