As the world’s woolly mammoth populations slowly dwindled during the last Ice Age, a unique subset living on Wrangel Island in the Arctic Ocean appeared to thrive – that is, until a “fairly abrupt” extinction event wiped out the massive beasts some 4,000 years ago.
Today, Wrangel Island is a small, isolated Russian landmass in the Arctic Sea. During the time woolly mammoths (Mammuthus primigenius) roamed the planet, this 125-kilometer-wide (78 miles) island was once part of the Bering Land Bridge that connected Beringia, a landmass that extended into the Bering Sea, with modern-day Alaska and Canada. As temperatures rose in the years following the ice age about 10,000 to 15,000 years ago, this bridge was slowly engulfed by increasing sea levels, essentially trapping some woolly mammoths on Wrangel Island. For 7,000 years, woolly mammoths thrived here despite species extinctions around the world.
Then, something mysterious happened. Radiocarbon dating of mammoth remains found on the island suggests the species died out abruptly. To determine the catalyst of such an event, researchers analyzed the isotopic composition of carbon, sulfur, and strontium in 77 wooly mammoth specimens, 52 of which were from Wrangel Island. This allowed them to document changes in diet that indicate how their prehistoric habitat might have changed. The results were then compared against previously published data for mammoths from Eurasia and St. Paul Island in Alaska.
Collagen and isotope compositions found in Wrangel Island mammoths did not change as the climate warmed 10,000 years ago and remained unchanged until the mammoths disappeared. But their results found differences in the carbohydrate and fat content of Wrangel Island mammoths when compared to their Siberian counterparts.
"We think this reflects the tendency of Siberian mammoths to rely on their reserves of fat to survive through the extremely harsh ice age winters, while Wrangel mammoths, living in milder conditions, simply didn't need to," said study lead author Laura Arppe, from the Finnish Museum of Natural History Luomus, University of Helsinki, in a statement, adding that this might have meant Wrangel Island mammoths were not as well equipped for extreme weather events that occur in the Arctic.
Wrangel Island mammoth bones also had traces of sulfur and strontium, suggesting that extreme weather events might have broken down rocks on the island, which in turn could have impacted the quality of freshwater sources. Altogether, the researchers suggest that Wrangel Island mammoths likely died from short-term events, like extreme rain or snow that might have covered the ground in a layer of ice, creating a barrier to their main food source. Further DNA analysis suggests that the isolated mammoths were also inbreeding.
“It's easy to imagine that the population, perhaps already weakened by genetic deterioration and drinking water quality issues could have succumbed after something like an extreme weather event," said study co-author professor Hervé Bocherens. The authors are quick to add that early humans may have played a role. The earliest evidence of humans on Wrangel Island was just a few centuries after the most recent mammoth death.
Understanding how the risk of extreme environmental influences and human behavior affects isolated small species populations can inform our understanding of the modern world, write the authors in the journal Quaternary Science Reviews.