For thousands of years after they had died out on the mainland, woolly mammoths survived on two Pacific islands. A new study concludes that the drier local climate meant that at least one island no longer had the freshwater supplies needed to support these massive beasts.
The combination of the ending of the last Ice Age and predation by humans was enough to wipe mammoths out from most of the territory they once dominated. However, Wrangel and St. Paul Island remained very cold and likely uninhabited by humans until the 18th century, allowing a small population of mammoths to survive half the current interglacial period.
Using five independent indicators, a team led by Professor Russell Graham of Pennsylvania State University concluded that the last of the St. Paul mammoths died out about 5,600 years ago. The Wrangel population survived another 1,600 years.
At just 110 square kilometers (43 square miles), one-seventieth of Wrangel's size, St. Paul is the more surprising mammoth refuge.
In Proceedings of the National Academy of Sciences, Graham and co-authors report on the mammoth's unlikely story from the point around 14,000 years ago, when rising sea levels isolated St. Paul from mainland Alaska.
The youngest St. Paul mammoth specimen the authors could find was 5,530 years old, which is just after the last mammoth DNA was found in island sediments. Pollen and marine species in the same sediments indicate a change around the same time, consistent with the disappearance of the only great grazer.
Between 7,850 and 5,600 years ago, lakes on St. Paul became shallower and the water more turbid, with substantial effects on the plankton living in them.
The authors conclude that the island was drying up, leaving the mammoths with too little water to slake their mighty thirsts. “Freshwater availability on oceanic islands is an under-appreciated, but important, driver of vertebrate mortality,” they write. “On Mauritius, a mass mortality event 4,150 years ago coincided with a severe drought that concentrated animals near increasingly toxic and saline lakes.”
Elephants need to drink 70 to 200 liters (18 to 53 gallons) a day. Despite the cooler climate of St. Paul, the authors think the mammoths, having evolved in even colder conditions, may have needed even more water. The island's only substantial lake may have been inadequate. There are signs of decreased water quality around the time of the extinction, possibly because the last mammoths desperately sought whatever water was available, destroying the vegetation around the lake and hastening their own end.
On the other hand, the paper rejects alternative theories, noting there is no evidence of either humans or polar bears on St. Paul at the time, nor volcanic eruptions, and most of the vegetation on the entire island remained stable through the period. However, the reduction in the size of the island that occurred around 3,000 years before the mammoths died out may have been a contributing factor, taking the population close enough to the edge that drying pushed them over.