The mammoths on St. Paul survived until 5,600 years ago, but the reasons for their extinction have long been a matter of speculation. Last August, a team led by Russell W. Graham of Pennsylvania State University ruled out all the leading candidates, including human predation, polar bears, increased winter snowpack, volcanic activity and changing vegetation.

The real reason, they concluded, after examining lake bed sediments, was simply a lack of fresh water. Elephants are heavy drinkers and mammoths, their close cousins, were probably even more so, because they were adapted to the cold but were trying to survive in the post-ice age climate. During dry periods, only one lake on St. Paul was available and this seems to have failed as thirsty mammoths destroyed the plant cover around its shores.

The mammoths of Wrangel, a much larger island, survived for some 1,600 years longer and seem to have met a different fate. A team led by Eleftheria Palkopoulou and Love Dalen of the Swedish Museum of Natural History gained a major insight into the population history of the woolly mammoth by analyzing the whole genomes of two individuals. One was a mammoth from the mainland, from the Oimyakon district of northeastern Siberia, that died some 45,000 years ago at a time when the species still flourished. The other was from Wrangel Island and perished around 4,300 years ago, a few hundred years before the final extinction.

From the amount of genetic variation in each genome, the Swedish team was able to calculate the effective population size — a genetic concept roughly equivalent to the breeding population — of the woolly mammoths at each time period. The Oimyakon mammoth’s genome indicated an effective population size of 13,000 individuals whereas that of the Wrangel mammoth was a mere 300.