Mammoth task of preservation

Published on Friday 4 July 2008 20:49

IF THERE is one iconic picture of Ice Age humans, it must surely be that of half a dozen muscled prehistoric cavemen surrounding an angry mammoth that they are trying to spear to death. Sadly, of course, this member of the elephant family that was unique to the northern hemisphere eventually became extinct, but it is sobering to remember that these magnificent animals were still living on Wrangel Island in the Siberian Arctic as recently as 3,700 years ago.

The classic explanation for the demise of these creatures was that they were hunted to extinction by humans invading the tundras of the north in the wake of the retreating Ice Age, in what is sometimes known as the "Pleistocene overkill".

The main evidence for this was that many large animals, including mammoths, disappeared from North America shortly after the first Native Americans arrived.

A more recent suggestion is that it was climate warming that made it difficult for these lumbering giants to find food. It has always been difficult to decide between explanations for past events of this kind, but an answer might now finally be at hand, thanks to the computer.

This has come about by a combination of better climate models allowing us to reconstruct past climates, and a better understanding of the maths of conservation biology.

In a paper recently published in the Public Library of Science Biology journal, David Nogus-Bravo (of Madrid's National Museum of Science) and his colleagues used powerful climate models to track the last 130,000 years and reconstruct the climate over the mammoth's entire range in Europe and Asia.

They used them to determine the climatic conditions that would have been found at all the sites where mammoths are known to have lived.

Their findings suggest a gradual increase in the size of the area with climate suitable for mammoths between 127,000 years and 42,000 years ago, followed by a long period of climatic stability during which the mammoth's geographical range extended down into southern China and even the area of modern-day Iran and Afghanistan.

But after 42,000 years ago, the climate began to warm precipitously, and by 6,000 years ago the mammoth would have been confined to the rim of the Siberian Arctic and a few isolated areas in Asia.

This marked reduction in the mammoth-friendly habitat would inevitably have coincided with a dramatic collapse in the size of the mammoth population. And it is at this point that humans become important.

Modern humans hunted mammoths since they first came across them after breaking out of Africa around 70,000 years ago. Nogus-Bravo and his colleagues used mathematical models from conservation biology to estimate the mammoth's susceptibility to hunting pressure under different kill regimes and population densities.

When mammoths were most abundant, between 40,000 and 20,000 years ago, human hunters would have had to kill in excess of one mammoth per person every 18 months to drive the mammoth to extinction.

But during the later phases, around 6,000 years ago, when mammoth populations were at their lowest ebb, it would only have taken kill rates of less than one mammoth per person every 200 years to wipe the species out. This is so low that even very occasional hunting would have been enough to tip the mammoths over the brink.

We know from archaeological evidence that hunting rates were high – early humans living in Ukraine 15,000-20,000 years ago made extensive use of mammoth bones for building shelters. In some cases, bones were used to weigh down the edges of tents.

And at Mezhirich, four huts were built with walls and roof supports made out of the leg bones, lower jaws, skulls and tusks of many mammoths, with bones from as many as 95 individual animals.

The lesson for us today is that while mammoths could easily absorb the hunting pressure from humans when they were abundant, their ability to do so changed abruptly once climate change caused a dramatic drop in their numbers. It remains an object lesson for us today, with the renewed threat of climate warming putting increasing numbers of rare species at risk.

&#149 Robin Dunbar is professor of evolutionary anthropology at Oxford University