Increase in human population killed off cave bears
European scientists have discovered that human expansion rather than climate change was the chief reason why cave bears became extinct in Europe around 24 000 years ago. They claim increased competition between humans and bears for land and shelter — in particular for the caves used by the bears for hibernation during the winter months — triggered the decline in cave bear population levels. Conversely, the brown bear has survived until today because it did not depend so heavily on the cave habitat. The findings were published in the journal Molecular Biology and Evolution.
To discover why the numbers of cave bears (Ursus spelaeus) started to decline in Europe, a team of scientists led by Dr Mathias Stiller from the Max Planck Institute for Evolutionary Anthropology in Germany studied mitochondrial DNA (deoxyribonucleic acid) sequences from 17 new fossil samples and compared them with the modern brown bear (Ursus arctos). The fossils were found in European deposits in Siberia (Russia), Ukraine, central Europe and Galicia on the Iberian Peninsula.
The scientists carried out a Bayesian analysis of statistical probability on the DNA sequences and found that human expansion, rather than climate change, was responsible for triggering the decline of the cave bear, a fearsome creature weighing on average 500 kilograms (kg).
'The decline in the genetic diversity of the cave bear began around 50 000 years ago, much earlier than previously suggested, at a time when no major climate change was taking place, but which does coincide with the start of human expansion,' said Dr Aurora Grandal-D'Anglade, researcher at the Institute of Geology at the University of Coruña in Spain and co-author of the study.
Meanwhile, radiocarbon dating of the fossil remains revealed that the cave bear ceased to be abundant in central Europe around 35 000 years ago, and definitively became extinct in most regions around 24 000 years ago. It should be noted, however, that it held out for a few thousand years longer in a few areas such as the north-west region of the Iberian Peninsula.
Dr Grandal-D'Anglade confirmed that the species' extinction could be 'attributed to increasing human expansion and the resulting competition between humans and bears for land and shelter'.
The scientists also made comparisons between the cave bear and its cousin, the modern brown bear, by analysing 59 cave bear DNA sequences from between 60 000 and 24 000 years ago, and 40 sequences from the brown bear, dating from between 80 000 years ago to the present day.
They concluded that climate change did have some bearing on the decline of the cave bear, whose definitive extinction broadly coincides with the last cooling of the climate during the Pleistocene period between 25 000 and 18 000 years ago. The researchers suggested this alteration in temperature may have led to a reduction in shelter and the vegetation that the animals fed on; the cave bear was largely a herbivore.
However, they deemed this change and the subsequent impoverishment of ecosystems merely the 'coup de grace' for the cave bear, noting that it was 'already in rapid decline'. They noted that the present day brown bear did not suffer the same fate and has survived until today as it did not depend so heavily on the cave habitat. 'Brown bears rely on less specific shelters for hibernation,' explained Dr Grandal-D'Anglade. 'In fact, their fossil remains are not very numerous in cave deposits.'
Max Planck Institute for Evolutionary Anthropology
Molecular Biology and Evolution
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