Brown bears have largely vanished from Europe over the past 12,000 years. One reason: warmer winters discouraged females from bearing cubs as they used energy to sleep rather than go into deep hibernation in the cold.
Researchers from Poland, Norway and Germany, who used fossil bone remains from 38 bear populations in mathematical models, said warmer winters since the last ice age had been a key factor in slashing their reproductive rates.
Paradoxically, female bears expended more body-fat energy to survive winters in slumber instead of hibernating more deeply and giving birth successfully, they wrote in the Scientific Reports journal.
Europe's first local brown bear extinctions had occurred between 7,000 and 5,000 years ago - in southwestern Europe.
Large-scale extinctions, also extending to the British Isles, began 2,000 years ago during the Roman Empire, suggesting complex impacts of humans and climate change.
Today, the brown bear (Ursus arctos) is confined to isolated populations in the Pyrenees, northern Scandinavia and eastern regions of Europe.
The modelling showed in "unprecedented detail" the decline in bear numbers during the Holocene, the scientific term for the era since Europe's last major ice age.
Researchers led by Jörg Albrecht of the Frankfurt-based Senckenberg Biodiversity and Climate Research Center (BiK-F) said brown bears had been present across the continent during the Late Glacial age.
Later, as Europe entered the warmer Holocene, "increasing winter temperatures facilitated human land use in regions with formerly unsuitable climates, thereby expediting overhunting and habitat loss,” said the researchers.
"Over the last 12,000 years winter temperatures in many parts of Europe have risen by two up to four degrees (Celsius, 35 to 39 degrees Fahrenheit),” said Albrecht.
The team, also based at Krakow, Poland and As in Norway, said its study had yielded perhaps the "most comprehensive glimpse” into species' extinctions and the complex interactions that led to them.
Looking for causes in a dichotomy of "human versus climate” was often insufficient to explain "mega-faunal” extinctions, they said.
"Complex direct and indirect effects of humans and changing environmental conditions” seemed to lie at the root of species loss around the world.
ipj/jm (epd, AFP)