Bears have stopped hibernating in the mountains of northern Spain, scientists revealed yesterday, in what may be one of the strongest signals yet of how much climate change is affecting the natural world.

In a December in which bumblebees, butterflies and even swallows have been on the wing in Britain, European brown bears have been lumbering through the forests of Spain's Cantabrian mountains, when normally they would already be in their long, annual sleep.

Bears are supposed to slumber throughout the winter, slowing their body rhythms to a minimum and drawing on stored resources, because frozen weather makes food too scarce to find. The barely breathing creatures can lose up to 40 per cent of their body weight before warmer springtime weather rouses them back to life. But many of the 130 bears in Spain's northern cordillera - which have a slightly different genetic identity from bear populations elsewhere in the world - have remained active throughout recent winters, naturalists from Spain's Brown Bear Foundation (La Fundación Oso Pardo - FOP) said yesterday.

The change is affecting female bears with young cubs, which now find there are enough nuts, acorns, chestnuts and berries on thebleak mountainsides to make winter food-gathering sorties "energetically worthwhile", scientists at the foundation, based in Santander, the Cantabrian capital, told El Pais newspaper.

"If the winter is mild, the female bears find it is energetically worthwhile to make the effort to stay awake and hunt for food," said Guillermo Palomero, the FOP's president and the co-ordinator of a national plan for bear conservation. This changed behaviour, he said, was probably a result of milder winters.