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. "The high Cantabrian peaks freeze all winter, but our teams of observers have been able to follow the perfect outlines of tracks from a group of bears," he said.

The FOP is financed by Spain's Environment Ministry and the autonomous regions of Cantabria, Asturias, Galicia and Castilla-Leon, where the bears roam in search of mates. Indications of winter bear activity have been detected for some time, but only in the past three years have such signs been observed "with absolute certainty", according to the scientists.

"Mother bears with cubs make the effort to seek out nuts and berries if these have been plentiful, and snow is scarce," Mr Palomero said, adding that even for those bears - mostly mature males - who do close down for the winter, "their hibernation period gets shorter every year".

The behaviour change suggests that global warming is responsible for this revolution in ursine behaviour, says Juan Carlos García Cordón, a professor of geography at Santander's Cantabria University, and a climatology specialist.

"Meteorological data in the high mountains is scarce, but it seems that the warming is more noticeable in the valleys where cold air accumulates," Dr García Cordón said. "There is a decline in snowfall, and in the time snow remains on the ground, which makes access to food easier. As autumn comes later, and spring comes earlier, bears have an extra month to forage for food.

"We cannot prove that non-hibernation is caused by global warming, but everything points in that direction."

Spanish meteorologists predict that this year is likely to be the warmest year on record in Spain, just as it is likely to be the warmest year recorded in Britain (where temperature records go back to 1659). Globally, 2006 is likely to be the sixth warmest year in a record going back the mid-19th century.

Mark Wright, the science adviser to the World Wide Fund for Nature (WWF) in the UK, said that bears giving up hibernation was "what we would expect" with climate change.

"It does not in itself prove global warming, but it is certainly consistent with predictions of it," he said. "What is particularly interesting about this is that hitherto the warming has seemed to be happening fastest at the poles and at high latitudes, and now we're getting examples of it happening further south, and heading towards the equator.

"I think it's an indication of what's to come. It shows climate change is not a natural phenomenon but something that is affecting not only on the weather, but impacting on the natural world in ways we're only now beginning to understand."

The European brown bear, with its characteristic pelt that ranges from dark brown through shades of grey to pale gold, has black paws and a tawny face. It has poor vision, although it sees in colour and at night, and if threatened rears on its hind legs to get a better view. It can live for up to 30 years. It has acute hearing, and an especially fine sense of smell that enables it to detect food from a long distance. It is carnivorous, but has a multifunctional dental system with powerful canines and grinding molars perfectly adapted to an omnivorous diet.

The animals would normally begin hibernation between October and December, and resume activity between March and May.

The Cantabrian version of the brown bear, a protected species, was once as endangered as the Iberian lynx or the imperial eagle still are in Spain, but is now recovering in numbers. Between 70 and 90 bears roamed Spain's northern mountains in the early 1990s; now 130 live there.

Other seasonal freaks

* The osprey found in the lochs and glens of the Scottish Highlands in the summer months, usually migrate to west Africa to avoid the freeze. This winter, osprey have been spotted in Suffolk and Devon. Swallows, which also normally migrate to Africa for the winter have been also seen across England this winter.

* The red admiral butterfly, below, which hibernates in winter, has been spotted in gardens this month, as has the common darter dragonfly, usually seen between mid-June and October, which has been seen in Cheshire, Norfolk and Hampshire.

* The smew, a diving duck, flies west to the UK for winter from Russia and Scandinavia. This year, though, they have been mainly absent from the lakes and reservoirs between The Wash and the Severn.

* Evergreen ivy and ox-eye daisies are still blooming and some oak trees, which are usually bare by November, were still in leaf on Christmas Day last year.

* The buff-tailed bumblebee is usually first seen in spring. Worker bees die out by the first frost, while fertilised queen bees survive underground between March and September. This December, bees have been seen in Nottingham and York.

* Primroses and daffodils are already flowering at the National Botanic Garden of Wales, in Carmarthenshire. 'Early Sensation' daffodils usually flower from January until February. Horticulturalists put it down to the warm weather.

* Scientists in the Netherlands reported more than 240 wild plants flowering in the first 15 days of December, along with more than 200 cultivated species. Examples included cow parsley and sweet violets. Just two per cent of these plants normally flower in winter, while 27 per cent end their main flowering period in autumn and 56 per cent before October.

Geneviève Roberts