Smaller animals in particular that can breed quickly, such as squirrels, some birds and insects, are showing signs of evolving new patterns of behaviour to increase their chances of survival. Scientists say that many of the genetic adaptations are to cope with changes in the length of the seasons rather than the absolute increases in summer temperatures.

Canadian Red Squirrel (Photo/Jason T. Fisher)

Larger animals and species that are slow to reproduce may on the other hand find it difficult to cope with climate change because they cannot adapt genetically as quickly as smaller, more fertile creatures that have rapid life cycles.

"Studies show that over the past several decades, rapid climate change has led to heritable, genetic changes in animal populations," said Christina Holzapfel, from the University of Oregon in Eugene.

Examples included Canadian red squirrels reproducing earlier in the year, German blackcap birds migrating and arriving earlier at their nesting grounds, and northern American mosquitoes living in water-filled leaves of carnivorous plants which can adjust their life cycles to shorter more "southern" day lengths.

William Bradshaw, professor of biology at Oregon, said that global warming is going at a faster rate at more northerly latitudes which is causing longer growing seasons, and less cold stress caused by extreme winter weather. "Over the past 40 years, animal species have been extending their range toward the poles and populations have been migrating, developing or reproducing earlier," Professor Bradshaw said.

"These expansions and changes have often been attributed to 'phenotypic plasticity', or the ability of individuals to modify their behaviour, morphology or physiology in response to altered environmental conditions," he said.

However, the scientists point out that in addition to these ad-hoc changes in behaviour, there is another type of evolutionary change at the level of the genes which is being caused by rapid climate change.

"Phenotypic plasticity is not the whole story. Studies show that over the past several decades, rapid climate change has led to heritable, genetic changes in animal populations," said Dr Holzapfel.

Writing in the journal Science, the researchers point out that there is little evidence to suggest that animals are changing genetically in order to adapt to the higher summer temperatures associated with climate change. Dr Holzapfel said that adaptations to changing seasons are likely to come first because this will have a more direct bearing on an individual's breeding potential.

"However, it is clear that unless the long-term magnitude of rapid change is widely acknowledged and effective steps are taken to mitigate its effects, natural communities that we are familiar with will cease to exist," she says.

* Global warming could be returning the world to the way it was four million years ago when sea levels were 80 feet higher than they are today, according to another study in Science.

© 2006 Independent News and Media Limited

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