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Mixed bear mating could get species through climate change

Jul 8, 2011, 9:19 GMT

Washington - Polar bears and brown bears are coming together again to survive the next major climate change, which is expected to have dire effects on their endangered populations, a study published Thursday said.

Melting arctic ice, the result of global warming blamed on massive carbin emissions, could force polar bears into the natural home of the brown bear, setting the two species up for more genetic mixing, according to the study in the twice-monthly scientific journal Current Biology.

'When they come into contact, there seems to be little barrier to them mating,' said Beth Shapiro, researcher at The Pennsylvania State University.

Researchers Shapiro and Daniel Bradley of Trinity College in Dublin examined the mitochondrial DNA of 242 brown and polar bear samples dating as far back as 120,000 years ago, to follow the bears' maternal lineages. Mitochondrial DNA passes unchanged from mother to child.

And the historic look showed that this isn't the first time the two bear species have teamed up to fight extinction.

Polar bears and brown bears have successfully mated several times in the last 100,000 years, creating a hybrid species that could have helped to carry both populations into the 21st century, the researchers said.

The last time the lines of the polar and brown bears likely crossed was approximately 22,000 years ago, during an ice age that made southern Ireland uninhabitable for the native Irish brown bear.

The cold spell eventually drove the bear to extinction, but not before sharing its DNA with the polar bear.

Researchers have found that the temperature change forced the Irish brown bear into the polar bear's habitat, providing for the integration of the two species. The maternal lines of modern polar bears have roots in the DNA line of the Irish brown bear.

'Hybridization between brown bears and polar bears occurred during a period of rapid climate change, when fluctuations in the amount and distribution of habitat in the North Atlantic would have provided ample opportunity for their ranges to overlap and therefore optimal conditions for opportunistic mating,' Shapiro and Bradley wrote.

They believe the hybridization could be the key to both bears' survival during periods of 'environmental deterioration.'

'Although the evolutionary role of hybridization is not yet completely understood' it 'may provide the means to transfer novel traits between species, providing a fitness advantage to hybrid offspring,' Shapiro and Bradley wrote.

But despite the bears' genetic mixing in the past, the researchers said that the 'brown and polar bears have remained evolutionarily distinct lineages over geological time, suggesting that they are likely to remain as such in the future.'

Shapiro and Bradley hope that their findings will lead the US Fish and Wildlife Service, which determines which terrestrial animals make the endangered species list, to protect the hybrid offspring of the species as well.

'It may be appropriate to reconsider protection of hybrids, because they may play an underappreciated role in the survival of species,' the researchers wrote.

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