From the mighty polar bear to the tiny house sparrow, many of Earth's species appear to be shrinking in size, a new study reports. And the authors think it's probably due to global warming, a little like wool sweaters that shrink when washed in hot water.

But other experts say that conclusion goes too far, blaming global warming for what may be natural changes.

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The new research was published online Sunday in the journal Nature Climate Change. It's based on a review of other studies and found that 38 of 85 animal and plant species showed a documented reduction in size over decades, including a type of Scottish sheep that is five per cent smaller than in 1985. Those studies looked at species over different time periods and in varying numbers.

The shrinking victims, according to the study, include cotton, corn, strawberries, bay scallops, shrimp, crayfish, carp, Atlantic salmon, herring, frogs, toads, iguanas, hooded robins, red-billed gulls, California squirrels, lynx and wood rats.

Two years ago, Scottish researchers made news with the shrinking sheep study. Several studies have shown that polar bears, which rely on sea ice during the summer, also are not as big. And this latest study said the house sparrow's weight has dropped by one-seventh from 1950 to 1990. A bird called the graceful warbler, also known as the graceful prinia, showed a 26 per cent weight drop during the same time period.

"There is a trend in a number of organisms across the board, from plants to big vertebrates getting smaller," said study co-author Jennifer Sheridan, a biology researcher at the University of Alabama. "The theory is as things get warmer, they don't need to grow as large."

Disagreement on cause

The latest research is the second major paper in as many months to discuss how animals are shrinking amid Earth's warming climate. In September, British scientists writing in the journal American Naturalist explained why cold-blooded organisms are affected by the "temperature-size rule," which causes them to grow faster but to a smaller adult size when the thermometer rises.

Most of the animals surveyed in the new Nature Climate Change paper are cold-blooded, so the warmer the weather, the faster their metabolism is and the more calories they burn, Sheridan said.

But Yoram Yom-Tov, a zoologist at Tel Aviv University whose studies Sheridan used in her research, said many species are shrinking and global warming can't be blamed exclusively.

"Changes in body size are a normal phenomenon," Yom-Tov wrote in an email. "When conditions are favourable, they increase in size or reproduce at higher rates, and when conditions are deteriorating, they do the opposite. I think that most species will adapt to climate change and survive. No need for alarm."

And Stanford biologist Terry Root, an expert in climate change, said the study's conclusions "seem kind of far-fetched." 

With files from CBC News