WASHINGTON, May 8 (-- Researchers from Denmark, the U.S. and China said Thursday they may have found the reason why polar bears can consume a fatty diet without developing high rates of heart disease like humans.
After a thorough genetic analysis of polar bears, the researchers found changes in several genes involved in cardiovascular function and fatty acid metabolism may protect the animals from the ill effects of a high-fat diet, mainly consisting of seals and their blubber.
One such gene, called APOB, is known to play a role in moving cholesterol from the bloodstream into cells, thus reducing the risk of heart disease, they reported in the U.S. journal Cell.
The researchers also compared the genome of polar bears to that of their closest cousins, brown bears, and found the polar bear is a much younger species than previously believed, having diverged from brown bears less than 500,000 years ago.
Previous estimates of the divergence time between polar bears and brown bears ranged from 600,000 to 5 million years ago.
"It's really surprising that the divergence time is so short," said senior study author Rasmus Nielsen of the University of California, Berkeley.
"In this limited amount of time, polar bears became uniquely adapted to the extremities of life out on the Arctic sea ice, enabling them to inhabit some of the world's harshest climates and most inhospitable conditions," he said.
What drove the evolution of polar bears is unclear, though the split from brown bears coincides with a particularly warm 50,000- year interglacial period that could encourage brown bears to extend their range much farther north, the researchers said.
When the warm interlude ended and a glacial cold period set in, a pocket of brown bears may have become isolated and forced to adapt rapidly to new conditions. As a result, up to half of the body weight of polar bears now consists of fat, and their blood cholesterol levels are high enough to cause cardiovascular disease in humans.
"The life of a polar bear revolves around fat," said Eline Lorenzen, one of the lead authors at the University of California, Berkeley.
"Nursing cubs rely on milk that can be up to 30 percent fat and adults eat primarily blubber of marine mammal prey. Polar bears have large fat deposits under their skin and, because they essentially live in a polar desert and don't have access to fresh water for most of the year, rely on metabolic water, which is a byproduct of the breakdown of fat."
The analysis in the study included blood and tissue samples from 79 Greenlandic polar bears and 10 brown bears from Sweden, Finland, Glacier National Park in Alaska and the Admiralty, Baranof, and Chichagof (ABC) Islands off the Alaskan coast.
The study also involved Danish researchers, led by Eske Willerslev of the University of Copenhagen, who provided polar bear blood and tissue samples and researchers at BGI-Shenzhen, China, including Shiping Liu, Guojie Zhang and Jun Wang, who sequenced the genomes and analyzed the data together with Lorenzen and Nielsen.
"The evolution history of polar bear demonstrated how quickly an organism could adapt to the evolutionary challenges," said Jun Wang. "With genomic data released for more species, we hope to reveal a more complete picture of how animals adapt to environment and how biodiversity forms."