By Dr. Becker
A recent study comparing the genomes of polar bears and brown bears suggests that the two species separated less than 500,000 years ago, making the polar bear a much younger species than we thought.
The study, published earlier in the year in the journal Cell,1 was a collaborative effort by a group of international researchers including biologists from the University of California, Berkeley. Genome sequencing also pointed to a number of unique genetic adaptations that may explain how polar bears are able to survive life in the high Arctic -- specifically, how the bears’ bodies are able to operate so efficiently on a tremendously high fat diet.
‘The life of a polar bear revolves around fat.’
“The life of a polar bear revolves around fat,” UC Berkeley researcher Eline Lorenzen said in a news release.2 “Nursing cubs rely on milk that can be up to 30 percent fat and adults eat primarily blubber of marine mammal prey.”
The polar bear’s vast fat reserves, which account for half his body weight, help him survive between meals, and also give him the ability to stay afloat as he swims.
Polar bears also store sizeable fat deposits under their skin, and since they don’t have access to fresh water most of the time, they must rely on metabolic water, which is a byproduct of the breakdown of fat. So the bears’ bodies are hydrated by burning fat.
Polar Bears Adapted to Arctic Life in Remarkable Ways
The team of Danish, Chinese, and American scientists analyzed the blood and tissue samples of 79 Greenlandic polar bears and 10 brown bears from Sweden, Finland, and Glacier National Park and the ABC Islands in Alaska.
They sequenced the genomes of the bears and analyzed them using a method that evaluates the length of DNA segments shared by the bears, which is how they discovered the relatively recent split between the two species.
The sequencing also revealed several genes that play a role in the polar bear’s extraordinary adaptation to spending most of his life on sea ice. They are genes involved in fat transport, fatty acid metabolism, and cardiovascular function, all of which help the bears live successfully on diet of blubber-filled marine mammals like seals. The genetic adaptations probably explain the bears’ plaque-free arteries and healthy hearts, despite a diet loaded with fat.
“Usually, the genes that evolve most radically in species are immune and defense genes. What’s surprising was the focus on cardiovascular function,” said UC Berkeley’s Rasmus Nielsen.
Just as surprising is that the polar bears adjusted to their super-high-fat diet in just a few hundred thousand years, which is record time, evolutionarily speaking.
In addition to these impressive physiological and metabolic changes, polar bear fur transformed from brown to stark white, and their bodies grew sleeker in comparison to brown bears.
Are Polar Bears Destined to Turn Back Into Brown Bears?
Scientists aren’t sure what drove the evolution of polar bears, though the split from brown bears occurred during a particularly warm interglacial period known as Marine Isotope Stage 11. The resulting climate change may have prompted brown bears to extend their range northward, and when the glacial cold ultimately returned, a population of brown bears could have found themselves isolated and forced to quickly adapt to new conditions.
But according to New Scientist:
“The split between polar bears and brown bears may soon go into reverse. Polar bears are losing habitat as the Arctic ice melts. That may drive them inland to brown bear habitat, where the two species will begin to hybridize, creating ‘Grizzlars’ that have apparently already been sighted.”3
"The polar bear could die out, not through lack of habitat, but through an increased rate of integration and gene flow between white and brown bears," says Nielsen.