You are how you eat: Diet changes altered genes of polar bears, study finds

New research is illuminating how polar bears evolved from their brown bear cousins — and it's all in their genes.

In a paper published June 17 in the Proceedings of the National Academy of Sciences, researchers detail how they discovered rapid changes in polar bears' genes in response to the shifting of their diets from vegetation to meat.

"It's this perfect system because the polar bear and the brown bear are really, really closely related … but their diets are incredibly different and their ecologies are incredibly different," said John Gibbons, assistant professor in the Department of Food and Science at the University of Massachusetts Amherst.

He explained they found that polar bears have significantly fewer copies of specific genes involved in diet than brown bears, adding "one of the coolest findings" was that polar bears had 88 per cent fewer genes involved with smell.

"What we're hypothesizing is that they don't need to smell as much," Gibbons said, noting polar bears "probably just use their smell to basically find seals and to find meat."

Brown bears, meanwhile, "have a really diverse diet," which includes berries, grasses, roots and bulbs, along with meat, he said.

Polar bears were also found to have fewer copies of a gene that encodes salivary amylase, which helps in the digestion of starch when animals chew plants.

Gibbons said similar research in humans has found more copies of salivary amylase genes in populations with higher starch diets than in those with low starch diets.

Researchers made the findings by comparing differences in gene copy numbers from 17 polar bears, nine brown bears and two black bears.

Brown bears and polar bears diverged less than 500,000 years ago, the paper says, and the polar bear evolved unique traits to adapt to the Arctic, including pigment-free fur that helps with camouflage. 

"In evolutionary terms, 500,000 years ago is like a blink of an eye," Gibbons said, noting polar bears and grizzlies can still reproduce with one another.  

The new research is also answering bigger-picture questions about how ecology and evolution shapes genes.

"It was almost too good to be true," Gibbons said of the findings.

"I think it's kind of an example of how the genome becomes like streamlined or optimized in response to selective pressure."

He explained that every additional gene costs energy, so cells become more efficient when genes that an organism doesn't need are removed. 

"Every individual genome is different. And you know evolution has been shaping all of our genomes since life began."

Gibbons said they are now interested in comparing closely-related human populations that have different diets to see if they will discover similar findings.