How polar bears got their white coat remains a scientific mystery, but newly published research suggests a way they could turn brown again.
One of the study's authors says that's what might eventually happen to some groups of modern bears as climate change alters their habitat.
"It's not something that happened in the past and might happen in the future — it's happening today," said Beth Shapiro, one of the authors of the study published in the journal PLOS Genetics.
Shapiro and her colleagues were looking for insight into the evolutionary past of bears. The researchers were trying to understand when black, brown and polar bears diverged from each other and developed their modern appearance.
Scientists still don't know when polar bears acquired their distinctive coat. The oldest remains only date from about 110,000 years ago, by which time the species was already well-developed.
"This 110,000-year-old bone probably isn't the earliest polar bear," said Shapiro. "We have no idea how early polar bears diverged from brown bears."
Ancient polar bears
But there is one group of brown bears that has shown signs of mixed heritage — the so-called ABC bears on the Admiralty, Baranof and Chichagof Islands off the coast of Alaska. Previous studies of some of their DNA have shown the ABC bears are more closely related to polar bears than they are to other browns.
Shapiro and her team decided to look more closely at the ABC bruins. They employed sophisticated DNA analysis to look deeply into their past.
What they found is that ABC bears are genetically close to polar bears because that's what they used to be.
Shapiro's paper suggests that the ABC bears are a remnant of an ancient population of polar bears that lived in the area during the last Ice Age. As that period ended and the ice slowly retreated, the ABC population got cut off from other polar bears.
"They couldn't mate with any more polar bears," Shapiro said.
"As the weather started to get warmer and warmer, brown bears — males, as it turns out — could swim across the channel that separated the ABC islands from the Alaskan mainland and colonize these islands. There they ran into this isolated, trapped population of polar bears and they started hybridizing with them."
Adapting out of existence
That scenario conforms to modern bear behaviour, in which it is the young males that set out from their birth range in search of new territory. It's also borne out in the DNA evidence. The team found the greatest percentage of polar bear genetic material in the female X chromosomes of the ABC bears.
"All of the DNA that's shared between mom and dad has about one to four per cent polar bear remaining in it," Shapiro said. "The X chromosomes have a lot more — up to around 15 per cent."
That isn't enough for the ABC bears to have maintained their original genetic identity, she said.
"In that particular location, they were converted into brown bears."
Shapiro said something similar is happening today as climate change reduces sea ice and changes the habitat of some polar bears to look more like that of their southern cousins.
"We've seen that polar bears are hybridizing with brown bears now, at the edge of their range in Canada.
"I guess it means if we destroy all their habitat, and the only habitat that's left for polar bears looks like brown bear habitat, then they're just going to hybridize with brown bears and turn into brown bears."
If that were to happen, it would take thousands of years, Shapiro hastened to add. But it does prove how adaptable polar bears can be — to the point of adapting themselves out of existence.
"Because they still retain the ability to hybridize with brown bears, they're still pretty environmentally adaptable, which is kind of a shame for polar bears."