Polar bear photo on left, grizzly bear photo on right

Photo: © Daniel J. Cox/Natural Exposures

Polar Bear-Grizzly Hybrids



29 Jan 2021

Ask the Experts

Q: What does the science say about polar bear-grizzly hybrids? Are they caused by global warming? Are they an answer for polar bears?

"There is no indication that interbreeding between polar bears and grizzlies is caused by climate change. Interbreeding will not help save polar bears because a hybrid is not a polar bear, and if the hybrid were to use sea ice, it would be at the same disadvantage as a polar bear."

Sightings of grizzly/polar bear hybrids in northwestern Canada may or may not be a symptom of global warming. And, even if it is, it has little relevance to future polar bear conservation.

The best available information suggests that polar bears separated from grizzly/brown bears about five million years ago. A recent fossil discovery in Svalbard indicates that polar bear dentition was essentially the same as it is now 120,000 years ago, suggesting that polar bears at that time were essentially the same as those of today.

Research shows that sporadic interbreeding between the two species occurred throughout the polar bear's history. We've known for many decades that polar bears and grizzly/brown bears can interbreed in zoos. It seems reasonable that in areas where their ranges are adjacent—that is, where the sea ice habitats occupied by polar bears and the terrestrial habitats occupied by grizzly/brown bears occur side-by-side—that some interbreeding takes place.

It is important to note that although the summer sea ice has changed greatly in the past few decades, it has changed much less during the polar bear's spring breeding season. And, projections show the extent of sea ice during the spring breeding season will change relatively little well into this century. This means that, at present, by the time the sea ice melts in summer and forces some polar bears onto land—where they might encounter grizzlies—the breeding season for both species is over or at least on the decline.

We must remember that regardless of whether interbreeding increases, it is very unlikely that this poses a threat to polar bears that is in any way on par with the threats of habitat losses and the polar bear's increasing inability to find sufficient food.

Some climate science deniers have speculated that interbreeding will be the salvation of polar bears and therefore we don't have to do anything about global warming to save polar bears. That, of course, is nonsense. In 50 or 60 years (without active and aggressive greenhouse gas mitigation) global temperatures will exceed anything that has occurred at any time during the evolutionary history of polar bears. Polar bears—with or without crossbreeding—simply cannot undo five millions years of evolution in 50 or 60 years.

Crossbreeding or not, with uncontrolled warming, we will see polar bears disappear. We may see grizzly bears expand their range as the climate warms, but that is not the issue we're concerned about. As interesting as it may be from a population genetics standpoint, and as much curiosity as it may generate in the public, the prospect of increases in hybridization between grizzly bears and polar bears is simply not a great conservation issue.

We cannot overemphasize that hybridization provides no solution to the polar bear's dilemma. And to the extent there may be increased hybridization, it probably will be of little consequence to polar bears facing dramatic declines in their habitat base. Polar bears are likely to starve out of their present ranges long before their genes are swamped by those of grizzly bears. If some polar bear genes persist in grizzly bears, after polar bears have disappeared from their current sea ice home, that fact will be irrelevant with regard to efforts to retain the magnificent and highly specialized life form we now know as the polar bear.

Discussions of hybridization aside, it is important to remember that by the time we allow the world to warm enough that the polar bears' sea ice habitat disappears, challenges to humans will be so great that no one will be thinking about polar bear conservation.

Dr. Steven Amstrup on the tundra

Photo: Jenny Wong

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