The Curious Case of the Grolar Bear
With changes in the Arctic climes come north-south species hybridization and the potential for strange new genetic brews
With changes in the Arctic climes come north-south hybridization and strange new genetic brews
When biologists Jodie Pongracz and Evan Richardson flew to Canada’s Arctic to capture and tag polar bears in the spring of 2012, they came across a startling sight — a polar bear strolling beside another bear with chocolate brown fur on its legs, a brown stripe down its back and a flat concave face and head that looked like it had been transplanted from a grizzly. This unlikely duo was sighted at latitude 73 degrees north, on a frozen arm of the Arctic Ocean near Victoria Island — hundreds of kilometres north of where grizzlies are normally found.
Two years later, Pongracz and Richardson captured the same animal and took a DNA sample. It was determined that the bear was a hybrid, or what some have dubbed a “grolar,” produced by the mating of a female polar bear and a male grizzly. This was not an isolated case. Grolars have been spotted in this part of the Canadian Arctic dating back to 2006, when a hunter shot one of these crossbreeds on Banks Island in the Northwest Territories. After viewing its carcass, the local Inuit allowed how they had no word for this creature in their language.
Some scientists have speculated that grolars and other hybrids may be the advance guard of a strange new genetic brew of Arctic species that will begin to emerge as global warming removes the icy barriers that once kept different species apart. Although the evidence remains sketchy, hybridizations have also been reported between harp and hooded seals, Atlantic walrus and Pacific walrus, narwhals and belugas, and right whales and bowhead whales.
There is no doubt that the reduction of sea ice in the Arctic is going to usher in profound ecological changes. Hybridization will be one of them. As Brendan Kelly, a marine biologist at the University of Alaska Fairbanks and executive director of the Study of Environmental Arctic Change, notes, “In the case of Arctic species, you’ve had a continent-size mass of sea ice keeping species isolated for thousands of years. If you remove that barrier, you can re-establish contact. Right now, the sea ice is melting quite rapidly, so we’re going to see a lot of previously isolated populations come into contact. We’re likely to see a lot more hybridization.”
In 2010, Kelly and two other American scientists, Andrew Whiteley and David Tallmon, published an article in the journal Nature in which they cited 34 opportunities for hybridization across 22 Arctic or near-arctic species. These were based on animals’ genetic compatibility and geographic range.
In certain cases, such as interbreeding between North Atlantic and North Pacific minke whales, the crossovers would not result in a loss of biodiversity, notes Kelly. In other instances, such as between bowhead whales and rare North Pacific right whales, whose population is estimated at fewer than 200, interbreeding could lead to extinction of the smaller population.
Not all cross-species matings produce viable offspring, but the chances are better in Arctic marine mammals, says Kelly, because their number of chromosomes has changed little over time. This is certainly true of cetaceans, says Carla Crossman, a marine mammal geneticist with Oceanwise, a global conservation program. She says there are 18 to 20 known crosses among cetaceans.
For now, most scientists don’t view hybridization as a major threat to biodiversity. “The dangers are real — if unqualified,” notes Marco Festa-bianchet, a professor of biology at Quebec’s Université de Sherbrooke, who co-chaired a group of wildlife experts with the Committee on the Status of Endangered Wildlife in Canada in 2010 that tried to come up with recommendations on how to deal with hybridization. “While we can generally predict that some species will move north, predictions of when, where and how are very imprecise.”
In recent years, red fox, white-tailed deer, Pacific salmon and killer whales have all begun appearing in the Arctic. The orcas’ big dorsal fins, which impede navigation in ice-choked waters, had previously kept them from the region, but that has changed with the warming climate. Aside from increasing the chances of hybridization, the sudden entry of an apex predator could have a major impact on the region’s ecology: orcas have been observed attacking and eating belugas and narwhals.
Barren-ground grizzly bears are also moving northward, and they seem to be thriving. As Andrew Derocher, a biologist at the University of Alberta and a world authority on polar bears, explains, “The climate has warmed, there is enough food for them, and there are fewer grizzlies being shot as the Inuit lifestyle shifts from hunting and trapping to a wage economy.” At the same time, polar bears are at risk: most of their hunting for seals is on the ice during winter, so now they are spending more time on land as the sea ice recedes, causing them to lose body weight and decline in numbers.
Bear hybrids in the Arctic would most likely be produced by male grizzlies travelling and mating with female polar bears because male grizzlies emerge earlier from winter hibernation and head out in search of food while the females tend to stay close to home. Such liaisons, explains Derocher, would not be casual encounters, as it takes several days to several weeks of frequent copulation to induce ovulation in female polar bears. How well-adapted such hybrids would be to Arctic conditions is unknown. Polar-grizzly hybrids in a German zoo exhibited behaviour associated with seal hunting but did not possess the strong swimming abilities of polar bears.
Another perplexing question — the source of this mysterious cluster of bear hybrids — was recently answered by DNA analysis. According to a study headed up by Pongracz and Richardson and reported in a 2016 issue of the scientific journal Arctic, all of the eight documented cases of hybridization — four first-generation hybrids and four second-generation “backcrosses” — can be traced to one female polar bear that mated with two grizzly males. That female and three of her offspring have since been killed, leaving the researchers to wonder if this spate of hybrids is merely a blip caused by the actions of one mate-less female polar bear, or if this type of unusual mating signals the start of the breakdown of species barriers.
“We don’t know if there are other hybrids running around out there,” admits Richardson, a polar bear research scientist with Environment and Climate Change Canada. “Three of the hybrid cubs (all of them male) may still be alive, and if they take after their father’s taste in partners, then more hybrids could be produced.”
The odds of these bears’ survival may be reduced simply because their curious features make them attractive trophies. “Their pelts can be sold for a good price,” says Derocher.
As for the question of whether we can expect to see more hybrid bears, Derocher is unsure. “What this means in the longer term is impossible to say. It’s very difficult to predict the path of evolution.”
What Derocher is more certain about is the bleak future of polar bears. He expects them to disappear from their present ranges long before their genes are swamped by those of grizzly bears. “The best estimates indicate that we’ll probably lose somewhere around two-thirds of the world’s population around mid-century, just based on the simple fact that we’re losing sea ice,” says Derocher. “No sea ice means no seals. And no seals means no polar bears.”a