Grolar and pizzly bears: What the family drama of interbreeding polar and grizzly bears reveals

Meet the protagonist of our story: a polar bear. Displaced by melting sea ice in the Arctic, it was forced to wander south, deeper into the Canadian Northwest Territories. Here our lady in white encountered some beautiful grizzly bears. She fell in love with both of them and had two cubs from each – three “grolar bear” daughters and a son. Thus began a remarkable dynasty, a lineage as closely linked as any in a Shakespearean tragedy.

The next phase was just as improbable. Once one of the daughters reached adulthood, she mated with her own biological father as well as her mother’s other grizzly suitor – essentially her stepfather. The result? Four cubs that were genetically his siblings, his children and his cousins, all at once.

In 2006, a hunter in the Canadian Arctic shot an animal with the physical characteristics of a grizzly bear and a polar bear. Genetic testing later confirmed it was a grolar, a member of this modern hybrid family. A decade later, when researchers revealed the intriguing relationships between these animals, the scientific community was baffled: offspring from cross-species matings are usually sterile, whereas here they were clearly fertile. Biologists have wondered whether this could be a prelude to the emergence of a new apex predator in the Arctic. Could these hybrids be an adaptive success story born from the chaos of climate change? Or are they an ecological harbinger of things to come? Today, thanks to new research, we can answer these questions.

As the Earth warms, the Arctic is warming four times faster than the rest of the planet, and polar bears (Ursus maritimus) are moving south. Their diet consists mainly of fat seals, making them dependent on sea ice as a hunting platform. With the volume of ice diminishing at an insurmountable rate, they are being pushed inland in search of new sources of food. Here they are increasingly likely to encounter their brown bear cousins, grizzly bears (Ursus arctos horribilis), which are moving north as warmer temperatures in the High Arctic allow them to expand their hunting and mating territories. This has sparked speculation that hybridization between the two species could become a regular phenomenon, giving rise to increasing numbers of grolars and “pizzly bears” (hybrids with a polar bear father and a grizzly mother). Some biologists even fear that gene flow from polar bears to the brown bear population could contribute to the former’s extinction.

However, the discovery of the Grolar dynasty suggests a more positive outcome: polar bears could adapt to the new world order. Documented grolars resemble grayish-brown polar bears with slightly larger legs and skulls, reminiscent of grizzly bear characteristics. These characteristics have led to speculation that the hybrids may be able to seek out a wider, more terrestrial range of food than their polar relatives, perhaps allowing them to hunt on land and not rely on diminishing sea ice. In some sense, pizzly bears might be a sad but necessary compromise, given current warming trends, paleobiologist Larisa DeSantis of Vanderbilt University in Tennessee said in a 2021 interview.

A happy family

However, a large-scale genomic study published last year paints a different picture. When researchers led by Joshua Miller of MacEwan University in Canada compared the genetic sequences of 371 polar bears, 440 grizzly bears and members of the grolar dynasty, they were in for a shock. Analysis indicated that there are no wild hybrids except for the grolars we know of. The confirmed rarity of this mating pattern now leads experts to believe that hybrids lack the essential skills needed to thrive in either parent’s environment. Indeed, recent studies demonstrate that they are not adapted to life in the Arctic because they do not have the unique morphology of the polar bear’s “non-slip” paws, nor are they fully equipped with the physical characteristics that grizzly bears possess for hunting, such as powerful forelimbs and shoulders. In other words, rather than being evolutionary adaptive chameleons, grolars and pizzlies are both poor polar bears and poor grizzly bears.

In the short term at least, it appears that such hybrids will remain rare and therefore will not give rise to any new species. But what about the long-term evolution of Arctic bears? Perhaps a look into the past could reveal what lies ahead. During the Pleistocene, a geologic epoch marked by repeated glaciations that began about 2.6 million years ago, the habitats of polar and brown bears also overlapped. Researchers at the University of California, Santa Cruz, have found that brown bears living today on three Alaskan islands emerged gradually during this period, following the dominant mating of male brown bears with the resident polar bear population. If this pattern repeats, it is possible that there will be a relentless erosion of polar bear species until only grizzly bears remain.

However, the Pleistocene was characterized by slow, cyclical environmental changes, a time frame that allowed for gradual adaptation. Today’s human-caused climate change is occurring at an unprecedented rate, so things may play out differently this time around. “The much bigger threat facing polar bears today is the loss of their ice habitat due to anthropogenic climate change. That’s something that will happen much faster than any threat from hybridization,” says Fiona Galbraith, a geneticist who has worked as a climate change consultant and now leads expeditions to the Canadian Arctic for Natural Habitat Adventures.

The Grolar dynasty is therefore an oddity, but it is also an emblem of the ecological collapse of our planet. Similar climate-induced hybridizations have been observed in other environments, including the appearance of the blynx, a bobcat-lynx hybrid in North America, and the coywolf, which some research suggests is the result of recent interspecific mixing between coyotes and eastern wolves. As with Arctic bears, these hybrids are the product of overlapping habitats created by rapidly changing conditions. Their success depends heavily on context, but many are poorly adapted to their new niche and are often sterile. As these hybrids become more common, there is a risk that this will lead to further loss of biodiversity and ultimately the collapse of entire ecosystems. “If the two species play similar roles in their ecosystem, then increased contact and hybridization [are] “It’s less likely to have a big impact on ecosystem functioning and conservation,” says John Whiteman of Polar Bears International. Otherwise, it could have major consequences. For example, grizzly bears often leave carrion behind for other animals to eat, whereas polar bears do not. “The loss of carrion could wipe out scavengers, with broad implications for everything from food webs to disease dynamics,” he says.

Returning to our initial dilemma – whether this grolar dynasty could be the next step in the evolution of the Arctic bear – the answer is now clear: it’s a grim “no”. Rather than observing adaptation in action, we hear the fading echo of an endangered species, the product of profound instability and environmental crisis. However, being the holiday season, perhaps we can imagine that this drama doesn’t have to end in tragedy. If the remaining ice holds and the snow remains, polar bear tracks could continue across the Arctic for many winters. It’s possible. However, this happy future depends entirely on us.