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

Meet our story’s protagonist: a female polar bear. Displaced by shrinking sea ice in the Arctic, she was forced to wander south, deeper into the Canadian Northwest Territories. Here, our lady in white encountered a couple of handsome grizzly bears. She fell for both of them and had two cubs by each – three “grolar bear” daughters and a son. Thus began a remarkable dynasty, a lineage as intertwined as any in a Shakespearean tragedy.

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

In 2006, a hunter in the Canadian Arctic shot dead an animal displaying physical characteristics of both grizzly and polar bears. Genetic tests later confirmed that it was a grolar, a member of this modern hybrid family. A decade afterwards, when researchers revealed the intriguing interrelationships between these animals, the scientific community was baffled: the offspring of cross-species matings are usually sterile, yet here they were clearly fertile. Biologists wondered whether this might be a prelude to the emergence of a new Arctic apex predator. Could these hybrids be an adaptive success story born from the chaos of climate change? Or are they an ecological warning sign of things to come? Now, with new research, we can answer these questions.

As Earth heats up, 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 blubbery seals, making them dependent on sea ice as hunting platforms. With the volume of ice declining at insurmountable rates, they are being pushed inland in search of new food sources. Here, they are increasingly likely to encounter their brown bear cousins, grizzlies (Ursus arctos horribilis), which are moving north as warmer temperatures in the High Arctic allow them to broaden their hunting and mating grounds. This has sparked speculation that hybridisation between the two species might become a regular occurrence, giving rise to growing 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 into the brown bear population might contribute to the extinction of the former.

Yet, the discovery of the grolar dynasty hints at a more positive outcome: that polar bears might be adapting to the new world order. The documented grolars look like greyish-brown polar bears with slightly larger feet and skulls, reminiscent of grizzly features. These traits led to conjecture that the hybrids might be able to forage for a broader, more terrestrial range of food than their polar parents, possibly allowing them to hunt on land and not be dependent on dwindling sea ice. In some sense, pizzly bears could be a sad but necessary compromise, given current warming trends, said palaeobiologist Larisa DeSantis at Vanderbilt University in Tennessee in an interview in 2021.

One happy family

However, a large-scale genome study published last year paints a different picture. When researchers led by Joshua Miller at MacEwan University in Canada compared genetic sequences from 371 polar bears, 440 grizzlies and members of the grolar dynasty, they had a shock. The analysis indicated that there are no wild hybrids except the grolars that we know of. The confirmed rarity of this mating pattern now leads experts to believe that hybrids lack the critical skills to thrive in the environment of either of their parents. Indeed, recent studies demonstrate that they are ill-suited to Arctic living because they lack the polar bear’s unique “non-slip” paw morphology and also aren’t fully equipped with the physical traits that grizzlies 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 grizzlies.

In the short term at least, it seems that such hybrids will remain a rare occurrence and so won’t lead to any new species. But what of the long-term evolutionary future of Arctic bears? Perhaps a look at the past might reveal what is in store. During the Pleistocene, a geological epoch marked by repeated glaciations that began about 2.6 million years ago, the habitat of polar and brown bears also overlapped. Researchers from the University of California, Santa Cruz, have found that brown bears living on three Alaskan islands today emerged gradually during that period as the result of male brown bears dominating mating with the resident polar bear population. If this pattern plays out again, it is possible that there will be a remorseless erosion of the polar bear species until only grizzlies remain.

However, the Pleistocene was characterised by slow, cyclical environmental changes, a timeframe that allowed for gradual adaptation. Today’s human-driven climate change is happening at an unprecedented rate, so things could work out differently this time. “The far bigger threat to polar bears today is the loss of their ice habitat due to anthropogenic climate change. This is something that will come about much faster than any threats from hybridisation,” says Fiona Galbraith, a geneticist who has worked as a climate change consultant and now leads expeditions in the Canadian Arctic for Natural Habitat Adventures.

So, the grolar dynasty is an oddity, but it is also an emblem of our planet’s ecological breakdown. Similar climate-induced hybridisations 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 interspecies mingling between coyotes and eastern wolves. As with bears in the Arctic, these hybrids are the product of habitat overlap created by rapidly changing conditions. Their success is highly context-dependent, but many are poorly adapted to their new niche, and they are often sterile. As such hybrids become increasingly common, there is a risk that this will lead to further loss of biodiversity and, eventually, the collapse of entire ecosystems. “If both species play similar roles in their ecosystem, then increased contact and hybridisation [are] less likely to have a big impact on ecosystem function and conservation,” says John Whiteman at Polar Bears International. If not, it could have major consequences. For example, grizzlies often leave carrion for other animals to feed on, whereas polar bears do not. “The loss of carrion could wipe out scavengers, with broad ripple effects for everything from food webs to disease dynamics,” he says.

Circling back to our original quandary – whether this grolar dynasty might be the next step in Arctic bear evolution – the answer is now clear: it is a bleak “no”. Rather than observing adaptation in action, we are hearing the fading echo of a disappearing species, the product of profound environmental instability and crisis. Being the holiday season, though, maybe we can entertain the idea that this drama doesn’t have to end in tragedy. If the ice that is left holds, and the snow remains, polar bear tracks could keep crossing the Arctic for many winters to come. It is possible. However, this happy ever after is entirely up to us.