The range of polar bears in northern Canada overlaps in some places with that of grizzly bears. Grizzlies (Ursus arctos horribilis), a subspecies of brown bear, migrate farther north in search of food and a territory, and males have been spotted on sea ice repeatedly for more than 100 years. So every now and then grizzlies and polar bears meet, occasionally even leading to successful mating. The offspring is then a mixture of both species. Is this interbreeding possibly even a response of the bears to climate change?
Hybridization in the Arctic and sub-Arctic has not only been observed in polar bears and grizzlies. There is also evidence of interbreeding between narwhals and belugas, bowhead whales and right whales, and Dall’s porpoises and common porpoises.
A 2010 study identified at least 34 possible hybridizations between separate populations, species and genera in 22 Arctic and sub-Arctic marine mammal species(Nature, B. Kelly, A. Whiteley & D. Tallmon, 2010). Hybridization between species is a threat to Arctic biodiversity because the more hybrids that occur as a result of the encounter of originally isolated populations or species, the more likely it is that rare species will become extinct because as genomes mix, their adaptations to habitat are lost. At the moment, however, quite little is known about how frequently hybridization actually occurs and what effects it has on populations. Even if mating between two species were to occur more frequently, it is not certain whether the offspring would be viable or whether pregnancy would occur at all. In Arctic marine mammals, however, the probability of this happening is quite high, as their chromosomes have changed little over time.
When polar bears and grizzlies meet, the encounter is usually aggressive. Unless it’s mating season, which for both species falls in the spring.
For the past 15 years, it has been known that both species successfully interbreed with each other. Recent DNA analyses even show that since the time about 500,000 years ago, when the species evolved separately, interbreeding between the two bear species has occurred repeatedly and both still carry ancient DNA of the other species.
The first genetically confirmed hybrid of a polar bear and a grizzly in the wild was a bear killed by a trophy hunter in 2006 in Canada’s Northwest Territories. Four years later, an Inuvaluit hunter in northwestern Canada shot another hybrid bear. DNA analysis revealed that this bear was the offspring of a female Pizzly, thus already the second generation, confirming that hybrid bears are not only viable but also fertile. In total, eight hybrids could be genetically proven so far, all descended from the same female: four offspring in the first generation and four more by backcrossing their offspring with grizzlies.
The names Pizzly and Grolar are composed of the English species names “Polar bear” and “Grizzly” depending on whether the father is a polar bear or a grizzly: Pizzly if the father is a polar bear, and Grolar if the father is a grizzly. Canadian wildlife officials proposed Nanulak from the Inuit words “nanuk” (polar bear) and “aklak” (grizzly bear).
Traits of Pizzlies and Grolars
Hybrid bears are distinguished by a mix of characteristics:
- the body is smaller than a polar bear’s but larger than a grizzly’s
- the shape of the head lies between the broad skull of the brown bear and the slim head of a polar bear
- the neck is about as long as that of a polar bear
- the humps on the shoulders come from the grizzly
- the soles of the feet are partly hairy
- the fur shows a pattern of hollow hairs like in polar bears and solid hairs like in grizzlies
- two hybrids from Osnabrück Zoo in Germany showed typical polar bear hunting behaviour, but were not able to swim as well as polar bears
With this mixture of adaptations perfect for their original habitat, the hybrids are not well equipped for either land or water and sea ice habitats. With the “little bit of everything,” they are a “pretty poor polar bear and a pretty poor grizzly,” according to Dr. Evan Richardson, polar bear researcher with Environment and Climate Change Canada.
What does the emergence of hybrids mean for polar bears?
Scientists are not in complete agreement as to whether bears are encountering each other more frequently because of climate change. However, it is known that grizzly bears have been sighted on the Arctic sea ice since at least 1885. Some scientists say that this is the reason for the interbreeding, while others believe that polar bears are expanding their range southward because of the warming of the Arctic.
However, if mating becomes more frequent, it may not bode well for polar bears. Evolutionary geneticist Eline Lorenzen of the University of Copenhagen and biologist Andrew Derocher, a professor at the University of Alberta in Edmonton, Canada, are concerned that the dominance of grizzlies could lead to the loss of polar bears’ unique genetic traits.
“Ultimately, one species will be integrated into the other, and it’s likely that it will be polar bears that integrate into brown bears,” Lorenzen says.
Whether this really happens remains to be seen. The crossbreeds are still so rare that, according to Derocher, there is no danger to polar bears from hybridization in the near future. In addition, pizzlies and grolars are not a protected species and are therefore popular among trophy hunters.
Julia Hager, PolarJournal
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