Despite the expectation that ‘grolar bears’, or ‘pizzly bears’, are on the rise, hybridisation between grizzlies and polar bears is actually very rare, according to new research.
Hybridisation between grizzly bears and polar bears is extremely rare, according to a new study. The two species of bear are genetically similar and have been known to hybridise in zoos, as well as examples recorded in the wild.
Researchers studied 819 wild grizzly and polar bears across Canada, Alaska and Greenland, encompassing four subpopulations.
Their investigation was a response to the expectation that the numbers of polar-grizzly hybrids (from the two bear species breeding) were increasing, as climate change causes the ranges of grizzlies and polar bears to expand and overlap.
But only eight hybrids were found to exist, amounting to one per cent of the sample. The results underscore the rarity of the phenomenon, according to the research team, which included representatives from Environment and Climate Change Canada, Polar Bears International, University of Manitoba, MacEwan University, Government of Northwest Territories and San Diego Zoo Wildlife Alliance.
Polar bears evolved from grizzly bears several hundred thousand years ago. Although ‘gene flow’ has occurred between the two species in the past, it appears recent hybridisation is restricted to a small group of polar bears and grizzly bears in the western Arctic.
Hybrids of grizzly and polar bears are often referred to as ‘grolar bears’ or ‘pizzly bears’ depending on the paternal lineage: a grolar bear is when the father is a grizzly and a pizzly bear is when the father is a polar bear.
How to genetically analyse grolar and pizzly bears
The researchers ‘Development of an 8K SNP chip to assess adaptive diversity and hybridization in polar bears’ study was recently published in the Conservation Genetics Resources journal, based on samples of polar bears, grizzly bears and hybrids collected between 1975 and 2015.
It’s the first-ever large-scale analysis of how often hybrids of polar and grizzly bears exist in the wild, made possible by developing a new SNP 8K genotyping (genetic sequencing) chip, called Ursus maritimus V2 SNP chip, to analyse samples from polar and grizzly bears in the lab.
The chip reads genetic samples, allowing rapid and reliable genome analysis and spots polar bear-grizzly hybrids with 100% accuracy.
The study confirmed the existence of eight known ‘grolar bears’ in the wild as descendants of one female polar bear. A single female polar bear mated with two grizzly bear males and produced four hybrid cubs. One of these female cubs mated with both of the grizzly bear males (one of which was her father) to produce four additional hybrids.
Is climate change causing hybridisation to increase?
Hybridisation between the two species is a recent occurrence (the study found the first recent case to be from 2006), suggesting that it is a result of warming temperatures causing grizzly and polar bear habitats to increasingly overlap.
Hybridisation is only possible in areas where grizzly and polar bear distributions cross over, which therefore excludes regions like the High Arctic.
Though uncommon, hybridisation is expected to increase as climate change continues, meaning that in the future, if global temperatures continue to rise, polar bears could become a thing of the past in the wild.
“We expect this to be the most likely outcome of frequent hybridisation,” says Dr Ruth Rivkin, postdoctoral research fellow with Polar Bears International, University of Manitoba and the San Diego Zoo Wildlife Alliance.
“We expect hybridisation will become more common. Grizzly bears are pushing further north into habitats that were previously only occupied by polar bears.
“We expect polar bear range will shrink, and that they will be limited to more northern parts of their range.
Is hybridisation good for bears?
“Hybridisation is probably not a good thing for polar bears,” says Rivkin. “While a small amount of hybridisation might help polar bears survive in warming environments, the most likely outcome of hybridisation is the loss of polar bears as a unique species.
“Our work shows that, to date, hybridization is a very rare event. But monitoring is needed as warming temperatures drive grizzly bears further northward into polar bear habitat.”
Main image: growling grizzly bear, also known as a North American brown bear. Credit: Getty
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