Climate change promotes inbreeding among polar bears | Polarjournal
Increased isolation between polar bear populations may increase inbreeding in the future. (Image: Heiner Kubny)

A study of polar bears in the Norwegian Svalbard archipelago found that their genetic diversity is up to 10 percent lower than it was two decades ago. Sea ice in the Arctic is melting faster than ever before. This makes the habitats for polar bears smaller and more isolated. The researchers said that as the ice melts, polar bears are less able to migrate from one area to another, leading to increased inbreeding.

Researchers at the Norwegian Polar Institute in Tromsø collected small tissue samples from polar bears living on Svalbards islands over two decades from 1995 to 2016.

Researchers collect various tissue samples, take a biopsy from a layer of fat and DNA samples from an anesthetized polar bear. The tissue and blood samples collected by the researchers at the Norwegian Polar Institute will also be analysed for contaminants and various health parameters. (Photo: Brutus Östling)

Since the generation time of polar bears is about 12 years, the study includes at least two generations, especially considering that the polar bears were already up to 20 years old in the first years of sampling.

“We were surprised to find such convincing evidence of loss of genetic diversity and gene flow in only about 20 years,” said Snorre Hagen of NIBIO (Norwegian Institute of Bioeconomy Research). In particular, they showed significantly lower genetic diversity in polar bears from north-western Svalbard, where sea ice loss was particularly severe, compared to bears from the rest of the Svalbard archipelago. Increased isolation between populations may increase inbreeding in the future

(a) Distribution of polar bears sampled in the present study across four geographical areas of the Svalbard archipelago with sample sizes indicated.
(b) Global surface air temperature trends since 1979 (linear trends in °Cel. for December – February).
(c) Trends in sea ice extent in the Barents Sea for April (typically the month with the highest ice extent in the region) from 1979 to 2018. (Graphic: Maduna et al. (2021) Proc. R. Soc. B.288)

“Both the extent and rate of loss of genetic diversity were alarming and concerning, as polar bears already have low genetic diversity and are facing increasing climatic pressures,” Hagen further opined.

Increased inbreeding is likely to further weaken polar bear populations. The offspring of related individuals are less likely to survive and also have lower fecundity. This effect is known as ‘inbreeding depression’.

“A decrease in genetic diversity and likely inbreeding depression may lead to a further reduction in survival and productivity, which could lead to an increased risk of polar bear extinction,” Snorre Hagen further indicated.

“Although sea ice loss leads to litters of “increased numbers of close relatives,” this is not technically incest,” Dr. Hagen said.

“Strictly speaking, incest is a concept of inbreeding that relates to humans and has legal implications. In animals and plants, we simply refer to mating between genetically closely related individuals as inbreeding, he said.

“Usually mating between close relatives such as brother and sister is prevented by various biological mechanisms, and in polar bears the generally high rate of migration ensures that relatives rarely meet and mate under normal circumstances.

“However, it is questionable how well they can really recognize each other. Especially if they were born as half or full siblings from different litters.”

Heiner Kubny, PolarJournal

Original study: https://royalsocietypublishing.org/doi/10.1098/rspb.2021.1741

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