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The stress of climate change seems to be making polar bears today age at a faster rate than those born just over half a century ago.

In Hudson Bay, Canada, at the southern edge of the Arctic, the ice-free season has lengthened by approximately , reducing the time that polar bears have to hunt and travel on the ice in search of their prey.

The polar bear (Ursus maritimus) population in the bay has been subject to annual sampling and monitoring since 1980. Since monitoring in the area began, the population appears to have halved to just over 600 individuals.

at the University of Manitoba, Canada, and his colleagues assessed the health of polar bears by looking at chemical tags that accumulate on their DNA, known as epigenetic markers. Epigenetic markers are influenced by environmental factors, including stress, and they regulate the activity of genes. Some of them can be used to estimate an animal鈥檚 biological age.

Using recent tissue samples and archived tissue samples from individuals with known chronological ages, Newediuk and his colleagues were able to look for differences in cellular ageing in polar bears over a 40-year period in which .

They found that individuals born since 2010 have a biological age that is on average 2.6 years greater than bears with the same chronological age born in the 1960s.

By analysing genetic changes in the population over time, Newediuk and his colleagues also found that the Hudson Bay bears don鈥檛 seem to be adapting to climate change.

鈥淲ith increasing exposure to harsh environments, declines in abundance, and little evidence for adaptive capacity, the western Hudson Bay polar bear population faces an uncertain future,鈥� they say in their paper, which has yet to be peer-reviewed.

at the University of Alberta, Canada, says that the study increases our understanding of the effects of climate change on polar bears.

鈥淲e have understood that stress increases in polar bears in response to reduced sea ice from monitoring of cortisol levels, but this new paper adds an additional layer of insight,鈥� he says.

Derocher says that polar bears face a suite of challenges alongside climate change, including disease, parasite exposure and pollution, which could all affect epigenetic ageing.

鈥淯ltimately, however, these sorts of genetic changes are likely less impactful than the direct mortality and loss of reproductive output that results from a longer ice-free period,鈥� he says.