Keeping track of polar bears through DNA footprints

Counting polar bears and snow leopards isn’t easy, but it’s critical if we are to monitor their survival, so a new approach – to extract DNA from their footprints – might help.

As the planet warms and ice caps melt, keeping track of vulnerable polar animals becomes ever more important. But current methods of following individual animals are often invasive and tedious.

Scientists have now developed a new tool – DNA analysis using skin cells shed in footprints left behind in the snow.

The research is published in Frontiers in Conservation Science.

It’s hoped the new method could help in critical population monitoring efforts. Important data about population size and connectivity which is currently missing might be made available.

“It is particularly challenging, to find polar bears in the Arctic, let alone count them and understand how they are coping with climate change,” says senior author Dr Melanie Lancaster of the World Wide Fund for Nature Global Arctic Programme.

Polar bears might be the largest land predator in the world, but that doesn’t make them easy to spot. They can weigh between 300 and 800 kg, but these arctic apex carnivores are vulnerable. Sea ice loss from climate change is listed as the single biggest threat to their survival. It’s estimated only about 26,000 individuals live in the world today.

The new method also means it is unnecessary to capture bears – stressful and dangerous for both bear and humans.

“Many Inuit express concern about invasive research methods,” says co-author Elisabeth Kruger of the World Wildlife Fund. “People are concerned about the welfare of the individual polar bear and the health and safety of people who may harvest the bear later. This is one of the reasons we are so excited about new methods like this – the person collecting the sample never needs to even see or be seen by the polar bear.”

Forensic techniques can be applied to tiny, degraded DNA samples. Sometimes, DNA can be left behind by an animal in passing – environmental DNA.

Faeces contains environmental DNA. But the quality is not great. For territorial animals – like the two other species involved in the study; lynxes and snow leopards – sampling faeces could affect the animals’ behaviour.

But skin cells left behind in snowy footprints also contain DNA.

“The tracks usually contain fresh cells, and the DNA is intact because of the cold ‘storage’ temperature,” explains lead author Dr Micaela Hellström of MIX Research Sweden AB. ”DNA that has passed the gut is much more degraded and therefore more challenging to work on.”

By collecting snow from individual tracks made by Alaskan polar bears and Swedish Eurasian lynxes in the wild and in captivity, and a captive snow leopard,  the scientists were able to confirm that the tracks provided accurate genotypes for individual animals.

The researchers melted and filtered the snow to collect environmental DNA. Although the concentrations of DNA retrieved from tracks sampled in the wild were very low, it was possible to retrieve nuclear DNA from 87.5% of wild polar bear tracks and 59.1% of wild lynx tracks. 13 wild polar bear samples could be genotyped, identifying 12 individuals.

11% of the lynx tracks could be genotyped, but when the scientists only looked at the tracks sampled by trained personnel, this rose substantially.

“We hope this method will be taken up by the polar bear research community, with the involvement of hunters, volunteers, and Indigenous communities, as a new way to collect information on polar bears,” Lancaster says. “We also hope the method will be expanded to other animals living in snowy environments – we have shown it works for lynx and snow leopards as a start.”