The proof is in the paw print: how researchers are monitoring polar bear populations

Researchers can now extract environmental DNA from paw prints left in the snow by threatened species, such as polar bears and lynxes, and use it to monitor their populations.

Polar bears and lynxes, like many arctic species, are vulnerable and elusive, making them very difficult to track and monitor. Rather than trying to these elusive animals, an international team led by Melanie Lancaster of the World Wide Fund for Nature (Solna, Sweden) developed a method for capturing DNA from skin cells from their paw prints in the snow. By analyzing this environmental DNA, researchers hope to gain more information about these species’ populations and how they are coping with climate change to aid conservation efforts.

There are a few reasons why physically capturing and tracking polar bears, for example, can be problematic. Firstly, it is dangerous and stressful for researchers. Secondly, individuals residing in areas near polar bears have expressed concerns about the welfare of the bears being captured and the invasive research methods that have been used in the past. Previous non-invasive environmental DNA studies of polar bears have relied on fecal samples; however, these samples don’t always offer high-quality DNA, limiting researchers’ understanding of the population they’re trying to monitor.

To provide a more suitable option to study these creatures, the team developed a method based on existing forensic techniques, which can be used to identify and analyze tiny, degraded DNA samples in skin cells gleaned from paw prints in the snow. “The tracks usually contain fresh cells, and the DNA is intact because of the cold ‘storage’ temperature. DNA that has passed the gut is much more degraded and therefore more challenging to work on,” commented first author Micaela Hellström (MIX Research Sweden, Uppsala, Sweden).

The team collected snow from the tracks of Alaskan polar bears and Swedish Eurasian lynxes both in the wild and in captivity. They then melted and filtered the snow, extracting environmental DNA before conducting microsatellite analyses. They also collected saliva, hair and mucus to confirm that the genotypes provided by the snow were accurate.

From 24 wild polar bears and 44 wild lynxes, the team managed to retrieve nuclear DNA from 87.5% of wild polar bear tracks and 59.1% of wild lynx tracks. Excitingly, the team succeeded in genotyping 13 of the wild bear samples, identifying 12 individual bears. Although the DNA concentrations in the wild samples were low, the team was still able to gain a great deal of genetic information.

Looking ahead, the researchers are hopeful that this technique will help inform the conservation of arctic species as it will allow us to better understand these populations and their behavior. “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,” concluded Lancaster.