By Ella Davies
Earth News reporter
The subordinate Arctic fox only barked at the intrusion
Russia's Arctic foxes are under threat from an expanding population of red foxes, according to scientists.
For the first time, a red fox has been observed intruding on an Arctic fox breeding den in Russia's far north.
The Arctic fox abandoned its den to the dominant intruder, leaving pups to fend for themselves.
Researchers say this is evidence that red foxes are expelling Arctic foxes as a warming climate allows them to survive much further north.
Arctic foxes (Vulpes lagopus) live in mountainous tundra habitats around the north polar region.
Previous studies have identified the foxes retreating from the southern edge of their range.
Scientists say that this is because red fox populations are expanding and, where the two overlap, red foxes dominate their smaller Arctic cousins.
Although direct observations of this competition had been made in Sweden, researchers from Russia and Norway wanted to find out whether Arctic foxes further north were under the same pressure.
Their findings were published in the journal Polar Biology.
"We were surprised to meet a red fox in our study area on the Russian Arctic tundra because this species is very rare in such northern territories," explained researcher Anna Rodnikova.
She and her team observed a red fox approaching an Arctic fox breeding den on the Yamal Peninsula, in north-western Siberia, during their summer study.
The Arctic fox was not at the den but when it returned it did so hesitantly, stopping 50m away and barking when it saw the red fox.
Although the red fox looked in poor condition, it was clearly the dominant one during this encounter.
After these initial defensive barks, the Arctic fox withdrew from the competition and did not return to the den.
"It was strange to see that the [Arctic fox] mother was so afraid of the red fox that it didn't try to protect the pups, which probably were inside the den," said Ms Rodnikova.
But the scientists said that it was unlikely that the red fox killed the pups and they found no evidence of their remains.
The team wrote in the paper: "The arctic fox pups were most likely hiding in the burrow system while the red fox was present and abandoned the den area after the encounter.
Researchers suggest that this encounter is strong evidence that red foxes are responsible for the arctic foxes' retreat.
Their study was part of a project using predators as indicators of changes in the Arctic.
Although their numbers are in the healthy hundreds of thousands, Arctic foxes are still considered an important indicator of climate change.
"We don't think that climate warming makes conditions directly more difficult for Arctic foxes," said Ms Rodnikova.
"Most likely climate warming allows red foxes to survive in severe northern conditions, so [they] have an opportunity to expand their range to the North where they dominate over Arctic foxes."
Red foxes are 25% larger than Arctic foxes but lack their adaptations to severely cold habitats including their superbly insulating fur coat and fur-covered feet pads.