Arctic foxes suffer while reds thrive in northern Canada
Arctic fox sightings in northern Canada are at an unprecedented low this winter, according to wildlife guides.
And, unusually, the number of red foxes has simultaneously surged in the area, on Hudson Bay.
The surprising pattern has prompted observers to question whether the elusive Arctic foxes are being driven out of their dens by invading red relatives.
"It stopped dead, turned and ran," says Tera Ryan, wildlife guide at polar expedition company Churchill Wild, describing the time she witnessed an Arctic fox's reaction to a red fox travelling away in the distance.
Amazing Arctic foxes
"In the Arctic you conserve energy... This was running for fear. He did not want to be seen by that red fox."
Famed for their bright white coats in winter, delicate Arctic foxes (Vulpes lagopus) are not much larger than a domestic cat. Yet with their thick, insulating fur and increased blood circulation they are adapted to thrive in some of the world's most extreme conditions.
Previous studies have indicated that larger and more aggressive red foxes moving northward may outcompete their Arctic cousins for food and even kill the smaller species when the two collide on the same territory.
Arctic fox populations naturally fluctuate from year to year depending on the availability of their main food source, lemmings.
But the wildlife guides at Seal River lodge on Hudson Bay have reported the lowest number of Arctic fox sightings for years, despite what they say is a good year for lemmings.
The team have reported an average of two Arctic foxes spotted near their observation lodge in the same day, whereas "it would not be unusual to see a dozen or more per day in an average year," says Churchill Wild's Mike Reimer.
"Last year we had Arctic foxes everywhere you looked and no coloured foxes. And this year is completely different.
"This year it's coloured foxes... we've got red, silvers, crosses. And we've had the odd Arctic fox try to come in and the coloured [ones] are much more aggressive so they drive them off."
But biologist Dr Jim Roth from the University of Manitoba in Winnipeg, Canada says that while it is "possible" a surge in red foxes this season could have a temporary impact on Arctic fox numbers, the dip is more likely to have been caused by another factor such as food resources, disease or parasites.
Dr Roth's annual observations of both Arctic and red fox dens around the Arctic town of Churchill show that the success of the two species is highly correlated.
"However, in 2011 Arctic fox den success was among the lowest ever recorded, while red fox den was among the highest," he says.
Dr Roth concluded that a different prey species such as snowshoe hare may have been abundant in red foxes' forest habitat but was not available to Arctic foxes hunting on the tundra terrain.
Despite this evidence, some experts believe that red foxes gradually moving further north are a major threat to Arctic foxes.
In Russia, reds have been observed taking over Arctic fox dens and scientists have occasionally found Arctic fox remains around some red fox dens.
"Being bigger, the red fox tends to exclude the Arctic fox from its habitat," explains Dr Dominique Berteaux from the University of Quebec, Rimouski (UQAR) in Canada.
"They occupy the same ecological niche and are in direct competition."
However, Dr Roth argues that in general, "changes in food availability and disease" are "more likely to have greater impacts" on Arctic fox numbers.
The change in red fox distribution, with the species pushing further northwards, has been associated with climate change in the Arctic.
Warmer conditions allow red foxes to travel further north as they are more likely to survive without the special adaptations of the Arctic species.
But Dr Berteaux, who has conducted a number of studies into Canada's Arctic foxes, believes we may actually be more directly accountable for the species' movement.
"Red foxes follow humans," he tells BBC Nature.
"In the last 60 years many villages have established in the Arctic and red fox benefit from the dump sites where they scavenge on human garbage."
Dr Bertaux simply summarises that the red foxes "have more food available now than in the past" but his fellow biologists continue to debate the issue.