The Washington Post
may have a harder time eluding predators in a world with less snow.
The best-preserved remnant of primeval European woods is the Bialowieza Forest along the border between Poland and Belarus. Wolves and rare European bison walk beneath the old-growth trees. Scurrying lower to the ground are Mustela nivalis, also known as least weasels.
Camouflaged fur helps the weasels avoid wolves and other predators. In the Bialowieza Forest, the least weasel comes in two subspecies. One wears a brown coat all year. The other sheds its brown coat each autumn to reveal a bright winter white, an adaptation to blend in with snow.
But there was no snow cover when biologist Karol Zub of the Polish Academy of Sciences Mammal Research Institute observed an entirely white weasel hunting rodents. Its fur stood out in sharp contrast to the green moss. “At this very moment,” Zub said, “I thought to myself: ‘Wow! If this animal is so well visible, it must be a perfect target for predators.”
His instincts were right. Snow no longer blankets the forest for as long as it once did, Zub and his colleagues wrote in a paper published Thursday in the journal Scientific Reports. And there are fewer white weasels.
The team captured 118 weasels in the forest between 1997 and 2007. “In our study, the densities of weasels are about four to five times lower than 10 years ago,” he said. As the number of days without permanent snow cover increased, the proportion of white weasels dropped precipitously.
This drop is a case of what wildlife biologists call camouflage mismatch: White fur, which should be a stealthy trait, becomes a vulnerability when there is no snow. “Camouflage mismatch may cause the local extinction of white subspecies,” Zub said, or at least a severe reduction in population.
L. Scott Mills, a wildlife biologist at the University of Montana who was not involved with this study, called it a “fascinating and important paper.” Mills has studied color mismatch in snowshoe hares, which also turn white in winter. The hares’ probability of surviving through the week decreased by as much as 7 percent when their camouflage did not match their environment, Mills and his co-authors reported in Ecology Letters in 2016.
Dips of just a few percentage points might not seem like a major problem, but the cumulative effect over a winter period can “seriously influence the mortality of this species,” Zub said.
There are 21 species known to change the color of their coats in winter, Mills said, including six species of hares, three weasel species, hamsters, lemmings and arctic foxes. He predicts the other coat-changers are faring about as poorly as the snowshoe hares and least weasels.
In the new work, the study authors analyzed weather data in the Bialowieza Forest going back to 1967. The average period with permanent snow cover decreased from about 80 days per year to 40 days. Historically, the snow cover vanished around March 16. But in recent years, it disappeared by Feb. 21, more than three weeks earlier in the season.
Zub and the other authors conducted a field experiment with mock weasels made out of toy rats purchased from Ikea, slimmed and elongated into more weaselly silhouettes.
They placed toy brown or white weasels in front of camera traps, in snow cover or on bare ground. Predators — buzzards, common ravens, gray wolves, raccoon dogs and red foxes — were far more likely to attack, sniff or otherwise harass the mismatched weasel models, particularly the white weasels in areas without snow.
The mechanism of natural selection, Zub said, is clear: Those animals that blend in live longer and reproduce. Those that clash with their environment are eaten.
A few weasels are adjusting. Normally, Zub said, the weasels molt into white in November. But now they are delaying their wardrobe switch. “About half of weasels we observed last winter started to molt at the beginning of January,” he said.
Life on Earth is reacting to climate change. Bears appear to be hibernating for fewer days. Tropical fish are swimming away from the equator in search of cooler waters. Some animals have moved up mountains to escape increased temperatures. Some plant species are creeping downward to embrace it.
But camouflage mismatch might be one of the most glaring effects. After all, it’s hard not to notice a flash of white fur in a dark forest.