Predator-prey relationships are a natural part of nature – unfortunate for the prey, but necessary at the scale of ecosystems. As a scientist who studied carnivores, I sometimes forget that I was also a kid who sobbed whenever a fictional fur-bearer died. 

This is why writers need editors – they remind us that not every eight-year-old is emotionally equipped for “nature red in tooth and claw.” And then they make us take that scene out of the book!

So consider this your content warning: today’s post is about murder weasels.

“Murder weasels” is not their real name, of course – in North America, we call them ermines, while in Europe they’re often called stoats. And all members of the weasel family are pretty murdery – wolverines will face down wolf packs to protect a kill, and fishers hunt porcupines by biting their faces until they bleed to death. But not every member of the weasel family does the murder while being. So. Darn. Cute.

I mean, just look at that adorable little serial killer. There is a reason humans keep pet ferrets, after all.

But back to the science…

Just how murdery does an ermine get? Here’s a deleted scene from the first draft of Polar: Wildlife at the Ends of the Earth:

The ermine undulates though the spaces beneath the snow, scenting for lemmings. Even here, beneath a drift 1.4 m deep, cool air sucks the heat from her long, thin body. She needs at least 1/4 of her weight in food each day, just to keep warm and keep going. And she is pregnant. Unless she finds food soon, the babies in her belly will starve before they’re even born.

Here’s another lemming nest, but like the others she’s found this winter, it’s abandoned. All is not lost, though, because the little ermine planned ahead. She bounds back through the tunnels towards last year’s den. In the summer, when the hunting was good, she stashed 153 lemmings in a cavity beneath the rocks. The entrance to her cache is narrow, so foxes couldn’t raid it, and the permafrost kept the meat cool and fresh. The ermine withdraws a morsel from her icebox, curls up in her fur-lined nest, and begins to eat.

153 lemmings killed and stored for later. 153!!!

Dr. Benoît Sittler studies wildlife in Greenland, where ermine are found as far north as 83 degrees latitude. He tells me that ermine caches are hard for scientists to find, but several at least as large as this one have been discovered. Though the scale of the slaughter seems excessive, caching is an essential adaptation. In Greenland – and throughout the Arctic – lemming populations boom to huge numbers before crashing into scarcity. Caching is a predator’s insurance policy – by storing excess prey during booms, they guarantee they’ll have something to eat when live lemmings are scarce on the ground. The adaptation is especially important for small animals – like ermines – that can’t store much body fat against the lean times.

There’s one aspect of the ermine’s behaviour that makes these murder weasels seem particularly ruthless. They run through tunnels under snowbanks, seeking out lemmings that have nested in shelter under the snow. They eat the lemmings, and then nest in the fur they’ve pulled off their victims. Which, on the one hand, yikes. On the other, this is also a highly practical survival strategy. While ermines’ white winter fur provides excellent camouflage against snow, it’s not much warmer than their brown summer coats. Lemming nests – and lemming fur – provide a little extra insulation during frigid polar winters.

What do you think? Are ermines serial killers or survival experts? I’d love to hear from you.

Five more sleeps until Polar: Wildlife at the Ends of the Earth hits bookstore shelves! Pre-order a copy from your local independent bookstore to reduce your carbon footprint AND support a cornerstone of your community. You can also order from your favourite online retailers.

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