Death takes many forms for the red squirrels of the Yukon: Birds of prey strike from above while silent lynx stalk the snows below. And sometimes, red squirrels slay their own.

In a study published this week in The Scientific Naturalist, researchers report that North American red squirrel pups often fall victim to attacks from nearby males. Sometimes these murderous males also eat the youngsters they kill.

Jessica Haines of the University of Alberta witnessed some of these aggressions, called infanticide, as part of the Kluane Red Squirrel Project near Kluane Lake in Canada’s Yukon Territory.

The first time Haines saw this grisly behavior was in 2014. A male squirrel she knew on sight—each is marked with bits of colored wire on the ear—ran up a tree to a female’s nest and sunk his teeth into a pup, which fell through the branches and died. A week later, Haines found another dead pup. Eventually, the whole litter would disappear.

View Images Scientists temporarily removed this squirrel pup from its nest during their research. The pups are tagged and tracked for the rest of their lives to learn about their lives and behavior. A small sample of ear tissue is DNA-tested to determine each squirrel's paternity. Photograph by Jessica Haines, Unversity of Alberta

Using DNA analysis, Haines and her coauthors determined that the squirrels doing the killing were not the fathers of the pups being killed. What’s more, dogged observations revealed that infanticide skyrockets in years when food’s abundant.

It might seem counterintuitive that more killings happen in “good” years, but it makes sense given that female squirrels are also much more likely to have a second litter of pups in years when there’s more food around. During the first round of breeding, females mate with multiple males, so there’s no good way for a male to know which pups are his. But if the male kills those pups, the female’s body stops lactating and she becomes ready to mate again, allowing him to guard her and ensure that he sires her pups later in the year.

Haines says she’s eager to show people the other side of an animal they think they already know so well.

“Squirrels also eat things like baby bunnies, baby birds, and birds’ eggs,” she says.

Nature Red In Tooth And Squirrel

Infanticide is not unheard of in the animal kingdom. In fact, some species, such as lions, have become infamous for some males killing all the young when they take over a pride.

But most people wouldn’t imagine fluffy, squeaky squirrels to be among the animals that kill infants of their own kind.

View Images This female red squirrel's pups were killed by a male. Biologist Jessica Haines says she often saw the mother sitting near her nest (right) around the time of the attacks, possibly trying to guard her young from being attacked. Photograph by Jessica Haines, Unversity of Alberta

Haines says people usually view squirrels as pets or pests. But the truth is that North American red squirrels are wild animals that do what they have to do to survive.

Sometimes, Haines says, the pup carcasses show signs of being eaten by the males. Other times, males could be seen stashing the carcasses away, presumably so that they could eat them later.

This sort of caching is common among squirrels, and the animals have been known to stockpile all sorts of interesting things in their middens, or store-rooms. Haines says she’s seen dried mushrooms as well as bunny legs—and one time, a jawbone from a coyote.

“It was close to the highway, so it could have been from roadkill,” she explains.

View Images Red squirrels appear to be able to tell in advance when white pine trees are going to have a "mast year," which creates a boom in seeds that the squirrels eat. Photograph by Jessica Haines, Unversity of Alberta

Many, if not most, squirrel species show evidence of infanticide, says John Koprowski, a conservation biologist at the University of Arizona and author of Squirrels of the World.

“Infanticide likely has a much more important influence on the evolution of behavior of animals than we currently appreciate,” he says.

Secrets of the Pines

While it’s clear that the red squirrels of the Yukon are committing infanticide and that the practice increases in “mast years,” which are years with booms in white pine seeds, one big mystery remains.

“The neat thing about squirrels is they actually can predict when a mast year is about to occur,” says Haines.

You see, females have more pups per litter in mast years. But the white pines don’t put out cones until the fall, and mating occurs the February or March before. The rise in infanticide also occurs before the pine cones have even sprouted. How, then, do the squirrels “know” that a boom year is coming?

View Images This squirrel pup, the sibling of one that was observed being killed by a male, was also attacked and killed. Photograph by Jessica Haines, Unversity of Alberta

Haines says the squirrels have a hunch. In mast years, it’s been found that plants have higher concentrations of hormones called gibberellins, so perhaps the squirrels are able to sense these compounds in the spring when they’re eating the trees’ buds.

It all just goes to show how much we still have to learn about these seemingly ordinary creatures. The Kluane Red Squirrel Project has been ongoing for nearly three decades, says Haines.

View Images This newborn pup had wounds similar to the bite marks seen on other pups attacked by adult males. Photograph by Jessica Haines, Unversity of Alberta

“And even after all that time, we’re still learning things about squirrels.”