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A live red squirrel pup temporarily removed from its nest so that researchers can give it ear tags for identification and perform genetic testing. A new study has found that squirrel pups are at risk of infanticide from adult males during years when food is expected to be plentiful.Jessica Haines/The Globe and Mail

The grisly fact that some mammals kill the young of their own species in order to gain an advantage is well-documented in biology. But it’s not something that Jessica Haines ever expected to witness in a serene and snowy forest in southwestern Yukon where she was studying red squirrels.

It was early 2014 and Dr. Haines, now a postdoctoral researcher at the University of Alberta, was conducting field work for her PhD. The work was part of a long-term study called the Kluane Red Squirrel Project, which has been running for more than 30 years in a white-spruce forest located off the Alaska Highway about two hours west of Whitehorse.

One day, while doing her rounds of individual squirrel territories scattered around the study site, Dr. Haines heard an incredible commotion in the trees. Following the noise, she came upon a tense standoff between a male squirrel and a female surrounded by her young. The animals were only metres away from her and easy to recognize because of the coloured tags that scientists had put in their ears for quick identification. Based on the tags, Dr. Haines knew that the male was intruding into the female’s territory.

Just then, the male lunged forward and killed one of the baby squirrels before her eyes. It was the first time any biologist had ever seen such behaviour in the boreal rodent, and Dr. Haines knew it.

“I was pretty shocked,” she said, reliving her feelings at making a discovery of such a gruesome nature. “It was exciting and kind of horrifying.”

Later, Dr. Haines found the body of another baby squirrel from the same litter. None of the original litter survived but the female gave birth to a second litter that season in which the babies were found to be the offspring of the male she had seen attacking the young.

Over the next several months, Dr. Haines was able to gather more evidence to show that what she had seen was not a freak event. Her findings, published Thursday in the journal Ecology, suggest that infanticide is a normal, if hard to spot, aspect of red squirrel behaviour.

More curious still, Dr. Haines and her colleagues propose a connection between the bloody practice and the ebb and flow of resources in the red squirrels’ forest environment. Only when there is likely to be plenty of food around do males target squirrel pups for a bloody assassination.

To understand why, said Dr. Haines, it helps to know that female red squirrels mate during a relatively narrow window in the late winter. Each female may mate with a number of males, so that the litters that are born about a month later can have multiple fathers. But if a mother squirrel loses her young, she can give birth a second time in the same breeding season.

A male red squirrel that a biologist observed killing young sired by other males.The Globe and Mail

If food is likely to be scarce come fall, the delayed start could be fatal for the young squirrels that are born late in the season. The squirrels eat the seeds of white-spruce cones, which mature in late summer and which they gather and cache to survive the fall and winter.

“It’s really important to get that food source, especially if you’re a juvenile,” Dr. Haines said.

But when food is bountiful, even late-bloomers can survive. At such times, Dr. Haines reasons, it could be worth it for a male squirrel to kill off a female’s young in order to improve his chances of fathering her second litter.

Reinforcing this picture, Dr. Haines and her colleagues have found that infanticide by male red squirrels is a practice that only seems to occur during so-called “mast years” when the white spruce are especially productive.

“What’s neat is the squirrels can actually predict the food availability in the fall,” Dr. Haines said. “They already know in the spring if it’s going to be a mast year.”

Exactly how the squirrels discern this cue from the white spruce is unclear, but the researchers say this is what must trigger male squirrels to engage in their ruthless killing of unrelated young, something which does not occur during non-mast years for the spruce.

The discovery adds a new twist to the circumstances in which biologists have seen infanticide arise in mammal species, including some that are much more closely related to humans.

“I’m fascinated by this report,” said Sarah Hrdy, an anthropologist at the University of California in Davis who first documented infanticide by male langur monkeys in India in the 1970s and who is known for her books about mothering and infanticide among primates.

She observed that the masting phenomenon occurs across a range of tree species in different settings, including the oak trees of California, where infanticide occurs among birds. “I think this will probably generate more research on the interconnections between infanticide and masting,” she said.

Dr. Hrdy also praised the Canadian study for capturing the phenomenon so convincingly among the red squirrels.

“It’s really, really hard to observe infanticide in the wild,” she said.

Dr. Haines said the finding added to her own appreciation for how complex squirrel behaviour can be.

“They are really neat animals to work with and they also have all this amazing biology that we’re still learning about,” she said.