Male red squirrels kill offspring of rivals, new University of Alberta study suggests

Wildlife biologist Jessica Haines of the University of Alberta was conducting routine fieldwork in Yukon in spring 2014 when she heard a commotion in the trees.

A male red squirrel had intruded on a nest of newborn pups, attacked one and killed it — right before her eyes.

"I was excited, on the one hand, but also kind of horrified and fascinated to be seeing this all at once," Haines said.

Haines had observed a phenomenon called sexually selected infanticide, behaviour previously undocumented in red squirrels.

In years when food is plentiful and female squirrels produce two litters of pups, male squirrels kill the offspring of their rivals.

Their motive for infanticide is a second chance at fatherhood, Haines said.

"The offspring are not his own, so he is reducing the success of his competitors," said Haines, a postdoctoral fellow in the Alberta school's department of biological sciences.

"And by doing that, he's also hoping to increase his chances of fathering offspring."

Haines was the first to observe and document the behaviour in red squirrels. Her research was published Thursday in the journal, Ecology.

After witnessing the attack, Haines used genetic testing to identify the killer. Haines also tagged the animals to monitor pups and gather evidence.

Many of the pups had injuries consistent with bite marks she saw on the dead pup, Haines said.

Mast years and shifting male behaviour

Squirrel infanticide is linked to fluctuations in white spruce cones, the main food source for red squirrels, Haines said.

It only happens during mast years — when spruce trees produce more cones more than usual.

During those plentiful years, female squirrels can have two litters instead of one.

Females breed in spring and cones mature in fall, so the squirrels predict future cone availability when they have the second litter, Haines said.

Despite the prevalence of sexually selected infanticide, more squirrel pups survive in years of food abundance than in other years.

The behaviour is more common among species that live in groups and experience more sexual competition, Haines said.

"We were surprised to see this behaviour in squirrels, because they live alone in separate territories and do not monopolize a single female during the breeding season," said Haines, who conducted her research in partnership with the Kluane Red Squirrel Project, a decades-long field study of red squirrels in Yukon.

"It's fascinating." 

With assistance from her colleagues in the North, Haines plans to continue her research on the mating habits of red squirrels.

Even after discovering their capacity for violence, she still has a soft spot for the chattering rodents.

"I fell in love with working with squirrels," Haines said. "This project has been going on for almost 30 years, and yet, we still are finding out new things about these squirrels."