I was walking in the Yukon one evening when I heard a commotion in the forest. At the time, I was working on my PhD under the supervision of Stan Boutin at the University of Alberta as part of the Kluane Red Squirrel Project. This meant that I spent a lot of time hiking on my study area monitoring the red squirrels that I was working with. It is common to hear squirrels calling because they are pretty chatty, but this commotion was different: this squirrel was very upset.
I walked quickly towards the calls, arriving to find a female squirrel yelling at her male next-door neighbor, who had intruded onto her territory. This is unusual, as squirrels live solitary lives and are usually respectful of each other’s boundaries. However, as I watched the male’s intentions soon became clear because within seconds of my arrival the male killed one of the female’s pups. I realized that I had just witnessed infanticide.
Infanticide is when an adult kills the young of their own species. I later found a second dead pup from the same litter, whose wounds were consistent with the infanticide that I had watched. The whole litter eventually died and their mother, the female I had seen yelling at the male, later had a second litter that summer. I was able to show using genetics that the male who killed the pup was not the father of any of the pups in the first litter, but that he was the father of all of the pups in her second litter.
This is an example of sexually-selected infanticide: this is when a male kills another male’s offspring in order to increase the chances that he’ll be able to father kids of his own when the female breeds again. I was fascinated by this behaviour, so I decided to explore it in more detail in our paper that was published today in the journal Ecology. I found evidence that this behaviour is linked to fluctuations in white spruce cones, the main food that red squirrels eat.
My colleagues previously showed that red squirrels can predict the future cone availability. The cones mature in autumn and at that time they can be harvested by the squirrels to be stored in a cache on their territory called a midden. Squirrel pups strike out on their own in the fall and during a bumper crop, called a mast year, they have a really good chance of surviving the winter because there are lots of cones for them to harvest and cache. Very few pups are able to store enough food in non-mast years so few of them will survive. Having access to cones is really key for pups to survive.
Squirrels are strategic and they can predict the future cone availability. During a mast year, the females will have more than one litter because they know that their pups will have access to cones once the fall arrives, and thus the pups will have a good chance of surviving. The females respond to masts in this way even though they breed in the spring but the cones aren’t available until autumn. In contrast, during non-mast years they will typically have only one litter.
I showed that litters die more frequently during mast years, suggesting that infanticide is more common during mast years. When litters die, their mother is more likely to have a second litter and will breed again sooner than if her litter had survived. So male red squirrels commit infanticide in mast years because the females will have that second litter, giving the males a second chance at fatherhood.
Post and photos by: Jess Haines