Differential mortality of females would cause them to be three times less numerous than males when the population reaches a low point
The intriguing cyclical variations of lemmings continue to surprise. The few answers that shed some light on this elusive phenomenon raise new questions that further thicken the mystery. This is the case of a study just published in the journal Ecology and Evolution by Dominique Fauteux, from the Canadian Museum of Nature, and Gilles Gauthier, from the Department of Biology at Laval University, both attached to the Centre for Northern Studies.
The two researchers used a database of lemming populations on Bylot Island, Nunavut, collected between 2004 and 2019 to try to better understand the causes of this population roller coaster, which causes lemming numbers to rise and fall in three- to five-year cycles. Lemmings are central to the terrestrial food chain in the Arctic. They are one of the main prey of arctic foxes, ermines, snowy owls and other birds of prey living at these latitudes.
During the study period, brown lemming populations went through 5 cycles that ranged from highs of 9 lemmings per hectare to lows of about 0.1 lemmings per hectare. The researchers found that the proportion of males and females was not stable over time. When densities are high, there are as many males as females," says Gilles Gauthier. On the other hand, when densities are low, there are three males for one female.
Females may be more vulnerable to predators when they are with their young in the nest they build under the snow cover.
By analyzing the data, the researchers deduced that higher winter mortality of females was the reason for the imbalance in the relative abundance of the sexes in the troughs. On Bylot Island, lemmings live almost 9 months of the year under the snow and there may be as many as 4 breeding cycles during the winter. It is possible that females are more vulnerable to predators when they are with their young in the nest they build under the snow cover," says Gilles Gauthier.
Among these predators, a key player seems to have been neglected until now: the ermine. Recent observations, made with an infrared camera system developed at Laval University, show that this small mammal can move through tunnels dug in the snow by lemmings.
Unlike arctic foxes, snowy owls or long-tailed jaegers, which can move on to other prey or migrate to sites where food is plentiful, ermine are highly dependent on the local abundance of lemmings," says Gauthier. We believe that there must be a crash in the ermine population for the lemming population to come out of a slump. Work being done at Bylot Island should allow us to test this hypothesis.
