In 1958, Disney released a documentary called White Wilderness, which showed the wildlife of the Arctic on the cinema screen. David Attenborough it ain't. The film is now notorious for containing faked footage of something that simply doesn't happen: lemmings committing suicide en masse.
Realising that the Arctic rodents did not collectively top themselves, the film-makers resorted to trickery. After producing footage of the lemmings migrating by placing them on a snow-covered turntable, they shipped some of them to a cliff overlooking a river and herded them over the edge.
The resulting footage shows hordes of lemmings plummeting off a cliff, with the culpable humans studiously out of frame. It helped cement the myth of lemming mass suicide in popular culture.
Yet had the film-makers looked a little closer, they would have found that lemmings really are bizarre creatures. Finally, the true nature of lemming behaviour is being revealed.
Suicide cult
Lemmings are small rodents, related to hamsters, gerbils and mice. There are over 20 species, all found in the far north, including Canada, Scandinavia and Russia.
Unlike many Arctic animals, lemmings do not hibernate through the winter. Instead, they forage along runs and tunnels dug beneath the snow layer. This allows them to carry on breeding even as temperatures drop to -20 °C, driving the population up. In most species the population grows for three years, then crashes to near-extinction in the fourth.
In theory, mass suicide would help to control a population that had grown too large for its food supply. But this doesn't work in practice as the behaviour could never evolve. Killing yourself for the good of the population is a selfless act, and any lemmings that selfishly refrained from suicide would outcompete their nobler neighbours. So a tendency not to commit suicide would spread through the population.
The absence of mass suicide means we must find another explanation for the fluctuating nature of lemming populations. Charles Krebs of the University of British Columbia in Vancouver, Canada, has come closer to finding the answer.
Predator banquet
The most obvious idea is that the lemming population gets so big there isn't enough food for everyone, so every four years most of the lemmings starve to death. There are lots of computer models showing that this could happen, but no real-life data on food shortages to support the idea.
Recently, it has been argued that the population crashes are driven by predators. As the population rises, the theory goes, more predators will be attracted to the region by the promise of food. Once there are enough of them, they will gorge themselves on lemmings, slashing the population.
Krebs says this is clearly a factor, but it is not enough to explain the fluctuations. The problem is that we don't know how much damage the predators do during winter, when the lemming population grows fastest, as, unsurprisingly, researchers do most of their field studies in the summer.
A more insidious idea is that male lemmings commit infanticide, killing pups sired by other males. At least they do in the lab: it is not clear how much infanticide goes on in the real world.
Killer males
This makes sense from the point of view of the males. After all, the pups are not theirs and so will not pass on their genes. It is a process that occurs in nature, with chimps and lions well known for killing other males' pups.
Infanticide could contribute to the lemming cycles because higher population densities will mean more encounters between females and their pups and unfamiliar males – who are liable to attack the pups.
It is still not clear how important infanticide is for lemming populations. If it turns out to be significant, these cute little rodents go through population crashes not out of selflessness, but because selfish males are killing each other's children.
