Mass Murder On the Tundra

Figure 1: The collared lemming, Dicrostonyx torquatus, is one of three lemming species found in Canada's Arctic.
There are no true mice in the Arctic, but there are seven species of lemmings and voles. Of these, lemmings (Figure 1) are the most famous, probably because of the myth that thousands of the roly-poly beasts periodically commit suicide by jumping off cliffs into the sea. The truth is no less fantastic. The species of lemmings and voles that occur on the arctic tundra do undergo huge fluctuations in numbers. With large numbers of young being born every year, and those young maturing quickly to reproduce themselves, populations can reach very high numbers. In these boom years, a myriad of predators benefit including wolverines, foxes, wolves, ermines, birds of prey, and even predatory fishes.

If natural disasters – a summer drought, a sudden melt of thick snow, an unusually harsh winter – do not kill them off first, overcrowding ensues. Aggression rises, and bloodshed and cannibalism come along with it. Once the hoards exhaust their food supply by eating all of the preferred plants in the vicinity, there are two subsequent phenomena: emigration-en masse, and a crash in population numbers (Figure 2). Lemmings are good swimmers, and emigrants readily enter any body of water they encounter; if the opposite shore is beyond reach, they simply perish. Population crashes are sudden and extreme, with carcasses strewn over the tundra.

Figure 2: Lemmings, and other tundra rodents, have extreme, cyclic oscillations in population size.

After more than twenty years of research into the cause of the extreme cycles in lemming numbers, theories abound but answers do not. Proposed explanations include combinations of starvation and predation. But there have been many observations that seem to indicate that these explanations are unsatisfactory. For instance, there have been population crashes in the absence of predators, and experiments where supplemental food failed to prevent a crash.

A new theory advanced by two British scientists suggests1 that the mass death is "murder"; it is caused by several species of plants that produce poisons that ordinarily prevent the lemmings from eating them. Their theory throws a twist on an older idea that fluctuations in lemming populations are caused by toxins in their food plants. However, the link between lemming cycles and their food plants seemed to be refuted by experiments in which no relationship was found between population numbers and the concentrations of toxins in their food plants.

Figure 3: The arctic blueberry, Vaccinium uliginosum.
The new theoretical model suggests that when the lemmings deplete their larder, they turn to non-preferred host plants for their nutrition. These alternate food sources respond to the assault by manufacturing lethal poison that cause the mass die-off. The authors of this model suggest that one prime agent of destruction is the crowberry, Empetrum nigrum, a widespread evergreen shrub (Figure 3). Another suspect is the arctic blueberry, Vaccinium uliginosum, but a positive identification must await experimentation.

It has been observed that lemming numbers remain low following a population crash for some time. This extended period of low numbers has not been explained by any previous theory. In the new model, it is proposed that the effects of the lethal poison are long term, affecting numerous generations and reducing breeding success. With such a lethal weapon, one might expect the lemmings to experience intense natural selection leading to the evolution of immunity to the poison. However, exposure to the toxin only occurs during a population peak because as long as the preferred food plants are available, the culprit species are avoided, or eaten in small quantities. The length of time between each "mass killing" likely prevents the evolution of resistance. The plants succeed in both killing their attackers and protecting themselves from being eaten, and evolution is unable to provide the lemmings with a means of self defence – the perfect murder!

1 Jensen SP, Doncaster CP. 1999. Lethal toxins in non-preferred foods: how plant chemical defences can drive microtine cycles. J. Theor. Biol. 199: 63–85.