Science: Millions & Millions of Mice

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As rodents increase, so do rodent epidemics. Nevertheless, epidemics are too erratic to explain the periodic ebb & flow of rodent population. Field mice flourish in dry weather, suffer in wet weather. But mouse cycles recur regardless of weather cycles.

The major mystery is how the animals maintain their fixed cycles in spite of all such interference. Zoologist Elton concludes that the master factor is still unknown. He believes it may prove to be of a hitherto-undetected meteorological nature, hints at possible interstellar influences.

Practical Importance. The discovery that mice erupt in cycles has important practical applications. In 1935 a U.S. zoologist was able to warn orchardists 18 months in advance of the worst mice outbreak New York State ever suffered, helped save thousands of fruit trees.

Awareness of rodent cycles also helps prevent diseases among human beings. In Norway, for example, lemming invasions are accompanied by outbreaks of "lemming fever"—a form of tularemia. Vole outbreaks in India stimulate the dread bubonic plague; in Central Europe, food poisoning ("ptomaine poisoning"); in Africa, a fever of men and sheep.

Mouse Harvests. Sometimes the mice cycles lead to cycles of disaster throughout nature. "Northward, beyond the line where crops will flourish," says Elton, "the mice themselves become a crop, harvested in turn by fox and trapper and trader." From the reports of the Hudson's Bay Company and of Labrador missionaries Elton has found that in the last century fox catches fluctuated in four-year cycles, one year after the cycles of voles in Labrador and lemmings in Ungava (subArctic Quebec).

When the rodents increase, foxes increase. So do hawks, snowy owls, martens and even caribou (because the wolves eat the rodents, which are easier to catch than caribou). When the rodents disappear, the foxes die off, and the snowy owls—"a mass of transfigured lemming" —emigrate to the U.S. Wolves attack the caribou herds again, and survival becomes harder for the caribou-hunting Indians.