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By Root 1254 0

a few rats into a fairly spacious but limited complex, gave them as much food and water as they needed, and did what he could to keep pollution within reasonable limits. The population did pretty much what he expected it to do: rose exponentially to a peak, then collapsed again. When the crowding became unbearable, the rats’ social system—such as it was—completely disintegrated. They fought continually and destructively, began to eat their own young, and showed every known symptom of environmental stress: ulceration, heart disease, hair loss … you name it, the observers saw it. It was never really intended as an experiment in the scientific sense, of course. If I remember correctly, Calhoun was working for the National Institutes of Health. It was a demonstration—a parable to supplement the natural parables of the lemming and the snowshoe hare.”

“I read about the snowshoe hare,” Lisa put in helpfully. “They’re responsible for the lynx cycle in Canada—and the lemmings are famous. There used to be a cinema ad that showed them pouring over a cliff, but I can’t remember what it was for.”

“It was an antismoking ad,” Miller reminded her. “People misunderstood the lemmings for a hundred years, just as they misunderstood the lynx cycle. The myth was that the lemmings were committing suicide, just like smokers who wouldn’t stop. There were all kinds of crackpot theories. One suggested that some atavistic instinct was forcing them to follow an ancient migration route to land that had been inundated by the sea. In much the same spirit, people tried to correlate the lynx cycle with the sunspot cycle, as if that would somehow provide an explanation. Even within the scientific community, there was a well-established myth of predator-prey cycles suggesting that the number of lynx pelts recovered by the Hudson Bay Company’s trappers varied cyclically because of the feedback effects of the trappers’ own activity, or because every time the lynx numbers increased, they sent the populations of their prey into steep decline. All nonsense, of course. The lynx population and the snowshoe hare population went up and down together—the population crashes that caused the hares to decline were entirely independent of the intensity of predation, but every time the hare population crashed, the lynx population crashed too.”

“But they can’t have been in the same situation as the experimental rats,” Lisa pointed out, glad for an opportunity to show that she was on the ball. “They had unlimited space.”

“That’s the curious thing,” Miller agreed. “You’d think so, wouldn’t you? The snowshoe hares had all of Canada, the lemmings all of Siberia and Scandinavia. You’d think that the limiting factor controlling their population size would be the availability of food—but it wasn’t. When the cases were actually investigated, it immediately became obvious that the peak populations could endure the winters, despite the scarcity of food. The populations didn’t collapse until the spring, when food was becoming much more abundant.”

He paused, inviting Lisa to catch on. She had to hesitate for six or seven seconds, but then she figured it out. “The mating season,” she said.

“Exactly,” Miller conceded, favoring her with a smile of pure but not particularly abundant generosity. “They could tolerate the density of population when their attention was fixed exclusively on the business of survival, but when the breeding season came around, the males became fiercely territorial. It wasn’t the absolute limitation of space that was important, but the perceived limitation. The competition for territory became so intense so suddenly that the animals couldn’t handle the consequent physiological stress. Their systems became permanently adremdinized. Snowshoe hares are relatively meek, so they just drop dead in droves, mostly from heart attacks. Lemmings aren’t—when they get into fighting mode, they simply can’t stop. The lemmings that died in the last couple of so-called lemming years were mostly killed on the roads, and human activity has had such a profound effect on their