Lemmings are small rodents with fur-covered paws who inhabit the arctic regions of North America and Europe. They feed on the crowberry and other sun-nourished tundra vegetation and, in turn, are fed upon by the arctic fox and snowy owl, thus completing a simple food chain. Lemmings give birth to at least one litter a year, with four to six kits in each litter, so they multiply quickly. But the tundra is vast and covered with lemming food. This increase in the lemming population affects the sex glands of snowy owls, It appears that when the snowy owl sees an excess of lemmings, its pituitary gland is stimulated and secretions are produced, which increases both sexual activity in the male and egg production in the female. So the more lemmings snowy owls see, the more reproductive activity there is, the more reproductive activity, the more snowy owl; the more snowy owls, the more lemmings eaten. A large lemming population increases the number of cubs in arctic fox litters, too, and foxes will also migrate into areas of lemming abundance. Therefore, when there are more lemmings, there are more lemming predators.
   But lemmings multiply much faster than anything that preys on them. A tundra region virtually empty of lemmings can, in the space of three years, become teeming full of them. Under ordinary circumstances lemmings live their entire lives in a restricted range of territory. They neither migrate nor roam. Thus their proliferation soon exhausts all available food and space; and their arctic home becomes a morass of hungry, scurrying lemmings running to and fro on top of each other and being eaten by an abnormal number of owls and foxes, as well as wolves, dogs, and bears. Then the lemmings suddenly begin to leave their home range in droves. Like a furry tide, they swarm blindly across the chilly steppes, heedless of the cold, deep lakes or the swift-flowing rivers, which leave thousands drowned along their banks. In places such as Alaska and Scandinavia, which are particularly hospitable to the breeding of lemmings, as many as eight litters may be born to a single female in one year. Thus, at the peak of the lemming cycle there are millions of the small rodents running desperately over the tundra, splashing through the lakes and rivers and sometimes swimming to their death in an apparently ritual procession out to sea.                      
   Starved, unsheltered, and worked up into a state of nervous exhaustion by overcrowding and fatigue, they die by the thousands so that the next year there are almost no lemmings, and precious few of the owls and foxes who eat them. Many of the foxes have died, other have run away. The snowy owls leave the arctic regions and reappear in the temperate forests of southern Canada searching for food. Undoubtedly, hunger is a major factor in the death of lemmings. But overcrowding may be equally important. Its a known fact that simple overcrowding among humans and rats causes extreme anxiety, with detrimental effects both on the nervous system and on the sex and adrenal glands. When lemmings run out of space, their adrenal glands swell and their sex glands shrink. Because males produce less sperm and females produce fewer eggs, there is little or no urge to mate; so there are very few pregnant lemmings. Of the young who are conceived during periods of severe overcrowding, many die in the uterus and many of those who are born alive die because their mothers cannot produce enough milk for them under the circumstances. Swelling of the adrenal cortex increases the production of hormones that suppress inflammation. Inflammation is the body's way of fighting infection, so a lemming with a swollen adrenal gland is vulnerable to harmful bacteria and viruses and the infectious diseases they breed. Since the lemmings are living in close quarters, epidemics spread rapidly through their ranks. Long-term hyperactivity of the adrenal gland can also lead to the eventual exhaustion of its ability to produce hormones. This allows inflammation to return but renders the lemming lethargic and indifferent to all situations of danger, risk, or adverse conditions. When lemmings have exhausted their adrenal supply, they fall easy prey to owls, foxes, heat, cold, and simple food shortage, Overcrowded lemmings die in droves.
   They die because there is no food left on their own territory, they die because other animals eat them at will, they die because they drown during their frenzies, they die because of plaques and ennui. And when they die, the owls and foxes die, too, or leave; and the arctic tundra, so lately the scene of vigorous life and carnage, is empty again, as suddenly as it was full. There are just enough lemmings left, hiding in the overgrazed crowberries, to start it all over.