A Self-Corrective for The Population Explosion?

What will come of the world's population explosion? Optimists talk of a limit brought about by voluntary birth control. Pessimists gloom about widespread starvation, plague, or the thinning effect of nuclear war. In the Bulletin of the Atomic Scientists, Dr. Hudson Hoagland of the Worcester Foundation for Experimental Biology suggests a third possibility. Nature, he says, has its own subtle systems for choking off excessive breeding.

Gland Trouble. When animals are overcrowded, says Hoagland, their increase often slows down even when they have plenty of food. Horrible things happen among jammed-up flour beetles. Females destroy their eggs; they turn cannibalistic and eat one another. Males lose interest in females, and though plenty of flour is left for food, the beetle population reaches a statistical plateau.

Mammals are much the same. The population cycles of jack rabbits in Minnesota seem to have little to do with the food supply. When the cycle approaches its peak, rabbits begin to die in horrible convulsions, with wild leaps and running movements. Their corpses are well nourished and show no signs of epidemic disease. But their internal organs are fat-clogged, degenerated and damaged by hemorrhages. Overcrowding seems somehow to upset the rabbits' pituitary and adrenal glands, causing their abnormal secretions to trigger a long chain of fatal troubles.

Only 40 years ago, a small island in Chesapeake Bay supported a few deer. They were given plenty of food, and they multiplied enthusiastically. But when the population reached one deer per acre, the animals began to die off. Their internal organs showed "adrenal stress," just like the Minnesota rabbits'.

When John Calhoun of the National Institutes of Health put wild Norway rats into a one-quarter-acre enclosure and fed them well, the normal rate of rat increase should have raised the population to 5,000 in 27 months. Instead it stabilized at about 150 adult rats. The females produced plenty of young, but they did not take care of them properly, and most of them died before maturity.

Social Stress. Anxious to learn how overcrowding does work, Calhoun put rats in four interconnected pens six feet square. Two of the pens were quickly pre-empted by boss male rats that kept harems of females and allowed no other males to mate with them. The harem females made proper nests, bore healthy young and raised them successfully. But in the other two pens, where no single males took charge, social stress was rampant. Some of the males gave the females no rest. Others turned homosexual or hid in corners. The females stopped making proper nests, and their young, born on the bare floor, died and were eaten.

Dr. Hoagland is not prepared to predict flatly that any of these unhappy effects will appear among the earth's human population as its density increases. But he makes some dark suggestions. Even though most inmates of crowded human slums escape to pleasanter places from time to time, he says, many slums show a social pathology (crime, delinquency, street gangs, psychotic behavior) as did the pens of crowded rats. In concentration camps, where no respite from crowding was possible, humans developed adrenal stress. In the future, Hoagland fears, if crowding gets out of hand, nature may strike with horrible and unpredictable ills to check further human increases.

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