Country Mice Rebounding After Chaste, Stressful Summer
By John Crouch, in the Amicus Curiae, College of William and Mary
Copyright John Crouch 1992
The nondescript jumble of buildings just beyond the Gradplex looks like the place where lawn mowers go to die. Actually, it is the place where Professors C. Richard Terman and E. L. Bradley explore a question of growing urgency to mankind: How do populations control their numbers?
The Laboratory of Endocrinology and Population Ecology is no typical lab. Its genetically diverse mice live off the land in woods around Williamsburg, mingling freely. Though provided with nests, they prefer to build their own. And though they live in what Dr. Terman calls "a mouse welfare state" where their population might be expected to expand exponentially, certain natural forces, not yet understood, induce them to limit their growth non-violently.
Dr. Terman is an animal behaviorist and a population ecologist. His interest in populations was sparked by lemmings. He wondered how most species avoid doing the lemming thing, and whether some species may meet similar fates as they increase. He found that when he released a few mice in a large enclosure and provided unlimited amenities, they did not increase to the point of suicide, fratricide, or cannibalism. They became inhibited.
More formal experiments with deermice in the "Pop Lab" established that this asymptote, or population plateau, occurred while much living space was still unoccupied. They also revealed that there is no particular density or absolute number at which population levels off. Four populations in identical cages stabilized at 7, 13, 29 and 47.
Nearly all mice stopped reproducing or failed to experience puberty, for no apparent reason. The few newborns were mothered so enthusiastically that it just wore them out. The celibate mice spent practically all their time in one huddle, as if trying to lose their individuality.
Dominant females from the populations' founding pairs began hoarding food, though Dr. Terman always provided more than enough. Three or four handmaidens helped stock the hoards. Any mouse could dine there, but the hoarders fiercely berated those who tried to carry food away.
Dramatic though these results were, their relevance to life outside laboratories was unclear. So for the last ten years, Dr. Terman has moved his research into natural environments, where he has observed the same trends.
Populations of white-footed mice around Williamsburg do not level off permanently, but they stop breeding from May to July, a time of plentiful food in which a pair of mice could have a litter every 25 days. Each offspring could be a parent after 45 days.
Instead, the mice's reproductive organs remain minute during these three months. If they are taken to the lab, however, they quickly develop and reproduce. Dr. Terman has not yet discovered what change in the mice's environment triggers the asymptote. He tried providing surplus food, but it produced no effect. Nor does the chastity result from the weather, because in August and September all their organs swell tenfold and do what they were designed to do. And just as in the early experiments, there is no typical population density at which breeding stops.
Dr. Terman and Dr. Bradley, an endocrinologist, suspect that the mice are sexually stunted by adrenalin which they produce in response to stress. In each population, one or two dominant, fertile mice seem somehow to induce stress in the others through subtle signals which do not even appear aggressive to human observers. Dr. Terman thinks that these cues are given primarily by touch, rather than by odor, sound or visual body language. He has put pairs of inhibited mice in cages where they can see, hear, and smell their neighbors, but not touch them, and each time they have developed and reproduced.
Key questions about the phenomenon remain unanswered. Why does this happen from May through July? How exactly do the mice stress each other out, if indeed that is what they do? More importantly, what kind of natural selection has favored the evolution of this behavior?
Dr. Terman stresses that his project is "basic research," and discourages law students and other humans from directly comparing their plight to that of his mice. His research reminds mankind, on the one hand, that scenarios in which trillions of future humans live in hives or die in droves may make for exciting science fiction, but they are poor science. There will be no millenarian Malthusian apocalypse, because individual people, like Terman's mouse colonies, hit the Malthusian wall in small ways every day.
Terman's work further discredits the notion that prosperity automatically produces overpopulation. On the other hand, it also warns us that species limit their numbers by countless and unforeseen methods. To Dr. Terman, the question is not whether human populations will be limited, but how.