On the Scottish island of Hirta, where the wind bends the grass and there is nowhere to hide, a flock of wild sheep lives. Since 1985, scientists have been recording who was born, who survived, and who left offspring. Over those years, the population has swelled to six hundred and seventy animals and shrunk to two hundred and eleven. These swings created a natural experiment: you can test how crowding and food shortages affect aging.
It used to be thought that in bad years older animals decline faster than younger ones, because they have less reserve strength. This idea was compared to a foundation: if a house stands on weak ground, a strong wind will shake it harder than a sturdy one. But in Soay sheep, the foundation turned out to be equally solid in any weather. High population density did lower average measures: survival fell, fewer lambs were born. However, the rate at which those measures worsened with age did not change in either males or females.
Researchers expected males to be more vulnerable to the environment, since in polygynous systems they spend a lot of energy on competition. But the data did not confirm this. Aging of survival and reproduction proceeded the same way at any density in both sexes. This means that the simple rule "males are always more fragile" is not universal. Perhaps the selective disappearance of weak individuals masks age-related vulnerability, while accumulated experience helps older sheep cope with difficulties.
So external stressors such as crowding and hunger lower the overall level of well-being, but they do not accelerate the aging process itself. The internal biological clock ticks at the same speed in good times and bad. This brings us back to the image of the foundation: wind can blow the roof off a house, but it will not make its walls crumble faster.
Today these results matter for ecologists who study wild populations and for gerontologists who seek ways to slow aging in humans. They remind us not to overestimate the role of external stressors. Old age, at least in Soay sheep, is not a house collapsing in the wind, but a foundation that stands firm until it exhausts its own resources.
Soay sheep have lived on the island of Hirta without human management for thousands of years, and their numbers fluctuate from 211 to 672 individuals, which makes the population an ideal natural laboratory.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.