Mortality in an alpine ungulate increases nonlinearly with snow depth.
This 45-year study on bighorn sheep reveals that while adult survival is typically buffered against environmental variation, it exhibits a critical non-linear threshold where extreme snow depths severely compromise population viability, a risk that has intensified as climate change increases the frequency of such extreme conditions.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
In the high, windswept mountains of the world, winter is not merely a season of cold; it is a physical barrier that reshapes the very possibility of life. For animals that live long lives, like the bighorn sheep, nature has often favored a strategy of resilience. The prevailing idea in ecology has long been that while young animals and the very old might struggle through harsh winters, the healthy adults in the middle of their lives are built to withstand almost anything. This concept, known as demographic buffering, suggests that prime-aged adults act as a shield for the population, maintaining high survival rates even when the weather turns severe, ensuring the group survives to reproduce another year. However, this assumption relies on the idea that environmental stress has a limit, and that animals can simply endure whatever nature throws at them. The question remains: what happens when the winter becomes so extreme that even the strongest adults can no longer hold the line?
Researchers set out to test this limit using a rare, forty-five-year record of bighorn sheep living on Ram Mountain in the Canadian Rockies. This isolated population has been watched closely since 1971, with scientists capturing and weighing the animals annually to track their health and survival. To understand the winter conditions these sheep faced, the team did not just look at simple weather reports. Instead, they used a sophisticated computer model to reconstruct the history of the snowpack, calculating exactly how deep the snow was, how long it stayed, and how heavy it became. They combined this detailed snow data with the decades of animal records to see how the snow actually affected the sheep, looking not just at average winter conditions, but at the specific moments when the snow became too deep to cross.
The study revealed that the old rule of adult resilience is breaking down, but only when the snow gets dangerously deep. For the young lambs and the oldest sheep, survival naturally drops as the snow gets deeper, which is expected. But for the healthy adults, the story is different. Under normal winter conditions, these adults are indeed tough; their survival rates remain high regardless of how much snow falls. However, the researchers found a sharp, invisible line in the snow. For female sheep, once the snow depth crosses 90 centimeters, their survival begins to plummet. For males, the breaking point is slightly higher, at 100 centimeters. Below these depths, the adults are fine. Above them, the snow becomes a mechanical trap, making it too hard to move or find food, and survival rates drop sharply. It is not a gradual decline, but a sudden collapse once the snow exceeds the animals' physical ability to cope.
This threshold effect is becoming a growing problem because the winters are changing. Over the forty-five years of the study, the frequency of these deep-snow events has surged dramatically. In the early years of the record, it was rare to have even a single day where the snow was deeper than 90 centimeters. By the end of the study, the probability of experiencing such deep snow in any given year had risen from a mere 8 percent to 87 percent. The snow is not just getting deeper on average; it is spending more days at levels that are lethal for the sheep. As a result, the population is seeing a steady, long-term decline in survival across all groups. The survival of lambs has dropped significantly, and even the survival of adult females, who are the most critical group for the population's growth, has fallen by a measurable amount over the decades.
The researchers also looked at how the snow affects the sheep indirectly. Deep snow forces animals to burn more energy just to move and dig for food, which leaves them in poorer condition by the following autumn. This poor condition then makes them more likely to die in the next winter. While this "carry-over" effect is real, the study found that the direct physical barrier of the deep snow is often the more immediate killer, especially for the oldest sheep. The findings challenge the comforting idea that adult animals can simply buffer themselves against any amount of environmental change. Instead, the data shows that their resilience is non-linear; it works well until a specific, critical threshold is crossed, at which point the system fails. As climate patterns shift and deep snow events become the norm rather than the exception, the safety net that adult survival once provided for these mountain populations is disappearing, leaving the species far more vulnerable than previously thought.
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