Year of Award

2025

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Ecology and Evolution

Department or School/College

Division of Biological and Biomedical Sciences

Committee Chair

Dr. Zachary Cheviron

Commitee Members

Dr. Jedediah Brodie, Dr. Angela Luis, Dr. Bret Tobalske

Keywords

subnivium, structural equation model, SEM, cold acclimation, winter, deer mice

Subject Categories

Other Ecology and Evolutionary Biology

Abstract

Despite global temperature increases, winter soil surface temperatures in some northern regions might actually be getting colder due to diminishing snowpacks exposing the soil surface to ambient winter conditions. Small mammal species in northern temperate areas often utilize a microclimate beneath the snow, known as the subnivium, to escape harsh winter conditions. Exposure to winter conditions without the subnivium as a buffer could cause small mammals to experience physiological stress and population declines. In this thesis, I explore the impacts of a declining subnivium from two complementary perspectives. In chapter 1, we used a structural equation model (SEM) to test for a relationship between small mammal abundance and subnivium quality. We expected a positive association between subnivium quality and abundance for species that use the subnivium over winter since it should buffer individuals and improve survival. As a result, we also expected abundance across species to covary if co-occurring species respond similarly to subnivium quality. We found that meadow vole abundance had a positive relationship with subnivium quality and a negative covariance with southern red-backed voles. Southern red-backed voles covaried positively with deer mice, neither of which showed a significant relationship with subnivium quality. These results indicate that not all small mammals are equally sensitive or resistant to changes in the subnivium, and it is important that future studies identify more sensitive species among the many that winter beneath the snow. In chapter 2, we used a thermal acclimation experiment to investigate spontaneous daily torpor (SDT) in deer mice (Peromyscus maniculatus) and how environmental variability affects torpor usage. A stable treatment, where the air temperature remained at a constant 0°C, was used to simulate subnivean conditions. The varied treatment cycled temperatures between -11°C and 11°C, which simulates soil temperatures exposed to winter conditions without a snow buffer. We show that deer mice use daily torpor in environments with reduced temperatures and shortened photoperiod. However, torpor use did not significantly vary between stable and varied treatments, implying that deer mice use SDT similarly in response to chronic and acute cold stress. Deer mice did display two phenotypes of torpor use: deep but short/infrequent bouts, and shallow but long/frequent bouts. While we have no evidence of deer mice altering torpor patterns in a variable thermal environment, these phenotypes may become exaggerated or diverge under new environmental stressors. Combining ecological modeling and experimental acclimation approaches allowed me to investigate how populations respond to changes in the subnivium in tandem with ways in which individual small mammals might respond and adjust their winter survival strategies.

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