Maria Stager

Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology, Ecology, and Evolution

Department or School/College

Division of Biological Sciences

Committee Chair

Zachary A. Cheviron

Commitee Members

Creagh W. Breuner, Lila Fishman, Tom E. Martin, Bret W. Tobalske


Junco, metabolic rate, phenotypic flexibility, population genetics, thermogenesis, transcriptomics


University of Montana


Individuals face many selection pressures that change throughout their lives. Phenotypic flexibility, the ability to flexibly and reversibly modify a trait value, is one way an individual can optimally match its phenotype to the prevailing environmental conditions. In this dissertation, I used juncos as a lens to understand the causes of variation in flexibility within physiological systems and among individuals. In my first chapter, I investigated how Dark-eyed Juncos (Junco hyemalis) alter mechanisms of heat production and heat conservation to cope with variation in ambient conditions. My results demonstrate the ability of birds to adjust thermoregulatory strategies in response to thermal cues and reveal that birds may combine multiple responses to meet the specific demands of their environment. To further explore the thermoregulatory strategies available to juncos, in my second chapter, I assess their potential use of non-shivering thermogenesis. My results indicate that muscular non-shivering thermogenesis is not an important mechanism of avian thermoregulation, potentially as a consequence of a tradeoff between the many demands placed on avian muscles. In my third chapter, I measured 20 additional physiological traits to explore the mechanistic basis of flexibility in complex phenotypes. I show that the relationships among traits contributing to whole-organism performance varied with the environmental context. Moreover, whole-organism flexibility in thermogenic performance was correlated with only a handful of subordinate phenotypes. In my fourth chapter, I identified drivers of variation in flexibility among juncos. To do this, I integrated measures of population genetic variation with assays of thermogenic performance and indices of environmental heterogeneity for individuals across the genus Junco. I find that native temperature heterogeneity correlates both with population genetic variation and the degree of thermogenic flexibility exhibited by an individual. In my fifth chapter, I present a review that considers the evolutionary implications of phenotypic flexibility and contrast those with developmental plasticity. I hypothesize that because these two processes experience selection distinctly, confer stability to populations differentially, and will likely evolve at different rates. Collectively, this work helps us understand the role of flexibility in adaptation and species’ resilience to environmental change.



© Copyright 2020 Maria Stager