Migration Energetics and Mitochondria: Modulation and Underpinning of Seasonal Flexibility in Mitochondrial Respiration in Migratory and Non-migratory Species

Abstract

The migratory phenotype consists of a suite of adaptations to support short- and long-distance movements. Numerous species make annual movements to and from more northern latitudes, taking advantage of the longer daylight hours and a high abundance of seasonal food resources during the summer. In eastern North America, many species face the challenge of the major geographic barrier posed by the Gulf of Mexico. Millions of birds maintain flapping flight across a roughly 800km stretch of water. For decades, researchers have studied migration to understand orientation and navigation, stopover biology, physiology, and energetics; however, despite significant advances in fundamental understanding of migration, many questions remain. From a plethora of studies, it is known that physiological adaptations serve as critical underpinnings of migration energetics supporting increased energetic output and efficiency. These adaptations can be observed across numerous hierarchical levels of biology from metabolic rates and whole-body respiration, tissue- and organ- specific changes, and cellular and even subcellular level adjustments. Nevertheless, gaps in the literature remain especially pertaining to organelle functionality within cells. Notably, the mitochondrion, which is responsible for producing over 90% of cellular energy in the form of ATP has received very little study in the context of migration. My goal was to investigate this critical gap in the literature to add to our current understanding of the migratory phenotype to further elucidate trait adaptation and migration energetics. My approach was to compare the mitochondrial physiology of closely related populations of birds that differed in migratory behavior. In Chapter 1, I first compared migrant and non-migrant populations of sparrows in genus Zonotrichia. In Chapter 2, I compared two species belonging to the family Mimidae. These comparative studies provided critical baseline information. For the Zonotrichia pair, mitochondrial respiration was seasonally flexible and played a critical role in migration energetics. For the Mimidae pair, results were likely confounded due to the migrants exhibiting mixed migratory strategies and due to other life history differences in the non-migrant. In Chapter 3, I investigated pectoralis ultrastructure and mitochondrial morphology as a potential mechanism underpinning patterns observed with Zonotrichia and found that mitochondrial size, number, and area are likely key mechanisms in the patterns reported in Chapter 1. Lastly, in Chapter 4, I expanded the Zonotrichia comparison to include another seasonal comparison within this genus. Unlike my initial predictions though, seasonal effects in this additional group were limited and variables such as ketones, demographics, and species-specific behavior likely played a larger role in the mitochondrial respiratory patterns reported. Collectively, this research provides some of the first documentation of seasonal flexibility in mitochondrial respiration and evidence of underpinning mechanisms.