The respiratory basis of metabolic rate and life histories

Oxygen fuels aerobic metabolism and as such, plays an important role in the physiology, ecology, and evolution of organisms. Traits related to oxygen acquisition (respiratory surface area) and use (metabolic rate) or the balance of oxygen supply and demand (or its mismatch, termed 'oxygen limitation') have been proposed to underlie broad patterns such as the temperature-size rule and the geographic distributions of marine species. Moreover, traits related to oxygen acquisition and use form the central focus of seemingly disparate macroecological theories that aim to explain and predict the structure and dynamics of ecological systems and how these systems and their constituents will respond to a changing climate. While these existing theories and oxygen-related explanations offer a compelling story, the role of oxygen in shaping biological observations, responses, and patterns is hotly debated. Further, much work in this area is experimental in nature and typically focuses on a single species in laboratory settings. Broader scale, macroecological research stemming from meta-analysis and modeling is needed to understand the generality of patterns. To that end, this thesis takes a macroecological approach and examines the generality of the relationships among traits related to oxygen acquisition and use, ecology, and life histories. First, I reveal that respiratory surface area explains patterns of metabolic rate across the vertebrate tree of life. Second, I uncover that larger-bodied, active, pelagic sharks have greater gill surface areas (respiratory surface area in fishes) for a given size compared to their smaller-bodied, less active, benthic counterparts. Conversely, the rate at which gill surface area increases with body mass is the same for all species, regardless of activity level, habitat type, or maximum size. Third, I test a central prediction of the Gill Oxygen Limitation Theory and find that across fishes, growth and maximum size more strongly relate to activity level than gill surface area. Collectively, my thesis highlights the complexities of integrating data across scales and illustrates that oxygen acquisition and use is tightly correlated with activity level, but the relationships with life histories are less straightforward. This body of work builds on existing theory while empirically testing relationships among oxygen acquisition and use, ecological lifestyle, and the life histories among fishes and other vertebrates.