ORCID
https://orcid.org/0000-0001-9217-5366
Date of Award
12-17-2024
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Life on Earth has adapted to extremely different temperatures worldwide. However, across distant branches of the tree of life, animals in similar temperature environments often share similar phenotypic traits. Specifically, traits that bear on the ratio between an individual’s surface area (e.g. appendage length) and its volume (e.g. body size) are particularly important for temperature regulation and are codified in canonical biogeographical rules. Although these global patterns of finding larger-bodied (Bergmann’s Rule) and shorter-limbed (Allen’s Rule) animals in colder regions are widely upheld, they also show substantial variation. It remains poorly understood why some animals conform to these biogeographical rules and others do not. This is especially important in the context of climate change, where ecosystems are heating rapidly and inflicting drastic changes on wildlife’s distribution, behavior, and morphology. In this PhD thesis I explore phenotypic adaptations to temperature gradients in birds through space and time. In each chapter, I uncover novel factors that explain conformity to the relationship between body size and temperature. In chapters 1 and 2, I take a macroecological perspective to understand temperature-phenotype relationships of all birds worldwide. These chapters help clarify longstanding ambiguity about the degree to which birds conform to Bergmann’s and Allen’s rules. Using complete morphological data of nearly all avian species, I show that families which show strong changes in body size exhibit weak changes in bill length and vice versa. Importantly, most families showed concerted, small net changes in both body size and bill size, suggesting that both Bergmann’s and Allen’s rule complement each other, and act in concert to allow subtle modifications of phenotypes that match with both Allen’s and Bergmann’s rules. I also discover additional roles of ecological constraint: in chapter 1 where I focus on resident birds, species with specialized bill shapes are more likely to show conformity to Bergmann’s rule, as changes to bill size are constrained by their impacts on foraging ecology. In chapter 2, where I focus on migratory birds that travel across continents and experience different climates, I show that taxa with longer migration distances and more seasonal niche divergence show dampened phenotypic responses. I also discover a novel seasonal tradeoff in rule conformity, where families that emphasize the temperature regime of the breeding range respond weakly to the temperatures on the non-breeding range. Together, these findings show that complementary conformity to Allen’s and Bergmann’s rules can result in subtle and simultaneous modifications of the thermally most-pertinent trait for homeothermy – the surface-area to volume ratio, confirming the validity of both rules, and offering and explanation why some taxa might not show the expected conformity to each individual rule. In chapter 3, I explore conformity to Bergmann’s rule through a temporal gradient of temperature change: global warming. While many bird species have rapidly decreased in body size as temperatures worldwide have warmed (in line with Bergmann’s rule), it remains unclear why some species are more sensitive than others to changes in temperature. Here I test a fundamental theory from cognitive ecology (the “cognitive buffer theory”) in the context of conformity to Bergmann’s rule. Using novel data on avian brain sizes, I show how larger-brained songbirds from North America show dampened decreases in body size compared to smaller-brained relatives. This clarifies some of the idiosyncratic responses of climate change and suggests how variation in behavioral flexibility can buffer species from directional selection on body size. In an appendix chapter, I apply similar techniques to investigate conformity to another fundamental biodiversity pattern (species-area relationships, SARs). Using macroecological databases on bird traits, I examine the role of dispersal ability on shaping SARs on island birds worldwide. I discover scale-dependent effects of dispersal ability: at small scales, increasing dispersal ability increases SAR slopes. At large scales, increasing dispersal ability increases SAR intercepts and decreases slopes. In summary, my thesis has contributed novel insights to fundamental global biodiversity patterns using collaborative synthesis approaches.
Language
English (en)
Chair and Committee
Carlos Botero
Committee Members
Jonathan Myers; Iván Jiménez; Jonathan Losos; Michael Landis
Recommended Citation
Baldwin, Justin Wheeler, "Avian Adaptation to Thermal Gradients" (2024). Arts & Sciences Electronic Theses and Dissertations. 3343.
https://openscholarship.wustl.edu/art_sci_etds/3343