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Date of Award
Doctor of Philosophy (PhD)
Brains control an organismճ ability to sense, remember, and respond to the frequently changing world. Brains are composed of multiple regions and systems, which are associated with different processes. These regions are homologous across all vertebrates yet vary greatly in size and shape across clades. While regions can function independently, they also interact extensively. These characteristics make it difficult to predict whether regions can change in size independently from other regions in response to selection (mosaic evolution hypothesis), or whether the brain evolves as a single concerted organ (concerted evolution hypothesis). Further, many traits such as cognition, behavioral flexibility, and survival are associated with overall brain size rather than the sizes of particular regions. Despite the potential fitness advantages of an enlarged brain, species with extreme encephalization, in which brain size greatly deviates from the allometric relationship between brain and body mass, are rare. One reason for this rarity is that increasing brain tissue is associated with energetic costs. Thus, evolving a large brain requires either a decrease in other energetic requirements (energetic trade-off hypothesis) or an increase in overall energy consumption (metabolic constraints hypothesis). In this dissertation, I aim to better understand the multiple forces that drive and constrain the evolution of brain regions and total brain size. I do this in African mormyrid electric fishes. Mormyrids are well known for having large brains and particularly large cerebellums; however, relative brain size and brain region scaling across mormyrid species had not been quantified before this study. I found that mormyrid species vary widely in relative brain size with multiple, independent lineages having extreme encephalization (Chapter 3). Brain region scaling primarily fits a concerted model of evolution within mormyrids, yet mosaic shifts were evident with the evolution of behavioral novelty, such as the electrosensory system (Chapter 2). When comparing the energetic costs of relative brain size, I found evidence to support the metabolic constraints hypothesis when comparing across mormyrid species (Chapter 3). However, I found that intraspecific energetic trade-offs and metabolic relationships varied among the three species studied. This suggests that the interspecific relationship between metabolic rate and relative brain size is not due to a direct constraint on brain size, and, instead, reflects a series of species-specific indirect constraints and adaptations that have resulted in macroevolutionary patterns (Chapter 4). Thus, in this dissertation I determined that brain region scaling incorporates aspects of both mosaic and concerted models; that as brain size increases, metabolic demand increases across species; and that this interspecific relationship is not due to direct physiological constraints but instead species-specific adaptations between evolutionary change in brain size and organismal energetics.
Chair and Committee
Bruce A. Carlson
Yehuda Ben-Shahar, Carlos A. Botero, Jason Knouft, Allan Larson,
Sukhum, Kimberley Varunee, "The Costs of a Big Brain: How Region Scaling and Energetic Costs Influence Brain Size Evolution in Weakly Electric African Fishes (Mormyridae)" (2018). Arts & Sciences Electronic Theses and Dissertations. 1655.
Available for download on Saturday, August 31, 2019