Roles of Behavioral Novelty and Organismal Energetics in the Evolution of Extreme Encephalization
Date of Award
Doctor of Philosophy (PhD)
Brains and their component brain regions vary widely in size and structure across vertebrates. However, extreme increases in total brain size relative to body size (extreme encephalization) and increases in specific major brain region sizes independent of other brain regions (mosaic brain evolution) are relatively rare. There are several hypotheses as to why, but the energetic cost of increased brain tissue and the regional interconnectedness in both brain development and function likely constrain how brains are able to change in response to selection. In this dissertation, I sought to address both how behavioral novelty relates to evolutionary changes in brain size and structure and how lineages that have evolved extremely large brains accommodate the increased energetic cost. Weakly electric fishes are excellent for addressing these questions for several reasons as these fishes evolved behaviorally novel active electrosensory systems along with several distinct neural innovations, which likely resulted in strong selective pressures. Further, multiple lineages independently evolved similar electrosensory systems. Previous studies found that African weakly electric fishes (Osteoglossiformes: Mormyroidea) evolved extremely large brains along with mosaic shifts in brain structure relative to other non-electric osteoglossiforms, yet the relationship to their behaviorally novel electrosensory system remained unclear. Looking across 870 ray-finned fishes, I found multiple shifts in brain-body scaling, including an increase in brain-body scaling at the base of osteoglossiforms with a subsequent decrease in one mormyroid lineage; however, these identified shifts were not associated with the evolution of any electrosensory phenotypes (Chapter 2). And yet, the brain region scaling patterns of electrogenic versus non-electric fishes were strikingly similar despite the considerable phylogenetic distance between them. I found convergent mosaic brain evolution associated with the evolution of electrogenesis and electroreceptor type in three electrogenic lineages, independent of differences in brain-body scaling (Chapter 3). When investigating means for supporting the increased energetic cost, I found increased expression of energy processing pathways in the brains of large-brained osteoglossiforms, as previously found in humans, and identified candidate molecular trade-offs between brain and other metabolically expensive tissues in large-brained species (Chapter 4). Overall, my work demonstrates that similar changes in brain structure have consistently evolved alongside similar novel behaviors independent of differences in brain-body scaling and that similar changes in gene expression have likely consistently evolved alongside extremely enlarged brain size. Taken together, these findings underscore a surprising degree of predictability in brain evolution.
Chair and Committee
Bruce A. Carlson
Yehuda Ben-Shahar, Jason R. Gallant, Michael J. Landis, Kenneth M. Olsen,
Schumacher, Erika Laurie, "Roles of Behavioral Novelty and Organismal Energetics in the Evolution of Extreme Encephalization" (2022). Arts & Sciences Electronic Theses and Dissertations. 2809.
Available for download on Friday, December 01, 2023