Life histories and brain evolution of sharks, rays, and chimaeras

The brain is perhaps one of the most fundamental organs in all vertebrates. It determines not only an individual’s ability to sense and process stimuli from the environment, but is also crucial in maintaining internal homeostatic processes as well as determining an individual’s cognitive abilities. Brains come at a steep energetic cost however, with neural tissue requiring ~20 times the energy of muscle tissue. With such an important role to play, the ‘expensive brain hypothesis’ was been established to understand the evolutionary correlates of brain size. Maternal investment, defined as energetic investment during development, is a strong underlying factor in brain size evolution where higher energy investment from mothers is associated with increased brain size. However, much of what we know about brains comes from studying birds and mammals, while generally overlooking other vertebrate classes. Despite their diversity, all jawed vertebrate brains are comprised of similar components, a pattern that first appeared in sharks, rays, and chimaeras (Chondrichthyans). Chondrichthyans are often disregarded as unremarkable from a comparative perspective, which overlooks their true diversity of life histories and ecological niches. This thesis seeks to understand the evolution of brain size and organization in relation to life history and maternal investment using chondrichthyans as a model system. First, I reveal the sequence of reproductive evolution, finding that egg-laying is ancestral and that live-bearing and additional maternal investment (matrotrophy) have evolved independently several times, and are correlated with increasing body size. Second, I find that the evolution of reproductive mode and ecological lifestyle underlie the evolution of both brain size and brain organization, such that shallowwater matrotrophic species have large brains that are predominantly composed of regions related to enhanced cognitive abilities, the telencephalon and cerebellum. Conversely, deepwater lecithotrophic species have small brains composed predominantly of medulla oblongata. Lastly, I find that similar patterns of regional scaling in mammals, birds and chondrichthyans differ from those of teleosts, agnathans, and amphibians, and I propose that differing reproductive strategies may underlie this variation.