Understanding the interplay between growth, body size, and fitness in individuals positions us to link population body size trajectories to population success. Individuals allocate excess energy from ingestion to growth. Thus, differences in energy availability can influence individual growth and body size at maturity. Body size is a critical determinant of reproductive success and survival, thereby influencing the overall fitness of an individual. However, quantifying how individuals allocate resources to growth, trade-offs between attaining large body size and prioritizing early reproduction or survivorship, and the direct fitness consequences of body size trajectories are difficult in wild populations, especially those that live in the marine environment, limiting access to data collection. My dissertation addresses these data gaps by investigating how marine mammals allocate energy to growth, the benefits and costs of attaining large body size, and the direct fitness associated with different body size trajectories. First, I conducted a comprehensive literature review of how marine mammals grow and how they allocate energy to growth, as well as provided an empirical estimate of the cost of growth in marine mammals (Chapter 1). I then focused on the potential life history benefits of attaining large body size, and the potential energetic and life history constraints placed on changing size trajectories (Chapter 2). I used stochastic dynamic programming to model three populations of North Sea harbor porpoises (Phocoena phocoena) of different size classes. Finally, I used empirical data from northern elephant seals (Mirounga angustirostris) to directly measure the annual and lifetime fitness consequences of deviating from expected body size (Chapter 3). Overall, this dissertation contributes a new biologically relevant understanding of the fitness consequences of investing in growth in marine mammals. Expanding our understanding of fitness consequences of variability in body size is critical in an environment that is experiencing rapid changes due to anthropogenic and natural stressors, often influencing the resources available to individuals to allocate to growth. Observations of shifting body size trajectories in response to environmental change have been documented in marine mammals, and understanding the individual fitness consequences and down-stream population consequences of these shifting size trajectories can aid in the management and conservation of these species.
