Evaluation of the somatotropic axis in two marine mammal species with diverse early life history strategies: Influence of physiologic age and nutrition on growth of harbor seals (Phoca vitulina) and Steller sea lions (Eumetopias jubatus)

Date of Completion

January 2008


Biology, Oceanography|Biology, Animal Physiology|Biology, Zoology




The somatotropic axis, including growth hormone, insulin-like growth factor (IGF)-I and IGF binding proteins, is a bridge between growth physiology, sex, developmental age and nutritional status in domestic animals. However, the importance of the somatotropic axis in growth and development of marine mammals has not been previously explored. Furthermore, developmental changes in the somatotropic axis in species that preferentially deposit lipid during periods of rapid growth, such as pinnipeds, have not been studied. Harbor seals and Steller sea lions represent two pinniped species with diverse early life history strategies, differential growth rates leading to differences in mature size, and contrasting expression of sexual dimorphism. Understanding hormonal regulation of growth in species with distinct developmental patterns will enhance our knowledge of the link between physiology, nutrition and life history of diverse species. ^ Parallel studies in captive (longitudinal data) and free-ranging (cross-sectional data) harbor seals and Steller sea lions were conducted through collaborative research efforts with several aquariums, research, and rehabilitation facilities across North America. The research objectives for this dissertation research are to describe the ontogeny and evaluate the affect of reduced nutrient intake and re-alimentation on components of the somatotropic axis, to investigate the relationship of the somatotropic axis to growth rate and composition of gain, and to compare the expression of the components of the somatotropic axis of harbor seals and Steller sea lions with respect to their differential mass specific growth rate, composition of gain, mature size, and early life history strategies. ^ Understanding growth, development, and nutrient allocation during times of reduced nutrient intake and re-alimentation may provide a unique perspective into the survival of wildlife. Environmental change caused by natural or anthropogenic affect may change the quality or quantity of prey available for wildlife. Nutritional stress can lead to decreased growth making animals vulnerable to disease and predation which may have substantial impacts on survival and hence population trends. Monitoring nutritional status of populations of marine mammals is essential to maintaining healthy sustainable populations and minimizing anthropogenic effects on population trends. The somatotropic axis appears to by a useful tool to evaluate nutritional status of pinniped populations. ^