Intraspecific variation in fatty acids in a deep diving marine mammal, the northern elephant seal.

The northern elephant seal (Mirounga angustirostris) is one of the most studied semi-aquatic mammals; recovering from the brink of extinction in the early 1900’s. For over 60 years, research on this species has been continuous since the formation of the Año Nuevo colony in California. Like all large ocean predators, northern elephant seals face uncertainty as the aquatic environment they inhabit responds to the impacts of anthropogenic climate change, over-exploitation of prey, and marine pollution. Northern elephant seals spend up to 8 months of the year foraging between two terrestrial fasts for breeding and molting. Lipids, and more specifically, fatty acids, deposited during foraging trips are used to fuel up to 98% of metabolic needs in fasting northern elephant seals. Each life history stage experiences age specific demands and sex specific demands driven by the evolution of extreme sexual dimorphism characteristic of this species. The goal of this dissertation was to characterize the fatty acid profiles in two metabolically active tissues, blubber and skeletal muscle, that are fundamental for surviving fasting periods and powering long foraging migrations. To gain an understanding of how fatty acids change across ontogeny, I collected blubber samples from fasting northern elephant seals across age groups and sexes to assess the compositions of the main energy substrate of this species and how it changes with the demands of each life history stage. In addition, skeletal muscle samples were also collected to identify skeletal muscle fatty acid profiles for the first time in northern elephant seals and assess changes in fatty acids across ontogeny and within sexes of this sexually dimorphic species. Lastly, blubber and muscle fatty acids were collected from translocated juveniles to better understand short-term diving behaviors on fatty acid metabolism in these tissues. I found fatty acids of blubber and skeletal muscle varied across ontogeny, highlighting the specific demands of each life history stage, the youngest age group of northern elephant seals were found to be the most dependent on one of the highest fat content milks found in nature to fuel their development and survival during their first foraging trip. Interspecific differences in skeletal muscle fatty acid profiles were driven by varying proportions of five major fatty acids, weaned pups and juveniles had significantly more abundant saturated fatty acids, influenced by the post-weaning development period in pups and limited foraging ability during juveniles first trip to sea. We observed contrasting proportional changes in skeletal muscle fatty acid classes of diving seals that had transit times <20 hours in comparison to seals that transited for >47 hours, while monounsaturated fatty acids decreased in the blubber of all translocated seals and blubber polyunsaturated fatty acids increased in the three younger seals. Northern elephant seals become more efficient divers as they develop over successive foraging trips during the first two years of life, influencing the fatty acid profiles of both fasting and diving seals.