Fasting physiology of the pinnipeds: The challenges of fasting while maintaining high energy expenditure and nutrient delivery for lactation

Abstract

Most animal species experience periods of food deprivation and many periodically forgo foraging in favor of other activities such as migration and reproduction. Pinnipeds-seals, sea lions, fur seals, and walrus-forage on marine resources but remain tied to land for reproduction. This separates energy acquisition from energy allocation for maternal investment, competition for mates, and for tissue maintenance and repair during pelage synthesis. Thus, pinnipeds regularly undertake energetically costly activities simultaneous with extended fasting at multiple life stages. This life history characteristic should favor physiological mechanisms for efficient fasting metabolism while supporting the energetic and substrate requirements of energy-demanding activities (e.g. lactation). Studies on a variety of pinnipeds have revealed highly efficient protein sparing despite high rates of energy expenditure and nutrient mobilization for lactation. Carbohydrate oxidation is low and high rates of lipolysis support nutrient delivery. Despite prolonged fasting and high rates of β-oxidation there is little accumulation of ketoacids. In contrast to numerous investigations on nonfasting adapted species, studies on phocid seals have revealed high rates of glucose production during fasting that exceed the needs for glucose dependent tissues and suggest high rates of carbohydrate recycling. Investigations into the hormonal regulation of fuel use and intracellular signaling pathways indicate adjustments to the typical mammalian regulation of gluconeogenesis. Together, these findings suggest alterations in the metabolic strategies for fasting exist in the pinnipeds compared to domestic and wild terrestrial mammals.