Thermoregulation in Steller Sea Lions: a modelling approach

Abstract

The Steller sea lion, largest of the otariids, is currently listed as endangered, in parts of its range, due to a substantial population decline over the past three decades. The most prominent hypothesis for this decrease is a reduced juvenile survival resulting from increased nutritional stress. Consequently, energetic demands are unable to be met by energy input. To better understand this relationship, an energetics model using relevant published data incorporating environmental conditions commonly experienced by sea lions in the wild has been constructed for otariids, focusing on Stellers. This predictive, individual-based physiological model allows the assessment of heat loss, from which costs of thermoregulation can be estimated for both aquatic and terrestrial habitats. The main aim of this system is to incorporate possible interactions between a number of factors, including the compensatory effects of the heat increment of feeding, the heat generated by locomotion, and the effect of body condition. Under the environmental conditions tested in water, Steller sea lions appear to require additional energy for thermoregulation, unless they are moving and generating additional heat as a result of the locomotion. While resting in air sea lions are within their thermal neutral zone during sunny conditions on clear days. However, when there is a lack of direct solar radiation, either at night or when it is cloudy, or under very cold conditions, sea lions must generate extra heat for thermoregulation. These costs are in part offset if the sea lion has recently fed and is therefore generating additional energy from the heat increment of feeding. For all the conditions tested, smaller sea lions, or those with poorer body condition, are most at risk from the effects of the cold and incur the greatest thermoregulatory cost. Using this model, the physical and environmental conditions under which sea lions utilize extra energy for thermoregulation can be established in addition to the key factors that may ultimately cause an energetic imbalance in Steller sea lions.