THERMOREGULATION OF OTARIIDS: A MODELLING APPROACH
At present there are a number of fields in the study of otariids which remain poorly understood. One such area is the energetic requirement of otariids under the varying conditions imposed by the environments, which they inhabit. It is for this reason, that the development of a predictive energetics-based model, to help identify sensitive parameters for otariid energetics, will greatly help in the further understanding of this family of pinnipeds. Energetic demands include a variety of obligatory costs such as maintenance metabolism, locomotion & foraging, and maintenance of thermal homeostasis, as well as facultative costs related to social activities, reproduction, parental investment, molt, and anabolism during ontogeny. Among the obligatory costs, thermal costs can be offset by skeletal muscle thermogenesis from locomotion and feeding i.e. thermoregulatory demands can be met by maintenance, locomotion and foraging costs. Since otariids forage in waters that range in temperature from a summer high of 35oC in the Galapagos, to a winter low near freezing (below 0oC for sea water) in the Bering Sea they are subject to a variety of environmental conditions, which in turn affects the partitioning of resources for these energetic needs. Among pinnipeds, only Northern fur seals - protected by thick pelage with dense underfur, walrus, and predominantly ice-breeding phocid seals - protected by very thick blubber layers ranging from 5 - 25 cm in thickness, forage in the low water temperatures of the polar regions. Otariids by comparison, have blubber layers of less than 2-3 cm in thickness and so arguably are not as well suited to these cold environments. Basal metabolic rate measurements are performed under standardized conditions on inactive and post-absorptive animals. For otariids, these determinations have only been performed for a few species, for example California sea lions. Based on these findings the metabolic rates for otariids are higher than those predicted by allometric regressions for terrestrial mammals. Utilizing this and other relevant published data a descriptive model will be constructed incorporating the environmental factors commonly experienced by otariids in the wild. Within the model all the components will be constructed using Matlab and Simulink modeling software (Mathworks, Natick, MA). This will make it possible to predict under what conditions, be it physical or environmental, the animals may be utilizing additional energy for thermoregulation. The model will account for both the physiology and behavior of the animals based on passive mechanisms and decision making processes. The main aim of this system is to allow for the possibility of interaction between the various factors involved. For example the heat increment produced from the activity may be enough to offset the need for the animal to expend extra energy in thermoregulation even though the animal is below its thermal neutral zone. This would explain why a number of otariids remain constantly active while in the cold water environment. Energetic based models provide researchers with the ability to identify key factors that will predict an energetic imbalance in foraging marine mammals and perhaps help to explain why for ceratin species such as the Steller sea lion nutritional stress may occur when energetic demands are not met by the energy input. This technique will ultimately permit predictions of survival likelihood's based on energetic balances for otariids.