Pollock Conservation Cooperative Research Center
2000 Awarded Research Projects
Thyroid hormones and plasma leptin concentrations during food deprivation and satiety: use as an index of metabolic condition in free-ranging Steller sea lions
Award: $51,388
Estimated completion: September 30, 2003
Abstract
If we wish to promote the recovery of Steller sea lions in Alaska, we must be able to measure how animals respond to perturbations in its environment. Until we can effectively assess the physiological or metabolic condition of individual sea lions in the wild we will not be able to accurately judge the success of management measures intended to aid the recovery of this species. What is needed is an index of body condition that reflects not only the cumulative effects of weeks to months of foraging effort, but one that is integrative and reflects the net energy balance at any particular point in time.
The primary goal of this study will be to develop an index or tool for the assessment of metabolic condition in free-ranging Steller sea lions based upon circulating hormone levels. We believe that we can develop such a tool through the measurement of a combination of hormones that are involved in the regulation of metabolism and food intake in mammals—thyroid hormones, cortisol and leptin. The value of this approach is that hormones represent an integrated response to a number of environmental and physiological factors that influence metabolism. Previous stud ies have used thyroxine and cortisol, along with several morphometric measurements to assess the well being of yearling Hawaiian monk seals that appeared to be malnourished. Their results suggest that a suite of measurements, including these hormones, provides a good indication of the physiology of a seal and its ability to adapt to suboptimal environments. Recent studies on terrestrial mammals have shown a close correlation between serum leptin levels and total body fat that could provide an index of body condition more easily monitored in free-ranging animals than the presently used deuterium dilution technique.
This study will build upon a preliminary study on the effect of food deprivation on serum leptin concentrations and utilize blood samples collected during previous studies to investigate the interrelationships between hormone levels, body fat content and nutritional state. Other environmental effects, such as circadian changes in hormone production and seasonal change in hormone levels, will also be addressed. Ongoing studies on free-ranging Steller sea lions will provide additional samples to test the application of our indices on wild populations.
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