Endocrine Explorations

To land-dwellers, a sea lion’s life looks fairly uneventful: swim, eat, sleep and reproduce. In fact, a sea lion leads a rather dramatic life. Sea lions and all marine mammals, for that matter live in environments that are continuously changing, particularly with regards to food supply. An animal’s ability to respond to these changes determines whether or not it will live to swim, eat, sleep, and reproduce another day.

Survival largely depends on how the animal regulates energy intake and expenditure when times get tough. Fortunately, they can defer these decisions to a powerful internal headquarters: the endocrine system. This complex system of glands secretes hormones that regulate the body, influencing an animal’s physiological responses in times of food shortages.

A team of Consortium researchers recently got to know the Steller sea lion’s endocrine system more intimately by finding out how hormone changes during periods of food shortage are influenced by an animal’s age, diet, and the season.

The research team comprised Tiphaine Jenniard du Dot, David Rosen, and Andrew Trites from the University of British Columbia Fisheries Centre; Julie P. Richmond from the University of Connecticut’s Department of Animal Science, and Alexander S. Kitaysky of the Institue of Arctic Biology (University of Alaska Fairbanks). Their study was published in Comparative Biochemistry and Physiology.

Honing in on hormones
The researchers focused on the three groups of hormones primarily involved in the endocrine response to nutritional stress: the somatotropic hormones including the insulin-like growth factor-1 (IGF-I), the thyroid hormones (T3 and T4), and glucocorticoids such as cortisol. They chose eight captive female Stellers (five three-year-olds and three five-year-olds), and reduced the animals’ food intake for significant periods in both summer and winter. One test group was given a smaller ration of its usual herring diet (a high-fat fish), while the other received a normal amount of fish but were switched to pollock (a low-fat, low-energy fish). After the test period, both groups returned to eating their normal herring regime.

The researchers found that the Stellers' levels of stress hormones (cortisol) and growth hormones (IGF-I) measured via blood samples changed as the animals lost and regained weight. In fact, they were able to link each hormone to a different strategy for preserving energy (lipid versus lean mass catabolism). Thyroid hormones, on the other hand, could not be linked to any physical changes they observed.

The researchers also found that young animals show greater hormone changes than their elders, and that the type of fish they received (high-energy or low-energy) made little difference on hormone levels.

Another step forward
The team concluded that studying a Steller sea lion’s cortisol and IGF-I levels could help them detect and better understand nutritional stress, as well as the energetic strategy animals use to cope with it. The study brought them yet another step closer to understanding how Steller sea lions cope with the unpredictable environments they call home.

March 3, 2010

Publication:
Changes in glucocorticoids, IGF-I and thyroid hormones as indicators of nutritional stress and subsequent refeeding in Steller sea lions (Eumetopias jubatus). Jeanniard du Dot T., Rosen D.A., Richmond, J.P., Kitaysky A.S., Zinn, S.A. and Trites A.W. 2009. Comparative Biochemistry and Physiology, Part A 152:524-534. abstract Physiological responses to changes in energy balance are tightly regulated by the endocrine system through glucocorticoids, IGF-I and thyroid hormones. Changes in these hormones were studied in eight captive female Steller sea lions that experienced changes in food intake, body mass, body composition, and blood metabolites during summer and winter. During a period of energy restriction, one group of sea lions was fed reduced amounts of Pacific herring and another was fed an isocaloric diet of walleye pollock, after which both groups returned to their pre-experimental diets of herring. Cortisol was negatively and IGF-I was positively associated with changes in body mass during periods of energy restriction (mass loss associated with increase in cortisol and decrease in IGF-I) and refeeding (body mass maintenance associated with stable hormone concentrations in summer and compensatory growth linked to decrease in cortisol and increase in IGF-I in winter). Cortisol and IGF-I were also correlated with changes in lipid and lean mass, respectively. Consequently, these two hormones likely make adequate biomarkers for nutritional stress in sea lions, and when combined provide indication of the energetic strategy (lipid vs lean mass catabolism) animals adopt to cope with changes in nutrient intake. Unlike type of diet fed to the sea lions, age of the animals also impacted hormonal responses, with younger animals showing more intense hormonal changes to nutritional stress. Thyroid hormones, however, were not linked to any physiological changes observed in this study.
Seasonal influence on the response of the somatotropic axis to nutrient restriction and re-alimentation in captive Steller sea lions (Eumetopias jubatus). Richmond, J.P., T. Jeanniard du Dot, D.A.S. Rosen and S.A. Zinn. 2010. Journal of Experimental Zoology 311A:144-156. abstract Fluctuations in availability of prey resources can impede acquisition of sufficient energy for maintenance and growth. By investigating the hormonal mechanisms of the somatotropic axis that link nutrition, fat metabolism, and lean tissue accretion, we can assess the physiological impact of decreased nutrient intake on growth. Further, species that undergo seasonal periods of reduced intake as a part of their normal life history may have a differential seasonal response to nutrient restriction. This experiment evaluated the influence of season and age on the response of the somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF-binding proteins (BP), to reduced nutrient intake and re-alimentation in Steller sea lions. Eight captive females (five juveniles, three sub-adults) were subject to 28-day periods of food restriction, controlled re-feeding, and ad libitum recovery in summer (long-day photoperiod) and winter (short-day photoperiod). Hormone concentrations were insensitive to type of fish fed (low fat pollock vs. high fat herring), but sensitive to energy intake. Body mass, fat, and IGF-I declined, whereas GH and IGFBP-2 increased during feed restriction. Reduced IGF-I and IGFBP with increased GH during controlled re-feeding suggest that animals did not reach positive energy balance until fed ad libitum. Increased IGF-I, IGFBP-2, IGFBP-3, and reduced GH observed in summer reflected seasonal differences in energy partitioning. There was a strong season and age effect in the response to restriction and re-alimentation, indicating that older, larger animals are better able to cope with stress associated with energy deficit, regardless of season.