Somatotropic Axis in Rehabilitated Harbor Seals: Hormones Associated With Nutrient Utilization and Growth
The metabolic hormones, growth hormone (GH) and insulin-like growth factor (IGF)-I, together with the IGF binding proteins (BP), are the primary components of the somatotropic axis, which is associated with growth rate, and accretion of protein and adipose. A model for the influence of nutritional status on components of the somatotropic axis has been developed in domestic animals. However, the importance of the somatotropic axis in growth of marine mammals has not been previously explored. First year survival of harbor seal pups is correlated to body mass, with larger pups having a greater survival rate.3 Therefore, pups with a greater growth rate leading to increased size may also have a greater rate of survival. Our hypothesis is that patterns of these metabolic hormones will be similar to the domestic animal model relative to their physiologic age and nutritional status. Therefore, the primary objective of this research is to evaluate the ontogeny of the somatotropic axis in rehabilitated harbor seal pups, including investigations to determine if these hormones are related to growth rate and nutrient utilization in harbor seals.
Animals were fed a fish-based formula (per body weight basis) for approximately 4 weeks and then weaned to a mixed fish diet. Blood samples and mass measurements were collected from healthy but abandoned harbor seal pups (<10 d of age; housed at The Marine Mammal Center, n=16, and Mystic Aquarium, n=4) at 2-week intervals throughout rehabilitation (approximately 8 weeks) and concentrations of GH, IGF-I and IGFBP-2 and -3 determined.2,4 Standard length, axial girth, and blubber depth (using ultrasound) were measured upon arrival and at release in animals at Mystic Aquarium. Longitudinal measurements of serum hormone data were analyzed as repeated measures with the mixed model analysis of variance of SAS.
Pups arrived at the rehabilitation facility thin and had been fasting for several days. During rehabilitation, feed intake and body mass increased (P<0.001), and pups gained an average of 0.8 cm of blubber. Concentrations GH were elevated at nutritional nadir (fasting upon arrival; 8.3 ng * ml-1) and then decreased with re-alimentation (1.1 ng * ml-1; P<0.001). In contrast, IGF-I concentrations were low when pups were fasting and increased with re-alimentation (43 ng * ml-1 vs. 202 ng * ml-1; P<0.001). IGFBP-3 increased (P<0.001) while IGFBP-2 tended to decrease (P=0.10) throughout rehabilitation.
Consistent with patterns observed in domestic species, GH increased with fasting. In domestic species, elevated GH spares body protein and facilitates the use of blubber stores for maintenance energy needs.1 As nutrient intake increased, GH and IGFBP-2 declined while IGF-I and IGFBP-3 increased, which was associated with total body mass gain and an increase in blubber thickness. These data, in conjunction with other projects, may be useful in developing a model to assess nutritional stratus in free-ranging pinniped populations. By investigating physiological factors that link nutrition and growth, we can assess the impact of decreased nutrient intake on growth and body composition that may have implications for survival.
The authors would like to thank the E. Brody, T. Norris, D. Wickham and F. Gulland for providing serum samples from harbor seal pups rehabilitated at The Marine Mammal Center, Sausalito, CA.
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4. Richmond JP, LM Mazzaro, SA Zinn. 2005. Evaluation of the somatotropic axis in harbor seals (Phoca vitulina) and Steller sea lions (Eumetopias jubatus): Hormones associated with nutrient utilization and growth in two pinniped species with distinct rates of development. Conference on the Biology of Marine Mammals, San Diego, CA. pp 237.