Abstract

Changes in the components of the somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF binding proteins (IGFBP), are useful predictors of nutritional status in domestic and marine mammals.^1,2 These metabolic hormones are associated with growth rate, and accretion of lean tissue and adipose, and are responsive to nutrient intake.¹ Concentrations of IGF-I generally increase while GH concentrations decline with increased nutrient intake, which is associated with accretion of protein and adipose. In contrast, during nutritional deficit, secretion of GH increases to stimulate metabolism of adipose and spare protein, while IGF-I decreases to spare nutrients for maintenance.

The objectives of this research were to evaluate the seasonal changes in nutrient intake, body condition, and components of the somatotropic axis in captive beluga whales. Our hypothesis was that body condition and serum hormone profiles would reflect seasonal changes in intake and energy status of captive beluga whales.

To address these objectives, blood was collected and body condition (axillary girth and blubber thickness determined by ultrasound) assessed monthly for 4 years in all three adult beluga whales (Delphinapterus leucas; one male, two females) housed at Mystic Aquarium. All samples were taken at 09:30 before feeding to minimize diurnal and ingestion effects on hormone concentrations. Intake was recorded daily. Seasons were categorized as winter (December to February), spring (March to May), summer (June to August), autumn (September to November). Serum concentrations of GH and IGF-I were quantified using radioimmunoassay.³ IGFBP were quantified using ligand blot.⁴ Longitudinal measurements of hormone data and body condition assessments were analyzed with repeat measures GLM.

All four years showed a similar seasonal pattern for intake, axillary girth, blubber thickness, GH, IGF-I, and IGFBP (P>0.05). Body condition indices generally reflected intake from the previous season. Peak intake occurred in winter followed by peak blubber depth in spring. Intake was least in summer followed by lowest blubber depth in the fall. Caloric intake, pectoral girth, and blubber thickness declined approximately 30% (approximately 10,000 cal, 12-14 cm and 28-30%, respectively), between peak and minimum caloric intake in winter and summer. Concentrations of GH were not affected by season (P = 0.69), but were greater in 2005 and 2007 (both 4.2 ± 0.55ng·ml^-1) compared with 2006 and 2008 (P < 0.01; 2.3 ± 0.46 and 2.1 ± 0.97 ng·ml^-1, respectively). In contrast, IGF-I concentrations were greater in the summer and fall (301.6 ± 19.4 and 241.7 ± 30.1 ng·ml^-1, respectively) compared with winter and spring (P < 0.01;176.9 ± 20.9 and 199.8 ± 19.1 ng·ml^-1, respectively), but the seasonal pattern among years was similar (P = 0.09).

Understanding seasonal fluctuations in feed intake and the resulting effect on body condition and metabolic hormones are important considerations for captive animal health so that deviations, that may indicate nutritional stress, caused by reduced quantity or quality of feed, can be detected promptly. Further, information gained from captive animals fed a known diet may provide information that could be useful in evaluating nutritional and health status of free-ranging populations of beluga whales.

Acknowledgements

The authors thank the husbandry and training staff at Mystic Aquarium, especially Jessica Morgan, Michaela Kluever, and J. Lawrence Dunn, for their assistance in this project.

References

1.  Breier B.H. 1999. Regulation of protein and energy metabolism by the somatotropic axis. Domest Anim Endocrinol 17: 209-218.

2.  Richmond J.P, J. Skinner, J. Gilbert, L.M. Mazzaro, and S.A. Zinn. 2008. Comparison of the somatotropic axis in free-ranging and rehabilitated harbor seal pups (Phoca vitulina). Zoo Wildl Med 39(3): 342-348.

3.  Richmond J.P., and S.A. Zinn. In Press. Validation of radioimmunoassays (RIA) for growth hormone (GH) and insulin-like growth factor (IGF)-I in phocid, otariid, and cetacean species. Aquatic Mamm 35(1): 19-31.

4.  Freake H. C., K. E. Govoni, K. Guda, C. Huang, and S.A. Zinn. 2001. Actions and interactions of thyroid hormone and zinc status in growing rats. J Nutr 131: 1135-1141.