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Seasonal variation in thyroid morphology and secretion in the white whale, Delphinapterus leucas

St. Aubin, D. J., Geraci, J. R.
Canadian journal of zoology 1989 v.67 no.2 pp. 263-267
Delphinapterus leucas, autumn, epithelial cells, estuaries, hormone secretion, seasonal variation, spring, subcutaneous fat, summer, triiodothyronine, whales, wintering grounds, Hudson Bay
Thyroid activity was examined in white whales, Delphinapterus leucas, during three phases of their annual cycle: spring migration from oceanic wintering grounds, summer occupation of an estuary in Hudson Bay, and return migration in fall. Circulating levels of thyroxine (T₄) and triiodothyronine (T₃) were significantly higher in summer. Histologically, thyroid epithelial cells were cuboidal during spring, became columnar during summer, and reverted to a cuboidal morphology in fall, changes consistent with increased synthesis and secretion of hormones during the estuarine phase. Small follicles, abundant in spring and fall samples, were relatively less numerous in summer specimens. Occupation of warm estuaries may be an important stimulus to thyroid hormone secretion, perhaps allowing the whales to mobilize subcutaneous fat stores for somatic growth in an energetically less demanding environment.