Abstract
Cortisol is a member of the corticoid family and is known as a stress hormone. In cetaceans, cortisol is frequently measured as a stress indicator. However, inappropriate estimation of cortisol concentration frequently occurs due to a lack of fundamental knowledge of corticoids in cetaceans. We carried out several experiments to establish standard methods of cortisol measurement in cetaceans. 1) Cortisol biosynthesis was confirmed in Antarctic minke whales Balaenoptera bonaerensis. Corticoids including cortisol, progestins, adrenal androgens and many other intermediates were synthesized from 14C-pregnenolone. 2) Serum cortisol concentrations in 15 cetaceans species were measured. Average cortisol concentrations in each species showed a wide range from 2.9 ng/ml in killer whale Orcinus orca to 39.9 ng/ml in striped dolphin Stenella coeruleoalba. A negative correlation between cortisol and body length was confirmed. 3) Diurnal changes in cortisol concentrations were investigated in well-trained Indo-Pacific bottlenose dolphins Tursiops aduncus and in killer whales. Cortisol concentrations in both species exhibited episodic fluctuations with decreases in the evening and increases in the early morning. Annual cortisol changes in killer whales were also investigated, and pattern varied according to sex. 4) Changes in serum cortisol concentrations following transport into captivity in a male rough-toothed dolphin Steno bredanensis were investigated. Cortisol concentrations were high on the day of transport, and continued to decline until stabilizing about four months later. 5) Twenty-one-deoxycortisol concentrations increased with time after transport, and a significant negative correlation between cortisol and 21 -deoxycortisol was demonstrated in a rough-toothed dolphin. This suggested that 21 -deoxycortisol is useful as a negative indicator of stress.
