Episodic and seasonal rhythms of cortisol secretion in male Eld's deer (Cervus eldi thamin)

Episodic and seasonal rhythms of cortisol secretion were evaluated in six adult Eld's deer (Cervus eldi thamin) stags. Plasma cortisol was measured in serial blood samples collected via remote catheterization every 10 min for 10 h within 2 weeks of the summer solstice (21 June), autumn equinox (22 September), winter solstice (21 December) and spring equinox (20 March), and in weekly blood samples collected from sedated stags. Cortisol was secreted episodically at a rate of approximately 0.6 peaks/h. Based on quantitative peak detection analyses of each 10-h data series, no overall seasonal differences (P > 0.05) were detected in the number of peaks (mean range, 5.7-6.2), maximal peak height (mean range, 30.1-40.8 nmol/l), mean peak height as per cent increase (mean range, 158-168%), mean interval between peaks (mean range, 80.1-88.6 min), mean peak width (mean range, 55.1-65.1 min) and mean peak area under the curve (mean range, 675-816 nmol/l min). Based on weekly blood sampling, spring cortisol concentrations were elevated (P < 0.05) compared with summer and autumn concentrations. However, when mean cortisol concentrations derived from the 10-h quarterly data sets were analysed, no seasonal differences (P > 0.05) were detected. The present study represents the first detailed confirmation of episodic cortisol secretion in any cervid. Results (1) indicate that Eld's deer stags lack a distinct seasonal rhythm of cortisol secretion and (2) clearly illustrate the need for frequent blood sampling in fully conscious individuals to ensure accurate assessment of adrenal status in cervids.