Diurnal periodicity of plasma Cortisol and corticosterone in desert bighorn sheep demonstrated by radioimmunoassay

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Abstract
A sensitive and specific radioimmunoassay (RIA) for Cortisol and corticosterone was validated for desert bighorn sheep (Oviscanadensis cremnobates) plasma. Statistical and physiological validation of assay accuracy, precision, sensitivity, and specificity demonstrated the RIA to be reliable. Coefficients for intra- and inter-assay variation were 1.20 and 2.35% for Cortisol and 3.50 and 3.97% for corticosterone, respectively. Sensitivity was 3.3 and 4.9 pg for cortisol and corticosterone, respectively. A distinct diurnal rhythm in plasma cortisol and corticosterone synchronous with photoperiod was found in free-ranging and captive bighorn sheep. A significant difference (P < 0.01) existed between the mean 24-h plasma cortisol concentration for captive (33.04 ± 5.97 ng/mL) and free-ranging (49.20 ± 8.17 ng/mL) bighorn. Corticosterone levels were not significantly distinct between captive and free-ranging bighorn. The mean scotophase cortisol concentration in free-ranging class III rams (60.97 ± 3.21 ng/mL) was significantly greater (P < 0.05) than the mean scotophase cortisol concentration (54.03 ± 5.97 ng/mL) of the free ranging group. Absence of group interactions in solitary rams may be sufficiently stressful so as to entrain increased plasma cortisol concentrations.