Adrenal responsiveness in domestic sheep (Ovis aries) to acute and chronic stressors as predicted by remote monitoring of cardiac frequency

Abstract

The concept of stress and the general adaptive syndrome as advanced by Hans Selye has received considerable attention during the past decade primarily in its interpretation of physiological changes associated with chronic stress. Our work with domestic sheep (Ovis aries) habituated to stalls and fitted with halters carrying indwelling electrocardiogram leads and jugular vein cannulas allowed us to remotely test heart rate and blood cortisol responses of these animals to graded stressors. A radioimmunoassay was validated on domestic sheep plasma. We were unable to identify significant alterations of the adrenal response test by sheep exposed to synthetic adrenocorticotropic hormone after 34 days of chronic stress, suggesting neither adrenal exhaustion nor hypersensitivity. As an indicator of acute stress, we obtained a correlation coefficient of 0.91 between heart rate and blood cortisol, which suggests that heart rate has a strong potential of being a reliable predictor of cortisol values. With a regression equation, the heart rate of observed free-living sheep monitored by telemetry could be used to predict plasma cortisol levels and that, in turn, to predict potential stress-induced changes in animal production, including immunity.