Use of telemetered heart rate as an indicator of acute stress in bighorn sheep (Ovis canadensis) ewes

Abstract

Increased exposure of bighorn sheep (Ovis canadensis) to human-associated disturbance is a primary concern of resource managers. However, quantitative methods to assess a free-living animal’s physiological response to acute environmental stressors are limited. I developed a technique for discrete long-term, long-range monitoring of heart rate in captive Rocky Mountain bighorn sheep (O. c. canadensis) and determined the usefulness of this technique as an indicator of stress. To monitor heart rate, I surgically implanted telemetry transmitters on the dorsolateral thorax of 15 captive adult bighorn sheep ewes. Twelve of 15 transmitters remained functional ≥1 yr. Heart rate data collected from the transmitters, using a telemetry receiver/datalogger, accurately reflected heart rate as measured with electrocardiogram tracings. Line of sight signal range was at least 800 m in 95% (37/39) of collections made from standing ewes, while data could be collected reliably (74%; 29/39) to 600 m from bedded ewes.

I examined the effects of controlled stressors on the relationship between telemetered heart rate and serum cortisol in these bighorn sheep. Ewes responded to graded stressors with a predictable rise in serum cortisol that was related to increased heart rate, but in many cases no overt behavioral responses were observed. Serum cortisol levels tended to increase linearly with heart rate for all ewes, although the magnitude of response varied with reproductive status (p < 0.001). Serum cortisol concentrations increased 0.11 ng/mL (SE = 0.017) for each 1 bpm in response heart rate in non-pregnant ewes and 0.09 ng/mL (SE = 0.023) for ewes in early pregnancy. In contrast, serum cortisol response of ewes in late pregnancy more than doubled to 0.23 ng/mL (SE = 0.030) for each 1 bpm in response heart rate.

I investigated the daily and seasonal variation in heart rate and serum cortisol in adult bighorn sheep ewes. I collected telemetered heart rate at about 1-min intervals, and blood for serum cortisol analysis at 2-hr intervals over a 24 hr period on the spring and fall equinox and summer and winter solstice, 1996. Heart rate varied by month of sampling (p = 0.007) with generally lower values in March and December. Daily variation was also observed with higher (p ≤ 0.009) heart rates during light than darkness. No daily (p = 0.63) or seasonal variation (p = 0.06) in serum cortisol was observed. Daily and seasonal variation in heart rate but not serum cortisol concentrations suggest that the predictive value of heart rate as an indicator of change in serum cortisol concentrations will vary slightly with time of day and season.

I conclude that remotely monitored heart rate can be used as a predictor of change in serum cortisol levels for ewes of known reproductive status and, when used in conjunction with overt behavioral observations, can provide resource managers with an improved assessment of acute stress in bighorn sheep.