Body temperature and heart rate variability, and their circadian rhythms in sheep as measured by biologgers

Heart rate (HR) variability (HRV) reflects the balance between the autonomic nervous system's sympathetic and parasympathetic branches, which regulate stress and relaxation. Wearable sensors record HR and body temperature (BT), to provide data that informed decisions can be made from on animal management and welfare. The purpose of this study was to investigate daily changes in BT and HRV, recorded by subcutaneous biologgers; specifically, to quantify the 24-hour circadian rhythm in BT (°C), HR (bpm), the Standard Deviation of the R-R intervals (SDNN), and the Root Mean Square of Successive Differences (RMSSD) (ms). Five ewes were implanted with a subcutaneous biologger, configured to collect data every 5 min for 7 d. Mean ( ± S.E.) BT (38.06 ± 0.01), HR (111.14 ± 1.68), SDNN (34.35 ± 2.90), and RMSSD (47.95 ± 4.21) were calculated. BT and HR were highest in the day (day: 38.15 ± 0.01 and 120.71 ± 0.43; night: 37.98 ± 0.01 and 102.47 ± 0.43; P < 0.001), and SDNN (day: 29.30 ± 0.87; night: 37.16 ± 0.67) and RMSSD (day: 39.01 ± 1.17; night: 53.53 ± 0.94) were highest at night (P < 0.001). BT and HR were positively correlated (P < 0.01), but both negatively correlated (P < 0.01) with SDNN and RMSSD. BT, HR, SDNN, and RMSSD presented a 24-h circadian rhythm, with acrophases (peak activity period) for BT at 1457 h and at 1223 h for HR, but those of SDNN and RMSSD were at 0350 h and 0327 h, respectively. In conclusion, biologgers detected the diurnal rhythmicity in BT and HRV in sheep, providing an option to use these physiological measures to assess an animal's health and welfare. • Variations in body temperature (BT) and heart rate variability (HRV) serve as indicators of stress. • Wearable sensors allow recording these variables without affecting the animal's behavior. • Subcutaneous biologgers detected a 24-h circadian rhythm in BT and HRV in lactating adult sheep. • There is a negative correlation between BT and HRV. • These variables should be taken into account when assessing animal welfare and health status.