Effects of stocking density on circadian patterns in locomotor activity, body temperature, and cardiac traits in ewe lambs

This study examined the effects of stocking density on the circadian patterns in locomotor activity (LA), body temperature (T), and cardiac traits in ewe lambs. Eleven animals were monitored for two consecutive 7-d periods under low (2.6 m² animal⁻¹) and high (1.3 m² animal⁻¹) densities. LA was recorded by triaxial accelerometers, and four lambs carried subcutaneous biologgers that measured T, heart rate (HR), and HR variability (HRV). Hourly means were analyzed by cosinor modeling to determine the MESOR, amplitude, and acrophase of 24-h rhythms, and the Circadianity Index (CI) was computed as an indicator of robustness. Mean T was lower and HR was higher low high density (T: 38.09±0.03 °C vs. 38.57±0.01 °C, p<0.001; HR: 78.6±2.3 vs. 73.5±0.9 bpm, p<0.001), but LA and HRV indices (SDNN and RMSSD) did not differ between densities. MESOR and amplitudes were similar at the two densities. T acrophase was more advanced at high density (1416±6.9 h) than it was at low density (2207±1.1 h, p<0.001), but the acrophases in HR and LA were similar. The CI confirmed moderate to high robustness in circadian rhythmicity (T: 0.46 vs. 0.29; HR: 0.50 vs. 0.47; LA: 0.16 vs. 0.17).  Cortisol levels in the feces did no differ between low density (86.42±2.62 ng/ml) and high density (79.62±4.26 ng/ml). The results indicate that an increase in stocking density influenced the temporal pattern in thermoregulation but did not markedly affect locomotor or cardiac rhythmicity, which suggests that short-term crowding has a limited influence on overall circadian organization in ewe lambs.