Physiological load and circadian organization of cardiovascular, autonomic and thermal functions in dry and lactating ewes assessed using biologgers

Context Lactation is a physiologically demanding period in sheep that is characterized by an increase in metabolic and cardiovascular requirements. Although physiological changes during lactation have been well documented, less is known about how lactation affects the circadian organization of cardiovascular and autonomic functions. Aim The objective of this study was to evaluate the impact of lactation on physiological load and circadian organization in adult ewes. Methods Five lactating ewes that were in the second to third week of lactation with lambs present, were compared to seven dry ewes. Subcutaneous biologgers recorded body temperature (T), heart rate (HR), and heart rate variability (HRV; standard deviation of normal-to-normal R–R intervals [SDNN] and root mean square of consecutive deviations between normal heartbeats [RMSSD]) every 30 min for 20 days. Raw physiological data and circadian parameters derived from 24-h cosinor models, including midline-estimating statistic of rhythm (MESOR), amplitude, acrophase, robustness, and circadianity, were evaluated. Key results Lactating ewes exhibited higher T and HR than dry ewes. SDNN was lower in lactating ewes, whereas RMSSD showed a non-significant tendency to be lower. Both groups exhibited robust 24-h rhythms in HR, although rhythms in lactating ewes had higher MESOR, amplitude, and robustness. Circadian organization of HRV was less consistent in lactating ewes, with fewer animals exhibiting significant circadian rhythms and greater inter-individual variability than in dry ewes. Dry ewes exhibited a lower physiological load and a more stable and coherent circadian organization than did lactating ewes. Conclusion Lactation in ewes is associated with an increase in cardiovascular and thermal load, a reduction in autonomic variability, and marked changes in circadian regulation, particularly autonomic rhythmicity. Implications Integrating circadian analysis with conventional physiological measurements provides a more comprehensive assessment of physiological status and might provide sensitive indicators of functional strain during lactation.