Plasma melatonin in domestic female Mediterranean sheep (Comisana breed) and goats (Maltese and Red Syrian)

The plasma melatonin nychtemeral profiles in Mediterranean ewes and goats were evaluated six times throughout the year. Melatonin levels were high throughout the night and generally below the assay detection limit during daytime. However, during long days, 30% of the last daytime samples had high melatonin concentrations. Plasma melatonin levels were higher in Comisana sheep than in goats, and higher in Maltese than in Red Syrian goats, with highly significant effect of the individual animal and high repeatability. Plasma melatonin was higher in April than in August. When there was a large difference between the duration of day and night, the plasma melatonin pattern and the light/dark cycle did not always match exactly, suggesting some form of superimposition and/or the prevalence of an endogenous rhythm. The difference found at similar scotoperiods with increasing or decreasing day length may be involved in the perception of the photoperiodic changes.