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Abstract
The aim of this study was to evaluate the circadian rhythm of melatonin in horses, sheep, and goats, and to compare this rhythmicity under the same conditions. Five female thoroughbred, five female Sarda sheep, and five female Sarda goats were housed in boxes equipped with an opening window. Animals were kept under a natural photoperiod and indoor ambient temperature (16–20 °C). Blood samples were collected every 2 h over 48 h period. Results showed a circadian melatonin rhythm with a nocturnal acrophase in the three species studied. In sheep and goats, acrophase was observed at about 2 h before respect to horses. All species studied showed a high robustness between rhythm 61.20 ± 7.57 and 83.60 ± 7.26, denoting a high percentage of stationarity of melatonin circadian rhythm. However, we can claim that each species has its own characteristics in melatonin secretion that must be considered in the exploration of circadian clock in different species.