Abstract

The aim of the study was to observe if individual variability in melatonin secretion exists in Sarda does and, if so, to what extent it was associated with the onset of seasonal ovulation. Blood samples were taken from 18 Sarda goats, every 2 h for 24 h, to determine the plasma melatonin secretion levels. To identify the reproductive cycles, serum progesterone concentrations were determined weekly, after introduction of males. Nocturnal plasma melatonin levels recorded during the second solstice (21 December) and first equinox (21 March) were higher that that obtained for the first solstice (21 June) and second equinox (21 September) (P < 0.01). The varying levels of plasma melatonin demonstrated two groups, those with high (Group A) and those with low melatonin secretion levels (Group B). An increase in plasma melatonin levels at night proved to be significant in the high secretion group of animals (P <0.01 for both equinoxes and second solstice; P <0.05 in first solstice). The low secretion melatonin group, showed a significant increase only during the second solstice and first equinox. A comparison between the nocturnal levels in the two groups was significant (P < 0.01) different. The animals with lower levels of melatonin showed an onset of cyclic ovarian activity about 12 days after buck introduction, while the other group started only 22 days later (P < 0.01). These results demonstrate that Sarda does are sensitive to a variation in melatonin secretion. The underlying reason is presumably genetic, given the high repeatability of plasma melatonin concentrations, recorded within each goat, in all four observations. The definite link between reproductive activity and melatonin model strongly suggests that goats with lower melatonin levels are less sensitive to photoperiod, and show a stronger response than to other stimuli, such as the male effect.

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