Variability of plasma melatonin level in pony mares (Equus caballus), comparison with the hybrid: mules and with jennies (Equus asinus).

In long-day breeders like horses, the length of nocturnal melatonin secretion is the main messenger of photoperiod. Previous studies have shown that the nocturnal jugular melatonin concentration is lower in horses, than in mules but is unknown in donkeys. The aim of this study was to estimate the inter-animal variability of plasma melatonin concentration in domestic mares and to compare this concentration with those observed in domestic jennies and in their hybrid mules. In the autumn, blood samples were collected at 22 h, 23 h, 0 h and 1 h during 2 nights at 3 weeks intervals, in 110 pony mares, 10 jennies and 6 mules maintained under natural photoperiod. Melatonin was assayed by a validated RIA method. The statistical analysis of the measures was done with a specific unbalanced analysis of variance model. The effect of species and individuals (nested under species) was highly significant. The mean melatonin concentration was 24 pg.mL(-1) in mares and was significantly lower than in jennies and in mules which were 90 pg.mL(-1) and 169 pg.mL(-1) respectively. The melatonin plasma concentration was higher in jennies than in mares. These results suggest that the melatonin concentration is genetically determined.