Variability of plasma melatonin level in pony mares (Equus caballus), comparison with the hybrid: mules and with jennies (Equus asinus) Daniel Guillaume, Luiz A. Zarazaga, Benoît Malpaux and Philippe ChemineauReprod. Nutr. Dev., 46 6 (2006) 633-639DOI: https://doi.org/10.1051/rnd:2006043