Inter- and intra-individual variation of oxytocin secretion in oestrous mares exposed to stallions

The neuro-peptide hormone oxytocin regulates several reproductive mechanisms in mammals, such as uterine contractions during parturition and milk ejection in the lactating mammary gland. Oxytocin may also influence behavior and behavioral strategies, e.g. pair bonding, social recognition, maternal behavior, trust building, or anxiety. Teasing oestrous mares by a stallion provokes the release of oxytocin. We therefore tested whether such elevated oxytocin levels reveal possible mate preferences as determined in typical preference tests. Thirteen mares were tested in two consecutive oestrus cycles with two different stallions each (from a total of seven) in order to take possible individual differences into account. The mares were exposed in a teasing barn, the stallion being free in a box with a window to the outside mare. Stallions were previously determined in a t-maze preference test (preference scoring by determination of a freely moving mare’s contact time with each of two stallions positioned in the right and left boxes at the end of a stable corridor during 15 min) as preferred or non-preferred by each mare. Mares entered each trial after having been confirmed to be in oestrus by behavioral observation and transrectal ultrasonography (follicle 35 mm and uterine oedema). Testing was performed 19 hours after ovulation induction with