High interindividual and intraindividual variation of oxytocin secretion in estrous mares exposed to stallions, but no significant link to mate preferences

Oxytocin is a hormone that may not only influence reproductive mechanisms in mammals but also their social behavior, including pair bonding. We therefore tested if the concentrations of oxytocin and other hormones reveal mate preferences of 13 mares in estrus. Each mare was first exposed to two stallions (haphazardly selected out of seven) and her behavior recorded. The mare was then returned to her box (i.e., no contact to stallions during that time). Approximately 4.5 hours later, venous blood samples were collected every minute during 30 minutes preceding exposure to one of the two previously used stallions, 6 minutes during exposure, and 30 minutes after exposure back in the mare's box. The procedure was repeated in the consecutive estrus cycle, with the difference that the mare was each exposed to the other of the two stallions during oxytocin measurements. In 20 of the 26 trials, oxytocin concentrations were significantly elevated during exposure to the stallion, without significant associations to cortisol and estradiol concentrations. We found no significant association between oxytocin secretion and preferences in the previous choice situation. While estradiol concentration showed a high repeatability over the two cycles, we found considerable intraindividual differences in oxytocin and cortisol plasma concentration among the two cycles. Partially, the variation in oxytocin concentrations could be linked to the time of ovulation, with lower oxytocin plasma concentrations in mares which ovulated later than expected. In conclusion, when teasing under experimental conditions, we found high interindividual and intraindividual variation among mares in the increase of oxytocin plasma concentrations, depending on the timing of ovulation. However, oxytocin levels seemed to be no predictor of mare preference.