Pre-ovulatory follicles and ipsilateral ovarian stroma are cooler than non-dominant antral follicles and stroma on the contralateral ovary of the mare.

Reports in several species have demonstrated that pre-ovulatory follicles (POFs) are cooler than neighbouring ovarian tissue, with cooling important for fertility. This study measured the temperature of POFs and compared it with other regions of the ovary, cervix, and rectum to determine if POF cooling occurs in mares. Fourteen oestrous cycles in 11 mares were monitored, and when an ovarian follicle ≥38 mm was present, deslorelin was given. Thirty hours later, rectal and cervical temperature measurements were taken. Mares were sedated and ovarian temperature measurements taken using a thermocouple probe in a 12G needle within a transvaginal probe. Measurements were made in the POF and a non-dominant follicle (NDF) on the contralateral ovary. In eight mares the temperature of the stroma in each ovary was also recorded. The data were analysed using a paired t-test. The POF was cooler than the contralateral NDF (p < 0.001) with a mean difference of 0.30 ± 0.17°C (n = 14). However, the stroma ipsilateral to the POF was cooler than the POF (p = 0.003; n = 8), whereas a NDF and stromal tissue within that ovary were statistically equivalent in temperature (p = 0.18; n = 8). The mare shows POF cooling, but unlike other species, the whole ovary is involved. This variation in cooling between ovarian compartments compared to other species could be related to the unusual structure of the equine ovary. Being cognisant of temperature changes within the ovaries around the time of ovulation in the mare could help our understanding of oocyte maturation and competence in vivo and in vitro.