Can body temperature be a tool to detect ovulation in the mare?

Abstract

Body temperature variations occur around ovulation in bovine (Suthar et al. 2011; Sakatani et al. 2016) and ovine (Barros de Freitas et al. 2018). Previous studies in the mare were unable to determine such a relationship (Ammons et al. 1989; Hasson and Rahawy 2022). Nevertheless, in another study, a decrease of body temperature within 6 hours after ovulation was highlighted (Bowman et al. 2007). Authors, however, performed a temperature monitoring only at fixed hours. The present work uses identification chips equipped with a temperature sensor (BioThermo™, Antelliq) implanted in the mare neckline to be able to follow temperature variations across 24 hours. Seven mares were monitored around ovulation (2.4±0.9 days before and 3.6±1.7 days after), two of whom were followed across two consecutive ovulations. Phases of the reproductive cycle were determined using daily transrectal ultrasonography. Briefly, three phases were identified: follicular phase (F) when identifying a dominant follicle with a diameter >30mm and uterine edema score at least “+” (scoring from “-“ to “+++”); ovulation day (Ov) being the first day (from 0:00 to 23:59) of corpus luteum appearance; luteal phase (CL) from the day after ovulation with a presence of corpus luteum. Body temperature was measured using a device produced for research-purposes only (Ikalogic, France) able to record temperature automatically every 10 minutes. A total of 4,780 temperature recordings (F n=2,410;Ov n=965; CL n=1,405) were obtained from the 9 studied ovulations and mean body temperature per half an hour was calculated for each mare in order to simplify the statistical analysis. To study temperature variations, paired comparisons between phases were performed (Ov vs F; Ov vs CL; F vs CL) using the temperature differences at each time set. Thus, using a linear mixed model (LMM) temperature differences were compared to zero. Individuals were set as random effect while time set was a fixed effect. Significant higher temperatures in the Ov phase compared to the F phase were identified between 4:30 and 8:00 with a minimum difference of 0.51±0.26°C and a maximum of 0.92±0.26°C. Moreover, temperatures were significantly lower during Ov phase compared to CL phase at specific time sets (from 10:30 to 16:00; 18:30 and from 20:00 to 21:00) with a minimum temperature difference of 0.29±0.17°C and a maximum difference of 0.60±0.16°C. These results are more time specific than in other farm species, probably due to the longer estrus duration in horses. In conclusion, temperature variations do occur around ovulation. A more complete study, however, studying temperature along the whole reproductive cycle is needed to better understand the underlying physiological processes.