Basal body temperature as a biomarker of reproductive state in ewes: A multi-method comparison

This study quantified body temperature dynamics throughout reproductive stages in crossbred ewes based on complementary monitoring techniques. Five ewes from a semi-extensive farming system were included in two sub-trials: animals were first assessed in the prepubertal stage and, subsequently, an oestrous cycle was induced by intravaginal progesterone sponges followed by eCG and prostaglandin; oestrous was confirmed by serum progesterone. Temperatures were recorded in five stages (prepubertal, pubertal, sponge, post-sponge, oestrous) based on eight measurements: rectal (RT), subcutaneous biologger (SCT) and infrared thermography (IRT) of vulva/axilla that were obtained either as unprocessed maximum pixel or as FLIR-processed maximum/mean. A linear mixed-effects model that included Method, Stage and their interaction indicated strong global effects (all p < 0.001). Estimated marginal means revealed a clear physiological trajectory: temperatures were highest in the prepubertal stage, generally decreased from the prepubertal stage towards the treatment-related stages, although the timing of the minimum value depended on the measurement method. Among the methods, RT yielded the highest estimated mean temperatures and the most stable measurements, SCT recorded slightly lower temperatures, vulvar FLIR-processed metrics were able to detect stage-related thermal changes, and axillary measures, especially unprocessed maxima, were more variable and environmentally sensitive. In general, thermal monitoring identified the expected reproductive-stage patterns. RT and SCT provided the most stable detection of reproductive-stage temperature changes, whereas vulvar FLIR-processed thermography represented the most promising non-invasive alternative. Axillary thermography was more variable and appeared to be more susceptible to environmental influences.