Lower reproductive success associated with locomotor stereotypies in mares

Stereotypies are often associated with suboptimal environments. However, their adaptive significance remains under debate. The aim of this study was to relate the occurrence of stereotypies in breeding mares to their reproductive fitness. The overall, first and second cycle conception rates, latency to first estrus, inter-cycle interval and the number of cycles per conception were compared between mares showing weaving (n=26) and control mares (n=31). Mares were mated by 9 stallions which were equally balanced between the two groups. Rectal palpation and ultrasound were used to monitor the follicular state of the mares and to confirm pregnancy. The relationship between the occurrence of stereotypies and reproductive efficiency was analyzed using a multivariate logistic regression. We used Mann-Whitney test to compare latency to first-cycle and inter-cycle intervals between stereotypic and control horses. χ2 tests were used to compare independent variables’ distribution between the two groups of mares. Stereotypic mares had a significantly lower overall conception rate (58% vs. 81%, P=0.034) and first-cycle conception rate (33% vs. 64%, P=0.005). Stereotypic mares showed a significantly higher number of cycles per conception (2.0±0.9 in stereotypic vs. 1.5±0.8 in control mares, P=0.037). There was no difference between stereotypic and control mares in latency to first estrus (45.7±25.9 in stereotypic vs. 45.7±23.2 in control mares, P=0.97), nor in inter-cycle intervals (26.1±7.7 in stereotypic mares vs. 23.9±6.6 in control mares, P=0.74). These findings suggest a lowered fitness in stereotypic mares.