Stereotypic behaviours and mating success in domestic 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 mating success. The overall, first and second cycle conception rates, inter-cycle interval and the number of cycles per conception were compared between stereotypic mares (n = 31) and non-stereotypic mares (n = 83). 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 mating success was analyzed using a multivariate logistic regression. χ2 tests were used to compare independent variables' distribution between the two groups of mares. Stereotypic mares had a significantly lower overall conception rate (55% vs. 84%, p = 0.0018), first-cycle conception rate (26% vs. 54%, p = 0.0214) and second cycle. This was still the case when only one type of stereotypic behaviour was considered. Thus, weaving mares (n = 26) had a lower overall (58% vs. 81%, p = 0.034) and first cycle (33% vs. 64%, p = 0.005) conception rate well as 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) as compared to non stereotypic mares (n = 31). There was no difference between weaving and control mares in inter-cycle intervals (26.1 ± 7.7 in weaving mares vs. 23.9 ± 6.6 in control mares, p = 0.74). These findings suggest a lowered fitness in stereotypic mares. To our knowledge, this is the first study relating the occurrence of stereotypies to fertility in horses. The findings that fertility may be impaired in stereotypic animals have important implications and deserve further consideration to elucidate the processes involved.