Three full sister mares with a stallion-like behavior and a high blood testosterone concentration

Abstract

Summary The testosterone concentration was very high in a 6-year-old maiden mare. The mare showed a typical male behavior. She appeared to have two full sisters with the same abnormal behavior. The mare's external genitalia were normal, whereas the intemai genitalia were abnormal: no cervix and no uterus, the gonads resembled a testis. The resting plasma testosterone concentration was 0.76 ng/ml. The results of the HCG stim- ulation test are shown in Table 1. The response to HCG administration was poor. Karyotyping ofS0 metaphase plates showed a normal female karyotype of 2N = 64 XX.