Evidences of Regular Estrous Cycles in Mules and Successful Use of These Animals as Recipients for Donkey Embryos

Mules are hybrid females, commonly considered as infertile, resulting from a cross between a donkey and a mare. Few recent studies address the particularities of the reproductive system of the mules. Here, we present data showing consistent evidences of regular cyclicity in mules and also the first report of two births of donkey foals after embryo transfer into mule recipients with spontaneous estrous cycle. Ovaries of mules (n = 72) were obtained at a local slaughterhouse. Clear evidences of cyclicity were found in 61.1% (44/72) of the ovaries, being 45.8% (33/72) corpora lutea and 15.3 (11/72) large antral follicles. After follicle aspiration, we obtained oocytes with regular distribution of granulosa cells and homogenous characterization of ooplasm. Because of the evidence of cyclicity in mules, two embryos collected from a donkey were transferred into two mules with spontaneous estrous cycles. Eutocic births of donkey foals occurred after 372 and 379 days of gestation. In contrast to the classical concept of infertile females, we found clear evidences that a higher percentage of mules can present ovarian activity and these animals may be considered for reproductive biotechnologies, such as embryo recipients.