Polyovulation in the South American Plains Vizcacha, Revisited: Coexistence of Spontaneous and Induced Ovulation

The South American plains vizcacha, Lagostomus maximus, is recognized as the mammalian species with the highest ovulation rate which can reach as many as 800 ova. This remarkable polyovulation phenomenon was first reported in the 1970s and has remained unchanged in its findings ever since. We conducted an extensive re-evaluation of ovulation in L. maximus by examining natural ovulation, ovulation induced through the administration of exogenous hormones and autologous seminal plasma, while also categorizing the morphological characteristics and maturation stages of the released ova. Our results confirm L. maximus as the foremost polyovulatory mammalian species, albeit with a mean of 154 ± 87, ranging from 29 to 326 oocytes per oestrous cycle. We observed the concurrent presence of spontaneous ovulation, serving as a mechanism to eliminate defective oocytes, alongside induced ovulation, referred to as "euovulation," that promotes the release of cumulus-oocyte complexes suitable for fertilization. In addition, we provide evidence for the involvement of seminal plasma in the process of induced ovulation and show that the "pseudovulation" phenomenon occurring at mid-gestation, which contributes to the formation of secondary corpora lutea, does not entail the release of oocytes into the oviducts. Our analysis also revealed the presence of spontaneous parthenogenetic oocyte activation and subsequent development, which occurred both during oocyte release in the ovulatory phase and within unruptured follicles in the ovary.