Characterization of oestrus and timed collection of oocytes in the grey short-tailed opossum, Monodelphis domestica.

A system of test-pairing was used to detect reproductive behaviour in the grey short-tailed opossum. This enabled timing and characterization of the development of pro-oestrous and oestrous behaviour, and facilitated collection of ovulated and unovulated oocytes. Oestrus was induced 8.5 days (n = 80, 95% confidence limits 7.56-9.21) after the introduction of a male. Timed examination of the ovaries by laparotomy indicated that ovulation occurred 14-16 h after the first onset of oestrous behaviour. The development of follicles was linked to pro-oestrous behaviour, and ovulation occurred in the absence of copulation. Vaginal exfoliative cytology indicated that pro-oestrous behaviour was associated with an increasing number of keratinized epithelial cells, and at the time of maximum receptivity to males, a heavy infiltration of polymorphonuclear leucocytes was seen. Oocytes were typically marsupial: large (approximately 250 microns in diameter), with a yolky vitellus and thin zona pellucida. An average of 6 oocytes were ovulated per ovary.