Species That Ovulate Primary Oocytes

In all domestic animals, ovulation occurs in the secondary oocyte stage, while in which species does ovulation occur in the primary oocyte stage?

Correct Answer: Bitch and Mare

Among domestic animals, the bitch (dog) and the mare (horse) are unique in that they ovulate primary oocytes — all other domestic animals ovulate secondary oocytes.

Oocyte Maturation Stages

• Primary oocyte: arrested in prophase I of meiosis — not yet undergone first meiotic division.
• Secondary oocyte: after first meiotic division — ready for fertilization; arrested in metaphase II.
• Mature ovum (ootid): after second meiotic division — only completes when fertilized by a sperm.

Bitch and Mare — Consequences of Primary Oocyte Ovulation

• After ovulation, the primary oocyte must complete first meiotic division (takes 2–3 days in bitch) before fertilization can occur.
• This accounts for the long fertile lifespans of sperm (4–6 days) and ova (4–8 days) in the bitch.
• In the mare, the oocyte completes first meiotic division shortly after ovulation in the oviduct.