Cycle:

• It is exhibited by all eutherian mammalian females except the order -primate.
• The mammalian females which exhibit one estrus cycle in one breeding season are called as monoestrous.
  Example: Dogs, Fox, Bats, Deer etc.
• The mammalian females which exhibit two or more oestrous cycles in one breeding season are called polyestrous.
  Example: Rat, squirrel, cow, pig, horse, cat, sheep rabbit etc.
• The oestrous cycle is studied with the help of vaginal smears.
• The duration of various phases of the oestrous cycle varies from species to species. In the rat, it is from 4 to 5 days and in the rabbit is 16 to 17 days.

The oestrous cycle has 4 phases –

1. Diestrus phase:

• It is also called as anestrus phase.
• It is the longest phase & its duration is of 60 to 70 hours.
• The ovary is without Graffian follicles.
• The uterine gland is thin, small & inactive.
• The vaginal smear shows leucocytes.
• During the non-breeding season, the female remains in the diestrus phase.

2. Proestrus phase:

• It is the smallest phase. Its duration is 1/2 to 5 hours.
• There is no growth of ovarian follicles.
• Vaginal smear exhibits epithelial cells.
• The uterine glands begin to increase in number & size.
• The thickness & contractability of the uterine wall begins to increase.

3. Oestrous phase:

• Its duration in the rat is 9 to 15 hours.
• It is also called a heat period.
• The basal temperature of the body is increased.
• Formation of Graffian follicles in the ovary.
• The vaginal smear shows the presence of cornified cells.
• The maximum level of estrogen in the blood.
• Ovulation takes place.

4. Post-Estrus or metestrus phase:

• Its duration is of 10 to 14 hours.
• The vaginal smear shows cornified cells & leucocytes.
• The maximum level of progesterone in the blood.
• Formation of Corpus luteum in the ovaries.
• If there is fertilization, the female remains in metestrus till the parturition.
• The oestrous cycle repeats if there is no fertilization and after parturition.