When do rats go into estrus? - briefly
Female rats become sexually receptive about 4–5 days after weaning, with estrus occurring during the dark phase of the light‑dark cycle and lasting roughly 12–14 hours. The cycle repeats every 4–5 days in mature adults.
When do rats go into estrus? - in detail
The estrous cycle of the laboratory rat lasts approximately four to five days and consists of proestrus, estrus, metestrus, and diestrus. Estrus, the period of sexual receptivity, typically begins after the proestrus phase, which is marked by a surge in luteinizing hormone and rising estrogen levels.
In a standard cycle, estrus commences around 24 hours after the onset of proestrus. For most adult female rats (8–12 weeks old), this translates to day 2 or day 3 of the cycle, depending on the exact length of the preceding proestrus. The estrus phase itself lasts 12–14 hours, after which the animal enters metestrus.
Key variables influencing the timing include:
- Age: Juvenile females (< 6 weeks) exhibit irregular cycles; mature adults show the described pattern.
- Strain: Common strains such as Sprague‑Dawley, Wistar, and Long‑Evans display minor differences; most complete a cycle in 4.5 days.
- Photoperiod: Constant lighting or altered light‑dark cycles can shift the onset by several hours.
- Nutrition and stress: Caloric restriction or chronic stress may prolong proestrus, delaying estrus.
Detection methods rely on vaginal cytology. During estrus, exfoliated epithelial cells are predominantly cornified, appearing as large, anucleate, polygonal cells under microscopy. A single sample taken in the early evening is sufficient to confirm that the animal is in estrus.
In practice, researchers schedule mating or experimental procedures based on the predicted estrus window: identify proestrus in the morning, anticipate estrus 24 hours later, and confirm with cytology in the late afternoon. This protocol ensures synchronization with the brief receptive period.