When is the first estrus in rats? - briefly
Female rats attain their «initial estrus» at about 5–6 weeks of age, indicating sexual maturity. Subsequent cycles recur roughly every 4–5 days.
When is the first estrus in rats? - in detail
The initial estrus of laboratory rats occurs during the third post‑natal week. Vaginal opening, the external sign of puberty, appears at 21–23 days of age; the first ovulatory cycle follows within 24–48 hours after this event. Hormonal profiles confirm the transition: a rise in luteinizing hormone (LH) peaks at the onset of estrus, while estradiol concentrations increase sharply during the preceding proestrus.
Key factors influencing timing:
- Strain differences: Sprague‑Dawley females typically reach vaginal opening at 22 days, whereas Wistar rats average 23 days; some hybrid lines may mature slightly earlier or later.
- Nutrition: caloric restriction delays vaginal opening by 2–3 days; high‑fat diets can accelerate it.
- Light cycle: a 12 h light/12 h dark schedule aligns the circadian rhythm with hormonal surges, stabilizing the onset age; constant darkness may shift the timing by one day.
- Environmental stressors: chronic stressors (e.g., overcrowding) extend the pre‑pubertal period.
Detection methods:
- Daily visual inspection for vaginal opening; the presence of a visible orifice indicates readiness for estrus monitoring.
- Vaginal cytology performed every 12 hours after opening to identify the cornified epithelial cell peak that defines estrus.
- Serum LH measurement, with a pronounced surge marking the exact moment of ovulation.
The first ovulatory event is typically observed at 25–27 days of age, corresponding to the first full estrous cycle. Subsequent cycles occur every 4–5 days, establishing the regular reproductive rhythm of the animal.