When does estrus begin in rats? - briefly
Estrus in female rats typically starts at around 30–35 days of age, coinciding with sexual maturity. The first proestrus phase often occurs within the first estrous cycle after this age.
When does estrus begin in rats? - in detail
The estrous cycle in laboratory rats commences shortly after sexual maturity, typically between post‑natal day 35 and day 45. Puberty is marked by the first vaginal opening, after which cyclic hormonal fluctuations appear. The proestrus phase, dominated by rising estradiol, precedes the fertile estrus stage, which is reached when luteinizing hormone peaks and ovulation occurs. In most strains, estrus lasts 12–14 hours and recurs every four to five days, forming a regular 4‑day cycle: proestrus, estrus, metestrus, and diestrus.
Key determinants of the initial estrous event include:
- Age: Most females exhibit their first estrus between five and six weeks of age; earlier onset may occur in genetically accelerated strains.
- Strain: Sprague‑Dawley and Wistar rats tend to mature slightly earlier than Long‑Evans or Fischer 344 lines.
- Nutrition: Adequate caloric intake and protein levels accelerate puberty; caloric restriction delays the first estrus.
- Photoperiod: Standard 12 h light/12 h dark cycles support normal timing; altered lighting can shift onset by a few days.
- Housing conditions: Group housing with adult males can induce earlier sexual maturation via pheromonal cues (the “male effect”).
Detection of the first estrus relies on vaginal cytology. A sample collected with a saline‑moistened swab shows a predominance of cornified epithelial cells, confirming the receptive phase. Hormone assays reveal a surge in serum estradiol and a subsequent luteinizing hormone peak within the same 24‑hour period.
In summary, the initial fertile phase in female rats emerges around five weeks of age, governed by genetic background, nutritional status, light regime, and social environment, and can be verified through cytological and hormonal markers.