At what age do rats go into estrus? - briefly
Female rats typically enter estrus at approximately «5–6 weeks» of age, corresponding to 35–45 days post‑natal. Onset may vary slightly with strain and environmental conditions, but sexual maturity is generally reached by the seventh week.
At what age do rats go into estrus? - in detail
Female laboratory rats typically reach sexual maturity between 5 and 7 weeks of age, at which point the first estrous cycle can be observed. The exact timing depends on strain, housing conditions, and nutritional status.
Pubertal onset is marked by vaginal opening, which normally occurs around post‑natal day 30–35 in Sprague‑Dawley and Wistar rats. After this external sign, the first estrus episode appears within 2–4 days, establishing a regular 4‑day cycle (proestrus, estrus, metestrus, diestrus). Hormonal profiles confirm this transition: luteinizing hormone (LH) surges and rising estradiol concentrations coincide with the initial estrus.
Key factors influencing the age of first estrus:
- Genetic background: some inbred strains (e.g., Fischer 344) mature slightly later, around 6–8 weeks.
- Photoperiod and lighting: 12 h light/12 h dark cycles support normal timing; extended darkness can delay puberty.
- Diet: protein‑rich diets accelerate vaginal opening, whereas caloric restriction postpones it.
- Body weight: attainment of a threshold weight (≈ 120 g for most strains) predicts onset more reliably than chronological age.
Methods for detecting the first estrus episode include:
- Daily vaginal cytology: presence of cornified epithelial cells indicates estrus.
- Monitoring LH surge via blood sampling: a sharp rise on the morning of estrus confirms ovulatory readiness.
- Observation of sexual behavior: lordosis quotient increases markedly during the first estrus.
In experimental designs, scheduling procedures that depend on reproductive status should begin no earlier than post‑natal day 35, allowing a buffer of several days to ensure the animal has entered a stable estrous pattern. Adjustments for strain‑specific maturation rates improve reproducibility and reduce variability in hormonal or behavioral outcomes.