When can you determine the sex of a rat? - briefly
Sex can be reliably identified at weaning, around 21 days of age, by visual inspection of the anogenital region. Earlier assessment is possible by measuring anogenital distance, though accuracy is lower.
When can you determine the sex of a rat? - in detail
The sex of a laboratory rat can be identified shortly after birth by examining external genital morphology. At birth, the anogenital distance (AGD) is already measurable; males exhibit an AGD roughly twice that of females. The male genital area presents a small, rounded genital papilla and a faint scrotal sac, whereas the female displays a shorter AGD and a distinct vaginal opening surrounded by labial folds.
Visual inspection becomes reliable by post‑natal day 1–2. By day 3, the differences in AGD and genital structures are sufficiently pronounced for accurate identification without specialized equipment. Confirmation is commonly performed again at weaning (approximately three weeks of age) when secondary sexual characteristics, such as testicular enlargement in males, become evident.
Alternative methods include:
- Molecular sexing: PCR amplification of the Sry gene from a tail‑snip or ear‑clip provides definitive results at any age, useful when morphological cues are ambiguous.
- Hormonal assays: Measurement of serum testosterone or estradiol can distinguish sexes, but requires blood sampling and is typically reserved for research protocols rather than routine colony management.
For routine colony maintenance, the recommended procedure is:
- Inspect pups on day 1–2 for AGD and genital papilla characteristics.
- Record sex determination in the breeding log.
- Re‑examine at weaning to verify initial assessment.
- Apply molecular confirmation only when visual identification is uncertain or when experimental design mandates genetic verification.
Accurate early sexing supports proper grouping, breeding strategies, and experimental design, minimizing the risk of mixed‑sex housing and ensuring compliance with animal welfare guidelines.