When can the sex of a rat be determined? - briefly
Sex can be identified by examining the anogenital region when pups are approximately 10–14 days old; prior to that, reliable determination requires genetic testing.
When can the sex of a rat be determined? - in detail
The sex of a laboratory rat can be reliably identified shortly after birth. By day 1–2, the anogenital distance (AGD) begins to show measurable differences: males display a longer distance between the anus and the genital papilla than females. Visual inspection of the genitalia becomes possible from postnatal day 3 onward, when the preputial fold in males and the vaginal opening in females are sufficiently developed to be distinguished without magnification.
For precise determination, the following procedures are commonly employed:
- Physical examination – gentle palpation of the abdomen and observation of external genitalia under a dissecting microscope; effective from day 3–4.
- Measurement of AGD – use of a calibrated micrometer to record the distance; provides quantitative confirmation and can be performed as early as day 1.
- Molecular sexing – PCR amplification of the Sry gene from a small tissue sample (e.g., tail tip); applicable at any age, including embryonic stages, but requires laboratory equipment.
When breeding colonies are managed, sexing is typically completed by the end of the first week to allow appropriate group housing and to prevent unwanted mating. Early identification also facilitates sex‑specific experimental designs and reduces the risk of confounding variables related to hormonal differences.
In summary, external characteristics become discernible within the first few days of life, while genetic methods offer confirmation at any developmental stage. Selecting the appropriate technique depends on the age of the animal, the required accuracy, and available resources.