How can you determine a mouse's sex?

How can you determine a mouse's sex? - briefly

Examine the anogenital region: in males, the distance between the anus and the genital papilla is greater and a visible urethral opening is present; in females, the genital opening is a simple slit close to the anus. Additional confirmation can be obtained by checking for the presence of testes in the abdominal cavity of males.

How can you determine a mouse's sex? - in detail

Identifying a mouse’s sex requires observation of external and, when necessary, internal characteristics. The most reliable indicators differ between juvenile and adult animals.

In young mice (up to about 10 days old) the anogenital distance (AGD) provides the primary cue. Males exhibit a noticeably longer AGD than females. Measure the distance from the anus to the genital papilla; a difference of 1–2 mm typically distinguishes sexes at this stage.

In adult mice the genitalia are distinct. The male presents a small, rounded penile opening (preputial gland) and a visible scrotum containing testes, which may be palpable even when not fully descended. The female shows a simple vaginal opening without a scrotum and a pair of nipples positioned laterally along the abdomen.

A practical examination protocol:

1. Restrain the mouse gently but securely, using a thumb and forefinger to hold the tail and a second hand to support the body.
2. Observe the ventral region:
   • Look for a scrotal sac and testes (male).
   • Look for a slit‑like vaginal opening without a scrotum (female).
3. If visual cues are ambiguous, palpate the area:
   • Testes feel firm and oval.
   • Female reproductive structures are soft and lack palpable masses.
4. Record the AGD measurement for juveniles, noting the longer distance in males.
5. For definitive confirmation, especially in neonates or when external features are compromised, employ molecular methods:
   • Collect a small tissue sample (e.g., ear punch or tail tip).
   • Perform PCR amplification of sex‑specific genes (Sry for males, X‑linked markers for females).
   • Analyze the product by gel electrophoresis or real‑time PCR to verify chromosomal sex.

When necropsy is feasible, internal examination of the gonads offers absolute certainty: testes are white, lobulated organs attached to the mesonephros; ovaries are smaller, reddish, and contain follicles.

Combining visual inspection, AGD measurement, palpation, and, when required, genetic testing ensures accurate sex determination across all developmental stages.