How to determine the sex of a house mouse? - briefly
Inspect the ventral side near the anus: males display a small, external penis and a noticeably longer anogenital distance, whereas females lack a penis and have a shorter distance between the anus and genital opening. This visual check provides a reliable sex determination for house mice.
How to determine the sex of a house mouse? - in detail
Identifying whether a domestic mouse is male or female requires careful observation of external genitalia and, when necessary, internal examination. The procedure varies with the animal’s age; newborns and juveniles present fewer visible cues than adults.
In adult specimens the most reliable external markers are:
- Anogenital distance – the space between the anus and the genital opening. Males exhibit a noticeably longer gap, often twice that of females.
- Genital papilla – a small, rounded protrusion located just ventrally to the anus. In males it is more pronounced and pointed, while in females it appears as a shallow, less distinct fold.
- Preputial glands – small, pigmented glands situated near the base of the penis. They are present only in males and may be visible as dark spots.
- Nipple development – adult females retain prominent nipples along the ventral surface; males have rudimentary or absent nipples.
For juveniles (up to three weeks old) the differences become subtle. The recommended approach is:
- Gently restrain the mouse on a soft surface, exposing the ventral region.
- Measure the anogenital distance with a fine caliper; a distance greater than 4 mm typically indicates a male.
- Examine the genital papilla under a magnifying lens; a slight protrusion suggests male, whereas a flat area suggests female.
- If uncertainty persists, perform a brief dissection under a stereomicroscope to view internal reproductive organs: testes in the abdominal cavity confirm a male, while ovaries and uterine horns confirm a female.
When handling live animals, use appropriate personal protective equipment and follow institutional animal care protocols. Record measurements and observations systematically to ensure reproducibility.