How can you distinguish a female mouse from a male? - briefly
The anogenital distance is shorter in «female» mice and they exhibit two pairs of visible nipples, while «male» mice have a longer distance and a distinct penile opening. Visual inspection of these characteristics reliably determines sex.
How can you distinguish a female mouse from a male? - in detail
Sex identification in laboratory and pet mice relies on external anatomical cues, developmental timing, and, when necessary, molecular analysis.
External morphology provides the quickest distinction. In adult specimens the distance between the anus and the genital papilla (anogenital distance) is markedly greater in males. The male presents a visible penile sheath and a rounded, often slightly larger, scrotal area, whereas the female shows a smooth, recessed opening without a sheath. Juvenile mice display a shorter anogenital gap; careful measurement with calipers can separate sexes before sexual maturity.
Nipple count offers another reliable marker. Female mice retain a full complement of eight pairs of mammary glands, visible as small protrusions along the ventral line. Males typically exhibit reduced or absent nipples, especially in the anterior pairs.
Scent glands differ between sexes. Males possess prominent preputial glands near the genitalia and larger perianal glands, both producing characteristic odors used in territorial marking. Females have smaller, less conspicuous perineal glands.
Body size and weight trends can assist identification but overlap considerably; males often achieve slightly larger body mass after puberty.
When visual assessment proves ambiguous, especially in neonates, palpation of the gonads is effective. Gentle abdominal pressure reveals testes in males, while females present ovaries that are less palpable. Dissection provides definitive confirmation, allowing direct observation of testes, epididymides, or ovarian structures.
Molecular techniques eliminate uncertainty. Polymerase chain reaction (PCR) targeting the Sry gene—a male‑specific marker on the Y chromosome—produces an amplification product only in males. Quantitative PCR or DNA sequencing can verify sex in embryos, early postnatal stages, or tissue samples where morphology is compromised.
In summary, accurate sex determination combines measurement of anogenital distance, inspection of genital morphology and nipple pattern, evaluation of scent gland development, optional palpation or dissection, and, when required, genetic assays targeting Y‑linked sequences.