How can I determine the sex of a field mouse? - briefly
Inspect the ventral region: males display a longer anogenital distance and a discernible scrotal pouch, whereas females have a shorter distance and lack a scrotum. This morphological distinction is reliable in mature individuals; younger mice may need dissection or DNA analysis for confirmation.
How can I determine the sex of a field mouse? - in detail
Determining the gender of a wild field mouse requires careful observation of external morphology and, when necessary, internal examination or genetic analysis.
External examination is the first step. The most reliable external indicator is the genital opening. In males, the opening is a circular orifice surrounded by a small, retractable preputial sheath; the distance between the anus and the genital papilla (anogenital distance) is noticeably longer than in females. In females, the opening appears as a vertical slit leading to the vaginal canal, and the anogenital distance is shorter. The presence of a visible testes, usually located in the abdominal cavity and detectable as a slight bulge beneath the skin in mature males, further confirms male sex.
Secondary characteristics can provide additional clues. Adult males often have larger, more prominent scent glands near the ventral surface, and their pelage may be slightly darker. Females may display more pronounced mammary tissue, especially when pregnant or lactating, though in non‑reproductive individuals the nipples are small and not always discernible.
When external features are ambiguous—such as in juveniles or subadult specimens—dissection offers definitive identification. After humane euthanasia, the gonads can be inspected: testes are paired, oval, and positioned near the kidneys; ovaries appear as small, almond‑shaped structures attached to the uterine horns. This method should follow institutional ethical guidelines.
Molecular techniques provide a non‑invasive alternative. DNA extracted from a small tissue sample (e.g., ear punch or tail snip) can be amplified using polymerase chain reaction (PCR) with sex‑specific primers targeting the Sry gene on the Y chromosome. Presence of the Sry fragment indicates a male; its absence, combined with amplification of an autosomal control gene, confirms a female.
A practical workflow:
- Visual inspection of the anogenital region; note distance and shape of the opening.
- Search for external testes or scent glands in mature individuals.
- If uncertain, perform a brief necropsy to locate gonads.
- For live or protected specimens, collect a tissue sample and run PCR with sex‑specific markers.
Combining morphological assessment with genetic confirmation ensures accurate sex determination across all age classes and physiological states.