How can I distinguish a male mouse from a female mouse? - briefly
Male mice show a longer anogenital distance and visible testes beneath the ventral skin when the abdomen is gently lifted; females have a shorter distance and no palpable testes. The presence of testes is the definitive indicator of sex.
How can I distinguish a male mouse from a female mouse? - in detail
Sex determination in laboratory mice relies primarily on external morphology, which becomes reliable after the first week of life.
In juvenile and adult specimens, the most direct characteristic is the genitalia. Males possess a visible scrotum and an elongated, rod‑shaped penis that protrudes from the ventral surface. The distance between the anus and the genital papilla (anogenital distance) is markedly greater in males, often exceeding 5 mm, whereas females show a short, rounded genital opening with a distance of 2–3 mm.
Nipple development offers a secondary cue. Female mice develop functional mammary glands with prominent nipples along the ventral line, especially evident during pregnancy or lactation. Males retain rudimentary, non‑functional nipples that are less pronounced and lack associated glandular tissue.
Behavioral observations can supplement morphological assessment. Males typically exhibit territorial marking, increased aggression, and mounting behaviors, while females display nesting activity and maternal instincts when pregnant.
When visual inspection is inconclusive, especially in neonates, additional techniques include:
- Palpation of the abdomen to detect testes positioned within the scrotal sac in males.
- Ultrasound imaging to visualize internal reproductive organs.
- PCR‑based genotyping targeting the Sry gene, which is present only on the Y chromosome.
Accurate sex identification is essential for experimental design, breeding management, and data interpretation. Combining external morphological criteria with confirmatory molecular tests ensures reliable discrimination between male and female mice.