Mouse sexing involves identifying the biological sex of a mouse. This skill helps prevent unplanned breeding, which maintains colony control. Accurate sex determination also supports proper housing, as male mice may exhibit aggression when housed together. Additionally, many scientific studies require sex-specific cohorts, making precise identification a foundational step.
Identifying Sex by Anogenital Distance
The most reliable method for determining a mouse’s sex involves examining the anogenital distance. This is the space between the anus and the genital papilla, the small projection from which the urethra emerges. In male mice, this distance is noticeably longer, and the genital papilla often appears more pointed.
Female mice have a much shorter anogenital distance, with their genital papilla appearing more rounded and located closer to the anus. Observing this difference is aided by a side-by-side comparison of a suspected male and female. This anatomical difference is present even in very young pups, making anogenital distance a consistent indicator.
Additional Visual Indicators
While anogenital distance is the most dependable method, other visual cues can support sex identification. Nipples are typically visible in female mice, appearing as small bumps along the ventral abdomen. In contrast, male mice generally do not display visible nipples, or they are extremely difficult to discern.
Adult male mice often appear larger and more robust than females, though this can vary across strains and individuals. A more definitive secondary indicator in adult males is the presence of clearly visible scrotal sacs containing the testicles, which descend around 2-3 weeks of age. This feature is absent in females and not apparent in very young males. These supplementary indicators should always be used in conjunction with anogenital distance for accurate sex determination.
Tips for Safe Examination
Proper handling is important when examining mice to minimize stress and ensure handler safety. Common methods include gently grasping the mouse at the base of the tail, allowing its forepaws to rest on a surface for support. Alternatively, the mouse can be cupped in one hand, or a gentle scruffing technique can be employed for more secure restraint.
Clear visibility of anatomical features is aided by good lighting. For examining very young pups, whose features are less developed, a magnifying glass can provide a clearer view. Sexing older mice is generally easier because their features are more developed. However, the anogenital distance remains a reliable indicator even in pups with careful observation.