How can a rat’s sex be determined? - briefly
Inspect the anogenital region: males exhibit a longer anogenital distance and palpable testes, while females have a shorter distance and a visible vaginal opening without testes. This visual and tactile examination reliably determines sex in laboratory rats.
How can a rat’s sex be determined? - in detail
Determining the sex of a laboratory rat requires visual inspection of the external genital region. The procedure can be performed on live or euthanized animals, but handling must minimize stress and injury.
First, restrain the rat gently with a proper holder or by scruffing. Place the animal on a clean, flat surface and expose the ventral area. The key anatomical landmarks are:
- Anogenital distance (AGD): The gap between the anus and the genital opening. In males, the distance is noticeably longer (approximately twice that of females) and the genital papilla is positioned farther from the anus.
- Genital papilla shape: Males display a rounded, protruding papilla with a visible preputial fold; females have a flatter, less prominent opening without a preputial sheath.
- Presence of testes: In adult males, testes are palpable within the scrotum, which appears as a pair of oval masses beneath the skin. Females lack these structures.
- Nipple pattern: Both sexes possess nipples, but the number and distribution are consistent across sexes and therefore not a reliable indicator.
For juveniles (post‑natal day 7–14), the AGD remains the most reliable criterion because testes have not yet descended. In such cases, magnification (a dissecting microscope or a handheld loupe) assists in distinguishing the subtle differences in papilla morphology.
A concise checklist for sex identification:
- Secure the animal and expose the ventral region.
- Measure AGD; record the distance.
- Observe the genital papilla:
- Rounded, protruding, with preputial fold → male.
- Flat, recessed, without sheath → female.
- Palpate for testes if the animal is older than three weeks.
- Confirm findings with a second observer when uncertainty persists.
Accuracy improves with practice and consistent lighting. Document the sex alongside the animal’s identification number in laboratory records to ensure traceability.