How to determine the sex of a rat? - briefly
Examine the rodent’s underside: males have a longer anogenital distance, a visible scrotal sac, and a small penis, while females show a shorter distance, a single opening, and lack a scrotum. Additional confirmation can be obtained by checking for the presence of prominent nipples in mature females.
How to determine the sex of a rat? - in detail
Identifying a rat’s gender requires visual inspection of the anogenital region, preferably when the animal is calm and properly restrained.
In adult specimens the male presents a clear separation between the anus and the penile opening. The distance, known as the anogenital gap, is roughly twice the width of the female’s gap. Beneath the penile opening a pair of descended testes can be seen as firm, oval masses on either side of the ventral abdomen; they may be concealed by fur in younger males but become distinct after puberty.
Female rats display a single opening (the vulva) situated close to the anus, with no visible gap. The ventral surface shows a smooth line without bulging structures. Nipples are present along the abdomen in both sexes, but their pattern does not aid sex determination.
For juveniles (under three weeks) the external differences are subtle. The recommended procedure is:
- Place the rat on a warm, flat surface with a soft cloth to reduce stress.
- Gently lift the tail to expose the ventral area.
- Observe the distance between the anus and the genital opening; a larger gap suggests a male.
- Look for any palpable bulges indicating undeveloped testes; use gentle pressure with a fingertip if necessary.
If visual cues remain ambiguous, a secondary method involves measuring the anogenital distance with a fine ruler; a male typically exceeds 5 mm, while a female measures around 3 mm at comparable ages.
Accurate sexing is essential for breeding programs, experimental grouping, and health monitoring. Proper handling, adequate lighting, and minimal restraint time reduce the risk of injury and ensure reliable identification.