How to determine the sex of an adult rat? - briefly
Examine the ventral genital region: males display a scrotum and a larger, recessed anogenital opening, whereas females have a single, round opening without a scrotum. Palpating the area can confirm the presence of testes in males.
How to determine the sex of an adult rat? - in detail
Identifying the gender of a mature rat relies on visual examination of external genital structures. The procedure should be performed with the animal gently restrained, preferably using a small animal restrainer or a gloved hand, and under adequate lighting.
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Anogenital distance – Measure the space between the anus and the genital opening. In males the distance is roughly twice that of females. A ruler or calibrated calipers can be used for precision.
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Genital opening morphology –
- Males: a circular, recessed opening (preputial orifice) surrounded by a thin, smooth skin fold. The opening is positioned ventrally and often accompanied by a small, visible scrotal sac containing testes.
- Females: a slit‑like opening (vulva) located more ventrally and close to the anus, lacking a surrounding fold.
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Presence of testes – In adult males the testes descend into a scrotum that appears as two paired, oval masses beneath the ventral skin. Palpation with a gloved finger can confirm their presence. Females lack this structure.
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Nipple pattern – Both sexes possess nipples, but in males the fourth pair (near the abdomen) may be less pronounced. This characteristic is less reliable than genital morphology.
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Preputial gland – Males often exhibit a small, creamy secretion from the preputial gland near the genital opening. The gland is absent in females.
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Behavioral cues (optional) – Males may display more frequent territorial marking and mounting attempts, whereas females show nesting behavior. These signs are supplementary and should not replace anatomical assessment.
Common errors to avoid
- Confusing the perineal skin fold with the preputial opening; the fold is present only in males.
- Measuring anogenital distance on a stressed animal; agitation can alter posture and distort measurements.
- Relying solely on nipple appearance; sexual dimorphism of nipples is subtle.
Best practice summary
Restrain the rat, illuminate the ventral area, assess anogenital distance, examine the shape of the genital opening, palpate for testes, and note any preputial gland secretion. Combining these observations yields a definitive determination of sex in adult rats.