How to distinguish male rats from female rats? - briefly
Males show a noticeable scrotum and an exposed penis, whereas females have a small, slit‑shaped anogenital opening without a scrotum. Females also develop larger abdominal nipples after puberty.
How to distinguish male rats from female rats? - in detail
Rats can be sexed by examining external anatomy, measuring specific body proportions, and observing behavioral traits. The most reliable indicator is the genital region. In adult males, the scrotum is visible as a pair of small, oval, pinkish sacs located just behind the anus; the testes may be palpable when the animal is warm. In females, the genital opening is a single slit without a scrotum, and the distance between the anus and the genital opening (the anogenital distance) is shorter.
For young pups, the anogenital distance provides a practical metric. Measure from the anus to the genital papilla; males typically exhibit a distance 1.5–2 times greater than females. The presence of a small, raised genital tubercle in males, compared to a flatter area in females, also aids identification.
Secondary sexual characteristics become evident after puberty. Males develop larger heads, thicker necks, and more pronounced scent glands on the forepaws. Females retain a more streamlined body shape and may exhibit nipples that become more visible during pregnancy or lactation.
Behavioral cues support visual assessment. Males often display territorial marking, aggressive posturing, and mounting attempts, whereas females tend to show nesting behavior and less overt aggression.
A step‑by‑step approach for accurate sex determination:
- Secure the rat gently to expose the ventral side without causing stress.
- Locate the anus and note its position.
- Identify the genital opening:
- Look for a pair of scrotal sacs (male) or a single slit (female).
- Feel for testes in males; absence of palpable testes indicates female.
- Measure anogenital distance if the animal is a juvenile:
- Use a fine ruler or calipers.
- Compare the measurement to species‑specific thresholds (approximately 5 mm for females, 8–10 mm for males in common laboratory strains).
- Observe secondary traits (head size, neck thickness, scent glands) for confirmation in adults.
- Record findings and, if needed, repeat the examination after a short interval to verify consistency.
When uncertainty remains, genetic testing (PCR amplification of sex‑specific markers) offers definitive confirmation, though it is typically reserved for research settings.