How can you distinguish a male domestic rat from a female? - briefly
Examine the ventral area: males display a longer anogenital distance and a visible scrotal sac with testes, whereas females have a shorter distance and a simple vulvar opening. The male’s abdomen may also appear slightly enlarged due to reproductive structures.
How can you distinguish a male domestic rat from a female? - in detail
Rats display clear sexual dimorphism that can be observed with a brief visual inspection. The most reliable indicator is the genital area located ventrally between the anus and the urethral opening.
- In males, a pair of small, almond‑shaped testes are visible beneath the skin when the animal is not neutered. The testes sit in a sac that can be gently palpated. The distance between the anus and the genital opening (the anogenital distance) is noticeably longer than in females, typically exceeding 1 cm in adult specimens.
- In females, the anogenital distance is short, usually less than 5 mm. No testes are present, and the ventral surface shows a smooth, uninterrupted line of fur. The genital papilla is a single, less pronounced opening directly behind the anus.
Additional characteristics support sex identification, especially in younger rats where gonadal development is minimal.
- Nipple pattern – Adult females develop a more extensive line of functional nipples along the ventral abdomen; males retain a faint, non‑functional ridge that may be difficult to see.
- Body size and head shape – Males often have a slightly larger overall body mass and a broader skull, while females tend to be leaner with a more tapered head. These traits are subtle and should not replace direct genital examination.
- Behavioral cues – Unneutered males may exhibit territorial marking, mounting, or aggression; females may display nesting behavior. Behavioral signs are variable and not definitive without physical confirmation.
When examining a rat, follow these steps to avoid injury:
- Secure the animal by gently wrapping it in a towel or using a handling tube.
- Turn the rat onto its back or side to expose the ventral surface.
- Locate the anus, then measure the space to the genital opening with a fingertip or caliper.
- Check for the presence of testes in males; in females, verify the short distance and absence of testes.
For neutered or spayed rats, the external genital morphology remains unchanged, but the testes will be absent. In such cases, rely on the anogenital distance and nipple pattern for accurate determination.