How can you distinguish a female rat from a male rat? - briefly
Female rats can be identified by a short anogenital distance and the presence of a vulva just below the anus, while males exhibit a longer anogenital distance and a visible scrotal sac containing the testes.
How can you distinguish a female rat from a male rat? - in detail
Sex determination in rats relies on anatomical, behavioral, and developmental cues that become apparent at specific ages.
In newborns, the most reliable indicator is the presence or absence of the anogenital distance (AGD). Measure the space between the anus and the genital papilla; males exhibit a markedly longer AGD—approximately twice that of females. The genital papilla in females appears as a small, flat, pinkish mound, whereas males develop a visible scrotal sac with paired testes that descend shortly after birth.
Around three weeks of age, the testes in males become palpable within the scrotum, and the scrotal skin darkens. Females lack a scrotum and retain a smooth ventral surface. At this stage, the nipples also provide a clue: females possess a full complement of eight nipples (four pairs) distributed along the ventral line, while males typically have only a few rudimentary nipples near the forelimbs.
In adult rats, secondary sexual characteristics are evident. Males display a larger, more muscular body, a broader head, and a pronounced scent gland (preputial gland) near the genital region. Females retain a narrower torso, a less developed scent gland, and exhibit estrous cycling, observable through vaginal cytology or changes in vulvar swelling.
For precise identification, follow this protocol:
- Measure AGD – use calipers; a distance >5 mm suggests male, <3 mm suggests female.
- Inspect genitalia – look for scrotal sac and testes in males; females show a simple vaginal opening without a scrotum.
- Palpate testes – gently feel the scrotum; presence confirms male.
- Count nipples – eight functional nipples indicate female; reduced or absent nipples suggest male.
- Observe secondary traits – larger body mass, prominent scent gland, and behavioral aggression are typical of males; regular estrous signs point to females.
Combining these criteria yields accurate sex determination across all developmental stages.