How to distinguish a male rat from a female rat? - briefly
Male rats exhibit a longer, more pronounced «anogenital distance» and a visible scrotum, whereas females have a shorter distance and no external testes. Females may also show slightly more evident nipples, especially during lactation.
How to distinguish a male rat from a female rat? - in detail
Male and female rats can be separated by examining external genitalia, body size, and secondary characteristics.
In newborns, the distance between the anus and the genital opening (anogenital distance) is the most reliable indicator. In males this gap is roughly twice that of females. The male opening appears as a small, circular slit, whereas the female opening is a vertical slit situated directly beneath the anus.
In juveniles and adults, additional features become apparent. Males develop a visible scrotum containing testes, which may be palpable even before fur fully covers the area. The testes descend into the scrotal sac around three weeks of age in laboratory strains. Females lack a scrotum and instead display a pair of small mammary glands along the ventral surface; these glands become more prominent during pregnancy.
Body size offers a secondary cue. Adult males typically weigh 20–30 % more than females of the same strain, though overlap exists and size alone should not be used as the primary criterion.
A practical procedure for sexing a rat includes:
- Restrain the animal gently by the tail and lift the hindquarters.
- Observe the anogenital region; measure the distance between the anus and the genital opening with a fine ruler if necessary.
- Palpate the area beneath the genital opening for the presence of testes in males.
- Note any enlargement of mammary tissue, especially in females approaching reproductive age.
When handling, minimize stress by using a quiet environment, soft gloves, and brief examination times. Accurate determination is essential for breeding programs, experimental design, and health monitoring.