How can you distinguish the sex of rats? - briefly
Inspect the anogenital distance: males have a noticeably longer gap and visible testes, while females show a shorter distance with a vaginal opening and functional nipples.
How can you distinguish the sex of rats? - in detail
Determining the sex of laboratory or pet rats requires direct observation of anatomical features, supplemented by age‑appropriate techniques when external cues are ambiguous.
External genital examination is the primary method. In adult males, the anogenital distance (AGD) is markedly longer; a visible scrotum contains two testes that descend into a sack posterior to the penis. The preputial opening is situated ventrally and is larger than the surrounding skin. In females, the AGD is shorter, the vulva appears as a small, slit‑like opening near the anus, and no scrotal sac is present. The mammary glands are also more prominent in females, especially during pregnancy or lactation.
When rats are younger than three weeks, sexual dimorphism is subtle. Reliable identification at this stage relies on the following steps:
- Measure AGD with a fine ruler; males typically show a distance at least twice that of females.
- Perform a gentle palpation of the lower abdomen; testes can be felt in males as firm, oval masses.
- Observe the presence of a penile sheath; it is absent in females.
- If uncertainty persists, collect a small tissue sample (e.g., ear notch) for genetic sexing via PCR amplification of the Sry gene, which is male‑specific.
Secondary characteristics may aid confirmation in mature animals:
- Females develop functional nipples and, during estrus, exhibit swelling of the vulvar region.
- Males may display larger head and body size, thicker neck musculature, and a more pronounced scent marking behavior.
Behavioral cues alone are unreliable for sex determination, as both sexes can exhibit aggression, nesting, and grooming. Therefore, anatomical inspection remains the definitive approach, supplemented by molecular analysis when visual assessment is inconclusive.