While it may be easy to properly sex adult rats, sexing babies is something many people find difficult. Rats are sexually mature as young as 5 weeks of age, so making sure you have them sexed right and separated by 5 weeks can help avoid the headache of unwanted or accidental litters.
1 Day Old – Male on the left, female on the right. Notice the male has a wider spacing between genitals and anus, and a slightly “swollen” look where the testicles will be. At this age it is still quite hard to tell the difference, since it is still subtle.
4 Days Old – Male on the left, female on the right.
5 Days Old – Male on the left, female on right. By now the differences are a bit more obvious. The spacing on the males is easier to differentiate from the females, and if you look closely you may be able to see the nipples on the females.
One Week old – Male on the left, female on the right. By now it’s getting easier to tell males and females apart. The spacing between genitals and anus is much larger than the female, and it’s easier to see where the testicles will be.
9 Days Old – Male on left, female on right.
12 Days Old – Male on left, female on right. If you haven’t been able to see nipples on your females before, it becomes quite easy now as the fur is starting to come in. In the picture of the female, you can see the nipples as rows of “dots” going up her sides. Only females have nipples, so if you see them, it’s definitely a girl!
15 Days Old – Male on left, female on right.
22 Days Old (3 weeks) – Male on left, female on right. By now the differences are becoming quite apparent.
3.5 weeks old (26 days) – Male on left, female on right.
