Is This Rat a Girl or Boy? Male and female rats are very easy to tell apart from one another, even from as young as three weeks of age. Males develop very large, distinctive scrotums (called goolies by most rat lovers) that you just can't miss seeing. Males also have no nipples, where as the females do. Here are two 5 day old rats being compared side by side. The easiest way to tell a male rat from a female rat at this age is to compare the distance from the genitals to the anus. A male rat will have almost twice the distance between the two. Here are two 3 week old rats being compared side by side. Even at this young age you can clearly see the development of the bulging scrotum on the male rat. Here are two adult rats being compared side by side. At full adult size the male's scrotum is huge, making them quite easy to tell from the females.