J. James Jasenau, Cecelia's Gerbils: a Story for Boys and Girls; a Manual for Adults, 2nd ed., Exposition Press, NY, 1971

Baby gerbils just born are the easiest to categorize into male and female groups. The little red dots on the underside of the female are the teats. If there are no little red dots, the gerbil is a male. After about four weeks, the sexes are easily distinguishable by the shape of the body where it meets the tail. On the male the body is tapered into the tail. The female, where the tail meets the body, is more like a lollipop.

Determining sex is most important in breeding. Many an amateur is waiting for the first litter from two males or two females.