It’s All About the Balls

by Lorien E. Menhennett

Female mouse pup –
completely smooth underneath.

I survived Week 1 of my supervisor being on vacation. My excitement for the week: learning how to definitively tell mouse pup females from males. I know what you’re probably thinking: boys have a penis, girls don’t. How hard could it be? Well, these little pups are so tiny (much smaller than a kiwi, as a point of reference), and probably weigh 10 – 15 grams. Itsy bitsy. The outward anatomy is sometimes hard to distinguish. (And I’m not the only one who has difficulty with this – I was looking at a sick mouse with the animal facility veterinarian on Friday morning, and she thought a male pup was a female at first! So I don’t feel so bad.)

Male pup – visible testicles.

I was splitting cages Friday afternoon (separating pups from their parents, males pups in one cage and females in another cage). From the outward genitalia, some were easy to tell, some not so much. Then I realized: the females are completely flat underneath, while if you hold the males long enough and let them squirm a bit, their testicles bulge out. Bingo! Definitive sexing. And my supervisor will be quite happy that I got the right mice in the right cages, because if you don’t, the males quickly impregnate the females.

It’s the little things, right?