How Can I Tell What Sex My Mouse Is?

Photos by Cait McKeown

When you buy a pair of mice, before you leave the pet shop gently lift their tails and compare their genitals. Look at the mice from the side as well as straight on. If the mice are the same sex they should look exactly the same. If you're not sure, try a magnifying glass or take them to a vet or humane society and ask them to check. Male mice can retract their testicles, but you can still see that the gap between the anus and the external genitalia is larger in the male.

Left: Male mouse (buck) aged three weeks, Right: Female mouse (doe) aged three weeks

Incidentally, if you are trying to buy a mixed-sex pair of mice, please think about what you will do with the countless babies that may result before you go ahead.

Some good photos comparing male and female mouse genitals are available on the Internet at the Yahoo Group Valerie’s Mice Paradise: http://groups.yahoo.com/group/valeriesmiceparadise. Look in the ‘Photos’ section on the left-hand side of the page, at the album entitled ‘Cait’s sexing mice album’. Some good line drawings accompany guidance on sexing mice at Reite’s Rodent Roadshow at http://members.aol.com/Reite/MouseGuide.html.

If the mouse is old enough to leave its mother and it is male, it will have testicles. These can be drawn up into the body but if you lift the tail slightly and look underneath you'll still see the slightly baggy scrotum (lovely!). The testicles will not remain drawn up inside the body for any length of time. Mice have very large testicles relative to their body size - an adult male's testicles are each about the size of a small peanut (minus the shell). Try putting the mouse's food into a high-sided bowl. The mouse will perch on the side of the bowl to eat. If it is a boy, its testicles will soon drop down and dangle for all to see ;-) Only female mice have prominent nipples, so it might be worth examining the mouse's tummy. Nipples are visible in tiny babies, but once hair grows they can be hard to spot unless the mouse has reared a litter.

How to Sex Mice
(© Andrea C. Jacobs, all rights reserved)
Sexing Male Mice:

• Male mice have a prominent bulge under the tail once they reach sexual maturity. This bulge (scrotum) holds the mouse's testes (testicles), where sperm is stored.
• Male mice have sexual organs that are greatly spaced. Notice the large space between the male mouse's anus and his sexual organ (penis), as opposed to the female's.
• Male mice do not have nipples.

Sexing Female Mice:

• Female mice have a small tear drop shaped vagina extending down from the anus.
• Female mice do not have a space between their genitals; the sexual organs and the anus are very close together and appear to be connected.
• Female mice have two rows of nipples running vertically down their bellies. This is the easiest, sure-fire way to sex mice.

Determination of Sex - The Scientific Way (Jackson Laboratories)
The sex of the newborn mouse can be determined from both the distance that separates the genital papilla and the anal opening and from the general appearance of the urogenital-anal region. The genital-anal distance in newborn males is generally 50% greater than in newborn females. In addition, the male genitalia are often more prominent, and in the pre-scrotal region below, a dark pigmentation is often visible. If a litter is large enough, it is likely to have both males and females. The simplest way to become adept at distinguishing gender is through pairwise comparisons of the pups - in each hand, a newborn pup can be held gently, but firmly, between the index finger and the thumb in an upside-down position. As neonates age, gender determination becomes somewhat more difficult. It becomes easier again at 8-10 days with the appearance of nipples along the ventral side of the female and at the age of weaning (18-28 days), when the penis has developed more fully in the male.

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