How to Tell if a Mouse is Male or Female

We believe that everyone who cares for small animals should know how to determine their sex. Known as “sexing,” this process is vital to prevent accidental breeding and social conflicts, as well as to plan routine healthcare.

Until you have practice and are confident identifying both males and females, we strongly encourage you to double-check an animal’s believed sex with us or another experienced source via photos before housing entire, sexually mature animals together.

At this stage, this article is primarily to aid in taking photos for sexing advice (from us or another knowledgeable source), but we will continue to add additional photos over time until it is a stand-alone sexing guide.

Important considerations

Mice are often missexed by sellers and guardians. This is important to remember when bringing new mice home.

Female mice must be kept in a same-sex pair or group. We suggest keeping between 3-8 mice in a group. Having a trio means that if one mouse passes away, there’s nobody left alone. Mice are highly social, and will become depressed if kept in isolation. Female mice tend to be easy to bond, and can also be safely housed with desexed males.

Male mice are extremely territorial towards other males, and we do not ever recommend housing entire, sexually mature males together — even if they’ve been together from a young age. They can quickly turn on each other with deadly consequences. Despite this, they are still highly social, and if possible, we suggest having males desexed so that they can live with females or other desexed males.

Mice can start breeding as early as 5 weeks of age, and their pregnancy is just 21 days long. They can become pregnant again as soon as they’ve given birth, and their litters are large. The average litter size is 6-8, but larger litters are not uncommon.

Whether intentional or accidental, housing sexually mature and entire male and female mice together will result in rapid breeding. These situations quickly get out of hand. If not properly separated, males will begin fighting with each other and females will be repeatedly impregnated.

If breeding occurs, the father (and any other adult male) must be removed immediately. All male babies will need to be separated from their mother and sisters at 4 weeks of age to the day. Every single male mouse will need his own cage.

We believe that everyone should take proactive measures to prevent accidental breeding. Learn how to sex your animals, double-check this with a knowledgable source, and make sure your caging is secure.

We do not support intentional breeding.

Sexing

Like many other animals, the primary method of sexing a mouse is via their anogenital distance — the distance between their anus and genitalia. Though not reliable indicators of sex, it’s worth noting that male mice tend to be larger with a thicker, longer coat, and that their urine typically has a strong musky scent.

If sexing juvenile or adult mice who tolerate handling, we suggest placing them on one hand while gently lifting their tail at the base with your other hand. Do not lift the mouse’s hind feet into the air or dangle them by the tail. You only need to be able to see under the tail. Do not attempt hold the mouse upside down. It won’t work and they will panic.

How to hold a mouse for the “lifted tail” sexing method.

Male mice will have a larger gap between their anus and genitalia than females, and the space will typically be furry (although the testicles themselves may be relatively hairless). You may see testicles, or they may be tucked up inside the body — male mice can do this when scared or cold. Males do not have nipples.

A male mouse. Note the large gap between his anus and genitalia.

Females will have a shorter gap between the anus and genitalia than males, and there will be a hairless line between the two. Females also have nipples, which may or may not be easily visible.

A female mouse. Note the short gap between her anus and genitalia, and the hairless line between the two.

If the mouse is very difficult to handle, you can place them into a container with a clear bottom and view their genitalia from this angle, though this is more difficult. Be sure that the mouse cannot jump from the container and injure themselves. Do not put a mouse into a sealed container with no airflow.

A young Mitchell’s Hopping Mouse in a clear container for sexing.

Sometimes, you might even be able to see testicles or nipples in a mouse when they are in their cage or carrier. As females lack testicles and males lack nipples, these are reliable indicators of sex on their own.

For baby mice (please note that we do not support intentional breeding), we suggest waiting until 7-10 days for sexing. This is the easiest time in a mouse’s life to determine if they are male or female, and it is safer than attempting to do so when they are younger. At this age, you can hold them upside down briefly to sex them, but try to minimise the time spent in this position and only tilt them as far as you have to to see properly.

If you want help with sexing juvenile or adult mice, you can send us a photo of the mouse’s anogenital region. We will not sex babies under 5 days old. Where possible, please make sure that your photo is clear and in focus. Here are some examples:

Recap

Mice are routinely missexed, and are rapid breeders. The most common reason we take in mice is due to out of control breeding, typically accidental missexings but also sometimes due to intentional breeding that got out of hand. We do not support intentional breeding, and believe that everyone who cares for mice should work proactively to prevent accidental breeding.

Male mice:

• Longer gap between their genitalia and anus than females

• Gap between the genitalia and anus is typically furred

• Have testicles, which may or may not be easily visible

• Lack nipples

• Must be housed alone unless desexed

Female mice:

• Shorter gap between their genitalia and anus than males

• Gap between the genitalia and anus is a hairless line

• Lack testicles

• Have nipples, which may or may not be easily visible

• Must not be housed alone — trios are recommended

We hope this article was useful to you! We will continue to add photos to it over time. We are always available for sexing advice for mice, rats, rabbits, or guinea pigs.