Sexing Rats: How To Identify Male vs. Female Rats

Angelina Childree, LVT
By Angelina Childree, LVT. Reviewed by Melissa Witherell, DVM on Mar. 14, 2024
Baby dumbo rat

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While sexing rats is similar to sexing other animal species, rats also have unique characteristics that make the process a little different. Fortunately, sexing rats is relatively easy once you know what to look for.

Why Sexing Rats Is Important

Rats, on average, have 6–12 litters of pups every year, with an average of 6–13 babies in each litter. This means that one rat can potentially have over 100 babies in a year! Rat reproduction can quickly become problematic for pet parents, so keeping rats in gender-specific groups is essential.

Female rats can have their first estrus (heat) cycle at 5 weeks old and reach sexual maturity between 3–4 months old. Captive-bred rats do not have a seasonal cycle, and their reproductive organs mature earlier than those in wild rats. A rats estrus cycle only lasts about four to five days, so rats are able to breed frequently—so frequently that they can even breed again within 24 hours of giving birth.

Rats are typically pregnant for about 21 days, but pregnancy may not be evident for the first 14 days as the mother’s belly expands.

How To Sex a Rat

Sexing rats is relatively easy, but it can be tricky in younger rats that have smaller anatomy.

Follow these steps to properly sex a rat:

  1. Firmly but gently wrap your thumb under one of your rats forearms and your index finger around the opposite shoulder.

  2. Cup your hand around your rat for support.

  3. Always pick up and carry your rat with two hands, and NEVER pick them up by their tail. 

  4. Once you are securely holding your rat, gently lift the base of the tail to view the genitalia.

  5. Now you are ready to identify the sex of your rat! 

Male Rat Anatomy

  • On a male rat, you will be able to see the penis in the center of the lower abdomen.

  • Below the penis, you will see the scrotum, which holds the testes.

  • The anus will be located centered behind the scrotum.

  • The testes descend at about 30–40 days old, so they can be harder to identify in younger rats.

  • Male rats do not have nipples, which can also be a helpful identifier as they grow.

  • The distance between the urethral opening on the penis and the anus (anogenital distance) is typically twice the length of that in a female rat.

Male rat anatomy
Male rat. Photo courtesy of Lauren Mick, LVT

Female Rat Anatomy

  • You can see the urethral opening in a female rat, like male rats.

  • Directly beneath the urethral opening will be a second, separate vaginal opening.

  • Further down the body will be the rats anus, but it will be much closer to the urethral opening than in a male rat.

  • Females will develop six pairs of nipples, visible at about 8–14 days old

  • Female rats often have longer tails than their male counterparts, but this is not an accurate way of sexing a rat.

Female rat anatomy
Female rat. Photo courtesy of Lauren Mick, LVT

How Do You Tell if a Rat Is a Boy or a Girl?

There are many ways to tell the difference between a male and female rat, but evaluating the anogenital distance (distance from the urethral opening to the anus) will allow you to differentiate between male and female rats as newborns.

Male rats have about double the anogenital distance than female rats. Additionally, female rats have a vaginal opening, creating a second opening under the urethral opening that is notably absent in male rats.

As pups grow and develop, you will be able to see nipples present on female rats but not on males. Once the male rats reach 1 month old, the scrotum will be more pronounced and may continue to grow in size with the rest of your rat’s body.

At What Age Can You Tell a Pet Rat’s Gender?

Using the anogenital method, you can identify a rats gender immediately. However, newborn rats should not be disturbed for about a week. Disturbing the environment can cause unnecessary stress for both mom and babies. Never touch a newborn rat unless it is a medical emergency.

Male and female rats should be separated into gender-specific groups as soon as they wean from their mother, as they will reach sexual maturity very soon afterward. Rats will typically wean around 21 days old.

Like other animals, the rats must remain in groups while young for social development. It’s important for young rats to play and explore with their littermates and learn how to share beds, hideouts, and toys.

Rats generally are ready to go to their new homes at around 6 weeks old, either as a bonded pair or as a new addition to your rat family!

References

Horatiu V. Florida International University. RATS -BIOLOGY & HUSBANDRY.

References


Angelina Childree, LVT

WRITTEN BY

Angelina Childree, LVT

Veterinarian Technician


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