If you’re the proud owner of rat pups, congratulations! It can be so much fun to watch a litter of rat pups grow up. One of the first things you’ll probably wonder about is the sex of your new babies.
At what age can you tell male and female rats apart? How can you tell if a rat is male or female? And when is it time to separate them?
While it’s possible to get male and female rats mixed up, in most cases it’s pretty easy to tell the difference. Male rats can reach sexual maturity as early as 6 weeks of age1, so it’s crucial to determine their sexes before that and separate the males from the females.
If you want to find out how to tell the sex of a rat pup, read right on.
Is it a Boy or a Girl?
At What Age Can You Sex Rats?
It’s possible to tell the sex of a rat pup pretty much from the day the are born.
However, you should avoid handling your rat pups this early, as this could cause a lot of stress for them and their mother. At worst, you risk stressing out the mother rat so much that she could resort to maternal cannibalism2, so refrain from interfering this early on.
A good time to start handling your pups and checking their sex is at around 2 weeks of age, or as soon as they have opened their eyes and start eating solid foods3.
This will allow for ample time to re-check the sex as your pups grow, until you should separate the males from the females for good at around 5 weeks of age.
How to Sex Rats
To determine the sex of your rat pups, you will need to pick them up to get a good look at their anogenital region.
This can be easier said than done, as baby rats are very active and will not stay still for long. In fact, it might take a few tries until you can start the sexing process. Be patient and handle your pups gently. If you restrain them too much this early on, they could grow to be fearful of your touch.
Never, ever pick a rat up by their tail!
This could cause them to lose the skin on their tail (“degloving”), which is a serious injury4. Instead, hold the rat in the palm of your hand and cover its body gently with your other hand to keep them from jumping off. You can use the thumb on your upper hand to gently lift the base of the rat’s tail and have a look. If you have a helper, great! One of you can hold the rat while the other checks the anogenital region.
When you are holding a rat pup, hover your hand above the cage bottom or another soft surface, so that the fall won’t be dangerous if a rat manages to escape.
Differences Between Male and Female Rat Pups
The most reliable way to tell male and female rat pups apart is by the anogenital distance, meaning the distance between their anus and their “genital papilla” (the hole a little further up their belly). The distance between these two spots is 1.5 to 2 times greater in males than in females5.
The genital papilla is also larger in males than in females.
You may need to check a couple of pups before you get a feel for the difference. Another approach is to check two rats together at a time, if you feel that you can safely hold them.
The absence of testicles is not a reliable way to determine a rat’s sex!
The testes remain retractable into the abdomen for the duration of the rat’s life5. However, if you do see a scrotum and testes, this is indicative of a male rat. The testes usually become visible at around 3 to 4 weeks of age6.
Female rats have bilateral rows of nipples showing from about 9 days of age. They will also have a faint “line” between their anus and their genital papilla, whereas this area is furrier in males6. But these are more subtle differences that should not be the principal indicators of your rat’s sex.
What To Do After Sexing Your Rats
Once your rats are weaned and you are sure of their sex, it’s time to separate the males. This will need to occur by 6 weeks of age at the very latest – 5 weeks is safer. Note that you will need to separate the male pups not only from their sisters, but also from their mother, as they will otherwise try to mate with her.
The female pups can stay with their mother until they will move into their permanent home.
Now is also the time to start planning the spay and neuter appointments for your rats, if you choose to go ahead with these procedures.
The earliest time for your pups to move into their new home would be at 6 weeks of age. It can be beneficial to wait a couple of days or weeks longer, as the period right after weaning is often a difficult time for a pup’s immune system. Also, your rats should not be too young when introduced into a new mischief, as they can’t fend for themselves yet.
Last but not least, keep in mind that if you have trouble sexing your rat pups, your veterinarian will be more than happy to help!
1. MSD Vet Manual. Breeding and Reproduction of Rats. 2020. https://www.msdvetmanual.com/all-other-pets/rats/breeding-and-reproduction-of-rats.
2. Reynolds RD. Preventing maternal cannibalism in rats. Science 1981;213:1146. https://pubmed.ncbi.nlm.nih.gov/7268426/.
3. American Fancy Rat & Mouse Association. Baby Rat Development from Birth to 6 Weeks. 2015. https://www.afrma.org/babyratdevdaily.htm.
4. Chastain CB. Animal Handling and Physical Restraint: CRC Press, 2017. https://books.google.ch/books?id=uDwPEAAAQBAJ
5. Johns Hopkins University Animal Care and Use Committee. The Rat. 2021. https://web.jhu.edu/animalcare/procedures/rat.html#reproduction.
6. American Fancy Rat & Mouse Association. Sexing 101 (sexing rats and mice) 2019. https://www.afrma.org/sexing101.htm.
Nina has a degree from the Veterinary School in Zurich, with a special focus on microbiological research. Nina has a passion for sports, nutrition and the outdoors and she loves all pets, but rats have a special place in her heart. When she’s not working or reading and writing about all things related to pet health, she loves to travel and surf.
As a small animal veterinarian, Nina is your go-to expert on pet health and nutrition.