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Sexing Guide

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Staff member
Mar 10, 2009
Coventry UK
This guide looks at
- When and how to check the sex to come to a 100% definitive identification.
- How to sex babies.
- How to sex adults, including neutered boars.

I How to sex a guinea pig

When do I need to check the sex?
- Please ALWAYS double check the gender with any new arrival or before you bond!
Mis-sexing is unfortunately not at all uncommon, whether that is shop piggies, breeder piggies, free-ads piggies and even some rescue piggies unless they come from one of our recommended guinea pig rescues. Even then, I have a quick check...
What to check and look out for in new guinea pigs (vet checks, sexing, parasites&illness)

- Please sex any babies before they are 3 weeks old!
Boars start making babies from 3-5 weeks (occasionally even sooner) and sows have their first season from 4-6 weeks of age (occasionally even earlier). That means that you need to remove any baby boars from their mother and sisters at 3 weeks!
More tips for how soon to split boars according to their individual development: https://www.theguineapigforum.co.uk/threads/sexing-separating-baby-boars-and-rehoming-babies.109391/

Newborn babies are often still rather indistinct but that often becomes somewhat clearer after 2 weeks. However there is a fair number of babies that are not textbook. Always have a hands-on look at the less obvious traits which are unequivocal before a baby reaches the 3 weeks cut-off line!

Please note that you can enlarge any pictures in this guide for a closer look by clicking on them.

How do I make sure that I have got the gender 100% right?
- Ask as many people as you can, but please make sure that they do not just guess. Especially online sexing from pictures of yours is often a guessing sport for the inexperienced rather than anything else!

- Asking your vet: Some experienced vets working with rescues are reliable, but many general vets have likely not sexed many or any babies before. Please always ask your vet how many guinea pigs they have sexed and whether they are guessing at the result or not before accepting their verdict blindly.
We have had member pregnancies and back-to-back pregnancies resulting from mis-sexed babies on this forum; it really happens!

- You are always welcome to post pictures of your piggies' bits in our sexing section, but please make sure that the pictures are clear and well-lit, preferably by holding a baby towards a source of daylight so the genitalia are fully in the light.
With wiggly babies it is of advantage to have one person holding the baby and another person taking the picture.
Please accept that we will ask you in any case to always conduct a hands-on check as well because sexing from pictures has its limits. We will not stop pestering until we have established the gender beyond any reasonable doubt.

- always double check yourself by looking for the presence/absence of a penis ridge and then pry the slit apart and look whether their is a fleshy seal or not. All boars have a penis ridge but no fleshy seal; their opening goes stright down into the anal sac. Sows obviously are flat in the area of a penis ridge but have a fleshy arrangement just below the surface of the slit. If both of these areas give you the same answer, then you can be 100% sure that you have the right gender. The presence of testicles and and expressable penis do obviously help.

Sexing can never be just a matter of guessing.
It needs to be a matter of being absolutely sure!

What are the secondary areas that require a hands-on check?


Both genders have them and they look the same, apart from individual skin colour variations. Nipples can be different colours and can differ in colour in the same guinea pig depending on pigmentation and markings in the skin.

Penis ridge
Boars have a small penis ridge under the skin just above the button while sows are completely flat. Please feel gently!
You can express the penis by pushing downwards towards the button on the sides of the ridge. This can sometimes be difficult, especially in babies. That is why baby sexing is generally done without expressing the penis when you are not experienced with boars.

Expressing a penis (with a nice build-up of smegma, which needs to be removed from time to time):
Boar care tips: Boar Care: Bits, Bums & Baths

While boars reach sexual maturity at 3-5 weeks old, their testicles don't descend until they are about 4-6 months old. Descending testicles generally manifest first as swellings on the side of the genitalia. Their final size can vary massively.
This process is usually accompanied by strong spikes of testosterone and signals the onset of the teenage months. Please see a vet if the testicles do not descend on both sides.

While in German speaking countries baby boar castration at 2-3 weeks has been widely practised for the last 3 decades (i.e. before a boar becomes sexually mature), this is virtually unknown in English speaking countries where vets prefer to wait until the testicles have fully descended.
Immature castrated babies can immediately return to their family and do not require separation; sexually mature castrates however have to wait a full 6 weeks after their neutering operation before they are 100% percent safe to go with a sow.
My own surprise baby Tegan is living proof that an over 5 weeks post-neutering boar (not one of mine, by the way) can still have the ability to father babies. Tegan is not the only case.
This recommendation follows RSPCA (UK welfare league) practice.
More information and pictures in this guide: Neutered / De-sexed Boars And Neutering Operations: Myths And Facts

In configuration, a boar penis sits above the slit, rather the like the dot of an "i".
A penis button is always round like a button.

When you peer into the slit, it goes straight down into the anal sac. This becomes much larger when the testicles descend and needs to be checked and if necessarily cleared regularly for the build-up of muck and in about 10% of older boars, for impaction.
Boar Care: Bits, Bums & Baths
Impaction Recovery - How To Help Your Guinea Pig.

When looked at, a sow knob is nestled in a "Y" shaped groove. A knob can vary enormously in size and can be as large as a penis button. It is however never quite round, especially when looked at from the side.

When you peer into the slit, you will find a fleshy arrangement just below the surface that is sealed in any sows that are not in season. It is quite unmistakable and by far the safest way of distinguishing boars from sows!

PS: A sow spay is not visible from the outside. It includes removal of the ovaries and usually the womb, too.
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Staff member
Mar 10, 2009
Coventry UK
III Sexing adult guinea pigs

Normal configuration and open slit
IMG_4964_edited-1.jpg IMG_6622_edited-1.jpg

Descending testicles
While boars can make babies from 3-5 weeks (which is the reason why they need to be separated from any sows at 3 weeks old), their testicles won't descend until they are about 3-4 months old. The descent of the testicles signals their sexual maturity. Many general vets are not aware of the difference between procreative and sexual maturity.
The descent of testicles signals the onset of the teenage months and is often accompanied by strong spikes of testosterone.
The testicles are fully descended at 6 months of age, which is the time when the testosterone output in boars is at an alltime high.
The hormones start settling down again once a boar reaches adulthood at around 15 months of age.
If your teenage boars are having problems, please read this guide here for your options: Boars: Bullying, Fighting, Fall-outs And What Next?

Descending testicles show first as a gentle swelling on the sides of the genitalia
(picture 3 months old boar)

Full adult boars
Normal configuration with fully descended large testicles and with pressure on the genitalia. Not all boars are equally endowed, but the testicles should always be clearly visible. Please see a vet if one testicle is not fully descended.
(picture: 6 months old boar with fully descended testicles)
IMG_3278_edited-1.jpg IMG_3470_edited-1.jpg
Large descended testicles seen from behind:
Warning To Vet Check Irregular Boar Bits

Neutered boars
Neutered boars have lost their testicles. This means that they look more like baby boars again once the stretched tissue of the anal sac has contracted again in the weeks following the neutering operation.
The expressed penis and slit arrangement still looks the same as in full boars.

DSCN2873_edited-1.jpg IMG_6617_edited-1.jpg

Please note that neutering does NOT change social behaviour or personality and CANNOT make fallen-out boars live together again! All it is does is to remove the ability to make babies.
More information on neutering: Neutered / De-sexed Boars And Neutering Operations: Myths And Facts

How to care for boar bits: Boar Care: Bits, Bums & Baths
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