MerryPip
Adult Guinea Pig
1 How do boar genitalia look like?
- Babies, boars with descended testicles and neutered boars (pictures)
- Nipples
- Sexing Guide link
2 Bums
- What is normal
- How to keep clean
- Impaction
3 Bits and Pieces (Penis and semen fluid)
- What is normal
- Removing smegma and dirt from the penis
- Penis prongs
4 Penis Problems to look out for
- 'Cauliflower willy'
- 'Boar glue' and sperm rods
- Prolapsed penis
5 Grease Glands
- What is normal
- How to clean
-What to look out for
6 Boar baths
- What kind of baths
- How often
- Bathing tips
Boars have a reputation for being a bit messy and difficult to care for, so here’s a guide to what’s normal, what’s not and what you can do to keep your boar(s) happy, healthy and comfortable in their bodies!
For our full information on boars, please see this link here: A Comprehensive Guide to Guinea Pig Boars
1 How do boar genitalia look like?
Baby boars
- Young boars don’t have very obvious testicles as these begin to descend when they reach 3-6 months old and about the same time as you will notice your piggy begins to show more ‘teenage’ hormonal behaviour.

- Baby boars become fertile from 3 weeks of age so this is when they need to be separated from mum and any sisters. They should be around 250g in weight.
Some vets will not sex babies until after 5 weeks or even later, but by then all baby boars have reached sexual maturity. Leaving baby boars with their family for too long can result in accidental pregnancy of both mum and sisters.
You can find more information about sexing babies and when to separate baby boars in this thread:
https://www.theguineapigforum.co.uk/threads/sexing-separating-baby-boars-and-rehoming-babies.109391/
Entire adult boars
-These are some pictures of uncastrated entire boars with their testicles descended to show some of the variation in what is normal as the size of the testicles can vary quite a bit from boar to boar!
- The testicles should be both descended by ca. 6 months of age.
- See a vet if the testicles do not descend evenly or if you notice an unusual swelling in the groin area. Warning About Boars Bits
Merrypip's Angus showing debris & hair around the penis

Merrypip's Hamish showing larger testicles than his brother Angus

Hamish from behind showing well descended testicles

Here is a little video courtesy of The Excellent Adventure Sanctuary for guinea pigs with special needs (TEAS) that is showing you how full boar genitalia look on moving boys; it also shows you clearly where the grease gland is located. It is the black dot on the back, just above the genitalia.
An undescended testicle after 6 months of age should be vet checked.
Warning To Vet Check Irregular Boar Bits
Neutered adult boars
- Neutering does neither change boar behaviour nor their personalities; it is NOT a surefire recipe for preventing fall-outs! It is only necessary if a boar is to live with sows.
As the success of a neutering operation very much depends on the experience and quality of a vet in order to minimise the risk of complications, it is recommended to do your research beforehand!
Freshly neutered boars also need a full 6 weeks post-op wait before they are 100% safe to go with a sow; there are babies from over 5 weeks post neutering-op boars. That is really how long semen in the tubes can stay live!
Wiebke's Maelog showing a mature male with testicles removed

Nipples
- As you can see in the previous picture, boars have two nipples that do not look any different from sow nipples, so you cannot use the presence of nipples to sex a guinea pig. The colour of the nipples depends on the skin colour; your boar can have two differently coloured nipples.
- Please see a vet if you can feel a lump under a nipple. Boars can get mammary tumours, too!
Sexing link
- If you are unsure of the gender, as sow knobs and penis buttons can look very similar in especially young guinea pigs, please feel for the presence or absence of a tiny penis ridge just under the skin above the penis button.
- You may want to also check that the inside of the slit below the button is straight and hasn’t got any female side flaps. It is not always easy for new owners to express a penis.
- If in doubt, you are always welcome to post a (preferably clear and well lit) picture of the outside of the genitalia as well as the slit in our no breeding Pregnancy and Sexing section for extra confirmation. It is visible once you have registered with the forum and signed our explicit forum rules.
Our illustrated sexing guide contains more pictures of boars and sows at different ages and also where to look and feel exactly for any secondary traits and how to spot them: Sexing Guide
- Babies, boars with descended testicles and neutered boars (pictures)
- Nipples
- Sexing Guide link
2 Bums
- What is normal
- How to keep clean
- Impaction
3 Bits and Pieces (Penis and semen fluid)
- What is normal
- Removing smegma and dirt from the penis
- Penis prongs
4 Penis Problems to look out for
- 'Cauliflower willy'
- 'Boar glue' and sperm rods
- Prolapsed penis
5 Grease Glands
- What is normal
- How to clean
-What to look out for
6 Boar baths
- What kind of baths
- How often
- Bathing tips
Boars have a reputation for being a bit messy and difficult to care for, so here’s a guide to what’s normal, what’s not and what you can do to keep your boar(s) happy, healthy and comfortable in their bodies!
For our full information on boars, please see this link here: A Comprehensive Guide to Guinea Pig Boars
1 How do boar genitalia look like?
Baby boars
- Young boars don’t have very obvious testicles as these begin to descend when they reach 3-6 months old and about the same time as you will notice your piggy begins to show more ‘teenage’ hormonal behaviour.

- Baby boars become fertile from 3 weeks of age so this is when they need to be separated from mum and any sisters. They should be around 250g in weight.
Some vets will not sex babies until after 5 weeks or even later, but by then all baby boars have reached sexual maturity. Leaving baby boars with their family for too long can result in accidental pregnancy of both mum and sisters.
You can find more information about sexing babies and when to separate baby boars in this thread:
https://www.theguineapigforum.co.uk/threads/sexing-separating-baby-boars-and-rehoming-babies.109391/
Entire adult boars
-These are some pictures of uncastrated entire boars with their testicles descended to show some of the variation in what is normal as the size of the testicles can vary quite a bit from boar to boar!
- The testicles should be both descended by ca. 6 months of age.
- See a vet if the testicles do not descend evenly or if you notice an unusual swelling in the groin area. Warning About Boars Bits
Merrypip's Angus showing debris & hair around the penis

Merrypip's Hamish showing larger testicles than his brother Angus

Hamish from behind showing well descended testicles

Here is a little video courtesy of The Excellent Adventure Sanctuary for guinea pigs with special needs (TEAS) that is showing you how full boar genitalia look on moving boys; it also shows you clearly where the grease gland is located. It is the black dot on the back, just above the genitalia.
An undescended testicle after 6 months of age should be vet checked.
Warning To Vet Check Irregular Boar Bits
Neutered adult boars
- Neutering does neither change boar behaviour nor their personalities; it is NOT a surefire recipe for preventing fall-outs! It is only necessary if a boar is to live with sows.
As the success of a neutering operation very much depends on the experience and quality of a vet in order to minimise the risk of complications, it is recommended to do your research beforehand!
Freshly neutered boars also need a full 6 weeks post-op wait before they are 100% safe to go with a sow; there are babies from over 5 weeks post neutering-op boars. That is really how long semen in the tubes can stay live!
Wiebke's Maelog showing a mature male with testicles removed

Nipples
- As you can see in the previous picture, boars have two nipples that do not look any different from sow nipples, so you cannot use the presence of nipples to sex a guinea pig. The colour of the nipples depends on the skin colour; your boar can have two differently coloured nipples.
- Please see a vet if you can feel a lump under a nipple. Boars can get mammary tumours, too!
Sexing link
- If you are unsure of the gender, as sow knobs and penis buttons can look very similar in especially young guinea pigs, please feel for the presence or absence of a tiny penis ridge just under the skin above the penis button.
- You may want to also check that the inside of the slit below the button is straight and hasn’t got any female side flaps. It is not always easy for new owners to express a penis.
- If in doubt, you are always welcome to post a (preferably clear and well lit) picture of the outside of the genitalia as well as the slit in our no breeding Pregnancy and Sexing section for extra confirmation. It is visible once you have registered with the forum and signed our explicit forum rules.
Our illustrated sexing guide contains more pictures of boars and sows at different ages and also where to look and feel exactly for any secondary traits and how to spot them: Sexing Guide
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