How to Tell If a Guinea Pig Is Male or Female
By Dr. Zubair Khalid, DVM, MS, PhD ·
You can usually tell if a guinea pig is male or female by gently checking the shape of the opening just below the tail and pressing lightly above it to see whether a penis emerges. Males have a small, round opening and a noticeable gap between the anus and the genital opening, while females have a Y-shaped opening with almost no gap. This article is educational and is not a substitute for veterinary diagnosis or treatment.
Quick Answer
- Look at the opening under the tail. A male has a small round dot. A female has a Y or slit shape.
- Check the gap. Males have a clear space between the anus and the genital opening. Females have almost none.
- Press gently just above the opening. On a male, a small pink tip (the penis) slides forward. On a female, the opening stays put.
- Feel for testicles. Adult males have two oval bumps low on the belly, though they can be pulled up inside the body and hard to find.
- Ask your vet if you are unsure. A wrong guess can lead to an accidental litter, so a quick professional check is worth it.
How to Tell If My Guinea Pig Is Male or Female: Step-by-Step
Sexing a guinea pig takes about a minute, but it needs a calm, steady hand. Two people make it easier: one to hold, one to check. Do this on a table or counter with a towel underneath so your guinea pig cannot fall.
Step 1: Set Up Safely
Place your guinea pig on the towel with its hindquarters facing you. Support the chest with one hand and keep the back end level. Guinea pigs are compact, short-legged rodents, and a wriggling animal can slip off a smooth surface [1]. If your guinea pig is very young, very pregnant, or clearly stressed, stop and let it settle before you try again.
Step 2: Find the Two Openings
Look at the area under the tail. You will see two openings: the anus closest to the tail, and the genital opening just below it. This is the single most useful landmark for sexing guinea pigs. Everything else is measured in published veterinary research.
Step 3: Read the Shape
On a female, the genital opening is shaped like a Y or a vertical slit, and the tissue between the two openings is thin and flat. On a male, the genital opening is a small round dot, and there is a visible strip of skin between it and the anus.
Step 4: Do the Gentle Press Test
Rest one finger just above the genital opening and press down and slightly forward, very lightly. On a male, this pushes the penis out through the opening. It looks like a small pink or whitish tip. On a female, nothing protrudes. You may see the opening widen slightly, which is normal.
Never pinch, squeeze hard, or use fingernails. If you press firmly and nothing happens, do not keep trying. Move on and check again another day, or have your veterinarian do it.
Step 5: Check for Testicles (Older Guinea Pigs Only)
In adult males, you can often feel two oval testicles in the lower belly area, just in front of the genital opening. They sit outside the body in a scrotal pouch. They can retract, especially if your guinea pig is cold, scared, or young, so their absence does not prove your pet is female.
Step 6: Confirm With a Second Check
Wait a few days and repeat the whole exam. Young guinea pigs are the hardest to sex, and a second look when the animal is a little older often makes the answer obvious. If the two checks disagree, treat your guinea pig as unsexed and keep it away from any opposite-sex guinea pig until a veterinarian confirms.
Guinea Pig Male vs Female: Comparison Table
| Feature | Male | Female |
|---|---|---|
| Genital opening shape | Small round dot | Y shape or vertical slit |
| Gap between anus and genital opening | Clear space | Very small or none |
| Press test above opening | Penis tip slides forward | Nothing protrudes |
| Testicles | Two oval bumps in adults, may retract | None |
| Body and head size | Often larger and heavier | Often smaller and finer-boned |
| Nipple check | No functional nipples | Two nipples on the lower belly |
| Best age to sex | Any age, easier after 8 to 12 weeks | Any age, easier after 8 to 12 weeks |
The size difference is real but not reliable on its own. In one study of guinea pig skulls, males had significantly larger and more heavily built skulls, while females had smaller, more slender skulls [2]. A separate study of the pelvic bone found the same pattern: males had significantly larger bone dimensions than females [3]. These are averages across many animals, so a large female can easily look like a male. Use the opening shape and the press test first, and treat body size as a hint only.
How to Tell If My Guinea Pig Is a Female: Extra Checks
If the opening looks like a Y and the press test shows nothing, you likely have a female. Two more checks can help confirm it.
Nipples
Female guinea pigs have two nipples on the lower belly, one on each side. They can be hard to see under long hair. Males may have small nipple-like bumps too, so this is a supporting clue, not proof.
Pregnancy Signs
A female who has been housed with a male may already be pregnant. A widening belly, a pear-shaped body, and increased appetite can show up in the later stages. Guinea pigs are social animals who naturally prefer living in groups, which is exactly why an accidental mixed-sex pairing happens so often [1]. If you suspect pregnancy, see your veterinarian. Pregnancy and birth carry real risks for guinea pigs, and your vet can advise on diet, handling, and when to expect the litter.
Behavior Is Not a Sex Test
Mounting, rumble-strutting, and chasing happen in both sexes. A female in heat may mount other females. A male may mount anything. Behavior alone will not tell you the sex of your guinea pig.
Sexing Guinea Pigs: Common Mistakes to Avoid
Most sexing errors come from a handful of repeatable mistakes. Here is what to watch for.
Mistaking a Retracted Penis for a Female Opening
A male guinea pig can pull the penis inside the body. When that happens, the opening can look flatter and less like a dot. The gap between the anus and genital opening usually remains, so check that spacing before you decide.
Pushing Too Hard During the Press Test
Firm pressure can hurt your guinea pig and make it struggle. Light pressure is enough on a male. If nothing appears with gentle pressure, stop.
Trusting Size Alone
Big guinea pigs are not always male and small ones are not always female. Skull and pelvic bone studies show group-level differences between the sexes, not a rule for every individual [2][3].
Guessing at a Young Age
Baby guinea pigs are small and the openings are close together. If you must know the sex right away, have your veterinarian check. Otherwise, wait a few weeks and recheck.
Housing a Mixed Pair "Just Until We're Sure"
This is the mistake that leads to litters. Guinea pigs can breed early and often. Keep any guinea pig of unknown sex in a separate cage, ideally in a separate room, until you are certain. If you are building a pair, read Guinea Pig Social Needs: Pair Housing, Introductions, and Signs of Conflict before you introduce anyone.
Assuming Two Males or Two Females Will Always Get Along
Same-sex pairs still need a proper introduction and enough space. Two males can live together, and two females usually can, but the pairing has to be managed. Sexing correctly is the first step, not the last.
Supplies and Setup That Support a Safe Exam
You do not need much, but the right setup keeps the exam short and safe.
- A nonslip surface. A folded towel on a table works well. Avoid holding your guinea pig high off the ground.
- Good light. A bright lamp or a phone flashlight aimed at the area makes the opening shape much easier to read.
- A second person. One person holds and supports the chest, the other checks the openings.
- A small treat afterward. A piece of hay or a favorite vegetable helps your guinea pig settle.
- A separate cage. If the sex is unclear, a spare cage is the safest tool you own. It prevents a pregnancy while you sort out the answer.
Once you know the sex, the next practical step is planning the housing and daily care. Guinea pigs have 20 teeth, including incisors plus premolars and molars, and those teeth grow throughout life [1]. A hay-based diet keeps them worn down properly [1]. If you are still deciding on a pet, Guinea Pig vs Rabbit: Which Pet Is Right for You? walks through the differences. If you already have guinea pigs and want to plan ahead, How Long Do Guinea Pigs Live? Lifespan and Care covers what to expect over the years.
Signs That Something Is Wrong
Sexing itself should not cause problems. Call your veterinarian if you see any of the following after an exam or at any other time.
- Bleeding or swelling around the genital opening
- A discharge that is thick, yellow, green, or foul-smelling
- Straining to urinate or passing small amounts often
- A swollen, red, or crusty opening
- Limping, hunched posture, or reluctance to move after handling
- Not eating or not passing droppings for several hours
- A male who cannot retract the penis, which can lead to injury or infection
Guinea pigs hide illness well. A guinea pig who stops eating is a sick guinea pig, and that is a same-day veterinary concern.
Limitations and When to Contact a Veterinarian
Sexing by appearance has real limits. Young guinea pigs, overweight guinea pigs, and guinea pigs with long hair around the rear are all harder to read. A retracted penis can mimic a female opening. A female with a prominent clitoris can look slightly like a male. No article can replace hands-on evaluation of your specific animal, and individual cases need a veterinarian.
Contact a veterinarian the same day if:
- You cannot determine the sex and you need to house guinea pigs together
- You see bleeding, swelling, discharge, or straining to urinate
- Your guinea pig stops eating or stops passing droppings
- You suspect a female is pregnant and she seems uncomfortable or unwell
Go to an emergency clinic now if:
- Your guinea pig is struggling to breathe
- Your guinea pig is collapsed, cold, or unresponsive
- A pregnant female is straining for more than a short time with no babies delivered
- There is heavy bleeding from the genital area
Schedule a routine visit if:
- You want your guinea pig's sex confirmed before pairing
- You are planning to breed, which should only be done with veterinary guidance
- Your guinea pig is a new pet and has never had a wellness exam
Frequently Asked Questions
Can I tell the sex of a baby guinea pig?
Yes, but it is harder. The openings are close together and the tissue is delicate. Use a bright light, keep pressure very light, and recheck at 8 to 12 weeks when the differences are clearer. If you need a firm answer before then, have your veterinarian check.
Do male guinea pigs always have visible testicles?
No. Testicles can retract into the body, especially in young, cold, or frightened guinea pigs. A male with retracted testicles may show no visible bumps at all. Rely on the opening shape, the gap between the anus and genital opening, and the gentle press test.
Is it safe to press above the genital opening?
Light pressure is safe when it is done gently and briefly. The goal is to see whether a penis tip slides forward, not to force anything. If your guinea pig struggles or seems painful, stop and let your veterinarian do the exam.
Can two guinea pigs of the same sex live together?
Yes, and guinea pigs are social animals who naturally prefer living in groups [1]. Same-sex pairs are the standard housing choice for most owners. Introductions still need care, so plan the pairing with the guidance in our social needs article.
What should I do if I am still not sure after checking?
Keep the guinea pig housed alone and away from any opposite-sex guinea pig, then book a veterinary visit for a hands-on exam. Do not guess and then house a mixed pair together. A single confirmed check prevents an accidental litter and the health risks that come with pregnancy.
Does a guinea pig's face or head shape tell me the sex?
Not reliably for an individual pet. Research shows males tend to have larger, more heavily built skulls and females smaller, more slender ones across groups [2]. Your guinea pig's head shape is a weak clue at best. Use the rear-end exam instead.
Are there health reasons to know the sex?
Yes. Some conditions are sex-specific or more common in one sex, and knowing the sex helps your veterinarian interpret symptoms and plan care. It also matters for housing, breeding decisions, and any future medical treatment. If you are unsure, ask your vet to record the sex in your pet's chart at the next visit.
Related Articles
- Guinea Pig Breeds Guide
- Guinea Pig vs Rabbit: Which Pet Is Right for You?
- Guinea Pig Names: Creative and Popular Choices
- How Long Do Guinea Pigs Live? Lifespan and Care
- Guinea Pig Social Needs: Pair Housing, Introductions, and Signs of Conflict
- Guinea Pig Names: Creative and Meaningful Options
References
- Description and Physical Characteristics of Guinea Pigs - All Other Pets. Merck Veterinary Manual.
- Sexual Shape Variation and Allometric Effects in Guinea Pig (Cavia porcellus) Skulls. Animals : an open access journal from MDPI, 2025.
- Sexual dimorphism in the right pelvic bone of the guinea pig (Cavia porcellus): Geometric morphometric analysis. Research in veterinary science, 2026.