You can tell if a guinea pig is male or female by gently examining the genital area. The most reliable method is applying light pressure just above the genitals: in males (called boars), a penis will visibly emerge, while females (called sows) have a smooth Y-shaped genital opening. This check matters more than you might think, because guinea pigs can become fertile surprisingly early, and pet stores frequently get the sex wrong.
The Gentle Pressure Test
The single most reliable way to sex a guinea pig is a quick physical check. Have a helper hold the guinea pig upright and close to their body, with one hand across the chest and one supporting the bottom. Sit the guinea pig on your lap or on a towel-covered table so it doesn’t slip. Do not flip the guinea pig onto its back, as this causes significant stress.
With the guinea pig secure, use your thumb to apply gentle pressure just above the genital area. In a male, the penis will protrude visibly from a sheath. If the male hasn’t been neutered, you’ll also see or feel obvious bulges on either side from the testes. In a neutered male, the penis still emerges but the testes won’t be present. In a female, no penis appears. Instead, you’ll see a Y-shaped slit. The shape of the genital opening alone is a strong indicator: males have a circular dot or “i” shape, while females have a distinct “Y.”
If your guinea pig is too stressed or wiggly for you to get a clear look, try again later when it’s calmer. Some guinea pigs simply won’t cooperate, and in that case a vet can confirm the sex quickly during a routine visit.
Other Physical Clues
Beyond the genitals, a few other physical features can help confirm what you’re seeing. Male guinea pigs have larger and more active perineal glands (the scent glands located near the base of the spine, just above the tail area). In males, these glands have clearly visible openings and produce a greasy, waxy buildup that contributes to a noticeable musky smell. Females have the same glands, but they’re smaller and produce far less secretion.
One common mistake: checking for nipples. Both male and female guinea pigs have nipples, so the presence of nipples tells you nothing about sex. Don’t rely on this as a clue.
Behavioral Differences Between Boars and Sows
Behavior alone isn’t enough to determine sex, but it can reinforce what you’ve found during a physical check. Boars tend to be more outgoing, curious, and vocal. They’re more likely to approach you, make noise outside of mealtimes, and display dominance behaviors like rumblestrutting, a low vibrating purr accompanied by swaying hips, especially around other guinea pigs. Unneutered males in particular can show territorial aggression toward cagemates.
Sows are generally more reserved and content to hide. They can certainly be vocal and social, but they’re less likely to strut or mount other guinea pigs. That said, every guinea pig has its own personality, so a bold female or a shy male isn’t unusual. Always confirm with a physical check rather than relying on behavior.
Why Timing Matters
Sexing your guinea pig early is important because they become fertile much sooner than most owners expect. Males can begin mounting females at just 3 to 4 weeks old. Females can become pregnant as early as 2 months (around 7 to 8 weeks). Full sexual maturity in both sexes arrives at roughly 3 to 4 months.
This means young guinea pigs from the same litter need to be separated by sex no later than 3 weeks of age. Separating them before 2 to 3 weeks can cause developmental problems, so the window is narrow. Guinea pig gestation lasts 63 to 72 days, and a male housed with a female can mate her again within hours of her giving birth. An accidental pregnancy is stressful for the sow and can lead to serious health complications, especially in young or small females.
Sexing Baby Guinea Pigs
Baby guinea pigs (pups) are harder to sex than adults because their genitals are small and less distinct. The gentle pressure technique still works, but you’ll need good lighting and a close look. In very young pups, the difference between a small protruding penis and a Y-shaped opening can be subtle. If you’re not confident in what you see, have a vet confirm the sex before the 3-week separation deadline.
It’s worth checking more than once. A single look at a squirming 2-week-old pup can easily lead to a wrong call, and one mistake means an unplanned litter a couple of months later. Many experienced breeders recheck at 2 weeks and again at 3 weeks before making final housing decisions.
What to Do if You’re Unsure
If you’ve tried the pressure test and still can’t tell, comparing two guinea pigs side by side often makes the differences obvious. A male’s genital area looks distinctly different from a female’s once you’ve seen both. If you only have one guinea pig, or if the animal is too young or too stressed for a clear examination, a vet visit is the simplest solution. Most veterinarians experienced with small animals can sex a guinea pig in seconds.