Left together in a single cage, a sexually mature male and female guinea pig can produce up to six offspring in just a matter of months. Looking at your cavies' genitals is the most reliable way to tell the difference between the boys and girls, as calls and behaviors between the sexes are very similar. If your pigs are young, undeveloped genitalia can make it difficult to tell them apart just by looking. Determine the sex of the guinea pig pups immediately, and confirm again when they're 2 weeks old.
The Importance of Knowing
By the time testicles on the male are obvious at 3 or 4 months of age, he has likely impregnated every female in the cage. Guinea pigs have no moral constraints about mating with family members, so plan to separate the young males as early as 3 or 4 weeks of age, before sexual behavior begins. Female guinea pigs can get pregnant as young as 4 weeks old, and young males at this age are happy to make that happen. Guinea pigs are happiest in pairs or groups, so it's helpful to know how to correctly identify sex before purchasing a guinea pig from a pet store, so you can get two of the same gender.
It's a Boy or Is It a Girl?
As you hold a guinea pig on its back, the critter's anal opening is closest to the tail. The genitals sit between the anus and the stomach. Males will have a greater distance between their anus and genitals than females, to allow for the development of testicles. The anal opening and the penis are clearly separated by skin. The circular tip of the penis and the straight line of the anal opening make the male guinea pig's anatomy look roughly like a small letter i. The female's genitalia lie adjacent to her anus. Gently spreading the folds will show a distinct shape like the capital letter Y.
Other Ways to Tell
If you're not absolutely sure from looking at the genital openings, you can gently extrude the male's penis for positive identification. With the male on his back, rest your thumb just above the genital opening. You will feel the shaft of the penis just beneath the skin. Press lightly on the skin, moving your finger slightly toward the genitals. The penis will slowly slide out of the genital opening. If light pressure does not cause a penis to slide out, or if you cannot feel a shaft beneath the skin, your cavy is a female.
A Second Opinion
Occasionally, guinea pigs can be difficult to differentiate. Swelling around a female's genitalia can look similar to testicles, and males can be mistaken for females when they pull their testicles up into their body when they are nervous. A veterinarian experienced in small animals will be able to give you a positive identification of the gender of your guinea pig. Beware of having anyone who is not specially trained with guinea pigs help you determine the sex of guinea pigs. Chances are, their guess will be no more accurate than your own.
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