Here's why:
* Scientific names are for species, not genders: The scientific name for a guinea pig is *Cavia porcellus*. This name applies to both males and females of the species.
* Sex determination is based on physical characteristics: You can determine the sex of a guinea pig by looking at the distance between their anus and their genitals. Males have a greater distance than females.
So, while there's no special scientific name for a female guinea pig, you can use the terms "female guinea pig" or "sow" to refer to one.