The answer to whether your baby Peruvian guinea pig's hair will grow longer is a resounding, "Yes, and how!" Your pint-sized pet's hair will grow and grow and grow until it reaches up to 50 centimeters (almost 20 inches) -- several times the length of your guinea pig's body.
Peruvian Guinea Pigs
Technically, all guinea pigs are "Peruvian" because this is where they were domesticated between 3000 and 6000 years ago. However, the name "Peruvian guinea pig" refers only to one specific breed selected for its enormously long hair. There is no such thing as a short-haired Peruvian guinea pig -- all members of the breed have 2 recessive genes for long hair.
Like most recessive genes, the long hair gene comes with some drawbacks; namely, these pigs are unable to groom themselves. Long-haired wild guinea pigs' coats become tangled until they can't see, eat, breed or evade predators. Pet long-haired pigs depend on you to keep them beautiful and functioning. Peruvian guinea pigs require daily brushing and regular trimming. That's "require" trimming. This isn't optional -- mats put your pig's life in danger.
Mats aren't just a cosmetic issue for Peruvian guinea pigs. Mats over the eyes block sight, and can cause irritation, pain, infections and blindness. Mats around the mouth can prevent eating. Males may get mats around the penis that not only end their romantic lives, but prevent urination. Females can become so badly matted they're unable to give birth or nurse their babies. Any pig can get matted around the anus, leading to terrible infection, not to mention ickiness and stinkiness. To put it bluntly, you'll have to shave your pig's butt regularly. Your guinea pig also needs her face trimmed, and pregnant mommies need their private areas and nipples shaved as well -- very, very carefully.
The biggest danger to Peruvian guinea pigs is flystrike. This tragic condition happens when a guinea pig becomes badly matted. Dirty skin or trapped feces give off an odor that attracts several fly species. These flies lay their eggs inside the mats. Maggots hatch and chew holes in the guinea pig. The guinea pig dies in great distress and pain. The best way to make sure your beloved pet never meets this terrible end is to brush her daily and regularly shave around problem areas. Take your guinea pig to a veterinarian or professional groomer, or have one teach you how to do this safely at home.