Buying another guinea pig?

Question
I currently own one pig, i dont know if its male or female but i think its male. However I have had him about 3 months and he seems happy, hes in a 100 gallon aquarium thats the length of a love seat, I also let him out to run around the house. I would like now with the big living area to give him a friend. Will he like another pig in his pen? Should I get the same or opposite sex? Thanks so much. I know they're social animals and I want my pig to have the best life possible. What would he prefer?

Answer
Nick --

Well, first things first.  Before you go purchasing another animal, you should find out the gender of the first.  Here's why.  Guinea pigs are prolific breeders (not quite as bad as rabbits, but still pretty bad) and you don't want to take the chance on having LOTS of babies.  What I would recommend is if you're not sure to take him to a reputable breeder or vet and have him checked for gender.  Then, you can determine which gender you want to get, if indeed you decide to get one.  They are indeed social animals, but they can do just fine on their own.  If you decide to get a female, and your animal is male, I would have your current pet neutered.  That way you don't run the risk of babies.  Otherwise, you can just get another male.  They are slightly territorial, but with a "territory" that large, they shouldn't have any problems with cooperating.  

One more thing.  You may want to consider some sort of ventilation for their housing situation.  Aquariums are really not ideal because they can increase the likelihood of lung problems.  You can either have a glass shop drill ventilation holes through the glass, or consider some sort of fresh air device (just make sure it doesn't move too much air, since guinea pigs catch cold easily.)

Hope this is helpful.  If you have any more questions, please feel free to contact me.  Good luck!

Lorena