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The guinea pig pair itself
Is the pair a mature, bonded pair with a known personality? Are you 100% certain of the sex? Remember, pet stores frequently missex animals and knowingly or unknowingly sell a pregnant female or a breeding pair. The more you know about the pair you are considering adopting or buying, the less risk you have of some of the other issues in this list. If you are considering getting two independent guinea pigs who aren't currently living together, then it's just about the same as the next section Adding a 2nd Cavy.
If the pair is young (under 6 months) and same-sex (siblings or otherwise), then you will be risking the fact that they MAY not get along in the future, especially as they progress through adolescence. If you get a male/female pair and the male is neutered or the female is spayed and they are currently living together, it is very rare that they will cease to get along at any point in the future. Any tiffs are usually temporary and they will settle right down again.
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