Guinea Pigs are social creatures and as such like living in herds. A Herd is a group of guinea pigs. Males with males, females with females or an altered male with females, any group of 2 or more. More information on Neutering can be found here: Neutering.
Introductions should always be done after a 3 week wait where you keep both new pigs a part for healthy safety. They should be done in a neutral area, not in their cages, a kitchen floor works REALLY well for this. Place down lots of food: Veggies like lettuce and bell peppers. Hay piles are a great idea too!
DO NOT place anything they can try and claim like a igloo, cuddle cup or a tube/tunnel. They may fight over them.
When you try to introduce guinea pigs you will notice some scary things, but never fear here is some sounds/actions with what it means/if its normal.
Rumblestrutting: a deep purr accompanied by a swaying motion, they will rock on their hind legs. Teeth Chattering: it is as it sounds and is a warning to the other guinea pig to be careful. Chasing: this is normal. Butt Sniffing: guinea pigs have scent glands located about where a tail would be if they had one. Nipping: as long as no blood is drawn this is acceptable. Humping: this is a normal dominance behavior and will happen with any combination of genders. Jumping: some pigs with jump at another pig, don’t stop this behavior unless a fight occurs.
Now those are all normal but the following is cause for concern: Hair Raised: guinea pigs will puff up the hair around their neck to appear larger and more intimidating. Showing Teeth/Yawning at another pig: this is a sign of possible aggression. Crouching: Your guinea pig will lower itself to the ground in a pounce
type position, get ready, they are thinking about jumping at the other
pig. Snorting: Sounds like a strong puff or hiss.
If you notice these have a towel or something else to cover the pigs because usually the following will happen: Bite attacks: no longer warning nips, they are lunges with intent to harm. Standing and facing each other: this is a clear, brief signal of their intent to launch
full attacks at each other. Separate if possible before the attack. Full battle: the pigs are locked together in a vicious ball of fur. This is very
serious. Separate immediately, but be careful. Throw a towel over them
and use a dustpan or something other than your hand to separate them.
Unintended bites from their very sharp incisors can cause serious
damage and hurts, A LOT!
While most "fights" are more or less only one
guinea pig showing another he is boss some times things can be to much.
Here are some suggestions on how to calm them down:
Clean out the cage. Removed everything, clean
everything with vinegar and water. If you can throw plastic houses,
toys, water bottles, and food dishes in the dish washer. When you put
everything back it will be even ground, meaning there is not smell to
mask and the "who's your boss" fight wont last as long.
Give them a bath together, Cavies that bath
together stay together. Bathing them will make them smell the same and they will have a common dislike, you the bather.
One thing that I personally learned was once
you start the introductions that's it! Do not remove them after a few
minutes, this only starts the whole " I my name is bob whats yours?"
all over again. Once you start the intro you must keep with it UNLESS THERE IS BLOOD. If they are having a royal smack down and blood coming from it then they probably will never be cage mates. Another
thing to think about is if you are planning on getting another male
for your male, get either a older one or younger one. This usually
helps, because who ever is old is usually boss. Though as I have
learned this may not always stay that way. Sometimes when the younger
male becomes a teen and hormones start coming in too strong and fights
break out, but again only remove if there is blood.
BLOOD: If you have two pigs who have fought to the point of bloody injury it is best to not try putting them in the same cage.