Two Male Guinea pigs both new to our household

Question
Guinea Pig: I am a new owner (48 hours) and have never owned one before but have been doing a lot of reading and listening to vocal sounds and their meanings.  I bought a male guinea pig 6 months old two days ago with cage, hay, pellet food, the necessary bedding, water bottle, etc. I had felt guilty about separating the pig I purchased from his cage mate. They looked like they were a good pair and the pet store clerk had confirmed this but told me not to worry about it. I could see they got along and that one was younger (smaller) and more submissive. I purchased the larger dominant one.

Over the next day I read about truly happy guinea pigs needing a companion of their own species. So last night - within 24 hours of purchasing my first pig, I went back into the pet store to purchase the mate that the guinea pig I had already purchased was previously housed with.  They had introduced a male of the same color but bigger (but I could not differentiate or remember which one the mate was) into the mate's cage.

So I then went back into the store to get the mate to bring him home as the companion. So mistakenly - I could not decipher between the two- as I had not paid attention and no clerk from the previous day was in the store to let me know which one I was to purchase. I ended up purchasing the non mate! I confirmed this today with the store. When I had removed the larger male from the store yesterday, the existing male who had been the male to my pig I had already bought chirped in alarm. So he's probably been through the emotional ringer at this point.

So I thought I was re-introducing two male pigs who knew each other and got along. but instead I put together two males who had never met (both young (theysay 6-12 months)- the new one being larger than the existing one and both look fairly full-grown). So far no actual fighting but all the territorial things are going on: rumbling (can't tell if it's the curious noise or the aggressive noise, some but not a lot of teeth chattering from my first pig, competitive eating, humping, chasing...but they do huddle together and hide together. They new pig has a nick in his ear but it's not fresh so I doubt it is from my existing pig.

The pet store confirmed the male still in the store was in fact the original companion but also now that 48 hours has past and the pigs will probably have forgotten about each other as well as now that the pig I have has been in his own cage for two days, there are no guarantees the reintroduction of the original mates will go smoothly. They may have to start from square one again.

I know my original pig is territorial and does not like having the other new male in his cage. He does raise his hair and warns him with grumbling all the time. The new larger male I believe is the submissive one but he keeps kicking bedding out of the cage. But now they are huddled together nicely looking like companions. The new male does not seem to mind the aggression and appears to be submissive as he does not do the same back - he also moves around the cage much more but scares easily at sudden movements or noises but seems very vocal and playful. The original pig seems grumpy and just stays under cover coming out only to grumble at the new one eating food or moving around.

This morning my idea was to take the new male back to the store and replace him with the existing mate still there. I thought I'd be bringing the two back together would make them both very happy but instead I uprooted the second mate from the store male, and turned around the world of my existing pig and made him first scared with a new home and no friend to being aggressive and territorial with a completely new male. Not sure if I should just leave things to work out themselves now or if I should in fact go get the original friend and see if that match is better - or would that just be adding too much insult to injury for all males involved?

What is your advice on me returning the new male and replacing him with the original mate to see if territoriality eases in my original pig. I would plan to change the bedding prior to reintroduction.

Thank you.

Answer
A 6 month old pig is considered a "senior" by weight and age. That's how they are classified when showing, breeding, etc. and does not mean geriatric as in 'old.'  

By 6 months the boars have reached their full sexual maturity. They begin reacting by hormones and instinct. They are territorial and hard wired to begin to defend what they decide is their territory.  This applies to any male animal and has nothing to do with whom they were raised and previously got along.

This applies to any male animals that are herding animals (as are guinea pigs). In a wolf pack the family unit stays together until a younger male starts showing signs of wanting to be in charge. Only the alpha male and female breed, the rest have to mind their behavior and not show any interest in any of the females, and that means none of them. That is how nature prevents inbreeding.

In a herd of horses, cattle, buffalo, zebras, hippos etc. those same rules apply. There is only one alpha male, period. If the alpha male is challenged he will fight for his right to remain number one. If the challenger loses the fight, he is banished forever from the herd. If the alpha loses, he is banished.

The same instinctual behavior in seen in guinea pigs. No matter whether they are brothers raised together in happiness or unrelated and content as youngsters the day comes when those hormones kick in. These boars will fight continuously until one of them is either severely injured or dies from fight related bites and abscesses.  

The exception to that is if one of the boars is beyond breeding age and has become docile. Even then sometimes the younger male will want to fight. It's not because he's mean or vicious, it's a hard wired hormonal driven response.

I use my older boars as nannys for the baby boars being weaned. They are not just tolerant, but actually cuddle and nurture the babies. But that's self limiting as the babies begin to come into their own.

Removing two grown brothers for even a day is enough to start that territorial battle when reintroduced. It's been my experience that introducing on neutral ground still does not change that. As soon as they are put together in a cage the fights begin.

If you really want to have two together you need to find a baby. But be advised that the time may still come when they will have to be separated. It's just the way nature works.

I always use stallions as the most obvious example of territorial attitudes. Two or three young colts have a wonderful happy time together until that day of active hormones arrives. They will fight in an attempt to not just show who's stronger, but to kill their competition.

Neutering at a young enough age will prevent that from happening. Neutering when a boar is already mature doesn't guarantee they will lose that instinct.  

Although cavies are herding animals and enjoy company that doesn't mean they can't also live a happy life all alone. Boars are in my opinion better pets than the sows because they don't get those periodic 'pms' visits (piggy mean syndrome) and are more consistent in their behavior. Once passed breeding age (4 years) the sows settle down like most old folks do. Boars do not. They never pass that stage of "this is my house, not yours."

To sum this up and make a long story short, you'd be better off just having one boar in your family unless you have two cages. That one boar will be a great companion and will look to you for attention and comfort. He'll give you many hours of happiness and fun.