Why do they need company?
Imagine living by yourself for your entire life and never having anyone to interact with - how would that make you feel? Guinea pigs that are kept on their own are more likely to become bored or even depressed. They are herd animals, meaning that they are very sociable, and thrive with company of their own kind!
A few of the many benefits that companionship provides:
Improved quality of life
Having a herd of guinea pigs with a neutered boar and several sows is similar to how they live in the wild, so their natural behaviours really show with that group combination. It is very interesting to see the way they interact with each other, and the various dynamics of a herd.
A significant benefit of housing guinea pigs in a herd is that they have a lot of freedom with who they want to be with. Having said that, some pairs of guinea pigs have a great bond.
When you have a pair of guinea pigs, if one passes away, you will be left with just one guinea pig, and as guinea pigs are herd animals, they can become very lonely or even depressed without company of their own kind - as a result you may then end up in a situation of either leaving the guinea pig on its own, or getting a new friend for the guinea pig (which can be complicated, especially if you have had a pair with a strong bond).
With a herd of guinea pigs, if one passes away, this may unsettle the group at first, but none of the guinea pigs will end up on their own, which is a great advantage of a group.
If you have the space, time and commitment, then I would really encourage owning a herd of guinea pigs. It can be a lot of work, but it is so rewarding. With guinea pigs, company is also a 'safety in numbers' thing because they rely on each other for security and can become scared on their own.
As they are very social creatures, they regularly interact and communicate with each other. They are much more confident little characters when they have a friend or more and their personalities really come through. If you have a lone guinea pig, I strongly reccomend that you get them a friend because guinea pigs should not be alone!
There may be some circumstances where are guinea pig can be particularly aggressive. In reality, if they are matched with the right partner(s) then they are likely to settle into place. A guinea pig on their own may seem okay, however it is difficult to detect signs of loneliness, and although they may not show it, there is a good chance they are experiencing it.
What are good guinea pig combinations?
Neutered boar with one/several sows
A pair or group of sows
Boars that have been brought up together
An older, more dominant boar with a young boar
There are some instances where it is possible to have a herd of males, providing that they have lots of space, although boar groups can be complex and problems are quite likely to arise.
Like with any group/pair, it is very important for the personalities to work well together - some piggies will get along very well, others may not like each other, it is just like with people! The main thing is that they are able to tolerate each other and this is essential for a group to work. Male groups are not recommended for new owners, and should only be attempted by experienced owners.
How do you introduce guinea pigs that have never met?
There are various methods for introducing guinea pigs that have never met before. I have had the most success by introducing guinea pigs via a split cage. This is a very simple, stress-free method, and all you will need is a partition. C&C cages are great for introducing guinea pigs who have never met before. This is because you can easily build a partition with just a few grids. Once the cage has a divider, you just put one guinea pig at either side of the partition.
If you are introducing multiple guinea pigs who have never met before, you can add more dividers. If introducing a pair or group to a new guinea pig, keep the pair/group together, then put the new guinea pig at the other side of the divider. Split cages are often very successful because it allows them to interact without doing harm to one another. You also get a clear indication of how they are going to get along judging by the way they behave. When new guinea pigs first meet via a split cage, it is common for them to display dominant behaviours - they do this to establish which guinea pig is boss, and which is submissive.
There is usually more aggression when the guinea pigs first meet, but once they have been together for a while, the dominant behaviours usually subside. I reccomend waiting about 1 week before introducing them properly. In the meantime, you may sit the guinea pigs together on your lap and hand feed them some veggies as a distraction - this is good positive reinforcement as the guinea pigs learn to associate each other with something positive. Little and often is good, so once a day for about 5 minutes at a time is ideal. Remember, if the guinea pigs start to show signs of aggression, put them back into the split cage as you want to always try and end on a positive.
After a week (or when you think is best), the next step I reccomend is introducing the guinea pigs in a neutral territory. It is very important that you pick a location where neither of the guinea pigs have visited before, otherwise they may become territorial and show more dominance.
For example: bedroom floor, bathroom floor, kitchen floor, pen on the grass etc.
You will need
Playpen or fully secure room
Towel or gardening gloves in case fighting gets serious and you need to separate the guinea pigs
Lots of food! Veggies, hay piles, grass, plants etc. This is vital for keeping them occupied
Kitchen roll and/or sweeping brush to clean up any urine/faeces in the house
Another person (optional, but useful) to keep an eye whilst you set up their cage
COMMON BEHAVIOURS DURING THE INTRODUCTION:
Butt sniffing - like how dogs greet each other, they often do this to unfamiliar guinea pigs
Humping - they may try to hump, and the guinea pig on top is trying to be the boss
Teeth chattering - usually a sign of dominance but sometimes out of fear
Rumble strutting - common dominant behaviour, they 'butt sway' and make a deep rumbling sound
Chasing - guinea pigs sometimes chase during introductions when trying to establish hierarchy
Pee spraying - especially with females, they do this to warn the other guinea pig(s) away
Lifting head high - they may try to lift their head high to show they are the boss
THINGS TO LOOK OUT FOR:
Nipping/drawing blood - guinea pigs may bite out of fear or aggression
Hair pulling - when sorting out the ranking, they sometimes pull out fur to show dominance
Launching - another indicator of aggression, they may launch/attack each other
Guinea pig dominance may seem scary, however it is best not to intervene. I would only separate the guinea pigs if fighting got very serious, and it could cause too much stress or even injuries if they are left together much longer. If all seems to be going ok, try to leave them for a few hours in the neutral territory to ensure they are going to get along.
What happens next?
This depends entirely on how the introduction went. If there was little to no aggressive behaviour, you may then put them together in the same cage. Remember that mounting, teeth chattering etc is very common initially. Ensure that the cage setup is quite simple - you don't want too much enrichment at this time. You may wish to provide two of everything, for example bottles, bowls, hide houses etc so they do not have to share. Make sure the hide houses have multiple entrances/exits so they can not get cornered (a towel/blanket draped over the cage works very well). It is also a good idea to introduce them in a clean cage with fresh bedding so the scent is neutral. Hay piles are great during the first few days as it provides a distraction and allows them to have their own space.
It is common during the first few days for the guinea pigs to show dominant behaviour. It can take a while for the guinea pigs to decide on the hierarchy. The common behaviours listed above may still apply to when the guinea pigs being put together in the same cage, so do not separate them if they are still showing this behaviour (as they need time to adjust). While dominant behaviours usually subside within a few days, sometimes the dominant behaviour will continue. Most commonly this is because the guinea pigs are both aiming for the boss role, and neither will back down, which can result in the guinea pigs being aggressive and possibly doing harm to one another.
What happens if...?
I am going to list different possible scenarios and the solutions to different introduction problems.
Scenario 1. A pair of male guinea pigs are both fighting for the boss role & neither will back down.
Solution 1. First of all, ensure that their cage meets the minimum dimensions (120cm x 60cm). Small cages may increase dominant behaviour. Also make sure they have two of everything, including bottles, bowls, hide houses etc, so they do not have to share. Some male pairs go through phases of dominance, and this often sorts itself out given time and the correct habitat. If they still fight, unfortunately there isn't a simple solution to this, and sometimes their personalities are just not right for each other. You could keep them in a permanent split cage so they can still interact with one another but not hurt each other. This solution has worked very well in the past with my male guinea pigs who would just not get along together. Alternatively, if you have 2 fully grown males that don't get along, you can introduce very young guinea pigs to each of the adult males so you have 2 pairs of guinea pigs; this situation is ideal as it means they can still fully interact and live together. Remember that neutering the males will NOT stop the dominant behaviour, so do not do this unless you are going to introduce a male to females.
Scenario 2. A pair of female guinea pigs are both fighting for the boss role & neither will back down.
Solution 2. Although males have a reputation for fighting, females can fight just as serious! The females tend to bicker more than doing physical harm to each other, but they can still do damage if they are showing very dominant behaviour. The best solution to this problem is to add a neutered male to the pair. Males definitely act as peace makers around females, so the dominant behaviour should subside and they should get along much better with the presence of a boar (but he must be neutered otherwise you may end up with more guinea pigs than you intended to have..).
Scenario 3. In a pair of males/females one of the guinea pigs is getting bullied by the other.
Solution 3. You have to determine wether they are actually getting bullied, or if they are just the submissive guinea pig. It is actually not a bad thing for one guinea pig to be bossy and the other to be submissive, because these pairs will generally be stable together long term. With a pair of males, if you think the guinea pig is actually getting picked on, then you may want to consider pairing them up with different partners, but usually this is not necessary because it is most likely just a sign of being submissive. With a pair of females, see 'solution 2'.
Scenario 4. In a group of females (+a male?), one of the guinea pigs is getting bullied by the others.
Solution 4. In this situation, you need to again, make sure that they aren't just behaving submissive. It is really common to have a variety of personalities in a herd, so some will be more dominant, and others will be quite laid back or submissive etc. If after a long time the guinea pig seems unhappy and you have evaluated the situation, and it seems they are getting picked on, you may want to separate them from the group and get them a friend of their own (either female or neutered male). I generally wouldn't advise separating them from the herd because it is possible they are just naturally submissive, but if you feel they are very unhappy, it may be the best thing to do. It is also important to remember that the hierarchy of a herd can change. Some things that can cause this to change include: when you add another guinea pig to the group, if the guinea pigs are getting older or starting to mature, if they move into a new accommodation, if you have changed their setup and much more. Different changes can cause the guinea pigs to behave differently, so give them time to and see how things go.
Scenario 5. In group of males, the guinea pigs are fighting.
Solution 5. Male groups should only be attempted by owners who have lots of experience. If you are in the situation where you have a group of males and they seem to be fighting, there are a few options. Firstly, is their cage big enough? Male groups need lots of space! If the cage is too small, this may provoke aggressive behaviour, so give them as much space as possible. Also, for each piggie you have, make sure they have access to a bottle, bowl, hide house etc so they do not have to share. If they still fight, you may need to separate. You can attempt to bond the guinea pigs into pairs - try to pair the more dominant guinea pigs with the more submissive guinea pigs. If this is not successful, I reccomend having a split cage so all the guinea pigs can interact but not do harm to one another. Also, if you have females guinea pigs nearby, this may exacerbate dominant behaviour.
Scenario 6. The guinea pigs will not cuddle up together and don't seem to be best friends.
Solution 6. Most guinea pigs like their own space, so if they do not cuddle up together, this does not mean they don't get along. Generally guinea pigs cuddle up to keep warm in the cold season as they use each others body heat for warmth, but in a warm setting, guinea pigs often like their own space.