Why Rabbits and Guinea Pigs Do Not Mix

This question has come up a lot recently in both the rabbit and guinea pig forums. So here is the long and full blown answer.
Diet
Guinea Pigs cannot create their own Vitamin C, therefore their pelleted diet needs to enriched with Vitamin C. Rabbits can produce their own Vitamin C so the diet is lacking extra Vitamin C therefore it is unsuitable for guinea pigs. Both rabbit and guinea pig pellet diets are made to suit the individual diets of both different species, therefore, mixing these diets or feeding to the wrong species will mean the diet then lacks vital vitamins and minerals.
Language
Guinea pigs use both body and vocal language. Rabbits cannot understand the vocal side of things, therefore the rabbit finds this frustrating. Rabbits are not very vocal at all, and they rely on body language to communicate. A lot of this is shown with the ears. As guinea pigs ears are not as big, or flexible, the piggy cannot understand the rabbit. This leaving the pig frustrated.
So, why do they understand humans then? Well, although it appears that they learn what words mean, this is probably more of an association with the sound we make. And if rewarded with a treat, the will respond to this. when we stroke our pets, it is like grooming, therefore making your pet feel good about itself.
Fighting
This is often the result of the communication difficulties between the two species. They simply cannot understand each other. The frustration can then lead to fights. These fights are not all one sided. Both the rabbit and the pig could end up bitten or scratched and seriously hurt. If a wound turns into an abscess it can be fatal, as can bites and scratches.
There is also the fact that rabbits have strong hind legs. When a rabbit gets excited, it performs an act called a "binky" (the equivalent of a guinea pig "pop corning") where the rabbit bounds to the air and kicks out. Rabbits also kick out on the take off to run fast. An unsuspecting piggy can get caught in the way and receive a nasty blow. This can seriously injure or kill the pig.
Sexual Frustration
While rabbits are not sexually attracted to guinea pigs, it does not stop them trying to hump the pig. This can be very annoying to the pig, as it is a form of bullying. When sexually frustrated, rabbits often nip and bite the victim of their frustration. A sexually frustrated rabbit will find anything suitable as a mate, a human, and guinea pig, a cuddly toy. Even furniture.
Can you stop any of the above?
The only prevention is not housing your rabbit with a guinea pig. There is no fix to all of the above problems that will almost definitely occur.
Pet Shops
So your now asking why do pet shops house rabbits and guinea pigs together. The answer, I don’t know. Probably because during a stay in a pet shop the staff feel the animals are too young to be affected by this, this is incorrect. The other possible explanation is because the stay in the store is so short. This is no excuse either.Pet shops who choose to house rabbits and pigs together are setting a bad example, and this is probably where most of the associations of keeping rabbits and guinea pigs together route from.
Conclusion
Don’t do it.Guinea pigs live with guinea pigs, rabbits live with rabbits. Of course, it is not that simple, but the bonding of pigs and pigs and rabbits and rabbits is a different topic completely, which can wait for another day.
Prevention is better than cure, therefore, prevent fatal accidents and huge vet bills by keeping these species separate.