Mice eating eachother, what went wrong?

[Oxylepy]

I bought my friend 2 mice a couple months ago and she's been keeping them as pets, well today she threw them some food and filled their water, then we went to the pet store for me to grab some supplies and a couple male mice for future breeding (I've no knowledge of people who breed around here and I'm impatient and wanted my males now ~_~ dont judge me ).

So we're out for like 5 hours and we come back with a ball for her mice to have to run around in, and I start bathing the rats (they were gross). Then my friend, Tina, is trying to get ahold of me (shes a couple rooms over) and one of her roommates is telling me what she's saying. I leave the rats in the bathtub and run in to verify what she claimed, and true enough there was a tail and a spine laying next to a bunch of fur.

It clearly wasn't the cat, since you wouldn't imagine it being in the little plastic tank it was in, and you'd imagine it being everywhere.

So yeah. they have been eating:

Fiesta Max Guinea Pig (I grabbed whatever was there)
16% Crude Protien
6% Crude Fat
14% Crude Fiber
12% Moisture

[GoingPostal]

Were they both females? Both males, opposites? Could be one was sick. Hard to say. Did she check on them before feeding? Maybe they had run out of food/water and turned on each other.

At my pet store they have a breeding pair of hamsters who have been together for a long time and had several litters and they came in to find the female with a huge hole chewed in her side and the babies nipped here and there. Babies were already a couple weeks old and not a spot on the male but it had to be him. Female was still alive though, super gross. I thought that was pretty bizarre.

[Oxylepy]

Both females, both have not encountered males, both have been around eachother for a while. She throws them food every day and changes litter regularly.

[Raptor]

It's your food, you basically starved them. Guinea Pigs have totally different nutritional requirements than mice. Guinea pig food will have more roughage, similar to rabbit food.

I keep mine on hamster/gerbil food and they do great.