Male Rats

You put two male rats together and one ate the other?

Did you do any sort of intro or did you just throw them in together? Rats are territorial. You have to introduce them to each other slowly. But in general they do not due well as singles (though there are rare exceptions). Neutered males also make great friends for females.

Happens on occasion, but using with babies or very, very young rats, and more is usually left, unless for some reason all the rats were hungry for dead rat (ick).

Perhaps the rats sense that they're ill or something. :?