Rats and mice cannot live together. Rats engage in muricide, a predatory behavior. Seventy percent of wild rats and 12 percent of laboratory rats hunt and kill mice.
Muricide is a stereotyped behavior, which means that most rats perform it in exactly the same way. Almost all rats eat the mouse after they kill it. Additionally, mice are stressed by the presence of rat odor; it makes them more easily startled than mice who are not exposed to rats or rat odor. Rat odor also has an effect on mouse reproduction. Female mice that can smell rat urine are likely to produce fewer litters than mice who cannot smell rats.