Rats eat nearly everything, including mice. The act of rats killing and eating mice is called muricide. This is a stereotyped behavior among rats. Muricide occurs due to hunger and environmental conditions, but mouse-killing is also affected by neurotransmitter signals in a rat's brain.
Rats attack and eat mice when food is scarce and they are starving. Wild rats tend to eat the carcass of their kill on the spot, including the neck and liver. Domestic rats often feed on its prey's brain. The predator-prey relationship is hard wired between rats and mice. Mice are more cautious when they smell rat urine, and the presence of rats affects a mouse's behavior and reproductive mechanisms.