Mice turn cannibalistic when they are stressed. This is not normal behavior for mice, though maternal cannibalism of babies is much more common than other forms. When babies are cannibalized, they are usually killed by adults beforehand. Cannibalism of dead adults occurs under conditions of starvation and overcrowding.
Mice are noted to be an infanticidal species. The killing and eating of pups by the mother is thought to be a defense mechanism, as it is particularly seen in mice bred for strong nest-defending behavior. Some lines of mice are more likely to kill pups than others. Infanticide is often associated with human handling, so sometimes mothers are anesthetized to prevent it.
Cannibalism under other conditions is considered to be unusual. Mice have been observed turning cannibalistic to survive cold winters, when dwindling supplies of food cannot support the animals' population. For example, dead mice caught in traps may become food to survivors. Under ordinary conditions, mice prefer to eat sweet and carbohydrate-rich foods rather than flesh.