Breeding Structure of the House Mouse, Mus musculus, in a Population Cage

House mice (Mus musculus), released in a population cage consisting of a series of nest boxes connected by narrow runways, formed small breeding units or demes in each of four runs. Generally, each deme was composed of a "dominant" male, several females and several "subordinate" males. The formation of these breeding units appears to be primarily the result of male territoriality. However, females were also found to contribute to the maintenance of such units. Once formed, demes remained stable over a considerable period of time in spite of factors such as high animal density in some territories. In older demes "dominant" males were replaced by their offspring. Interdemic migration, once demes were established, was rare and appeared to be possible only through females. Attempts to introduce mice into established demes of five to eight animals failed unless at least four mice were introduced simultaneously. The existence of stable demes may explain a number of enigmatic situations encountered in natural populations of Mus musculus.