Social Behaviour and Density Regulation in House Mice Living in Large Enclosures

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Abstract
Growing populations of house mice were studied in 2 large enclosures (each 26 .times. 14.8 m). Emphasis was placed on the possible role of social organization in influencing gene flow and density regulation. Populations were initiated with 15 pairs of half-wild adults; food and water were supplied in excess. Colonization of the enclosures took between 1-4 days depending on the complexity of the substrate. There followed the rapid establishment of 8 territorial groups in each enclosure by the 24 surviving mice. Both populations grew rapidly, and one leveled off at about 2000 individuals after 8 mo.; the other population was kept lower by predation, escapes and removals. A combination of reproductive stunting and neonatal mortality strongly influenced growth rates in both populations. The social groups proved to be extremely cohesive. Successful immigration was rare and weaned young almost invariably returned to their home group after a period of exploration. After 10 or 11 mo., there were still 8 social groups in each enclosure and their boundaries were only slightly modified from initial conditions. Social groups varied greatly in density and in the frequency of 5 pelage colors. Three attempts to introduce new mice into the existing social fabric were attempted. A new social group was successfully established only when the introduced mice were allowed to establish themselves in a disputed area between group territories. Otherwise, none of the adult males and only some of the adult females were able to join established groups. A group of known emigrants was mostly male, adult and contained more albinos than expected by chance. House mice exhibit both cohesive social groupings and a relatively rapid rate of gene flow. The latter is accomplished primarily be extensive dispersal and formation of new social units. In the absence of constraints imposed by essential resources., the territorial system restricts the number but not the size of social groups in a given area. In dense groups, other social factors cause the accumulation of reproductively stunted individuals and high neonate mortality.