1. Social groups typically form due to delayed dispersal of adult offspring when no opportunities for independent breeding exist, or the costs of dispersal are higher than the costs of remaining philopatric. Ecological constraints are thought to be a main reason for group-living in animals. 2. Reproductive competition within groups can induce high costs of philopatry, and is thought to be a main reason for solitary living. 3. Experimental manipulation of reproductive competition is difficult. One solution is to compare sociality between periods with and without reproductive competition. 4. Here, we show empirically in a 8-year field study that striped mice (Rhabdomys pumilio) of both sexes were group-living during the breeding season when population density (PD) was high but solitary living when PD was low, supporting the ecological constraints model. 5. After the breeding season, in the absence of reproductive competition, the positive correlation between PD and percentage of group-living striped mice was absent. Almost all striped mice were group-living even under very low population densities. This supports the reproductive competition model. 6. Ambient temperature, food availability and predation pressure, did not influence sociality. 7. In captivity, the costs of reproductive competition in communal groups include female infanticide and aggression between females. 8. We conclude that group-living is favoured by constraints imposed through habitat saturation and by its benefits (improved thermoregulation by huddling, group-territoriality and predator avoidance), and that reproductive competition is a major force favouring solitary living in striped mice.