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This study aims to elucidate the social system of the wild cavy (Cavia aperea), the feral ancestor of the domestic guinea pig, whose behavior under natural conditions is almost unstudied. Therefore, a population of C. aperea was investigated for a 6-month period in its natural habitat in southeastern Brazil. The animals' space use was examined via radiotelemetry, social interactions were recorded using direct observations, and genetic relationships were analyzed via DNA fingerprinting. Additionally, the distribution of plant cover, food resources, and predation risk was recorded to investigate the impact of different ecological factors on evolution of the social system. In the study period, a low population density was detected and a strong predation pressure existed, which resulted in a high mortality rate of C. aperea. Spatial distribution of wild cavies was strongly associated with areas of dense ground vegetation. Within these areas, small groups consisting of 1 male and 1-2 females occupied stable home ranges that overlapped only slightly with home ranges of adjacent groups. Social interactions were restricted mainly to individuals of the same group, and initial analyses of paternity indicate that the females' offspring were sired by the respective group male. The social system and spatial organization of C. aperea are regarded as adaptations to high predation pressure because in dense vegetation small group size reduces the risk of detection by predators. Moreover, habitat use, social interactions, and paternity point to a single-male system in this low-density population of wild cavies. Key words: Carla aperea, habitat use, predation risk, radiotelemetry, single-male system, social system, spatial organization, wild cavy, wild guinea pig