This book comprises a long-running research about the spacing behaviour and social and genetic mating system of the corn mouse Calomys musculinus. This book aims to make known several aspects of the behavioral ecology of this species that inhabits Pampean agrarian ecosystems of central and north western of central Argentina. The species is a good settler of disturbed habitats and shows a wider habitat and trophic niche than other coexisting rodent species. Besides this species is very important in relation to its role as reservoir of the Junín virus, the etiological agent of the Argentine Hemorrhagic Fever.