Where animal home ranges overlap extensively, objectively identifying exclusive areas within individual ranges has been difficult, particularly in species lacking overt territorial behaviors. By analyzing the overlap between successively smaller core areas among individuals in a population of the long-lived Australian skink, the sleepy lizard (Tiliqua rugosa), we objectively determined exclusive areas within animal ranges. Using 4-year radio tracking data, we found that ranges consisted of relatively large sally zones (mean 66–80% total range), around home ranges with multinucleate cores strongly associated with key refuge sites. Total range and home range area varied significantly among years, being smaller in a drought year. Total ranges overlapped extensively between and within sexes, but for both sexes, intrasexual overlap of inner range cores rapidly approached zero, suggesting intrasexual territoriality. Intersexual inner core overlap reflected this species socially monogamous mating system. But, male overlap of female ranges and inner cores was consistently higher than female-male overlap. Refuges and/or mates may be defended resources within these core areas, although aggressive behavioral interactions were rarely observed. In the extensively overlapping sally zones, males shared space with females other than their principal partner. In productive years, with larger home ranges and more extensive overlap, some lizards associated with extra partners, suggesting that males opportunistically use sally zones for polygyny. Consequently, we suggest that benefits to females from male association may change with environmental quality, such that if food resources are low, monogamy may be favored if females increase foraging efficiency as a consequence of male vigilance during pairing.