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We tested hypotheses based on philopatry, kinship, and ecological constraints to explain sociality in a semifossorial desert rodent, the great gerbil, Rhombomys opimus. Data were collected in the field in Uzbekistan in the spring and fall of 1996 and 1998-2004. Population densities fluctuated dramatically with high turnover in both males and females to reveal that dispersal and social structure were density dependent. Fewer gerbils dispersed at higher densities and members of family groups dispersed together. A majority of females lived in groups at high densities, but as population densities declined, proportionally more females were solitary. DNA analysis revealed that group-living females were genetically similar, whereas solitary females visited by the same male, as well as adult males and females in the same family group, were usually not genetically similar. Reproductive success as measured by the number of emergent pups and survival of juveniles during the summer drought was not related to group size or whether females were philopatric. A majority of females in family groups reproduced, and all females engaged in cooperative behaviors. We accepted three hypotheses to explain fluctuations in group formation in the great gerbil: variation in food abundance and distribution, habitat saturation, and kinship. We conclude that great gerbils are facultatively social. Flexible social behavior may be adaptive in unpredictable desert conditions. Females live solitarily under conditions of limited food and high mortality that disrupt social behavior and group formation and share territories with female kin under favorable conditions for survival and reproduction when kin groups can be maintained. Males adjust to the distribution of females.