This paper discusses the bearing of variations in certain aspects of social structure on the survival and reproductive success of individuals. Possible factors limiting the female’s size and frequency are discussed. The consequences of the male’s ability to sire and look after several litters at once and the effects of the resultant sexual selection on social structure are considered. The relations between environmental variables, particularly resource distribution and predation, and aspects of social structure are discussed and their bearing on reproductive success and individual survival evaluated. The conclusions reached are illustrated by reference to selected species, and the types of information required for a fuller evaluation of the ideas presented are listed.