Recent research on space use and social behavior of small mammals has revealed pronounced differences in the degree of territoriality between species and between sexes within species. Hypotheses to explain these differences have been based on optimality approaches. Leading hypotheses are that the spatial distribution, abundance and renewal rates of food resources determine whether females defend territories, and that the spatial and temporal pattern of availability of females determines whether males are territorial. Other hypotheses invoke resources other than food, or maintain that territoriality in females deters infanticide. This review briefly summarizes these hypotheses and evaluates recently collected evidence from comparative and experimental studies. Copyright © 1990. Published by Elsevier Ltd.