Extensive observations of foraging female lions reveal that two group sizes maximize foraging success during the season of prey scarcity: one female and five or six females. Foraging success does not vary significantly with group size when prey is abundant. Female lions live in fission-fusion social units (prides) and forage only with members of their own pride. If lion grouping patterns were primarily related to group-size-specific feeding efficiency, females in prides containing fewer than five females should forage alone when prey is scarce, whereas females in larger prides should forage alone or in groups of five or six. However, extensive data on the grouping patterns of radio-collared females show that females in small prides most commonly forage in as large a group as possible, even at the expense of foraging efficiency. Females in large prides most often forage in intermediate group sizes of four or five. However, mothers keep their cubs in a creche and form highly stable maternity groups that are effective in defending the cubs against infanticidal males. Most large prides contain a creche involving four or five mothers, and in the absence of a creche, large prides show no preference for any group size. Females also compete aggressively against neighboring prides, and larger groups successfully repel smaller ones in territorial disputes. Small prides appear to be excessively gregarious in order to compete against larger neighboring prides.