Sibling care is found in diverse taxa, including invertebrates, birds, and primates. Understanding why siblings cooperate is critical to unraveling the evolution of social behaviour. In the small carpenter bee Ceratina calcarata one daughter will often forage with her mother on behalf of siblings. This under-provisioned dwarf eldest daughter (DED) is smaller, suggesting maternal manipulation, though it is not known if aggression plays a role in her behaviour. To test the effect of social environment in this system observation nests were used to determine frequencies of foraging and interactive behaviours in an undisturbed control, and two treatments, mother absence and mother and DED absence. Activity within the nest increased under both treatments. Aggression peaked in the absence of mother and DED, suggesting mothers and DEDs retain social order. In the absence of mothers foraging behaviour declined. DEDs thus reduce aggression, but do not substitute for mothers as foragers. Regular daughters are capable of foraging but do so infrequently. Mothers are primary foragers, but DEDs provide an important insurance mechanism for assured fitness returns through cooperation and sibling care.
