In a dwarf mongoose family group, guarding is performed almost exclusively by subordinate males. The amount of time this social category invests in vigilance varies with group size. When too few guards are present, although mean vigilance time/guard increases, the vigilance systems becomes inefficient and gaps occur between guards. The effectivity of vigilance is high, 9 3% of incipient raptor attacks being aborted by the guard giving warning beforehand. For ground predators, vigilance efficiency is 100% when guards are present. For dwarf mongoose family groups, predation mortality is shown to be the major cause of reduced reproductive success. The number of young surviving is positively correlated with group size. Group size is shown to be a function of the efficiency of group protection mechanisms such as vigilance and group attack of ground predators. Maximal fitness gains at the level of the individual are attained at a group size lower than the observed mean group size. This finding is related to the concept of differing optimal group sizes for dominant and subordinate animals. The trend towards larger group sizes than necessary for optimal fitness is considered a means of reducing costs to subordinate animals involved in group protective roles.