  The evolution of parasite virulence and host defences is affected by
population structure. This effect has been confirmed in studies focusing on
large spatial scales, whereas the importance of local structure is not well
understood. Slavemaking ants are social parasites that exploit workers of
another species to rear their offspring. Enslaved workers of the host species
Temnothorax longispinosus have been found to exhibit an effective
post-enslavement defence behaviour: enslaved workers were observed killing a
large proportion of the parasites' offspring. Since enslaved workers do not
reproduce, they gain no direct fitness benefit from this 'rebellion' behaviour.
However, there may be an indirect benefit: neighbouring host nests that are
related to 'rebel' nests can benefit from a reduced raiding pressure, as a
result of the reduction in parasite nest size due to the enslaved workers'
killing behaviour. We use a simple mathematical model to examine whether the
small-scale population structure of the host species could explain the
evolution of this potentially altruistic defence trait against slavemaking
ants. We find that this is the case if enslaved host workers are related to
nearby host nests. In a population genetic study we confirm that enslaved
workers are, indeed, more closely related to host nests within the raiding
range of their resident slavemaker nest, than to host nests outside the raiding
range. This small-scale population structure seems to be a result of polydomy
(e.g. the occupation of several nests in close proximity by a single colony)
and could have enabled the evolution of 'rebellion' by kin selection.
