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Sterilisation of parasites prevents host reproduction, thereby diverting host resources to their own benefit. Previous theory predicts that parasites should evolve maximum virulence, yet hosts are often incompletely sterilised. Whereas prior attempts to resolve this paradox have sought evolutionary explanations, we present theory and experiments showing that incomplete sterilisation can arise from ecologically driven fluctuations in parasite load. The African ant-plant Acacia drepanolobium reproduced more when occupied by small colonies of the sterilising symbiont Crematogaster nigriceps. In nature, small colonies result from interference competition between ant colonies; these territorial conflicts thus provide intermittent windows of opportunity for host reproduction. Our mean-field model shows that numerical insufficiency of parasites can produce partial sterilisation of host populations, creating the appearance of reduced virulence even if ants have evolved to sterilise completely. This general framework helps explain both the apparent ubiquity of partial sterilisation parasitism and the ability of these symbiotic associations to persist. [ABSTRACT FROM AUTHOR]

Competition is an important evolutionary force behind population regulation and community structures. The degree of competition symmetry (competition hierarchies) between species determines coexistence, exclusion, or niche differentiation. Intraspecific competition, however, is an important component in dictating levels of symmetry/asymmetry between species and should be accounted for when attempting to understand interspecific evolutionary or ecological outcomes. Some social parasites compete for access to a common host and, thus, adhere to both parasite-host and predator-prey coevolutionary and ecological dynamics. Here I examine tripartite evolutionary dynamics with respect to intraspecific competition of two sympatric slave-making social parasites in the family of ants that conduct "brood raids" on a shared host species and use the captured brood as a functional work force. Slave-makers were challenged with either a conspecific or heterospecific parasite and the impact on hosts and parasites was evaluated. Within-species raiding occurred frequently in the "prudent" slave-maker but was almost nonexistent in the "virulent" slave-maker. Although intraspecific raiding led to larger single prudent slave-maker colonies, the preservation of two independent virulent slave-maker colonies had an early and a more devastating effect on the host. However, in response to synchronized raiding phenologies the prudent slave-maker emerged as the better direct competitor against the virulent slave-maker. Sex ratios were male biased in the virulent slave-maker colonies but numerically equal (hence female biased) in prudent slave-maker colonies. Combined, these results suggest that interspecific parasite interactions have produced ecological shifts in both slave-makers and attenuated the coevolutionary arms race between the prudent slave-maker and the shared host in sympatric populations. [ABSTRACT FROM AUTHOR]