Competition for a shared host: Inter- and intra-species interactions of two social parasites and the effect on tripartite co-evolutionary dynamics

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. Intra-specific competition, however, is an important component in dictating levels of symmetry/asymmetry between species and should be accounted for when attempting to understand inter-specific evolutionary or ecological outcomes. Some social parasites compete for access to a common host and, thus, adhere to both parasite-host and predator-prey co-evolutionary and ecological dynamics. Here we examine tripartite evolutionary dynamics with respect to intra-specific competition of two sympatric 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. Social parasites 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’ parasite but was almost non-existent in the ‘virulent’ parasite. Although intra-specific raiding led to larger single prudent parasite colonies, the preservation of two independent virulent parasite colonies had an early and a more devastating effect on the host. However, having effaced raiding phenologies, the prudent parasite emerged as the better direct competitor against the virulent parasite. Sex ratios were male-biased in the virulent parasite colonies but numerically equal (hence female-biased) in prudent parasite colonies. Combined, our results suggest inter-specific parasite interactions have produced ecological shifts in both parasites and attenuated the co-evolutionary arms race between the prudent parasite and the shared host in sympatric populations.