An apparent mutualism between Afrotropical ant species sharing the same nest

Social insects have a highly developed nestmate and species recognition system that is quite effective at keeping out unwanted intruders. Rare cases of “parabiosis,” however, are known in some ants where two species apparently live peacefully alongside each other within the same nest. Here, we report on such an association between the tiny Afrotropical ant Strumigenys maynei and the large ant Platythyrea conradti. We demonstrated that both ants peacefully share the same arboreal nests in spite of the presence of clearly distinct nestmate recognition cues. Because of the large size differences, we hypothesized that each of the two species would benefit from specializing in carrying out particular tasks, in analogy to the size-related division of labor observed in species with size-polymorphic workers. In line with this theory, we found that the tiny ant S. maynei was highly efficient at nest defense against intranidal arthropods and alien ant intruders, whereas the large ant P. conradti was highly skilled in nest engineering. We argue that the described association is likely mutualistic, as P. conradti clearly benefited from the supreme defense capabilities of S. maynei, and that, conversely, S. maynei took advantage of small prey thriving in the organic nest material collected by P. conradti. SIGNIFICANCE STATEMENT: Associations between social insects are typically parasitic in nature. A few cases, however, are known of beneficial associations between social insects. Here, we report such a rare association between two Afrotropical species that share the same nest even though they lack matching colony odors. The large ant Platythyrea conradti benefited from the presence of Strumigenys maynei as this tiny, but highly aggressive, ant was much more efficient in attacking intranidal and extranidal enemies. S. maynei in turn took advantage of P. conradti as this ant constructs a unique nest which attracts suitable prey.