Molecular phylogeny, ecology and multispecies aggregation behaviour of bombardier beetles in Arizona

Aggregations of conspecific animals are common and have been documented in most phyla. Multispecies aggregations are less common and less well studied. Eight species of Brachinus beetles —famous for their unique, highly effective, chemical defense—regularly settle together form large diurnal multispecies aggregations in dark, moist areas in riparian habitats in the Sonoran Desert Region. Here, we document these multispecies aggregations and investigate the incidence and dynamics of aggregation behavior. Analysis of species composition of 59 field-collected aggregations revealed that 71% contained more than one species, eight species regularly co-occurred in aggregations, and no two species showed a preference to aggregate with one another. We provide the first phylogenetic analyses of participants in multispecies aggregations, and find that Brachinus species found together in aggregations are not each other’s closest relatives but rather are dispersed throughout the phylogeny of the genus. Further, we find no tendency for species to aggregate with close relatives more frequently than distant relatives. Laboratory experiments on B. elongatulus showed that it chose to settle in occupied shelters over empty shelters. Experiments with B. hirsutus and B. elongatulus showed that B. hirsutus prefers to settle under shelters housing heterospecifics over conspecifics. Our findings suggest that these multispecies aggregations do not form by chance, but rather are initiated by a genus-wide aggregation cue associated with the presence of individuals already in a shelter, which is likely to be chemical and potentially tactile in nature.