Abstract
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.
Highlights
Aggregations of conspecifics in the context of foraging, mating, or breeding are common in the animal kingdom
These eight species are widely distributed throughout the tree
The molecularbased phylogeny confirmed our species identifications, initially based on morphology, showing that eight species were collected in the multispecies aggregations including Brachinus elongatulus, B. mexicanus DEJEAN, B. hirsutus, B. favicollis ERWIN, B. lateralis DEJEAN, B. costipennis MOTSCHULSKY, B. gebhardis ERWIN, and B. phaeocerus CHAUDOIR
Summary
Aggregations of conspecifics in the context of foraging, mating, or breeding are common in the animal kingdom. Individuals of several carabid genera regularly settle together to form multispecies aggregations in France (Jeannel, 1942) These aggregations often include three species of the bombardier beetle genus, Brachinus Weber (Jeannel, 1942; Wautier, 1971). Virtually all aggregations contain multiple Brachinus species, and sometimes other riparian ground beetles including Chlaenius Bonelli, Agonum Linneaus, Platynus Bonelli, Panageus Latreille, and Galerita Fabricius (Fig 1) These multispecies aggregations are Mullerian associations, as the species display similar aposematic coloration. Multispecies aggregations might randomly form due to limited availability of suitable shelters and/or beetles might have a clumped distribution during the night that increases the likelihood of multiple individuals sharing a shelter during the day
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