Even for the armed, there is safety in numbers

Bombardier beetles, members of the genus Brachinus, are uniquely equipped to defend themselves but still prefer to seek safety in numbers, US entomologists have discovered.

A study in the journal PLOS ONE reports that up to eight species – often only distantly related – were found living together, with each mating at different times of the year in a strategy likely intended to avoid accidental hybridisation.

The nocturnal bombardiers are fearsome opponents, famous for shooting a hot noxious liquid from their rear ends with an explosive noise that in itself is enough to deter many would-be attackers. For that reason, scientists are surprised that they feel the need to form groups rather than competing for food and territory.

Jason Schaller and colleagues from the University of Arizona decided to investigate by sequencing DNA from all bombardier beetles in 59 sheltering groups collected in the Sonoran Desert Region of Arizona, conducting what they say is the first phylogenetic analysis for such a multi-species grouping.

They also filmed beetles choosing from a selection of artificial shelters in the lab. Individuals preferred to join existing groups rather than shelter alone, but most showed no preference for exactly which species they sheltered with.

Around 71% of groups contained at least two species and 21% had three or more.

One possible explanation is that it takes around 24 hours for the bombardiers to recharge their noxious chemicals after use, so hanging out with others ensures there is always a colleague armed and ready.

The bombardier beetle belongs to the ground beetle family, Carabidae, which contains hundreds of species. Most are carnivorous and hunt on the ground or in trees.

“This research lays the groundwork for future explorations into the nature of multispecies aggregation behaviour within this group and among terrestrial arthropods in general,” the authors write.