Army ants are named for their aggressive hunting behavior, and they’re also fierce defenders of their colonies. But this hasn’t stopped several beetle species from the family Staphylidae which have evolved to infiltrate roaming army ant colonies and live in them as parasites. A new study published in the journal Current Biology finds this capacity evolved not just once, but at least a dozen times in beetle species that are only distantly related.
“These beetles represent a new and really stunning system of convergent evolution,” says Joseph Parker, a research associate in the Museum’s Division of Invertebrate Zoology and co-author on the study. “It’s an elaborate symbiosis, which has evolved in a stereotyped way, multiple times from free-living ancestors.”
Ant-colony invaders physically resemble their targets, but their body shape isn’t the only thing these devious beetles have changed. These grifters also pass the smell test, copying ant pheromones, and even mimic ant behavior to gain the acceptance of their hosts.
Once they’ve earned the trust of the colony, these copycat beetles march alongside their formic counterparts. Some smaller species may even hitch rides on larger ants. And they dine on meals hunted down by army ant soldiers and intended for ant larva—and sometimes, even on the larvae themselves. According to Parker, the new study shows that this practice has been occurring for a surprisingly long time.
“What’s exceptional is that this convergent system is evolutionarily ancient,” says Parker. Although most other systems of convergent evolution—think the beaks on Darwin’s finches—are a few million years old at most, this newly discovered example extends back into the early Cretaceous and has arisen with shocking frequency since then. “It raises the question: why has evolution followed this path so many times?”
Parker and his co-authors suggest that the beetles’ last common ancestor, which lived more than 100 million years ago, probably possessed traits that would allow its descendants to readily evolve into army ant parasites. One such trait: glands that secrete defensive chemicals, which are present in the mimic beetles’ free-roaming relatives.
Since rove beetles are predators, the brood of an army ant colony is an attractive food source. These traits, along with the rove beetle’s body plan—flexible and able to readily evolve into an ant-like shape to deceive hosts—enabled the beetles to repeatedly infiltrate army ant societies.
“There’s been this explosion of ants over the past 50–60 million years that must have radically changed terrestrial ecosystems,” says Parker. “They presented this huge niche for exploitation that these beetles were equipped to exploit, and they did so multiple times in parallel.”