Sneaky beetles evolved disguise to look like ants, then eat them

Munetoshi Maruyama and Joseph Parker

By Andy Coghlan

It’s one of the sneakiest ploys that has ever evolved. Rove beetles blend seamlessly into army ant societies, but instead of helping out, they devour the young of their unsuspecting companions.

The deceit is so successful that it has independently evolved in at least 12 parasitic rove beetle species – a phenomenon called convergent evolution.

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The beetles’ entire body shape evolved to resemble the army ants they prey on, and they smell and act like the ants too. They even go marching on raids with them.

“What we found is that multiple times, the ancestors of these rove beetles adapted to __life inside army ant colonies,” says Joseph Parker at Columbia University in New York. “Each time, their body shape and behaviour underwent the same radical changes.”

Parker discovered the phenomenon with his colleague, Munetoshi Maruyama of Kyushu University Museum in Fukuoka, Japan. He says the finding challenges arguments by famous palaeontologist and author Stephen Jay Gould and others that completely different creatures would evolve if the evolutionary clock was restarted from scratch.

Instead, the findings suggest that evolution may take the same predictable path whenever a certain scenario arises. In this case, beetles first prey on army ants directly, but later evolve to sneak into the army itself.

Parker and Maruyama painstakingly collected several species of rove beetle over many years by tracking diverse army ant colonies around the world. Some were new to science.

Even though the beetles typically accounted for only 1 in every 5000 ants, the pair became expert in picking them out from the crowd. “You need good eyes, but before long your vision sharpens and you get better at spotting them,” says Parker.

Mingling with the ants

Taku Shimada

DNA analysis of the beetles allowed him and Maruyama to assemble a family tree showing how all the species were related and to estimate their divergence from each other. The creatures seem to have last shared a common ancestor 105 million years ago, around the same time that our ancestors diverged from mice.

Parker says the length of time since the common ancestor in this example of convergent evolution is remarkable, because most known examples from elsewhere in the animal kingdom — including marsupials, cichlids, stickleback fish, and mammal teeth and ears – happened in the past few million years.

Feels like an ant

The beetles are remarkably adept at mimicking the shape of the ants, which have disproportionately long hind legs, tiny constricted waists and a rear body segment that protrudes upwards at an angle.

“The ants are blind and use chemical and, we believe, tactile signals to recognise nest mates, so our evidence indicates that the beetles mimic the ant shape to ‘feel’ like a real one,” says Parker.

“The most remarkable thing is the fact that this exceptional body shape has independently evolved at least 12 times, indicating a certain level of predictability in the evolution of ant-associated rove beetles,” says Christoph von Beeren at the Technical University of Darmstadt in Germany.

The selective forces that drive this evolution remain uncertain and deserve further investigation, says von Beeren, whose team recently discovered a tiny beetle that looks just like an ant abdomen – helping it to hitch a secret ride on the back of army ants.

The beetles have also evolved to smell like ants, developing hydrocarbon-based scents on their outer body surface that mimic those on ants. “The beetles are often seen grooming the ant bodies, presumably to physically procure these chemicals from their hosts,” says Parker.

Ant cleaners?

But over the millennia, have the beetles evolved to earn their keep by serving some useful purpose for the army ants?

“This is something we’re very interested in,” says Parker. “Maybe the beetles produce something in their various glands that’s in some way beneficial to the ants. Perhaps they also feed on things like mites attached to the ants’ bodies, and this kind of effect might keep the colony’s parasite burden down.”

Taku Shimada

How the rove beetles seek out mates among the army hordes is also a mystery.

“If their main chemical profiles match those of the host ants, maybe they use alternative chemicals to recognise mates,” says Parker. “The likelihood is that they breed within the army ant colony, but there’s much we don’t know, including where they lay their eggs, where their larvae develop, and what the larvae even look like.”

Journal reference: Current Biology, DOI: 10.1016/j.cub.2017.02.030

Read more: New beetle species bites army ant’s butt and hitches a ride