A Social Parasite’s Sophisticated Mimicry

An ant colony is an insect fortress: When enemies invade, soldier ants quickly detect the incursion and rip their foes apart with their oversize mandibles.

But some invaders manage to slip in with ease, none more mystifyingly than the ant nest beetle.

Adult beetles stride into an ant colony in search of a mate, without being harassed. They lay eggs, from which larva hatch. As far as scientists can tell, workers feed the young beetles as if they were ants.

When the beetles grow into adults, the ants swarm around them, grooming their bodies. In exchange for this hospitality, the beetles sink their jaws into ant larvae and freshly moulted adults in order to drink their body fluids.

“They’re like vampire beetles wandering in the ant nests,” said Andrea Di Giulio, an entomologist at Roma Tre University in Rome.

Dr. Di Giulio and his colleagues have now uncovered a remarkable trick that the beetles use to fool their hosts. It turns out they can perform uncanny impressions, mimicking a range of ant calls.

Dr. Di Giulio and his colleagues study a species of ant nest beetle called Paussus favieri, which lives in the Atlas Mountains of Morocco, where it infiltrates the nests of Moroccan ants, known as Pheidole pallidula.

Like many ant species, Pheidole pallidula makes noises by rubbing its legs against ridges on its body. The meanings of these signals vary from species to species; leaf-cutting ants summon bodyguards for the march back to the nest; in other species, a queen trills to her workers to attend to her.

Scientists have found that Pheidole pallidula ants make three distinct sounds, each produced by a different caste: soldiers, workers and the queen.

Dr. Di Giulio and his colleagues noticed that the unwelcome guests, the ant nest beetles Paussus favieri, also have ridges on their bodies, suggesting that they too can make sounds. This was odd; related beetles that don’t invade ant nests don’t have this kind of anatomy.

So the scientists set the beetles on top of microphones and listened carefully. The beetles began to sing, as the researchers had suspected they could. But to their surprise, the beetles didn’t produce just a single call.

Instead, they made three distinct kinds of sounds, each closely resembling the one made by a different ant caste.

“We were expecting only a very simple sound,” said Dr. Di Giulio. “We were stunned by finding such a complex pattern of signals.”

He and his colleagues wondered if the beetles were mimicking the ants in order to trick them. They installed a miniature loudspeaker in a chamber and buried it in sand, then put ants in the chamber and played beetle calls through the speaker.

The ants didn’t respond by attacking. Instead, they crawled toward the sound to investigate, waving their antenna in a pattern they use only to detect fellow ants. Then “they started digging to the speaker, like they were rescuing somebody,” said Dr. Di Giulio.

The scientists published their findings this month in the journal PLOS One.

“The ability of ant nest beetles to fully integrate into ant society has fascinated and mystified scientists for decades,” said Wendy Moore, an entomologist at the University of Arizona who was not involved in the study. “The results are wonderfully surprising.”

Dr. Di Giulio suspects that the ant nest beetles combine the sounds with other forms of deception. They grow large antennae that are packed with glands, for example, which may release chemicals that mimic the ones made by ants.

But Dr. Di Giulio can’t yet say why the beetles have evolved such an elaborate repertoire of ant calls.

It is possible that they use different calls to manipulate the ants in different ways. The beetles may mimic soldier calls to ensure they don’t get attacked, for example. The queen call, on the other hand, may elicit royal treatment.

Scientists refer to ant nest beetles as social parasites. Instead of infecting a single host’s body, they hijack an entire society.

Dr. Di Giulio said the new findings demonstrate just how impressive parasites can be.

“When we think about parasites, we are reminded of unpleasant, simplified creatures,” he said. “But in this case, the parasite is actually more complex than related beetles that are not parasites.”