An ant colony is a safe haven in an unforgiving world, offering food and protection for the insects that built it.
“They are well-defended fortresses,” said Rachelle Adams, an evolutionary biologist at Ohio State University who studies ants and their communication.
Other insects have discovered this fact, too.
“It’s such a concentration of resources that it makes sense that other organisms want to get in,” Dr. Adams said. But first those bugs have to figure out how to sneak inside and blend in without being detected and killed by the ants.
In a study published Wednesday in the Annals of the New York Academy of Sciences, researchers show how some butterflies hack the secret code of the colony, essentially learning to speak the ants’ language as a way to find shelter there.
Gossamer-winged butterflies in the family Lycaenidae evolved so that baby caterpillars can use both acoustic and chemical communication to trick ants. The ants then care for them in the colony until they become butterflies and fly away.
Scientists have studied this behavior for centuries. But how exactly the caterpillars break the ants’ code has remained a bit of a mystery. Some caterpillars create a sugary substance to lure ants to them when they’re outside the nest. They can also produce sounds and pheromones to mimic ants, with some even imitating ant queens to try to get royal treatment in the colony.
Once in the nest, either the caterpillars are fed by ants, or they eat the ants’ young.
For some butterfly species, this gambit is the only way they’ll survive.
The overall effect is that of a baby crying on a doorstep. And the rhythm of those cries might be key to breaking the colony’s code, the new study finds.
Previous work on ant-caterpillar communication focused more on frequency and tone, ignoring rhythm and timing. But rhythm matters.
“Think about humans,” said Chiara De Gregorio, an ecologist at the University of Warwick in England who led the new study. “It’s not just about what we say, it’s how we say it.”
To study rhythm in ant-caterpillar communication, Dr. De Gregorio collaborated with Francesca Barbero, an entomologist at the University of Turin in Italy who studies ant-caterpillar interactions. Dr. Barbero’s team collected two species of ants and nine species of butterflies from the wild, including butterflies that need to have their caterpillars taken in by ants to survive and those that don’t.
In the lab, the researchers used tiny, hypersensitive microphones to record ants and butterflies, and then pored over graphs of the recordings, measuring the pattern and timing of tens of thousands of sounds the insects had made.
The researchers hypothesized that ants and butterflies with tighter relationships would have more similar rhythms and tempos in the sounds they made, with the caterpillars driven by evolutionary pressure to sync up with the ants.
“These butterflies need to have patterns really similar to the ants that are taking care of them, because otherwise they will not survive,” Dr. De Gregorio said.
Indeed, caterpillars that relied more heavily on ants for survival produced sounds and vibrations that were much more similar to the ants’ sounds than caterpillars that didn’t need the ants’ assistance.
The sync-up seen between ants and the caterpillars that rely on them could reflect an “evolutionary arms race in communication,” with ants trying to keep caterpillars out and caterpillars trying to get in, said Dr. Adams, who was not involved in the study.
Dr. De Gregorio was also interested in how simple or complex the rhythms were. Complex rhythms, or a series of beats that aren’t as simple as a ticking metronome, are relatively uncommon in nature; they’ve been observed primarily in humans and primates, she said. Earlier observations of caterpillar-ant communication led the team to suspect the rhythms were complex, and that’s what they found, most commonly in caterpillars that depend on ants.
“It’s a big step forward in understanding how they can fool the ants,” Dr. Barbero said.
It’s one of the first instances of complex acoustic rhythms known in the insect world — although, the experts pointed out, scientists are only really starting to look.
Konrad Fiedler, an entomologist at the University of Vienna who was not involved in the study, said it offered quantification of longstanding ideas about caterpillar communication.
“It’s really a demanding thing to break this code,” he said, “and the fascinating thing with this new paper is that rhythm and speed are integral parts of code breaking.”
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