Large Blue's queen 'impressions' fool ant colonies

6 February 2009

Rory Bremner is famous for his impressions, but would they be good enough to infiltrate a royal household? Probably not, which makes the social-climbing antics of the Rebel's Large Blue butterfly all the more amazing.

Rebel's large blue butterfly

According to research in the American journal Science, the endangered butterfly's pupae and caterpillars mimic the sound of queen ants to trick worker ants into cleaning and feeding them in preference to their own offspring.

Like our own towns and cities, ant colonies are complex hierarchical societies with highly developed communications systems. And, again like our own, these societies' success attracts unwanted attention from other species.

While ants fiercely defend their nests about 10,000 species have evolved adaptations to infiltrate ant societies and feed on their rich resources.

The main ways ants communicate seem to be chemical signalling and touch. But now an international team of researchers has found that queen ants make distinctive sounds. These sounds seem to confer a higher social status, with ants preferentially feeding the caller in times of shortage and protecting them from danger.

Ants and caterpillar

Ants tend to a butterfly pupa, mistaking it for a queen ant.

Social butterflies

The Large Blue butterfly's caterpillars secrete chemicals and use begging skills to infiltrate host ant colonies. The ants feed them until they reach 98% of their ultimate weight before the caterpillars form pupae and turn into butterflies 11-23 months later.

Using recordings made with a specially manufactured microphone, the research team played back the calls of Rebel's Large Blue caterpillars to nests of its host ant workers. The results demonstrate that once an intruding caterpillar has been admitted and accepted as a member of the host ant society, the mimicking of adult ant acoustics, particularly of queens, allow it to advance its seniority towards the highest attainable position in the colony's hierarchy.

Sound in information exchange within ant colonies has been greatly underestimated.

- Dr Francesca Barbero, University of Turin

Lead author, Dr Francesca Barbero from the University of Turin says, "Our new work shows that the role of sound in information exchange within ant colonies has been greatly underestimated."

Co-author Dr Karsten Schönrogge from the Centre for Ecology & Hydrology says, "Our experiments showed that in response to the caterpillar's sounds, the worker ants protected them in a similar way to how they protect their own queens."

Social climbing

Previous research by co-author Professor Jeremy Thomas, now at the University of Oxford, had concluded that when an ant colony is disturbed, the caterpillars are rescued by the worker ants in preference to ant larvae. Moreover, if food is scarce, nurse ant workers killed their own brood and fed it to the social parasite.

Due to habitat changes, the Rebel's Large Blue butterfly has become an endangered species. It is restricted to a few meadows within mountainous areas of Europe where a specific type of ant co-exists with the gentian plants needed by the young caterpillars.

Thomas says the study is the final piece of the jigsaw explaining how the caterpillars can out-compete their host ant's larvae. The new findings will play a key part in designing a conservation strategy.

He adds that "There is an urgent need to investigate whether acoustical mimicry has evolved among other rare social parasites."

The research was carried out by an international team from the University of Turin (Italy), the Centre for Ecology & Hydrology (UK), and the University of Oxford (UK).