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Brain chemical turns tree ants into aggressive soldiers

Angry ants Soldier or nurse? Worker roles in Australian green tree ant society are influenced by a chemical in the tiny insect's brain.

Differences in the levels of the chemical octopamine in the brains of two types of worker ant correlate with the level of aggression they show, finds a US-Australian study published in the Royal Society journal Proceedings B.

And the researchers have hard evidence supporting the correlation: tweak an ant's octopamine levels and their behaviour changes accordingly -- "so that's cause and effect," says co-author Professor Simon Robson of James Cook University.

Robson, who studies ants in colonies living on the Townsville university campus, collaborated with Boston University doctoral student Franne Kamhi and her colleagues.

Like many other social insects, green tree ants (Oecophylla smaragdina), have sterile workers which look different and do different jobs, even though they all hatch from the queen's unfertilised eggs, explains Robson.

The two main types of green tree ant worker are: 'majors' which are slightly bigger and more aggressive, taking on a 'solider role' and defending the territory; and the smaller 'minors' that do the bulk of the work in the colony, he says.

The researchers compared major and minor worker ants and measured how they behaved when exposed to intruder ants, either from a different green tree ant colony, or from a totally different species.

The major ants behaved much more aggressively than the minors towards the intruders, particularly when the intruders were from a different species.

Delicate research

Kamhi then measured the levels of various neuro-chemicals in the green tree ants' brains, including dopamine, serotonin and octopamine.

"It's very delicate work!", says Robson. "Franne had to dissect out tiny ant brains…so you have to watch your coffee intake…and make sure you have a steady hand!"

The study found a clear correlation, with higher concentrations of octopamine in the brains of the soldier ants than in the minors.

"Kamhi then did some very clever experiments where she modulated the levels of octopamine," says Robson.

By getting the ants to ingest an agent that blocked the receptor sites for octopamine in the soldiers, she showed their levels of aggression reduced.

On the other hand, painting octopamine onto the bodies of minors made them more aggressive.

"So what we've done in this study is we've actually shown the chemical that seems to underlie the [aggressive] behaviour. That's the key and it's the first time that link has been shown," says Robson.

Green tree ants, also known as weaver ants, are found in tropical Australia. A closely-related brown form exists in South East Asia.

"I think that green tree ants could be a really good model system for lots of behavioural [research] because they have some of the most complicated behaviour within the social insects," says Robson.

Related: Ant antennae key to sorting friend from foe