29 February 2012

Warring ants remember rivals’ smell

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Ants may communicate amongst themselves about the odour of rival colonies and adjust their aggression tactics accordingly, researchers have suggested.
ants odour rivals memory

Two green tree ants 'tasting' each other – are you friend or foe? Credit: Flickr/Mr.Chan

SYDNEY: Ants may communicate amongst themselves about the odour of rival colonies and adjust their aggression tactics accordingly, researchers have suggested.

A recent study into the territorial aggression of ants has found that tropical weaver ants from northern Australia, also known as green tree ants, share a collective memory for the odour of ants in rival nests. This finding suggests that after having encountered enemies in the field, ants are able to spread their knowledge to others in the nest.

“We were able to provide a very compelling case, I think, that information is being transferred around the nest,” said Mark Elgar, professor of evolutionary biology at Melbourne University and co-author of the study in the current issue of Naturwissenschaften.

“It’s not a novel idea that ants have some kind of corporate knowledge or collective memory,” he added. “For example, the nutritional status of the colony – when they’re short of carbohydrates or proteins – can be transferred to all the workers, simply by the state of individual larvae.”

Juicy gossip between ants

Ants are capable of memorising different odours, particularly those used as signals of colony identity, so they can identify colony mates from rivals by odour alone. In the past, ants were thought to have a genetic aversion to the odours of other colonies – the more distant the genetic relation, the stronger the aggressive response.

New research conducted by Elgar and his team now suggests that aggression is caused by social interaction, and may have little to do with genetics. The more often individual rivals met in the field, the more aggressive entire colonies became towards each other. The reaction was collective and specifically targeted to the colony, suggesting that ants can broadcast tactical information across colonies of over 50,000 individuals.

Elgar suggested that when ants taste each other, which they do regularly by patting each other with their antennae, they might be able to pick up and memorise any changes in their normal odour.

“If you imagine two workers have a good old argument or even a fight, some of the odour is going to rub off on their bodies. Maybe those workers present themselves to others, saying, ‘get this smell, that’s one you’ve got to remember.’ But it’s completely in the realms of fantasy,” he said.

The hive mind

Elgar and his team took ants from 12 nests and challenged them with intruders from colonies that were either familiar or unfamiliar to them, based on previous introductions. They gauged the ants’ reactions, measuring aggressive behaviours such as recoiling and mandible flaring, nipping, biting and grappling.

“We found that the ants were much more aggressive towards individuals from the colony that they had had multiple invasions from. But it wasn’t as if they were generally angry with all of the invasions and were lashing out at everybody,” said Elgar. “They were directing their aggression to ants from the nest they had been interacting with.”

Statistical analysis found it unlikely that each ant sampled could have encountered the rivals first-hand in the field, since one nest can contain over 50,000 individuals. The researchers suggest that the ants must have learnt about their rivals from an indirect source, such as their nestmates.

“We haven’t excluded the possibility that this information is being acquired either first or second hand,” said Elgar.

Chemical warfare

Another possible explanation for widespread odour recognition, said Elgar, is that many of the dead rival workers are taken back to the nest, and so there may be a little pile of dead workers.

“When the workers walk past they think, ‘oh yes, we should look out for that smell’. That is a possibility. I don’t think it’s a compelling one, because the odour would have to stay on the ants for a long time and not change as they decompose,” he added.

The study takes only the first steps to understanding more about communication in ant colonies, commented Chris Reid, a PhD student at the University of Sydney. “It’s important to remember they haven’t yet found out exactly what the mechanisms are. There aren’t a whole lot of results, but the implications are very interesting. It’s similar in lots of ways to a human immune system. While the mechanisms are completely different, the outcome is similar in that threats to a colony are recognised, the information is passed throughout the colony and the colony remembers the threat, so they can respond more quickly and dramatically in the future.”

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