The relationship between the tropical acacia plant and ‘guard’ ants that defend it from predators has long been a fascinating example of symbiosis in nature: the ants feed on the acacia’s sugary nectar, and in turn aggressively sting and bite other animals that would eat and damage the plant. But it turns out that this arrangement might not be as friendly as previously thought. New research reveals that the acacia plant actually produces a chemical that drives the ants into a defensive frenzy–alternately persuading them to fight to protect it and banishing them from its flowers when convenient.The old adage about truth being stranger than you-know-what certainly applies here. Scientific research has now shown us a case where a plant is deploying a chemical drug on a legion of ants to get it to do its bidding. If that were the plot of a sci-fi film, we’d call it mind control. But as usual, the actual evolutionary science is more interesting than any number of b-movies.
Researchers at the University of London have been studying how the ants and acacias could have co-evolved when they began to understand how truly complex their relationship was–and how vital the ants are to the plants’ survival. So much so that the plants can ‘persuade’ the ants to attack other creatures–anything from spiders to giraffes–on their behalf.
An acacia ant faces off against a more formidable foe
When Ants Attack
The BBC spoke with Dr. Nigel Raine about his findings on the subject, and he explained how the ants greatly assist the acacias.
“They guard the plants they live on,” said Dr Raine. “If other animals try to come and feed on the rich, sugary nectar, they will attack them.” In Africa, one type of ant-guard, known as Crematogaster, will even attack large herbivores that attempt to eat the plant.
And yes, that includes mammals vastly larger than them: “If a giraffe starts to eat the leaves of an acacia that is inhabited by ants, the ants will come out and swarm on to its face, biting and stinging,” he said. “Eventually, the giraffe will get fed up and move off.”
Living the Good Life of an Acacia Guard Ant
In return, the ants get more than just access to the rich, sugary nectar of the plant. They also get protection, and in some cases, a home customized specifically for them–the acacia provides a hollowed-out, reinforced structures for the ants to nest in. The acacia also provides the ants with a sort of easy-access VIP pass to the nectar, preventing them from making the longer trip to the flower to feed.
These ‘extrafloral nectaries’ on the acacia’s stem serve a distinct purpose to the plant as well–they keep the ants away from the nectar in the flower, which is needed to lure pollinators like bees to ensure the plant’s survival. It’s a win-win–the ant gets a ‘bribe’ to stay out of the flowers and easy feeding, and the plant still gets its protection. That is, when it’s still convenient . . .
The Acacia’s Ant-Controlling Chemical
When the temptation potentially becomes too much–like when the acacia needs to produce extra pollen to draw in pollinators–things get ugly. The acacia, as the BBC notes, resorts to ‘chemical warfare’. It produces a chemical that’s physically repellent to the ants, keeping them out of the flower and driving them into a frenzy.
Dr Raine and his colleagues found that the plants with the closest relationships with ants – those that provided homes for their miniature guard army – produced the chemicals that were most effective at keeping the ants at bay.
The chemical is thought to be in the pollen itself–and when it’s carried off by the bees and hummingbirds, the ants return, no longer repelled.
The acacia and its guard ants no doubt have a fascinating symbiotic relationship–but the surprising use of chemicals to govern that relationship could perhaps have even more fascinating ramifications for the study of other natural pairings. What else out there is being duped and tamed by chemicals?