Acacia ants on their host tree (Crematogaster mimosae, Acacia zanzibarica). Image credits: Felix A. Hager and Kathrin Krausa.

You could hardly find a more impressive alliance than the one between acacia trees and acacia ants — they even share the same name because their bond is so strong. The trees provide food (in the form of nectar) and accommodation, and the ants protect the trees against would-be nibblers like elephants or giraffes. The trees even provide outposts in the form of hollow spikes, that the ants use to take refuge while defending the trees.

Now, a new study has revealed how ants are capable of telling when such a mammal approaches, and how they’re even capable of differentiating it from wind, sounding the alarm and starting to actively patrol the plant when nibblers are nearby.

This comic depicts how acacia ants are tipped off to the presence of herbivores by vibrations that run throughout acacia trees when an animal (elephant) gets too close or begins to chew. As a result, the insects begin patrolling the acacia’s branches more actively. The ants don’t react when the trees’ movements are caused only by the wind. Image credits: Felix A. Hager and Kathrin Krausa.

Researchers observed first-hand that whenever they got too close to acacias, ants would come running to their defense. But they also realized that they weren’t sure how ants know when to start the defense. The traditional theory says that odor is the main tell, but that just didn’t seem to tell the whole story: they had a hunch that vibrations also play a part.

“We often inadvertently touched the acacia branches and backed off because of the very fast and disruptive attacks of ants that swarmed on us,” says Kathrin Krausa, co-author of the new study. “It struck us that it was assumed that odors associated with plant damage alert the ants. As biotremologists studying vibrations, we felt that this is only half of the story.”

To test their theory, they measured vibrations in acacia induced by two different factors: wind and a browsing goat. They then replicated these vibrations using special devices, observing how the ants behave in the absence of any other cues other then vibrations. The ants didn’t care much about the wind, but whenever the goat-like vibrations were produced, they started actively patrolling the acacia tree and getting ready for defense.

“If an ant detects vibrations due to an elephant nibbling at its tree, it needs to find the attacker as soon as possible and decide in which direction to go,” Krausa says. “We were impressed by the ants. Spread all over the tree, they made the right decision and walked toward the vibration source to fight back against the attacker almost every time.”

Caught in the act: the research team in the Kenyan savanna: Felix A. Hager, Peter Mwasi Lombo, Kathrin Krausa, and a helpful goat. Image credits: Felix A. Hager and Kathrin Krausa.

This isn’t the first study to indicate the intricate ways through which the acacia tree and ants achieve an impressive level of interdependence. Previous studies have found that the ants protect the trees from pathogens and also destroy any threatening plant growing nearby. However, the tree is actually the driving force of this relationship — while we tend to think of plants as inflexible and static, the acacia tree paints a completely different picture. It evolved to produce a sweet nectar that’s addictive to the ants and also changed to enable the ants to better protect it. The passive plant, it turns out, deals an extremely addictive substance to the ants, which then go to great lengths to protect it.

However, it’s the first time this type of vibration study has been carried out.

“We’ve just started to understand this mode of communication,” Krausa concludes. “There is a lot of work waiting for us!”

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