Leafcutter ants remember past infections, strengthening colony defenses

A recent study has discovered a surprising behavior in lemon leafcutter ants (Atta sexdens). These ants not only exhibit social immunity – the ability to detect and remove pathogens to protect their colony – but also appear to remember past infections and respond more aggressively to familiar threats.

The research, led by São Paulo State University (UNESP), reveals that leafcutter ants can recognize a pathogenic fungus they have previously encountered, even 30 days after the initial exposure. 

When re-exposed, they intensify their cleaning behaviors and recruit more workers, a phenomenon researchers suggest could be considered a form of “social immune memory.”

A superorganism’s defense mechanism

“If we think of the colony as a superorganism, these behaviors would be like the immune system,” said first author Aryel Goes, a former bioscience student at UNESP. “In the same way that our defense cells attack invaders, such as viruses and bacteria, the workers fight infections that could harm the colony.”

The study shows that, unlike individual immune systems in animals, ants respond to infections collectively. 

“What we’re showing is that the social immune system of ants can store information about previous infections with the same pathogen, a phenomenon seen in our immune cells, but which occurs at a behavioral level in ant colonies,” Goes said.

To test this idea, the researchers conducted several experiments to observe how ants responded to repeated exposures to the same pathogen.

A stronger response to familiar threats

In one experiment, the team exposed ant colonies to a pathogen, then reintroduced it after seven days and again after a month. The ants’ response became significantly stronger over time, with the most intense reaction occurring at the 30-day mark. 

More ants participated in cleaning activities, both grooming themselves and removing the harmful fungus from the fungi they cultivate for food.

Lemon leafcutters belong to a group of ants that maintain a mutualistic relationship with fungi, cultivating them inside their colonies as a food source. In return, the fungi provide essential nutrients for the ants.

As part of the study, researchers tested four different pathogens – two harmful to the ants themselves and two that targeted their cultivated fungi. 

In another experiment, the scientists exposed separate colonies to a pathogen, then reintroduced it after seven days and again after 60 days. This time, the ants’ responses were noticeably weaker compared to those that encountered the pathogen after 30 days.

“Unlike our immune system, which in some cases responds expressively to a specific pathogen even many years after initial exposure, ants’ social immune memory has a shorter time frame, so they need to be constantly exposed to the pathogen for the information to persist,” explained Goes.

Do ants recognize specific pathogens?

The researchers also tested whether the ants’ memory-like response was pathogen-specific. In a third experiment, they exposed the ants to one pathogen, then introduced a different pathogen after seven days. The ants’ response to the second pathogen was much weaker than their reaction to the original one.

The following week, the researchers reintroduced the first pathogen. While the ants reacted, their response was only moderate – similar to their first encounter with the pathogen, but far less intense than when the pathogen had been reintroduced after 30 days.

Rodrigues suggested that this behavior could be a strategy to prevent accidentally spreading a new pathogen within the colony. “One hypothesis is that the lemon leafcutter reduced its hygienic response to avoid spreading the new pathogen to the rest of the colony. 

This is because cleaning altruism, in which ants lick each other to get rid of pathogens, can also spread new contaminants. That’s why they would have been more careful with a second infection that was completely new to the colony,” he explained.

Do ants respond to unknown substance?

To ensure that the ants’ response was indeed triggered by pathogens and not simply by exposure to an unfamiliar substance, the researchers applied a harmless solution to five colonies as a control. 

The ants in this group showed the weakest response of all the experiments, suggesting that the ants can distinguish between different substances and respond more aggressively to harmful pathogens.

The study raises the possibility that social immunity in leafcutter ants could increase the efficiency of pathogen removal, improving the colony’s overall survival. However, more research is needed to determine whether this actually qualifies as a form of immune memory.

Implications for pest control

Leafcutter ants, including Atta sexdens, thrive in open environments and have adapted well to agricultural landscapes, making them a common pest in monoculture farming. 

Their strong cleaning behaviors pose a challenge to biological control methods that rely on natural pathogens rather than chemical pesticides.

“Lemon leafcutters are adapted to open areas, so they benefit from monocultures and are a pest. Their cleaning behavior is one of the obstacles to effective biological control without the use of chemical insecticides. Understanding the social immunity of leafcutters can help to develop more effective and ecologically correct controls,” Rodrigues concluded.

The study is published in the journal Proceedings of the Royal Society B.

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