Ants swallow their own acid to avoid getting sick

Ants swallow acid that is secreted by their poison glands to improve their gut microbial communities and prevent infection. According to a new study from Martin Luther University Halle-Wittenberg (MLU), the acidic environment that ants create in their stomach can kill harmful bacteria that is ingested with food, which reduces their risk of getting sick.  

“Animals continuously encounter microorganisms that are essential for health or cause disease. They are thus challenged to control harmful microbes while allowing the acquisition of beneficial microbes. This challenge is likely especially important for social insects with respect to microbes in food, as they often store food and exchange food among colony members,” explained the study authors.

Numerous species of ants have a special gland in their abdomen that produces formic acid. Dr. Simon Tragust said that scientists have long assumed that the acid was used for the sole purpose of warding off predators like insects and birds. In a previous study, however, Dr. Tragust showed that ants can also use formic acid to disinfect their brood and prevent the spread of harmful fungi. 

The current investigation was based on the observation of some puzzling behavior. “Whenever ants swallow food or water, they start cleaning their hindquarters afterwards,” said Dr. Tragust. He explained that this behavior did not seem to be linked to digestion, considering that they cleaned their hindquarters even after they had only consumed water. 

After conducting several experiments, the team discovered that the ants were disinfecting themselves on the inside. “When the ants were able to access the acid, their chances of survival increased significantly after eating food enriched with pathogenic bacteria,” said Dr. Tragust. 

Given that ants pass food from their mouth to the mouths of their nest mates, the benefits of cleaning were found to extend beyond the individuals. If the ant passing food around had previously ingested the acid, the recipients had a lower risk of getting sick. According to Dr. Tragust, this behavior may reduce the spread of infection within the ant colony.

“In addition to improving their own survival, the ability of donor ants to access their poison also improved the survival of receiver ants without access to their poison following trophallactic exchange of pathogen-contaminated food,” wrote the study authors.

“Acidic crop lumens might therefore act as a barrier to disease spread in formicine ant societies, alleviating the cost of sharing pathogen contaminated food and counteracting the generally increased risk of pathogen exposure and transmission associated with group living.”

The findings also provide new insight into why some ants have very few bacteria in their digestive tracts. “Acid swallowing acts as a filter mechanism, structuring the ant’s microbiome,” explained Dr. Tragust. He said that acidic stomachs are only known to occur in humans and a few other vertebrates. Humans produce acid directly in the stomach, but the results are the same – the stomach acid kills germs in food and influences the gut microbiome.

The researchers conclude that the ability of formicine ants to access their own poison and filter out the harmful microbes they ingest may help to explain the ecological and evolutionary success of this group of insects.

The study is published in the eLife.

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By Chrissy Sexton, Earth.com Staff Writer