Anyone who annoys ants is known to get burning acid from their rear end. But apparently the secretion is not only used by the insects as a weapon against major troublemakers, but also against bacteria in their food: They swallow their own acid and thereby disinfect their stomachs, according to a study. In this way they can suppress the spread of pathogens in their colonies. In addition, the acid “medicine” influences the intestinal flora of the ants, the scientists report.
In addition to biting tools and spines, many species of ants are also chemically armed: They produce what is known as formic acid in a special gland in the abdomen, which can be released if necessary or even sprayed widely. Popularly it is called “peeing on” when the insects treat us with this burning substance.
“For a long time it was assumed that this acid only served as a defense against enemies, for example against insects and birds,” says Simon Tragust from the Martin Luther University in Halle-Wittenberg. In early studies, however, he and his colleagues were able to show that ants also use the acid for other purposes: They also use it for disinfection when caring for the brood. The insects thereby suppress the spread of harmful fungi in their nests.
As the researchers report, the basis of the current study was an observation of a conspicuous behavior of their test ants: “Whenever they have eaten food or water, the insects increasingly began to brush their rear end,” says Tragust. So the suspicion arose that they were taking in liquid from their acid glands and swallowing it. To investigate this assumption, the researchers carried out experiments with ants of the species Camponotus floridanus.
Ants make their stomachs sour
The scientists fed some test animals with sugar solutions whose acidity (pH value) was precisely known. In some of the animals, they had blocked access to their acid glands so that the secretion could not be absorbed. The researchers then recorded the pH values of the stomach contents of all test animals. It was shown that the acid content of the stomach contents of the control animals was significantly higher than that of the specimens that did not have access to their gland. The researchers conclude that the ants actually acidified the stomach contents after their meal by ingesting a portion of secretions from their abdomen.
But what is the purpose of this behavior? “It didn’t seem to have anything to do with the decomposition of food elements, because observations show that the ants also take up the acid after they have only drunk water,” says Tragust. So the assumption was made that the acid could have a disinfecting effect. This function of acid in the stomach is also known from other living things. In these, however, it is produced in the stomach itself. In order to track down a possible antimicrobial effect of acid uptake, the researchers first carried out laboratory tests: They produced nutrient solutions with acid contents that correspond to the stomach contents of the ants after ingesting their glandular secretions. They then infused them with a bacterium that causes diseases in ants. It showed that the typical acid content in the stomach of ants kills the pathogens.
Disease protection and design of the intestinal flora
The researchers then confirmed this result through infection tests with ants, in which some of them had no access to their glands due to a blockage. “If the insects were able to absorb acid, their chances of survival increased significantly if they ate food that was enriched with pathogenic bacteria,” said Tragust, summarizing the results of the experiments. The experiments show that ants disinfect themselves from the inside by ingesting the secretion.
This beneficial effect is not limited to the individual animals, as further experiments have confirmed. As the researchers explain, it is very important in ant colonies that pathogens are not passed on, because the animals transmit food to their nestmates from mouth to mouth. “This is a great potential source of infection,” says Tragust. “If the ant that passes on food has previously consumed acid, the second ant has a lower risk of getting sick, the experiments showed. “In this way, behavior at the ant colony level reduces the spread of infection,” says Tragust.
Finally, the researchers emphasize the importance of the ants’ intestinal flora as a further aspect of their own acid absorption. It was already known that acid-loving species from the Acetobacteraceae group are typical of the healthy bacterial community in the intestine of insects. Through experiments, the researchers were able to confirm that these microbes, in contrast to the pathogens, cope well with the typical acid content that is caused by ingesting the glandular secretion. “Acid uptake is therefore also a filter mechanism that structures the microbiome of ants,” explains Tragust.
Source: Martin Luther University Halle-Wittenberg, Technical article: eLife, doi: 10.7554 / eLife.60287