Ant colonies rely on intimacy. Living in one means being part of a superorganism, with each ant acting like a cell in a larger animal. And according to a new study, it also means communicating — even voting — via mouth-to-mouth fluid exchange.
Known as trophallaxis, this process is common among social insects. "Food is passed to every adult and developing ant by trophallaxis," explains Laurent Keller, an ecologist at Switzerland's University of Lausanne and senior author of the new study. "This creates a network of interactions linking every member of the colony." But as Keller and his colleagues report in the journal eLife, trophallaxis is also a form of communication. Ants famously communicate by smell, yet swapping spit seems to play a key role, too, giving ants incredible abilities to control their colony.
"A lot of researchers consider trophallaxis only as a means of food-sharing," says co-author Richard Benton, a professor at the Center for Integrative Genomics (CIG) in Lausanne, in a statement about the findings. "But trophallaxis occurs in other contexts, such as when an ant is reunited with a nest-mate after isolation. We therefore wanted to see if the fluid exchanged by trophallaxis contains molecules that allow ants to pass other chemical messages to each other, and not just food."
Using Florida carpenter ants, the researchers isolated and analyzed these fluids. They found a wide array of specific proteins — including many that seem to be involved in regulating the growth of ants — along with hydrocarbons, microRNAs and a juvenile hormone that regulates insect development, reproduction and behavior.
The larvae fed by these ants were twice as likely to complete metamorphosis and become large worker ants. The hormone seems to provide a boost toward healthy adulthood, according to CIG researcher Adria LeBoeuf, who says it can give adult ants powerful collective influence over the development of their colony.
"This indicates that juvenile hormone and other molecules transferred mouth-to-mouth over this social network could be used by the ants to collectively decide how their colony develops," says LeBoeuf, who was first author of the new study. "So, when the ants feed their larvae, they aren't just feeding them food; they are casting quantitative ballots for their colony, administering different amounts of growth-promoting components to influence the next generation."
Along with growth proteins and juvenile hormone, the researchers also identified molecules and chemical signals in the fluid that help ants recognize their nest-mates. That includes the first evidence of chemical cues in trophallactic fluid that are known to give ants a colony-specific odor, helping them distinguish friend from foe.
"Overall, we show that liquid transmitted among ants contains much more than food and digestive enzymes," LeBoeuf says. "Our findings suggest that trophallaxis underlies a private communication channel that ants use to direct the development of their young, similar to milk in mammals."
If nothing else, discoveries like this illustrate how much we still have to learn about ant society. But revealing ants' secrets can also provide broader benefits, since they're often a rich source of inspiration for biomimicry. And as LeBoeuf points out, studying ants can also help us shed light on the biology of other animals, possibly even humans. There are many pitfalls in comparing creatures as different as ants and apes, but investigating the quirks of social insects can at least spur us to look at our own behavior with fresh eyes. We may recoil at the idea of trophallaxis, for example, but previous research has hinted at evolutionary reasons why we kiss.
"This opens the possibility," LeBoeuf says, "that the oral exchange of fluids, such as saliva, in other animals might also serve previously unsuspected roles."