The ant and the freeloader

An infection makes parasites of those affected, and wrecks harmony in the ant colony

Jessica Ledgard

What would you do if you could not just live longer but retain your youth? Would you take up on the offer, especially if your gain was the loss of your family and friends?

As a team of scientists discovered, this Intro-to-Ethics-type dilemma is the puzzling reality in some Temnothorax nylanderi ant colonies.

In many ways, the ants, found in Central Europe, lead humdrum lives. Yes, they do build their nests inside acorn shells but otherwise exist within the conventional social structures displayed by other ant species. Workers organize around a queen and follow the usual trajectory of going from nurses to foragers as they get older.

The Trojan poop

Occasionally, however, while bringing bird feces, that delectable feeding choice of their larvae, the ants sometimes accidentally also cart along some eggs of the tapeworm Anomotaenia brevis. The Temnothorax are one of the hosts in their life cyle.

That is when things take an unusual turn. As they grow, the infected worker larvae remain young and supple, retaining the yellow color of youth. They spend their days pampered by other workers, who bring them food and care for them, just as they would for their queen. In fact, the infected ones were not behaving like the industrious ants Aesop compared so favorably with a profligate grasshopper.

“We originally wanted to study how tapeworm infections might affect ant behavior,” explains Susanne Foitzik, professor of evolutionary biology at the Johannes Gutenberg University in Germany. “So the findings were initially perplexing for us, too.”

Why were the infected ants so plump and healthy? Was it because of the care they were receiving from other workers?

“We originally wanted to study how tapeworm infections might affect ant behavior. Parasites are supposed to harm the host, but in this case the hosts were living longer. They even seemed to be thriving,” she explains.

The test

At this point, the study was only examining behavior. So the team removed the infected ants from some nests and added infected ones into healthy nests, and discovered that the behavior of healthy workers changed in both scenarios.

“We also found some statistical differences in the survival rate of non-infected workers. The supposedly healthy ants were dying quite quickly,” says Foitzik. The study, however, lasted just four weeks, hardly enough time to figure out what was going on.

The team eventually monitored dozens of colonies for three years. Their findings corroborated their initial observations: over time, every single healthy worker died, while a large percentage of the infected ones were doing quite well, thank you. This was particularly remarkable because all of the tapeworm-ridden ants were alive when the nests were first collected. This meant that some of them could have been potentially alive for years before the study got going.

The siren song of the infected

Although their initial thought was that the infected ants were living longer because of the care they were receiving, they started to suspect that their extended lifespan was not just the result of being pampered.

They discovered that down-regulating a specific gene in the ants caused the eventual death of infected workers, leading them to believe that the parasite probably affects the hosts on a molecular level. According to Foitzik, this was somewhat frustrating, because although they had previously shown the effect of down-regulation, they could not include that nugget in the published paper as they could not provide enough evidence for it in this study.

“It was fascinating, but we just couldn’t show the downregulation process,” she says. “We have all this interesting data that we have not been able to publish because it doesn’t comply with what scientific reviewers expect.”

There was also the question of why healthy workers were straining so much to care for their tapeworm-carrying siblings.

“We were never able to see any begging behavior from the infected hosts, but we did discover that the chemicals of the infected workers are more appealing to the healthy ones,” Foitzik says.

The team realized that even though the parasites were not harming the hosts, the uninfected ants were literally working themselves to an early death. The tapeworms, they believe, turn the infected ants into a chemical magnet, forcing other workers to pamper them, even at the expense of their own lives.

Moreover, the whole colony was affected. The sex ratio was off: a telltale sign of distress. Ants are not just individual insects, for a colony of them makes up a sort of “super-organism.” What affects one ant affects them all. The researchers understood that the parasites had infected the entire society, warping its labor and caste division, and making the ants work essentially for them through their hosts.

Although the healthy workers died faster, and there were clear signs of overall stress in the community, infected colonies did not have a shorter lifespan than uninfected ones.

“This is puzzling, but not surprising,” says Foitzik. “Extending the lifespan of a worker only makes sense if the colony will also go on;, otherwise why do it? But, still, something was amiss. What is the parasites’ endgame and how do they benefit?”

A diabolical strategy

Foitzik and her team have a theory. For the life of the tapeworms to come full circle, the infected ants need to be ingested by a woodpecker. Once in, the parasites mature into adults and produce eggs, continuing the life cycle.

When the researchers cracked open the colonies, to their surprise they discovered that the infected ants just stood there, like a deer in headlights.

“Usually, parasites get hosts to leave their homes and expose themselves, becoming easy prey,” says Foitzik. “In this case, however, they stay in the nest.”

She believes this is because the woodpeckers feed on the acorns in which Temnothorax ants live. By never leaving the nest, the infected workers are less susceptible to predators other than woodpeckers.

Foitzik says she is been surprised by the attention the research has received.

“This project was part of the Ph.D. research of one of my students. And then her Ph.D. was over and she left to do a postdoc somewhere else. So I was left with the responsibility, and just wanted to get the paper out! But the response has been great. I guess other people also find parasites fascinating.”

Jimena Ledgard is a writer, journalist, translator and communications consultant based in Lima, Peru, She is a Chevening alumna and graduated from the London School of Economics