Sick ants invite self-sacrifice to save colony: “Come and kill me”

Sick young ants emit an odor to tell workers to destroy them to protect the colony from infection, scientists said Tuesday, adding that queens do not appear to commit this act of self-sacrifice.

Many animals hide their illness for social reasons. For example, we know that sick humans are at risk of infecting others and so can still go to the office or pub.

Ant colonies, however, act as a “super-organism” that ensures everyone’s survival, in the same way that infected cells in our body send a “find me and eat me” signal, according to an Austrian-led team of scientists.

Ant nests are “a perfect place for an outbreak because there are thousands of ants crawling on top of each other,” Erika Dawson, a behavioral ecologist at the Austrian Institute of Science and Technology and lead author of a new study called “Altruistic Disease Signaling in ant colonies,” told AFP.

When adult worker ants contract a disease that could spread throughout the colony, they leave the nest and die alone. In contrast, young ants, called pupae, are still trapped in a cocoon, making this type of social distancing impossible.

Scientists had already understood that when these pupae are terminally ill, a chemical change occurs that produces a particular smell. The adult worker ants then gather together, remove the cocoon, “bite holes in the pupae and insert poison,” Dawson explained. The poison acts as a disinfectant, killing both the colony-threatening pathogen and the pupae.

For the new research, the scientists wanted to know if the pupae were “actively saying, ‘Hey, come kill me,'” Dawson said.

“Altruistic act”

First, the scientists extracted the smell from the diseased pupae of a small black garden ant called Lasius neglectus. When they applied the scent to healthy brood in the lab, workers still destroyed them.

Next, the team conducted an experiment showing that diseased pupae only produce this smell when worker ants are nearby, proving that it is a deliberate signal of destruction.

“While it’s a sacrifice – an altruistic act – it’s also in their own best interest because it means their genes are going to survive and be passed on to the next generation,” Dawson said.

However, there is one member of the nest who does not sacrifice himself. When queen pupae become infected inside their cocoons, they do not send out a smelly warning signal, the team found.

“Are they cheating the system? Dawson said the team asked that question.

However, they found that “queen pupae have much better immune systems than worker pupae, and so they are able to fight infection – and that’s why we think they weren’t signaling,” she said.

The study authors note that sick queens face a conundrum.

“By alerting others to destroy them, queen pupae would risk losing future reproductive opportunities if they survive the infection,” the authors write. “On the other hand, by spreading their infection to their colony, they could incur high indirect fitness costs.”

Dawson hopes that future research will determine whether queen pupae sacrifice themselves when it becomes clear that they will not be able to overcome their infection.

The study was published in the journal Nature Communications.

Scientists have already studied how ants communicate. A Stanford University study published in 2012 showed that harvester ants transmit information to determine how many ants to send to a particular food source. The researchers concluded that ants communicate in much the same way that data travels on the Internet, dubbing it “anternet.”

The researchers also concluded that other sick species were practicing social distancing, including guppies, bats and mandrills. Bees have been observed to use tactics to avoid getting sick, including completely evicting sick bees from their hives.

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