Star Metro - 'Come and kill me': sick ants invite destruction to save colony

Many animals conceal illness for social reasons. For example, sick humans are known to risk infecting others so they can still go to the office -- or the pub.

Ant colonies, however, act as one "super-organism" which works to ensure the survival of all, similar to how infected cells in our bodies send out a "find-me and eat-me" signal, according to an Austria-led team of scientists.

Ant nests are a "perfect place for a disease outbreak to occur because there are thousands of ants crawling over each other," Erika Dawson, a behavioural ecologist at the Institute of Science and Technology Austria and lead author of a new study, told AFP.

When adult worker ants get an illness that could spread through the colony, they leave the nest to die alone.

Young ants, known as pupae, in contrast are still trapped inside a cocoon, making this kind of social distancing impossible.

Scientists had already figured out that when these pupae are terminally ill, there is a chemical change that produces a particular smell.

Adult worker ants then gather around, remove the cocoon, "bite holes in the pupae and insert poison," Dawson said.

The poison acts as a disinfectant, which kills both the colony-threatening pathogen and the pupae.

For the new research, the scientists wanted to figure out whether the pupae "were actively saying: 'hey, come and kill me'," Dawson said.

- 'Altruistic act' -

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

Then, the team conducted an experiment showing that the sick pupae only produce the smell when worker ants are nearby, proving it is a deliberate signal for destruction.

"While it is a sacrifice -- an altruistic act -- it's also in their own interest, because it means that their genes are going to survive and be passed on to the next generation," Dawson said.

However, there is one member of the nest that does not sacrifice itself.

When queen pupae are infected inside their cocoons, they do not send out the smelly warning signal, the team found.

"Are they cheating the system?" Dawson said the team asked themselves.

However, they found that the "queen pupae have much better immune systems than the worker pupae, and so they were able to fight off the infection -- and that's why we think that they weren't signalling", she said.

Dawson hopes future research will investigate whether queen pupae sacrifice themselves when it becomes clear they will not beat their infection.

The study was published in the journal Nature Communications.

N.Fournier--SMC