Scientists have found that sick young ants release a smell to tell worker ants to destroy them and protect the colony from infection, though queens do not seem to commit this act of self-sacrifice.
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 meet their friends.
Ant colonies, however, act as one “superorganism”, meaning that the ants work together to ensure the survival of all, according to an Austria-led team of scientists. An ant colony’s response to sickness is similar to how infected cells in our bodies send out a “find-me and eat-me” signal.
Ant nests are a “perfect place for a disease outbreak to occur because there are thousands of ants crawling over each other”, said Erika Dawson, lead author of a study published last month. Dawson is a behavioural ecologist at the Institute of Science and Technology Austria.
When adult worker ants get an illness that could spread through the colony, they leave the nest to die alone. In contrast, young ants, known as pupae, 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. The smell signals worker ants to destroy both the colony-threatening pathogen and the pupae (see graphic).
For the new research, the scientists wanted to figure out whether the pupae “were actively saying: ‘hey, come and kill me’,” Dawson said.
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.
“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,” Dawson said.
She 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.
What are some other ways that ants sacrifice themselves for their colony?
Every night, colonies of the Brazilian ant Forelius pusillus perform a routine act of self-sacrifice. As the sun sets, most ants stay inside the nest while one to eight workers remain outside to seal the entrance completely with sand.
Because these ants live on bare soil in tropical areas where many other aggressive ant species live, they are vulnerable to attack. By concealing the nest, the outside workers ensure the safety of the colony, even though they are blown away by the wind, attacked or lost by morning.
In the Borneo rainforest, worker ants of the Colobopsis explodens species protect their colony by performing an explosive final act of defence. During battles with rival ants, these workers rupture their own bodies, which causes them to die but also sprays a toxic fluid on their enemies.
