It knows when to take its medicine Power and Syred/Science Photo Library

It’s a nasty poison, but for an ant fighting off a dangerous fungus, it could be the only hope. For the first time, ants have been seen self-medicating – on food rich in hydrogen peroxide.

Large, dense colonies of social insects like ants and bees can be particularly vulnerable to parasite infections and fungal diseases. One way to manage this might be to ingest otherwise harmful substances to fight the infections, but conclusive evidence of this behaviour in insects had been elusive.

Nick Bos and his colleagues at the University of Helsinki, Finland, have now shown that ants choose to eat hydrogen peroxide if they have a dangerous fungal disease – and are more likely to survive as a result.


First, his team demonstrated that hydrogen peroxide is usually harmful for ants of the Formica fusca species. They fed them one of two diets – either a simple honey-based solution, or this same solution spiked with hydrogen peroxide.

They showed that healthy ants given the spiked diet had a mortality rate of around 20 per cent, compared with around 5 per cent for those who got the harmless solution.

But when they tried the same thing using ants infected with the fungus Beauveria bassiana, the opposite happened. The death rate of these ants fell from around 60 per cent for those on the ordinary diet to 45 per cent in those given food laced with hydrogen peroxide.

Deliberate dose

When the ants were offered a choice, the healthy ants tended to avoid the spiked food, says Bos. But the infected ants ate more of the solution containing hydrogen peroxide, and chose their dosage carefully.

When the toxic solution was weak, the infected ants tended to opt for equal amounts of this food and the pure food. When a stronger solution was offered, they only fed on it around a quarter of the time.

“It is not known yet how ants know they are infected, but it’s very clear that they do somehow change their behaviour once they are,” says Bos. Some ants close to death will leave the nest to die in isolation, so they must somehow know they are ill, he says.

“I think this is good evidence of self-medication,” says Jessica Abbott of Lund University in Sweden. “They showed that the ants deliberately ingest hydrogen peroxide when infected, and that doing so increases the survival of the ant and decreases the fitness of the parasite.”

Aphid medicine

Hydrogen peroxide is a strong source of reactive oxygen species (ROS), chemicals which can harm the body but also help defend against invading pathogens.

Bos and his team think that in the wild, these ants might choose to ingest ROS for medicinal purposes by eating aphids or decaying dead ants, both of which can contain these chemicals. Although the drop in mortality in medicating ants may seem small, in a busy nest it could make all the difference.

“In ant colonies there are hundreds to thousands of individuals and even a small increase in the proportion of surviving individuals really can make the difference for the entire colony,” says David Baracchi of Queen Mary University of London. He says that social insects face intense pressure from pathogens because of a combination of the insects’ high densities and low genetic diversity, and the constantly high humidity and temperatures in their nests.

“It is natural that they have evolved amazing mechanisms to counteract microorganisms, and self-medication is one of those,” says Baracchi. Earlier this year, he and his colleagues showed that bumblebees may do something similar, possibly using the nicotine present in the nectar of some flowers to fight parasites.

Baracchi believes that self-medication is likely to be widespread throughout animal kingdom. “We just have to keep looking for this behaviour.”

Journal reference: Evolution, DOI: 10.1111/evo.12752