Ant queens have good reasons for eating their own babies

When black garden ant queens notice their young are sick, they eat them before the illness spreads to the rest of the nest.

A cannibal queen may not win any “mother of the year” awards, but the strategy could be an effective way to protect her kingdom, research suggests. The findings provide insights into the evolution of “filial cannibalism”, the practice of parents consuming their offspring.

Ants and other colony-dwelling social insects can thwart the spread of diseases by having workers self-isolate when sick or by removing infected nestmates. These “social immunity” duties are well known, write Flynn Bizzell and Christopher Pull at the University of Oxford. But ant queens start their colonies alone, so how do they defend against disease as they establish and grow a nest?

To find out, Bizzell and Pull collected newly mated black garden ant (Lasius niger) queens and brought them into the lab. Once the ants started laying eggs and establishing fledgling colonies, the researchers took the larvae away from the queens and exposed some of them to spores of the lethal Metarhizium fungus, which infects wild ant nests. After those larvae had time to develop infections that would become fatal, but were not yet contagious, the team returned all the larvae to their mother.

The queens ate 92 per cent of their sick young, but only 6 per cent of the uninfected larvae, showing they could detect the infection and intervene. Failing to catch the infection could have disastrous consequences. When the team exposed colonies to very infectious larval cadavers sprouting with spore-producing fungi, all the broods died. And only 20 percent of the queens survived, even after they sprayed the corpses with acidic, antimicrobial venom.

Despite these risks, the queens that eat their infected larvae seem to avoid harm. The queens may be swallowing their own antimicrobial venom to make their guts hostile to fungal spores, the researchers suggest. They base this conclusion on previous observations of worker ants swallowing venom and the team’s observations of queens grooming their venom gland openings.

“If the queen gets infected and dies, the colony dies,” says Sebastian Stockmaier at the University of Tennessee, Knoxville, since she is the only reproductive individual. So, it makes sense that an evolved strategy for dealing with disease would emphasise the survival of the queen.

Eating the sick babies yields other benefits too. The researchers found queens that ate their sick young went on to lay 55 per cent more eggs than those that didn’t, suggesting they had recycled those caloric resources. This advantage, plus the removal of disease risk, might illustrate a way filial cannibalism could evolve in some species, the researchers argue.

Joël Meunier at the University of Tours in France wonders if offspring hatched after their older siblings are eaten have immune systems that better protect against the fungal infection. If so, proving this could reveal “dual benefits” of filial cannibalism, for both mother and offspring.

The findings suggest the behaviours necessary for caring for young and for disease protection in fledgling colonies overlap. As a result, Bizzell and Pull argue that worker ants’ disease-preventing behaviour could have evolved from the kind of generalised parental care seen in many types of insects.