Queen Ants Use Hygienic Cannibalism to Protect Their Colonies

Ant mothers are masters of tough love: they have no opportunity to indulge their unwell young.

US researchers have discovered that when a newly hatched black garden ant larva encounters a pathogen, its queen will suck up and swallow the entire grub-like body before it can become infectious.

Hygienic cannibalism in black garden ant nests

This form of 'hygienic cannibalism' prevents the queen and other larvae from becoming ill while they are enclosed together in a nest. It also leaves the mother with additional energy to lay eggs in her next breeding attempt.

"Ant queens start their colonies alone and essentially starve themselves to raise their first workers," explains biologist Flynn Bizzell from the University of Oxford.

"The queens who produce the most workers have the best chances of survival, so being able to eat and recycle infected larvae back into brood production means valuable resources are not wasted."

Bizzell and fellow University of Oxford biologist Christopher Pull found that worker ants in established colonies do not display the same inclination towards cannibalism.

In certain ant species, workers and soldiers have even been observed giving medicine to sick or injured nestmates, or carrying out life-saving procedures. Yet such interventions are not viable in the nursery of a black garden ant (Lasius niger), particularly during a colony's earliest days.

At this susceptible stage, queens cannot simply remove poorly larvae from the nest. They are shut inside with their young, in a crowded space, and have nobody else to assist them.

How queen ants stop infection spreading

The danger of infection spreading in these conditions is too great for a queen to overlook. Once she identifies a pathogen in the nest, she eliminates every infected larva she can locate as quickly as she can.

The researchers found that, where necessary, a queen will cannibalise 92 percent of infected larvae. Queens that do this are able to lay 55 percent more eggs in their next attempt.

For their recent experiments, Bizzell and Pull exposed five larvae for each queen to infectious spores from a fungal pathogen.

The larvae were kept apart for 24 hours, allowing the fatal infection to develop, though they were not contagious at that stage. After the infected larvae were placed back in the nest, Bizzell and Pull observed the queens' response.

For hours, the mothers chewed their young. They ate nearly all the ill larvae but spared most of the healthy individuals.

The fungal pathogen appeared not to affect the queens, even after they had consumed it. Bizzell and Pull believe the ants may somehow be defending themselves internally.

Before and after queens were seen eating their offspring, the biologists noticed that some ants groomed a gland on their abdomens. The gland produces an acidic antimicrobial venom; by ingesting this venom, the ants may be able to neutralise the pathogen in their guts.

The danger of late-stage fungal infection

In additional trials, queens were given sick larvae that had already died and started growing fruiting bodies containing infectious spores. The queens groomed the corpses and sprayed them with venom from the gland.

This phase of the infection is evidently highly hazardous for the queen, however. Almost 80 percent of queens died from the infectious disease even after spraying the larvae.

When a queen survived, her brood did not: all were lost to secondary infections. Taking early action to stop the disease spreading is therefore crucial.

The results indicate that queen ants consume their young as an adaptive response to illness, safeguarding their own survival and that of both their present and future offspring.

There is nothing quite like a mother's care.

The study was published in Current Biology.