Ants kill the sick to save the rest

Ants practice infanticide to protect the health of the colony.

The invasive garden ant (Lasius neglectus) takes the drastic measure of killing sick colony members, even pupae, when they are infected with a pathogenic fungus. Invasive garden ants are so named because they are originally from Turkey, Iran and Uzbekistan and now inhabit urban areas throughout Europe. 

A new study in the journal eLife, led by researchers Christopher Pull and Sylvia Cremer at the Institute of Science and Technology Austria, discovered that the ants can detect and eliminate infections developing inside the bodies of their nest mates even before they have shown external disease symptoms. 

“We found that the ants are able to smell and single out sick colony members very early on in the infection process,” explains Cremer. 

“They then perform what we have termed ‘destructive disinfection’, the killing of the sick animal and the fungus, to prevent the pathogen becoming contagious and spreading to nest mates.” 

Cremer and Pull have previously studied the behaviours that invasive garden ants perform to prevent infection in contaminated colony members, referred to as sanitary care. They found that the ants look after colony members carrying the pathogenic fungus Metarhizium brunneum by intensively grooming them and thus removing spores that threaten the health of the colony. 

The recent discovery of destructive disinfection takes the colony’s sanitary protocols a step further –  colony members are cared for when possible, but sacrificed if necessary. 

“The ants produce formic acid that can kill the fungus, but it needs to enter the pupa’s body for it to work,” says Pull. 

“During destructive disinfection, the ants therefore remove pupa’s silk cocoon and bite holes in its body. They then spray their formic acid through these holes, so that it enters the pupa’s body and kills the pupa along with the fungus.”

There are other examples of ants engaging in public health measures in order to save a colony from disease. 

In carpenter ants (Camponotus aethiops), individuals who are infected by the Metarhizium fungus change their behaviour radically. In 2011, a study in the Journal of Evolutionary Biology showed that infected ants isolate themselves from other colony members, act aggressively towards any other ants they encounter, and spend from day three post-infection outside of the nest until death. 

In another study reported in BMC Evolutionary Biology, ant queens of the species Lasius niger will carefully dismember and then bury the corpses of their sister queens who have died from fungal infection. Carrying out this hygienic action reduced the risk of the surviving queen falling ill seven-fold. 

Cremer and Pull believe their recent discoveries on invasive garden ants’ social immune defence responses demonstrate striking parallels between ant colonies and the immune systems of vertebrates. 

Like the immune system of a body that specifically targets and eliminates infected cells, ants destroy the infected pupae to stop the pathogen completing its lifecycle, thus protecting the rest of the colony. 

Hence, in an analogous fashion, the same principles of disease defence apply at different levels of biological organisation. “The ability to detect and destroy harmful elements was likely necessary for the evolution of both multicellular organisms from single celled life and superorganisms from individual animals,” Cremer says.