By Matt Walker
Editor, Earth News
One for all...
Ants live altruistically, but some die altruistically too.
When ants of the species Temnothorax unifasciatus get sick, they abandon their nest, walking far away from their relatives to die alone.
They perform this act of heroism to prevent the illness that is killing them from spreading to the colony.
The discovery, published in Current Biology, is the first time that such behaviour has been shown in ants or any other social insect.
Professor Jurgen Heinze and PhD student Bartosz Walter decided to investigate the ants' behaviour after becoming interested in anecdotal reports of dying individuals deserting their families.
Such behaviour has been reported in dogs, cats, elephants and even people.
But because it happens occasionally, it cannot be quantifiably studied.
So Prof Heinze decided to set up an experiment to study the phenomenon in ants, which he also noticed would occasionally leave the colony for no apparent reason.
The researchers exposed a colony of Temnothorax unifasciatus ants reared in their laboratory to the spores of a lethal parasitic fungus called Metarhizium anisopliae.
Most of the workers who died from the fungal infection permanently left the nest hours or days before death, and died in a foraging area far from their nest mates.
"Our study suggests that infected ants at least in some species walk away from a colony and die alone, rather than risk infecting others," Prof Heinze told the BBC.
Crucially, the researchers were able to rule out the possibility that the fungus itself caused the diseased ants to walk away.
Many parasites manipulate their hosts in order to increase their own transmission.
Flu viruses make people cough, while in ants, one cordyceps fungus effectively turns its victims into zombies, impelling ants to climb up a stem where they die.
From this elevated vantage point the fungus's fruiting body grows out of the victim's body, allowing the spores to be more widely distributed by the wind.
But by exposing the ants to CO2, the researchers artificially reduced their lifespan.
Uninfected ants who survived this treatment, but still knew they would die prematurely, also left the nest before death took hold, proving the fungus itself did not drive them out.
"This simple experiment demonstrates that nest leaving is not caused by parasite manipulation but by the imminence of death," says biologist Michel Chapuisat of the University of Lausanne, Switzerland, who studies the evolution of insect societies.
"Another interesting finding was that the workers left the nest voluntarily and were not carried away by other workers," he writes in the same issue of the journal.
By choosing to face death alone, the ants were making a truly altruistic act.
That is important because the exact opposite has been found in the bumblebee, another social insect.
Bees infected by fly larvae move out of the hive into colder air.
But in doing so, the cold temperatures slow the lifecycle of the parasite.
So the infected bees are actually trying to extend their own lives, rather than save their nest mates.
The heroic act by the terminally ill ants is the latest in a line of extraordinary behaviours discovered among social insects.
In order to help protect close relatives, termites have been found to explode during fights, bees die after stinging, while the members of another species of ant have been found to condemn themselves to death by sealing in a nesting colony from the outside.