© National Academy of Sciences/PNASPupae that had been coated with the chemicals that signal life - dolichodial and iridomyrmecin - were ignored by workers
Life really stinks for Argentine ants. New research shows that while alive, the ants produce two odoriferous chemicals that prevent their compatriots from immediately carting their bodies away to the 'morgue'.

Within minutes of their death, however, the conspicuous absence of these chemicals prompts workers to remove the carcasses, explaining how the foraging ants are able to detect and dispose of their dead before infectious pathogens and pungent chemicals fill the corpse.

The work overturns a long-held idea - first suggested by ant expert E. O. Wilson - that it is the buildup of fats after death that encourages workers to collect the dead.

Dong-Hwan Choe, an entomologist at the University of California, Riverside, says the accrual of fats can't explain why some ants and bees dispose of their dead well before the chemicals reach a high level.

Sign of life

In laboratory experiments, Choe and colleagues found that workers do indeed pick up and dispose of ants more quickly when the ants have been dead for at least an hour.

© Dong-Hwan ChoeAn Argentine ant worker carries a dead nestmate
They believe that live ants constantly produce chemicals that encourage their bodies to be discarded and also chemicals that override that signal. When they die, the "override" chemicals disappear, causing workers to cart away the ants' carcasses.

It's not yet clear which chemicals encourage body disposal, but evidence for this comes from the fact that when Choe's team coated pupae with fats extracted from live ants, workers carried them to the refuse pile within 10 minutes.

But his team did identify two molecules, dolichodial and iridomyrmecin, that seem to override the disposal signal, preventing ants from carting one another away. The molecules are present on the cuticles of live Argentine ants, but not on insects that have been dead for more than 40 minutes.

When the researchers painted the chemicals onto pupae - which don't produce them - worker ants ignored the pupae.

Widespread myth

The emission of chemicals that send out a signal saying "I'm alive" speeds the removal of corpses and the pathogens they could soon spread, says Choe. It also prevents ants that are tainted with the chemicals from their dead brethren from being buried alive.

Deborah Gordon, an entomologist at Stanford University, says the research will put paid to the notion that it is the accumulation of chemicals after death that signals workers to carry corpses away. "This is an exciting result that helps to dispel a widespread myth that persisted despite the lack of evidence," she told New Scientist.

She adds: "It seems to me very possible that there are similar chemical changes in other social insects and I hope this work inspires people to look."

Journal reference: PNAS