Colonies of Argentine ants spreading around the world might be real-life invasions of the living dead.
Normal, still-breathing adult workers carry chemicals signaling “Dead ant — haul to burial pile” on their outer covering, proposes Dong-Hwan Choe of the University of California, Riverside.
What prevents awkward mistakes about who’s really dead are two additional compounds also found on the covering of living ants, Choe suggests. These compounds temporarily inhibit responses to the death cues by signaling, “Wait — still alive so far,” Choe and his colleagues report online May 4 in Proceedings of the National Academy of Sciences.
Tests of ant reactions to chemicals applied to ant pupae provide the first experimental evidence for this “not-dead-yet” signaling system, Choe says.
Undertaker cues in ants have fascinated biologists studying the evolution of social living. “Being able to quickly remove dead individuals and other possible sources of pathogens and contaminants is extremely important for all animals living in societies, including us,” says Patrizia d’Ettorre of the University of Copenhagen. “Think about all the effort and money that we invest daily in waste management.”
Understanding life or death cues might also help in devising new ways to control Argentine ants, Choe says. The small ants have been moving into new territories worldwide and disrupting native ecosystems. He’s investigating ways to spread ant-killer from the dead ants to the colony mates that haul them away.
Earlier experiments had searched for a death cue that appeared on a corpse after death. Anesthetized ants typically get left alone until they come around, so stillness alone doesn’t trigger the undertakers. Yet a worker that really does die inside the nest gets hauled outside to a pile of mixed rubbish. But researchers haven’t yet agreed on what chemicals are involved and how they work.
Choe and his colleagues dropped fresh and not-so-fresh ant corpses into a lab colony and found that ants that had been dead for only an hour triggered about the same strong urge to remove them as ants that had been dead for 24 hours. Such a speedy response seems awfully fast for decomposition chemistry to trigger disposal, Choe says. So he and his colleagues experimented with the notion that, instead, some “still alive” cue fades fast.
To test different chemical signals, the researchers took advantage of the ants’ natural tendency to retrieve young ants at the pupal stage that somehow end up outside the nest. Pupae don’t have the same surface chemistry as adults, so researchers doused them with various signaling brews.
When treated with chemical extracts of ants that had been dead for an hour, 18 of 20 pupae were hauled to the refuse pile as if they were dead adults, suggesting that chemicals did play a role in the removal.
The team also treated the pupae with two compounds found on living ants and freshly dead ants but not found on ants dead an hour or more. These two compounds, dolichodial and iridomyrmecin, naturally dissipate quickly, the team found.
When doused with these compounds, the pupae were ignored. They were neither disposed of nor promptly brought back to the nest, suggesting other adults treated them as if they were indeed living adult workers, Choe and his colleagues say. Eventually, the workers did retrieve the pupae, as would be expected when an override signal — the dolichodial and iridomyrmecin — faded away, the researchers report.
Walter Tschinkel of Florida State University in Tallahassee says the idea is plausible but hasn’t been proven. “They did not put the last nail in that coffin,” Tschinkel says. Using pupae didn’t seem an appropriate test to him. He says he’d like to see what happens to dead adult ants treated with those compounds.
