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Species: Cardiocondyla genus ants
Habitat: Mainly open arid habitats around the world

Paint a target on his back. Instead of dispatching their young competitors directly, adult male ants smear them with bodily fluids, leaving the youths with a bulls-eye marking them for assassination by worker ants.

“They let the workers do the dirty job of finishing off all the rivals,” says at the University of Regensburg in Germany.

Insects that live in colonies – such as ants, bees and wasps – generally operate as a superorganism. The entire group benefits when each member supports and safeguards their collective society.

“To some extent, they all have the same interest,” says at the University of Vermont in Burlington. “But not completely.” That’s because some resources are in short supply.

Unlike most ants, which seek out mates in swarms of eligible insects from many colonies, ants in the genus Cardiocondyla breed within their nests. By staying home, those males pit themselves against one another in order to reproduce with their nest’s queen or queens.

Formidable mandibles

Researchers already knew that males of some species of Cardiocondyla use their formidable mandibles to pierce or crush the soft bodies of young adversaries. And some daub other adults with gut secretions that lure worker ants into killing the marked individual.

But scientists hadn’t previously seen adult males targeting young ants – recently emerged from a pupa – with their kiss of death.

Heinze and his team collected 10 colonies of a single Cardiocondyla species from Queensland, Australia, ranging in size from about 10 to 80 individuals, then brought them back to the lab for observation. Each group contained up to several dozen ants, but at most only a single adult male, presumably because other males were killed as soon as they emerged. After finding the dismembered corpses of 11 young males in the nests, the researchers scanned colonies by eye and video camera, hoping to catch the culprits in the act.

The scientists witnessed four assaults. Adult ants clutched victims in their mandibles, then dabbed the juvenile with a combination of faeces and other substances from the gut. In three of the cases, this attracted worker ants, which bit the besmeared individual and pulled it apart. The fourth youngster was ignored by the workers, allowing him to mount a counter-attack against the older male: he painted a target on him, prompting workers to kill the elder ant.

“We know that in other species, older males sometimes fail to kill rivals and then they will be overthrown,” says Heinze. “So obviously age is important here.”

Chemical warfare

It’s not unusual for ants to perpetrate the kind of chemical warfare seen in Cardiocondyla, but in most cases it’s females competing against one another, says Helms Cahan, since males typically don’t have a reason to fight with their nest-mates.

“This is a really unusual male-centric version of what lots of different kinds of social insects do to manipulate others and destroy their competition,” she says.

The researchers aren’t sure why the male ants don’t just slay their adolescent adversaries themselves – as other Cardiocondyla species do – but Heinze speculates that it may be because this species has shorter mandibles or weaker muscles.

The study is based on only a few observations from a small number of colonies, says Helms Cahan. That makes it difficult to know how rare this kind of lethal “kick me” sign is within the species. Heinze and his team hope to remedy this problem by testing more populations in Queensland and Papua New Guinea.

Journal reference: Entomological Science, DOI: 

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