Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: Ectatomma ruidum

Habitat: The forests of north and central Costa Rica

You might have heard of soldier ants, worker ants and queens. But robber ants?


“It’s not an average day when you discover that a whole new caste of ants exists,” says Terrence McGlynn from California State University in Carson, but work in Costa Rica suggests that some ants devote their lives to stealing food and resources from neighbouring nests. “We are proposing that this is a new caste.”

We’ve known for about 20 years that some of the ants in two species – Ectatomma ruidum and Messor aciculatus – behave as robbers. What McGlynn and his colleagues have discovered is that E. ruidum thieves devote themselves to the job, forming a distinct behavioural caste. “Ballpark, about 5 per cent of a colony are thieves, at least in our site in Costa Rica,” he says.

McGlynn and his team followed the activity of 47 individuals they identified as thieves to see how they behaved. They found that the thieving ants always displayed a distinctly sneaky behaviour, taking food morsels from one nest and bringing them back to their own.

The thieving ants are not merely foraging ants that get their food from other colonies, says McGlynn. “When you watch thieves, they act like they’re working to avoid detection,” he says. “When disturbed, they’re more likely to freeze, and then move in the opposite direction. If you grab them, they drop their stolen food. Once they get in the clear, they scamper more quickly, while regular non-thieving foragers will hang onto their food, as they know it was rightfully acquired.”

Their behaviour is clearly different from your typical worker ant, says co-author Hope Jahren at the University of Hawaii, Honolulu. “They know they are doing something wrong. Guilty little ants,” she says.

Sometimes the thieves get caught, but the consequences are not severe. “When the thieves are caught – which happens once in a while – they get removed by ants who act like bouncers. They just pick up the thieves, who protest not a bit, and carry them off about a metre or so and let them go. The next day, they’re thieving like normal,” says McGlynn.

Smelly signature

It is not just behaviour that makes these thieving ants unique. Previous work has shown that E. ruidum ants identify outsiders using the odours that they carry on their exoskeleton. “Each colony has a unique odour, says McGlynn. “Thieves, however, have an odour signature that is halfway between their own colony and the one that they’re stealing from. This, apparently, is close enough for them to sneak into other colonies, but still be accepted by their own nest mates.”

But why hasn’t this thieving behaviour evolved in other ant species? “It might be that in this species the ability to tell nest mates from non-nest mates is degraded, allowing thieving to be more common,” says Tomer Czaczkes at the University of Regensburg in Germany.

“Of course, another ever-present possibility is that this sort of behaviour is, in fact, common, but has so far been missed. Most species of ants in the tropics have never been studied at all.”

McGlynn and his colleagues plan to learn more about E. ruidum thieves before they examine other species, though. “Now that we’ve described how this is a distinct caste, we can begin to explore the genomic and developmental pathways that make thieves steal and be sneaky,” he says.

Journal Reference: Animal Behaviour, DOI: 10.1016/j.anbehav.2015.08.024

Image credit: Benoit Guenard