Queen ants reproduce, soldiers defend, and workers forage. Now, researchers say they’ve discovered a whole new caste of foragers within ant colonies in Costa Rica: thieves. These ants specialize in removing food items from neighboring colonies and bringing them back to their own nest, according to new findings published in Animal Behaviour. And they act suspiciously too.
Ant colonies have such effective mechanisms for preventing robberies that thievery within the same species has only been documented in two species. One of these is Ectatomma ruidum, a ground-nesting tropical ant in Central America with colonies of up to a few hundred workers. Their diet consists of nectar and other arthropods – which can be gathered by tracking prey and nutrients within the home range or by entering other nests, waiting for, and then removing newly arrived food items carried in by foragers. Each colony receives food from less than 10 of these robber ants.
Besides their obvious specializations in acquiring purloined goods, are thieving foragers who retrieve resources from nearby nests different from normal, non-thieving foragers who collect resources from the surrounding leaf litter? Anecdotal evidence suggests that burglar ants are also sneaky: They’re fast, they hide, and they drop food and switch course to make themselves harder to follow. After all, if thieves function as a distinct type of forager, they should display a separate set of behaviors to avoid being caught in the act.
So, a team led by Terrence McGlynn from Cal State Dominguez Hills studied dozens of E. ruidum colonies from June to August of 2014 at La Selva Biological Station on the Caribbean slope of Costa Rica. While thieves are morphologically indistinguishable from non-thieving foragers, the team did observe multiple instances of robber ants behaving differently, especially after pulling off a successful heist.
When returning to their nests with stolen items, thieves had fewer encounters with other ants, and they were more likely to pause during movement or release food when grasped. They also walked faster through the burgled victim's home range than they did in their own home range. And when perturbed, thieves were more likely to reverse the direction they were moving in. These behaviors seemed to reduce the likelihood of being detected, and isotope analyses suggest that both thieves and non-thieves were accessing the same food items, just with different behaviors.
“When the thieves are caught – which happens once in a while – they get removed by ants who act like bouncers,” McGlynn told New Scientist. “They just pick up the thieves, who protest not a bit, and carry them off about a meter or so and let them go. The next day, they’re thieving like normal.”