Blatant thievery among colonies of tiny tropical ants may allow them to prosper in part because the species is more peaceful than the vast majority of its closely related relatives, according to a University of Colorado at Boulder study.
The odd practice of food theft among neighboring colonies of the ant species Ectatomma ruidum is fostered by "chemical camouflage" used by larcenous ants to enter nearby nests, said CU-Boulder biology Professor Michael Breed. Success also depends on the vigilance of guard ants posted at nest entrances to eject intruders.
Chemicals are the primary comunication tools of ants and bees, Breed said. Each member of an ant or bee colony appears to carry a distinct aroma from a colony-specific hydrocarbon compound on their bodies -- an evolutionary strategy that alerts colony members to interlopers trying to gain entry to their nest.
Unlike most ant and bee guards, which kill would-be intruders, E. ruidum guards simply wrestle intruders away from the nest entrance and set them down. But the skirmishes -- similar to bar bouncers ejecting unruly patrons -- ironically transfer the correct chemical password for nest entrance from the bodies of the guards to those of the interlopers, ensuring successful nest entry by intruders on their next attempt.
"If the guard ants killed the intruders, this system of larceny would come to a crashing halt," Breed said. "It may be that this species is physically incapable of killing individual members, or it may just be a kinder, gentler ant species."
E. ruidum ants generally live in small, shallow nests in the ground, each hosting from up to 200 ants and a queen ant. But Breed is doubtful that the thieving ants -- found from Central America to the Amazon -- are aware of their deceptive behavior. "I don't think we can give these ants credit for being cognitive," he said.
In a field study by Breed and three CU-Boulder students in fall 1998, a number of different E. ruidum colonies were observed on Barro Colorado Island in Panama.
The researchers used tiny pieces of cheese placed at the entrances of individual nests, initiating a string of thefts by ants that rippled from colony to colony.
Foraging ants generally took cheese pieces into their home nests within minutes, said Breed. Since thieving E. ruidum ants from different nests were usually lurking in the home nest, a tresspasser almost invariably took the cheese particle out of the home nest and into its own nest within minutes.
Interestingly, the same cheese bit was often observed being taken from the second nest by yet another larcenous ant, which took it to a third nest. At times, a single bit of cheese passed through a half-dozen nests in a relatively short period.
"There was a lot of cheese moving around these colonies," said Breed. "On the surface, it looks like counter-intuitive behavior with no simple explanation."
A paper on the 1998 work in Panama authored by Breed, CU graduate students Terrence McGlynn and Erin Stocker and 1998 CU graduate Anne Klein will appear in an upcoming issue of the international entomological journal, "Insectes Sociuax."
A 1992 study by Breed and several CU students in Costa Rica first confirmed the use of chemical camouflage by E. ruidum to enter neighboring ant colonies. The effectiveness of the chemical cues and their use as a camouflage was later replicated in subsequent laboratory experiments.
To test nestmate recognition by E. ruidum guard species in the 1998 Panama study, individual foraging ants from several colonies were captured, then chilled into a state of immobility. Individual chilled ants then were placed either on their own nest or on another nest entrance, said Breed.
In the majority of cases, the immobilized ants placed on the nests of foreign colonies were dragged away from the nest entrance. However, at one site furthest from the home nest, the guard ants rejected only eight of the 21 interlopers.
Breed speculated that vigilance by guard species had fallen off either because food was not a limiting factor or because other threats to the colony were of greater importance.
"If research is to be educational, students need to be involved in the whole process, from the hypothesis and design to the actual experiments and results," said Breed, who regularly involves graduate students as well as undergraduates from the Undergraduate Research Opportunities Program in his work.
"And if they have substantial input, they should get credit for the work when it is published. Teaching and research can work together rather than being at odds."