Unwelcome Guest Ants Protect Colony From Future Invaders
September 10, 2013

Unwelcome Guest Ants Protect Colony From Future Invaders

April Flowers for redOrbit.com - Your Universe Online

Because their loyalty can be bought, mercenary soldiers are notoriously unreliable and may turn against their employers before moving on to the next dirty job. A new report from the Centre for Social Evolution at the Department of Biology, University of Copenhagen, shows that fungus-farming ants are different, however. The study, published in Proceedings of the National Academy of Sciences (PNAS), reports that permanent parasites that are normally a chronic social burden protect their hosts against a greater evil.

"Our experiments show that the scouts can detect whether or not a host colony has a cohabiting guest ant colony before deciding to initiate a raid so the guest ants serve as an effective front line defense," said Rachelle Adams, SI Molecular Evolution Postdoctoral Fellow at the University of Copenhagen.

Ants do not usually contract infectious diseases, however, their societies are often invaded by social parasites; insects that exploit the resources of ant colonies for their own benefit. Some of these social parasites escape detection by using the social immune system of the hive against itself by producing bar-code like chemical recognition labels similar to the host's own. Others use obnoxious chemicals, or brute force, to infiltrate or usurp host colonies. Megalomyrmex, a particularly devious ant genus, produces alkaloid-based venoms to repel and poison their hosts and adversaries.

The research team discovered a surprising story of ant warfare between three parties, reminiscent of dramas in human history and literature: The victims are peaceful fungus-farming ants that by a remarkable strike of evolutionary misfortune have two other ants as natural enemies.

One of those enemies is an agile raider whose scouts are always on the lookout for new farmer-colonies and recruit their nestmate warriors for swift strikes. These ants kill or chase away the defenders, then pillage and plunder the brood and crop of the farmer ants. The invaders move on after a few days in search of a new colony to usurp - not unlike the hordes of Ghengis Khan that laid waste to Asian and European settlements in the middle ages.

The second natural enemy of the farmer ants are, paradoxically, their powerful protectors as well. Megalomyrmex is a highly specialized ant genus that uses its alkaloid poison to permanently move in with a farming host colony to exploit its fungus farm at relative leisure. This permanent invasion is a lifelong burden for the farmers, but they do survive and realize some reproductive success. When mobile raiders threaten the host colony, however, having the guest ant lodgers is an asset as the guest ants rise to the defense of their hosts.

The researchers used laboratory experiments to show that the guest ant defenses are so effective that they not only kill raiders, but their mere presence greatly decreases the probability of a raid. The results of those experiments revealed that the scouts can detect whether or not a host colony has a cohabiting guest ant colony before deciding to initiate a raid. This means the guest ants serve as an effective front line defense, explains Dr. Adams.

In laboratory settings, when a Megalomyrmex worker discovered an invading raider, the worker returned to the colony to excite the other workers. One by one, they came out and overpowered the invaders, preventing a raid.

The findings help to explain why guest ant parasitism is common in the Panamanian sites where the colonies were collected, which is a very unusual situation as socially parasitic ants are normally very rare.

The researchers said that their findings illustrate how sophisticated and subtle co-evolutionary processes driven by natural selection can be. In the world of ants, not only is the idea that "the enemy of my enemy is my friend" valid, but that that natural selection can maintain lesser evils when that helps prevent greater harm, similar to the well-known example of sickle-cell anemia being maintained in areas where potentially deadly malaria is endemic, but not elsewhere.

Dr. David Nash, biologist at the Center, said that this kind of interaction, where being a foe or friend depends on a the presence of a third party, are probably far more common than we realize, and may be fundamental for the co-evolution of interacting species.