Mercenary ants defend agricultural society

From Stri.org

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Often superior to citizen soldiers, mercenaries have played an important role in human conflicts since ancient times. A research team working at STRI discovered that a species of agriculturalist ants, Sericomyrmex amabilis, hosts a species of better-armed mercenary ants, Megalomyrmex symmetochus, who come to their rescue when their fungal gardens are invaded.

“Newly mated queens of the parasitic mercenary ants stealthily enter and establish their colonies in the gardens of the fungus-growing host ants,” said Rachelle Adams from Jacobus Boomsma’s lab at the University of Copenhagen. Adams is lead-author of the report published last week in PNAS.

With co-authors from Copenhagen and from the Department of Chemistry at the Virginia Military Institute, she found that the parasitic mercenary ants use their potent chemicals called alkaloids to defend host colonies against the raiding predatory ants, Gnamptogenys hartmani. The raiders can take over Sericomyrmex fungal gardens and nests.

During an attack, the mercenaries proved to be much more efficient than the host ants at killing the raiding predators. Even a moderate number of parasitic guest ants can provide protection against predatory attacks, effectively reducing host ant mortality.

However, the host ants pay a high price for the help. The mercenaries hamper host colony growth by feeding on the brood–the eggs and larvae–and by clipping the wings of host virgin queens, possibly to retain them as an additional work-force rather than let them disperse.

In addition, the authors show that raider ant scouts prefer to recruit to the colonies of the fungus-farming ants whose odor indicated that no mercenary ants were inside.

The inspiration for this project was a direct outcome of the University of Copenhagen and STRI supported graduate course, Tropical Behavioral Ecology and Evolution, offered in 2011, 2013 and planned for 2015. Two Copenhagen students from the 2011 course are junior authors on the study.

Adams, R.M.M., Liberti, J., Illum, A.A., Jones, T.H., Nash, D.R. and Boomsma, J.J. 2013. Chemically armed mercenary ants protect fungus-farming societies PNAS http://www.pnas.org/cgi/doi/10.1073/pnas.1311654110

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