6 November 2009

Orphaned army ant workers help their neighbours

Pictures © Daniel Kronauer

Workers carry ant brood in a colony
emigration. Pictures © Daniel Kronauer

Tropical army ants are renowned for their large societies of several million individuals, their massive swarm raids that overwhelm much larger animals than themselves, and their frequent colony emigrations to new hunting grounds. The huge army ant queen is of central importance to the colony, because she alone produces the eggs that ensure reproduction and growth. But after she suddenly dies colonies tend to disappear quickly. This has puzzled naturalists for a long time, but it now turns out that their demise is due to all these orphaned workers joining a neighbouring colony.

In an article published in the Proceedings of the Royal Society B this week1, researchers from the Centre for Social Evolution at the University of Copenhagen and the Museum of Comparative Zoology at Harvard University show that orphaned army ant workers of the African species Dorylus molestus, instead of being doomed to die shortly after their mother, often join and work for foreign colonies in the vicinity. Also the distinct colony odours of a resident and joining colony become homogeneous in a matter of days, indicating that the adopted workers become fully integrated and can no longer be recognized as aliens.

That residents are eager to accept foreign work force is easy to understand, but the seemingly altruistic behaviour of the joining workers towards strangers is puzzling, unless there is also something to be gained for them. The new study shows that this is very likely the case, so that these joiners make the best of a bad job in terms of passing their genes on to the next generation. The key to the solution is that where most other orphaned ants can produce a brood of sons after their mother-queen's death, the efficiency of this final process is vanishingly small in army ants. At the same time, any army ant is likely to be somewhat related with any of its neighbouring colonies because new colonies bud-off from existing ones. The new study suggests that it is this unusual combination of factors that makes orphaned workers voluntarily join their next-door neighbours.