Large societies of ants and humans have standing armies with professional soldiers, but smaller societies usually rely on conscription when they are threatened. Leaf-cutting ant societies of intermediate size have evolved the peculiar practice of turning daughter queens that failed to mate and disperse into life time nest-defenders, reminiscent of mythical female warriors whose normal reproduction was impaired by their unusual military careers.
New research of the Centre for Social Evolution at the Department of Biology, University of Copenhagen shows that some societies of leaf-cutting ants produce daughter queens that can either leave the nest on wings to mate and found a new colony (as the large majority of ants do) or become virgin queen soldiers when their normal reproductive future fails. These societies are too small to maintain a specialized soldier caste, but since queens are considerably larger than workers they make good soldiers in the event of an attack.
Female soldiers are uncommon in human history, but normal in ant societies, in which only the females work and defend the nest. Mythological societies such as the Amazons living around the Black Sea and the Valkyries or shieldmaiden in ancient Germanic and Nordic sagas emphasize that women-soldiers start their careers as virgins, and Herodotus claims that no Amazon girl was allowed to marry before she had killed a man in battle. Also otherwise, it is hard to escape the notion than these women remained somewhat reproductively handicapped for the rest of their lives. Hippocrates writes that Amazons have no right breasts, so the muscles of their right arm and shoulder could develop more strongly. Other versions of the myth claim that male residence and normal sexual relationships were forbidden in Amazon territory, so that reproduction required very special measures.
In the ant-societies the scientists observed virgin daughter queens in soldier roles in the field (Panama), and they decided to experimentally force virgin daughter queens to stay home by removing their wings to see how this would change their behaviour.
Wingless daughter queens modified their behaviour in two different ways: they began to take care of the colony’s brood and were unusually motivated to fight with intruders, unlike normal virgin queens that try to get away and leave their worker sisters with the dangerous defence tasks. Hiding from danger makes perfect sense, because ants pass on most of their genes to future generations by only a few “shy” but fertile queens that are well protected by many sterile and aggressive workers.
The switch in daughter queen behaviour is remarkable, because they change from a queen-like to a worker-like behavioural program by the simple event of losing their wings. It is as if they know that wing-loss implies that they will stay virgin forever and never be able to reproduce as queen, so all they can do is offer a bit of help to their intact queen sisters who can still pass on the colony’s genes. In all other ants studied so far, failed queens are simply killed and cannibalized by their sisters to recycle the resources that are stored in their bodies. CSE researchers speculate that one of the reasons why such cannibalism does not happen in leaf-cutting ants is that these ants farm fungi for food in underground gardens so they have lost the ability to digest meat.
It normally only pays ant societies to maintain a specialized caste of soldiers when their societies are big, similar to large human nations tending to have more massive armies with better tanks and warships. Soldier ants can therefore be observed in warmer climates but not in North-west Europe, where ant nests only become moderately large, so the normal workers need to take care of defence tasks. It seems that the fungus-growing ants have discovered a third way to secure effective colony defence by making failed queens adopt defender roles for which they were never intended, but for which they are well suited due to their body size. It is interesting that developing an ant soldier career is directly connected to impaired reproduction, similar to what myths tell us about the few historical societies from which women were known to pursue military careers.
Nehring, V., Boomsma, J.J. & d’Ettorre, P. (2012): Wingless virgin queens assume helper roles in Acromyrmex leaf-cutting ants. Current Biology 22: 671-673 doi:10.1016/j.cub.2012.06.038.
Dr. Volker Nehring, Centre for Social Evolution, University of Copenhagen. Currently at University of Freiburg, Germany: Volker.Nehring@biologie.uni-freiburg.de, Phone +49 761 203 8346
Professor Jacobus J. Boomsma, Centre for Social Evolution, University of Copenhagen, Denmark: JJBoomsma@bio.ku.dk, Phone +45 353 213 40 or +45 204 367 71
Professor Patrizia d’Ettorre, Centre for Social Evolution, University of Copenhagen, Denmark & Laboratory of Experimental and Comparative Ethology, University of Paris 13 – Université Sorbonne Paris Cité, France: Patrizia.dEttorre@leec.univ-paris13.fr, Phone: +33 665 163 200