Zoologger: Spinster queens become commoners

Zoologger is our weekly column highlighting extraordinary animals ? and occasionally other organisms ? from around the world

Species: Acromyrmex echinatior
Habitat: Casually swapping lifestyles in wild areas from Mexico to Panama

There are few things more galling than finding oneself in reduced circumstances. If you grow up wealthy and privileged, it's awful to suddenly be on the breadline, your only source of amusement being a constant stream of cheap jokes about racial stereotypes. Someone even made a TV show about it.

If you're in that situation, console yourself with the knowledge that it could be worse ? your fellows might have written you off as a waste of space and decided to kill you. That's what usually happens to ant queens that don't manage to leave their home colony.

Callous as this behaviour might seem, it makes evolutionary sense. Ant queens are only good for one thing ? making more ants ? so if they're not doing it the colony has no use for them. But the failed queens of one species have found a way to live out their lives: as humble workers.

Ant farms

Acromyrmex echinatior ants live in one of the most complex societies on Earth. They are leaf-cutter ants: they collect huge volumes of leaves from the area around their nest, and cultivate a special fungus on them. The ants eat the fungus and nothing else. Leaf-cutter colonies often have several different worker castes: the larger ones collect the leaves and the smaller ones tend the fungal gardens.

Like honeybees, some termites and the famous naked mole rats, leaf-cutters are eusocial. In each colony only one female, the queen, reproduces. All the workers are sterile and work to preserve the colony. At first glance it seems like the workers have a raw deal, but because they are all closely related they are actually preserving their own genes.

Once a colony is established, it starts producing new queens. Their destiny is to leave the colony, find a male and mate with him, then found a new colony.

In most eusocial species, being a queen is a make-or-break existence. You either leave to start a new colony, or you die. The queen's body contains valuable nutrients: if they're not being used, she will put her genes to best use by giving the nutrients back to the colony.

Working girls

But not in colonies of A. echinatior, or the closely related A. octospinosus. Watching colonies in the wild, Volker Nehring of the University of Freiburg in Germany noticed several unmated queens living seemingly untroubled alongside the main queen.

Wondering what the virgin queens were getting up to, Nehring captured some newly-hatched queens and removed their wings, preventing them from leaving on their mating flights. The grounded queens began behaving like workers, taking care of larvae and attacking ants from other colonies. A. echinatior colonies don't have soldiers, and queens are bigger than workers so they are better than them at seeing off intruders.

The queens of other eusocial ants have lost the ability to behave like workers, so they can't adapt under such straitened circumstances. However, A. echinatior queens have retained their worker skills because they take an active role in setting up their own colonies.

While the virgin queens may only stay on sufferance, Nehring thinks they are quite safe from the other workers. Because leaf-cutter ants only eat fungus, they may have lost the ability to digest other foods. As a result, the workers probably couldn't eat the virgin queens even if they tried.

Journal reference: Current Biology, doi.org/jcc

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