When ants stir up a rebellion

8 December 2012

It has long been known that certain ants keep other ants as slaves. However, Professor Dr. Susanne Foitzik of the Institute of Zoology at Johannes Gutenberg University Mainz (JGU) has discovered that these slaves have what it takes to rebel. And she can explain how this ability has developed during the course of evolution.
 

Ants, ants, everywhere ants. They appear to be omnipresent in the Evolutionary Biology division at Mainz University. There are watercolors of ants decorating the otherwise bare corridor: That one could be a bulldog ant, those two over there are perhaps leaf-cutting ants. Ants even adorn the signs on doors to the various offices. "Of course, I've got a slave-maker ant on my door," says Professor Dr. Susanne Foitzik with a smile.

The evolutionary biologist has been working with these insects for 14 years now. Since at least the beginning of the 19th century, it has been known that there are many kinds of parasitic slave-maker ants. However, it was in 2009 in the US state of New York that Foitzik was witness to something that had not previously been observed: She saw the slaves fighting back. They killed their oppressor's offspring and, as a result, limited the size of their nests.

Slave-makers in a nutshell

"The fundamental problem is understanding why something like this happens. The slaves have no offspring and they don't do it so that they can go back their old nest. In most cases, it will have already been destroyed anyway." It is not immediately apparent how they benefit from this behavior. It is an absurdity in terms of the rules of evolution.

Protomognathus americanus is only a few millimeters in length. The nests of this ant species are correspondingly small. They can even be found inside a hollow acorn or a nutshell. Until Foitzik reported her observations, experts assumed that such nests occur only very sporadically within ant communities. However, this was not consistent with the New York community: every tenth one belonged to the slave-maker ants. The other ants were thus subject to considerable evolutionary pressure and the risk of falling victim to the parasites was immense.

"The slave-maker ants carry out five to ten raids a year," Foitzik explains. The nests of the ant species Temnothorax longispinosus are their targets. The slave-makers kill the adults and take the eggs back to their nest. Once they hatch, the slaves follow their usual pattern of caring for the brood so that, in effect, they work for their parasites and raise their offspring.

Slaves will care for the brood of the oppressor ...

The slave-maker Protomognathus americanus is no longer capable of this. Each of its nests has one queen, three to six slave-maker ants that are essentially only designed to carry out the raids, and about 30 slaves that do all the other work.

In the summer of 2011, Foitzik and her staff collected some 2000 nests of the ant species Protomognathus americanus and Temnothorax longispinosus from 15 states in the US and Canada, including New York, West Virginia, and Ohio. Each nest is accommodated on a slide measuring just a few centimeters. Foitzik can easily position these under a microscope. The nest chamber itself is smaller than a postage stamp. "Can you make out the slave-makers? They are slightly larger and have broader heads." Difficult to see with only the naked eye.

... but only if the chemistry is right

The slaves thus care for the brood of the other species as if it was their own. At first, they seem to be unaware of the difference. But as soon as the larvae pupate, the situation changes drastically: The slaves then begin to neglect the pupae or even tear them apart. In the populations observed in New York, only 49 percent of the slave-maker pupae survived.

"Kill everything with the wrong scent." This is one of the most fundamental rules in the ant world. The pupae bear a chemical marker on their skin and the slaves become aware that the pupae do not smell like their own.

Foitzik's focus is on the evolutionary interaction of different species, i.e., their co-evolution. In each case, one species exerts pressure on another species. The greater the pressure, the greater the need to respond in order to survive. In New York, Protomognathus americanus is fairly common. The host species Temnothorax longispinosus is thus subject to major evolutionary pressures.

A web of strategies and counter-strategies

"The ants of neighboring host nests are often closely related to each other," the evolutionary biologist explains. When the enslaved ants keep the nests of their parasites small by killing their pupae, this is clearly of no use to their own nest, which has almost certainly been destroyed. But it does benefit the closely related ants in neighboring nests in that it keeps the demand for slaves low so that raids are less frequent.

Strategies and counter-strategies determine the co-evolution of ants. The more detail Foitzik provides, the more complex and fascinating the picture becomes. She has identified regional differences: "In New York State, the host ants are much more aggressive when confronted with slave-makers." They fight back furiously and can thus bring more of their brood to safety. In West Virginia, however, slave rebellions happen more often than in New York.

And there is still another factor: Protomognathus americanus tries to modify the scent of its pupae to resemble as closely as possible that of the scent of the pupae of the host species. The smaller the difference between the chemical profiles, the less likelihood there is that the slaves will notice the difference and thus rebel.

Of ants and men

"All ants live in social systems," Foitzik explains. "They have developed many skills that humans have also developed, but via completely different routes. They use agriculture – some species grow mushrooms in their nests. Other species are nomadic and roam around keeping aphids as humans keep cattle. And there is warfare in the ant world. I am fascinated by this interplay of similarities and differences."

It is not difficult to see this. In addition to pictures of her children, an ant sculpture adorns her desk, a favorite subject for a photo whenever the press visits. On the shelf is relevant literature, of course with ants on the cover, and finally we are back to the corridor with the watercolors. Which artist actually created them? "They are mine," Foitzik replies after some hesitation. "Otherwise, it would be so bleak here."