Each year in late summer, amid the leaf litter of forests of the northeastern United States and Canada, the slavemaking ant Protomognathus americanus raids the nests of a tiny ant known as Temnothorax longispinosus. The slavemakers kill the Temnothorax adults and steal away their helpless pupae, holding the young captive and raising them to serve as the next generation of slaves. This dramatic struggle is disconcerting for its obvious parallels to oppression and slavery in human societies. But it is perhaps even more unsettling because it suggests that these activities, which are widely condemned in our societies, have biological underpinnings.
“The slavemakers completely rely on their slaves. They are fed by slaves, and the only thing they specialize in is leaving the nest and attacking host nests,” explained Sebastian Pohl, a biologist at Ludwig Maximilian University (LMU) in Munich, Germany. Pohl and his colleague Susanne Foitzik, a professor of behavioral ecology at LMU, reported in a recent issue of the journal Animal Behaviour that slavemaking ants purposefully raid large Temnothorax colonies—a surprising discovery given that attacking large colonies comes with very high risks for raiding slavemakers.
The slavemaking ant Protomognathus americanus. (Photo courtesy of Sebastian Pohl)
Before slavemakers raid a nest, a scout is sent to locate and evaluate host, or Temnothorax, colonies to determine which one would be the most ideal to attack. Once a nest is chosen, the slavemaking ants descend upon it in full force, fighting and slaying Temnothorax workers and carrying pupae back to the slavemaker nest. The host pupae later become imprinted on the slavemaking colony, and by the time they have grown into adult ants, they faithfully forage and work for Protomognathus.
Which Nest To Raid?
To understand how Protomognathus decides which host nest to attack, Pohl and Foitzik began by studying the ants in the laboratory. “In our first experiment, we used host colonies with the same number of workers but with differing numbers of pupae,” Pohl said. “We thought Protomognathus would prefer the nest with more pupae, but we found that they didn’t prefer either.”
In the second experiment, Pohl and Foitzik varied the number of host workers and kept the number of pupae the same. “We thought that they would take the nest with fewer workers, but they actually attacked the nest with more workers,” Pohl explained.
To learn more information about the slavemaking ants’ raiding behavior, Pohl and Foitzik decided to take a closer look at the biology and ecology of both Protomognathus and their Temnothorax hosts in their natural environment. This meant traveling to the Huyck Preserve in New York state, just southwest of Albany, which was the collection site for the ants they used in their laboratory research.
The Huyck Preserve in New York, showing the natural habitat of slavemaking Protomognathus ants. (Photo courtesy of Sebastian Pohl)
“The slavemaking ants, and especially their host ants, are actually very common [in the northeastern United States],” Foitzik explained. “They inhabit deciduous forests made out of oak, maple, or hickories. They live in the leaf litter layer, and their nests are found in acorns, hickory nuts, or little branches or sticks on the forest floor.”
At the Huyck Preserve, Pohl and Foitzik noted several intriguing observations. “We know a lot about their ecology because we can manipulate their resources and the frequency of slavemaking nests,” Foitzik described. “These field manipulations show a strong influence of the slavemaker on host populations. Host colonies in areas with slavemakers are less dense and smaller and raise fewer new workers but more sexuals (i.e., males and queens). Males and queens are winged and can fly out of areas with high slave-raiding risk.”
Reducing Costs and Maximizing Benefits
Precisely why Protomognathus attacks the larger host nests is not entirely clear, in part because there is not much information available on the cost of scouting activity. Slavemaking colonies usually contain only 2 to 5 workers, which carry out scouting activities.
“A single slavemaker scout detects the host and may face the host colony alone,” Pohl said. That means one slavemaking worker battling against anywhere from 60 to 100 Temnothorax workers. Based on this observation, and on what is known about the high fatality rate among young queen slavemaking ants that attempt to single-handedly take over a host nest, scouting is considered to be a risky and potentially costly endeavor for both the individuals and the colony.
An acorn containing a colony of slavemaking ants. (Photo courtesy of Sebastian Pohl)
Hence, sending out fewer scouts reduces the costs associated with raiding. In addition, attacking a few, large host nests is likely to yield sufficient numbers of pupae to replenish the slave workforce in the Protomognathus colony. Looking at the long-term impact of raiding behavior, Pohl and Foitzik concluded that the overall risk reduction associated with fewer raids actually maximized benefits to the slavemaking colony over the long run, facilitating the successful production of offspring and allowing the colony to prosper.
A Secret World of Small Societies
Many questions remain about the behavior of Protomognathus, but Pohl and Foitzik hope to soon uncover new information about the factors that trigger scouting and raiding activity. “We are gaining insight into a secret world of very small societies,” Foitzik said.
And although ant slavery resembles the slavery systems of human societies, there are actually many differences. For starters, as Foitzik noted, “Protomognathus enslaves ants of a different species.” In other words, what we currently describe as slavemaking behavior in Protomognathus actually more closely resembles domestication. “It is similar to us using horses or cows for our work,” Foitzik added.
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A regular Britannica Blog feature written by the encyclopedia’s own Kara Rogers, Science Up Front goes behind the headlines to bring researchers’ stories of discovery centerstage. Begun in 2009 to highlight the ingenious work of pioneering scientists and to bring greater accuracy to science reporting, Rogers goes straight to the source, exploring the latest advances in science, from medicine to nanotechnology to conservation, through first-hand interviews with researchers. The series covers all things science, so check back regularly to see who’s up on Science Up Front.
