BY LYNN LOVE
Twenty-two years is nothing in evolutionary time, but it is long enough to figure out some things about evolution if you are a diligent scientist. That is exactly what Huyck Preserve Research Associate Joan Herbers did. In the process, she spurred a generation of research on slavemaker ants, expanding the field of co-evolution studies and, along with several of her scientific protégés, made some important discoveries.
Now a professor [emeritus] at The Ohio State University, Herbers was a young assistant professor at the University of Vermont in 1979. A colleague invited renowned biologist Tom Eisner to lecture and from him Herbers learned that that there was a hidden gem of a field station just 170 miles away that offered research grants and free summer housing—a boon for any young scientist earning an entry level salary.
She spent that summer and the following one, 1980, poking around the Huyck Preserve with her kitty litter pan—“essential for collecting specimens!—searching for something interesting to study. Her memory of the “aha” moment is vivid: “When I snapped open that first twig and saw those ants spill out, I knew I had my project.” What she saw that got her excited was a colony of two- to three-millimeter ants, then called Leptothorax longispinosus [now called Temnothorax longispinosus]. (Non-scientists know them as acorn ants; Herbers called them leptos.) In one twig colony there were two queens and in another nearby, three. The more she collected them, the more Herbers found the queen numbers to vary. It seemed random, but probably wasn’t. She realized that if she simply pursued the reasons why, a substantial project would unfold before her.
By 1981 and for nearly a decade, Herbers explained queen behavior in acorn ants in a series of research publications, all based on work she did at the Huyck Preserve, as well as Mallett’s Bay State Park in Vermont and Watoga State Park in West Virginia. Because the leptos in West Virginia never had more than one queen, compared with the multiple queens found in the nests in New York and Vermont, Herbers eventually determined that a combination of hard-wired genetics and variable environmental factors determined how the ants organized themselves. The research earned her several promotions and she eventually left Vermont for a new job in Colorado, with a rich archive of data. She also took on a brilliant young postdoctoral fellow named Susanne Foitzik.
Although Herbers focused on leptos, time and again she found “harpies” in the mix. The harpie is similarly-sized (formerly known as Harpegoxenus americanus, now known as Protomagnathus americanus or Temnothorax americanus) to the tiny acorn ant. Every summer, the two types compete for resources among the forest leaf litter. Food, water, and shelter are basics that all animals seek. In the case of the harpies, add “labor.” The tiny acorn ants, the leptos, with colonies of just 30 to 50, are the prime targets for the harpies. The harpies evolved in such a way that they do not raise their own young or forage for food and water. Instead, they take slaves. They go out and kidnap the larvae of acorn ants, carry them back to their own acorn (or twig or hickory nut), and harness the power of the worker ants once they hatch. The leptos don’t even know they were born into slavery.
Foitzik, a young German researcher, was already interested in social insects and had worked on leptos in Europe as a graduate student with Juergen Heinze. She met Herbers in the summer of 1996 on a specimen-collecting trip in the United States. “She impressed me,” says Herbers. After a year of staying in contact, Foitzik inquired about doing a postdoc in Herbers’ lab, and the two turned their attention to slavemaker ants—the harpies. “I told her I thought the interaction between leptos and harpies could be interesting,” says Herbers. “She was the one who figured out how to ask the right questions and collect the right data.”
Initially, Foitzik explains, the two researchers thought that the slavemaker ants, with their peculiar ways, might be too rare to reveal anything general or important. After the researchers mapped large sections of the forest at the Huyck Preserve over the summer of 1998, however, they found that slavemakers were very common. Things got interesting from that point.
In a series of publications beginning in 2001, Foitzik and Herbers laid new foundation for the understanding of slavemaker ants. They documented large numbers of them at the Huyck Preserve and compared their presence in Albany County with colonies in Vermont, West Virginia, and Ohio. The research showed that slavemaker raids took place frequently during July and August. Perhaps most important, Herbers and Foitzik also discovered that where slavemakers live, slave-ant populations are powerfully impacted. “Slavemakers raid between five and 10 times a year,” says Foitzik. “Host colonies seldom survive these raids.”
Slavemakers attack and kill adult slave ants when they invade a nest, then collect the booty—unborn slaves—to take back to their own nest. The raids change the demographics of the slave populations in the areas where slavemakers are active. “We showed that host nests were smaller, had fewer queens and had a higher relatedness,” says Foitzik. In other words, the leptos’ very survival is compromised by the presence of slavemakers.
Once they documented how heavy the toll was on slave populations, Foitzik and Herbers unwittingly opened another avenue of inquiry. How were the slaves coping with the pressure?
Despite the bleak circumstances for slave ants under siege at the Huyck Preserve, the two species are far from doomed; extinction of slave populations would only mean trouble for slavemakers. Instead, the two species are co-evolving as players in what’s known as an “evolutionary arms race.” A classic arms race has no other purpose than keeping up. If one player gets an improved new weapon, the other gets a bigger one, or a better defense. The adjustments keep going to maintain a kind of equilibrium. The same is true in nature when the survival of two or more species is closely linked.
Slavemaker ants at the Huyck Preserve are “killing machines” according to Herbers. “They’re mean and nasty and leave no survivors.” Turns out, slave ants at the Huyck Preserve are also nastier than those found elsewhere. They bite the heads and bodies of the raiding slavemakers at a higher rate than any of the other sites Herbers and Foitzik study. Casualties are high even for the raiding ants.
The same species of slavemakers in West Virginia leave survivors at least half of the time, Herbers explains. The interesting difference is that the raiding ants consistently fail to block the entrance to slave nests when they arrive, which allows some workers and an occasional queen to escape, after which they can join or start a new colony elsewhere. The slave ants there also fight back less ferociously. Why the difference?
One explanation for the unmatched behaviors is that the sites represent different stages of co-evolution. There’s probably a longer interaction between the two species in West Virginia, Herbers explains, perhaps because that land was never glaciated. Slavemakers there already may have reached a point of overexploitation of their host population. The weaker aggression might be the result of both species backing off for the greater good. The Huyck Preserve ants may have not reached that point yet. Like the variation in lepto queen numbers Herbers observed early on, adaptive flexibility is a byproduct of evolution. Studying multiple sites allows for a potential snapshot of the process in evolutionary time.
“The Huyck Preserve is a hotspot for understanding how these two species co-adapt,” says Foitzik. Stable conditions in the Preserve’s forests mean that the leptos, the slave ants, are at their optimal abilities, she says. They are abundant and healthy. Slavemakers in New York, because of their forcefulness, “represent the very height of the negative impact of a co-evolutionary process,” explains Herbers.
Because the site is so robust, and Herbers and Foitzik’s groundwork so solid, a new generation of grad students and postdocs continue to arrive every summer to study slavemaker and slave ants. Many come directly from Germany, where Foitzik has a well-established laboratory at the University of Mainz. Their efforts are constantly refining our understanding of the co-evolution of the slaves and slavemakers.
In 2009, for example, Alexandra Achenbach, a student of Foitzik’s, revealed an interesting new defense in slave ants. They rebel. The enslaved workers care for slavemaker larvae as if they were their own. At a certain point, however, probably because the developing ants have a stronger chemical signature, or smell, that is different from what the slave ants identify with, they stop caring for them. Then they kill up to two-thirds of the slavemaker brood, either by tearing their developing bodies apart, or by pushing them outside the nest where they die of neglect. The mortality rate is even higher for unhatched queens in the care of slave workers. This sabotaging activity keeps the slavemaker nests smaller, which limits their raiding behavior and therefore their impact on the surrounding slave target nests.
Andreas Modlemeier and Tobias Pamminger, two students who also work with Foitzik, are repeat visitors at the Huyck Preserve. [In 2011 and 2012], they each published research about the range of behaviors among the slaves. While it’s true that slaves at the Huyck Preserve exhibit more aggressive behavior than slave ants elsewhere, the degree varies. Modlemeier is interested in the evolution of colony “personalities” among the slave ants, and Pamminger is creating a more detailed map of the co-evolutionary arms race by focusing on slaves.
Sebastian Pohl, another grad student, worked on a project at the Preserve, the results of which surprised even Foitzik. Pohl wanted to learn the criteria that drive slavemakers’ decision to raid a particular slave nest. When offered the choice of bigger versus smaller nests—and with them, bigger versus smaller risks of fatalities—the slavemakers consistently opted for the bigger risk. The reason is linked to nest size; a larger nest usually contains more larvae, which become the slavemakers’ future work force. “I would have thought the slavemakers prefer many battles with smaller nests,” says Foitzik.
Christine Johnson, who had been a postdoc with Herbers, came to the Huyck Preserve for several summers after she discovered a type of slavemaker that had never been seen before in Ohio. She wanted to compare the interactions among slaves and slavemakers in New York to those she examined in Ohio because she specializes in community dynamics—how the ants in one locale behave differently than those found in other locales. Based on her two summers here, along with her own new data from Black Rock Forest in Cornwall, New York, she has identified some additional factors that contribute to slave ant behavior, she says. Johnson [wrote] up the results for publication.
Finally, the very fact that Herbers and Foitzik have created an enterprising system for studying the co-evolution of leptos and harpies means that young researchers at the Huyck Preserve hail from other labs, too. Julie Miller, a graduate student from Cornell University who conducted her first experiments on slavemaker ants this summer is one example. Modlmeier and Pamminger even steered her in the right direction. “I would have been wandering around the Preserve forever unless they had showed me where to look,” she says. Though only her first season, Miller already wants to return [the following] summer to observe ants, as well as collect more specimens to take back to the lab. “There seem to be some general principles of collective decision-making for raids,” she says.