Back atcha.
From the Same Ant Species, Different Behavior By Louis Jacobson
Special to The Washington Post
Monday, November 26, 2001; Page A13
For more than a century, scientists have known that certain species of ants subsist by raiding other ant colonies and turning their offspring into slaves. But researchers have known relatively little about how these "slavemaking" ants actually operate in the wild -- until now.
Joan M. Herbers, a biologist at Colorado State University in Fort Collins, and her laboratory colleague Susanne Foitzik have spent the past few years studying ant colonies near Albany, N.Y., and White Sulphur Springs, W.Va. At each location, Herbers and Foitzik observed one slavemaking species -- known as Protomognathus americanus -- as well as the one or two species it enslaved. The enslaved species all belong to the genus Leptothorax.
What Herbers and Foitzik discovered is that the enslaver ants in West Virginia behaved much differently than the enslaver ants in New York -- even though the ants in both locations belonged to the same species. In turn, the enslaved ants in West Virginia also behaved differently than did those in New York.
Peter Nonacs, a biologist at the University of California at Los Angeles who calls Herbers's work "solid" and "interesting," notes that slavemaking ants interest scientists because they practice several noteworthy behaviors. Ants, like humans, are social animals, with large numbers of individuals alternately cooperating and fighting with one another, Nonacs said. In addition, few species aside from humans take slaves (or, to be precise, domesticate another species for their own benefit, as the ants do).
Herbers's findings also provide new insights into how parasitic species behave and evolve. Parasitic behavior has attracted a great deal of interest recently because it sheds light on many human scourges, from mosquitoes to the influenza virus.
"Scientists understand that evolution is pretty darn important, yet we really don't understand how it works, especially with regard to how parasites and hosts evolve," Herbers said. "Slavemaking ants and their hosts are easy to manipulate in the field, yet they give us good insights into how evolution operates."
Only about two dozen ant species -- about 4 percent of those known worldwide -- display slavemaking behavior. The niche is highly specialized, much like a butterfly species that drinks one kind of nectar and cannot survive on any other. The risks inherent in specializing so narrowly have turned many slavemaking ants in Europe into endangered species.
In North America, slavemaking ant species range from northern Alabama to Quebec, but no further west than Iowa. Slavemaking ants live only in climates that are cold for part of the year, probably because their raids are precisely aligned to the changing of the seasons and could not be easily timed in places with long-lasting tropical summers.
The enslaved species of Leptothorax cluster in small, enclosed spaces, such as acorns. The individuals are tiny -- no larger than two millimeters, which is slightly smaller than their P. americanus enslavers.
The slavemakers are fundamentally incapable of caring for themselves. "They're degenerate," Herbers said. "They're not hardwired to do all the things that ants normally do. The only thing they can do is raid."
And raid they do. When P. americanus ants descend upon a Leptothorax colony, they create such a big commotion that the Leptothorax adults are forced to abandon their offspring. The slavemakers scoop up the larvae and pupae and carry them back home, where they are raised into a life of slavery. When the unwitting slaves die, the cycle of raiding begins again.
Herbers and Foitzik have provided biologists with a subtler understanding of this process. After sampling DNA from captured slaves and from colonies of the same species nearby, the scientists found that the New York slave ants -- unlike those in West Virginia -- left behind few close relatives. Such results suggested that the New York raiders succeeded not only in stealing their neighbors' offspring but also in slaughtering the colony's mature members, including the ones capable of reproducing.
This ferocity was confirmed when the scientists allowed raids to be conducted in their laboratory. New York slavemakers, when allowed to raid Leptothorax ants that hailed from either New York or West Virginia, "came in with machine guns blazing," Herbers said. "They killed everybody they could." West Virginia slavemakers, by contrast, allowed the mature Leptothorax ants to escape. "It was like a gentleman's argument," Herbers said.
At the same time, the New York Leptothorax ants -- primed by the vicious battles their species is used to fighting at home -- battled back ferociously, even against the relatively benign assault of the West Virginia slavemakers. The West Virginia Leptothorax ants, for their part, tried to flee just as they do in West Virginia -- rather than battling to the death with the murderous New York slavemakers.
What could account for such different behavior by members of the same species? The answer, Herbers and Foitzik said, has to do with supply and demand. In New York, the slavemaking species has been growing in number, while the populations of enslaved species have been largely stable. In West Virginia, both predator and prey have reached a relatively stable equilibrium.
The pressure of a rising population, Herbers suggests, has intensified the raiding behavior of New York slavemakers and, in turn, spawned more aggressiveness among the ants being raided. By contrast, the stability of the West Virginia populations has made the situation there "evolve into something resembling detente," Herbers said. "The slavemakers would be out of a job if their behavior became much worse. The only way to explain our data is that there has to be some evolutionary give-and-take between a parasite and a host. The metaphor really seems to be one of an arms race."
Herbers and Foitzik also found that Leptothorax ants living in "bad neighborhoods" -- those with many slavemaking ants in the immediate vicinity -- used different reproductive strategies than those who lived in "good neighborhoods" with few slavemakers around. In bad neighborhoods, Leptothorax colonies put most of their efforts into producing reproductive offspring, rather than sterile workers. In good neighborhoods, the colonies focused on expanding their reach by producing workers.
Herbers has been awarded a $300,000 National Science Foundation grant to continue her research. Her immediate plan is to allow slavemakers from New York and West Virginia to raid Leptothorax species they never see in the wild to gauge whether the behavioral tendencies demonstrated in earlier experiments will be repeated.
© 2001 The Washington Post Company |
