Sex on Six Legs_ Lessons on Life, Love, and Language From the Insect World - Marlene Zuk [87]
Just such geographic variability in ants was the subject of a study by Susanne Foitzik, now at Regensburg University in Germany but formerly another postdoctoral scholar working in Herbers's laboratory. Foitzik and others have recognized that the ants are a good way to study ways that a host and parasite can influence the evolution of each other, in what's called a coevolutionary arms race. After all, one wouldn't expect the host, or exploited species, to just sit back and take it—for example, we evolved an entire immune system to resist the attacks of viruses and bacteria. Other kinds of hosts of social parasites show varying degrees of defenses against the parasite; some cuckoo and cowbird hosts recognize and reject the interloper's eggs, while others seem to be oblivious to the gigantic size of the parasite chick relative to their own offspring and valiantly stuff food into the cuckoo chick's gaping maw at the expense of their own reproduction.
Foitzik and her coworkers looked at the ways that the slave, or host, species varied in its ability to defend itself against the slave makers. They were interested in whether the defense mechanisms were the same in different places, regardless of the intensity of the raids by the slave makers, or whether each pair of host and parasite populations evolves a unique way of interacting, with a new arms race in each locale. They compared colonies of a raiding species and its victims in the Huyck Preserve in New York state with those in West Virginia. More and larger colonies of the slave-making species occur in New York, which should make the pressure on the host species more severe, since they are being raided more frequently. The slave-making ants in turn can kill the queens of their hosts without too many repercussions, since many colonies of potential victims are also present.
The scientists found that the coevolution between host and parasite was in fact different in the different places; in New York, a guard ant was more likely to be found protecting the host nest entrance, and in turn the New York slave-making ants took more of the brood from the nests they raided. The host defenses were also more aggressive to the initial scouts sent out by the raiding parties. "Ironically," write the researchers, "these host ants are probably killed by enslaved conspecifics [members of the same species] that accompany ... workers on raids, rather than by the slave-makers themselves." The defenses, however, weren't unique to a particular set of nests, supporting the idea that universal defense mechanisms evolve throughout the population.
The idea that the hosts could defend themselves against the raiders wasn't given much credence until recently, and it's tempting to speculate that the lack of exploration of the idea came from people clinging a little too tightly to that slavery analogy. Slave rebellions are risky and scarce. But it's commonplace to imagine a host and parasite, for example, the worm inside the gut of a mouse, continually evolving ways to attack or defend against each other.
Whether you think of it as piracy, parasitism, or slavery, capturing live individuals of another species and benefiting from their labor requires a complicated set of behaviors. How did such a practice evolve? Charles Darwin offered the first potential explanation in The Origin of Species, proposing that the ancestral slave makers first took the pupae as prey. When some of the pupae accidentally escaped detection back in the host nest and became adult workers, they were not perceived to be foreigners and, hence, began doing their normal ant activities, which made the colony as a whole prosper.
Another possible route to the evolution of piracy is via the territorial battles that commonly take place between colonies of the same species. Ants and other social insects usually have very strong