Every summer, blood-red ants of the species Formica sanguinea go on a mission to capture slaves. They infiltrate the nest of another ant species, like the peaceful F. fusca,
assassinate the queen, and kidnap the pupae to raise as the next
generation of slaves. Once the slaves hatch in their new nest, they
appear none the wiser to their abduction, dutifully gathering food and
defending the colony as if it were their own.
Scientists have long wondered how such slavemaking behavior evolved.
Now, new evidence suggests that today’s slave snatchers started out as
temporary parasites—ants that laid their eggs in the nests of other
species and then used those workers as part-time caregivers for their
own offspring.
The evolution of enslavement in Formica ants has long eluded
scientists, largely because they didn’t know how species in the genus
were related. So Jonathan Romiguier, a molecular biologist at the
University of Lausanne in Switzerland, and colleagues sequenced and
meticulously mapped the genetic relationships of 15 Formica
species to create the most robust family tree to date. The tree includes
major branches for slavemakers, species without slaves, and parasitic
species that exploit foreign workers on a temporary basis.
The order of those branches tells the story of how enslavement
evolved. By tracing their way down to the base of the tree, the
researchers gleaned that the ancestors of all Formica ants
formed colonies without recruiting slaves.
Parasitic ant species soon
arose, in which queens laid their eggs in neighboring nests and enlisted
the resident workers to care for their broods. Then another branch
diverged, and it was there that the full-blown master-slave relationship was born, the researchers reported late last month in BMC Evolutionary Biology. Because the slavemakers are lumped together with the parasites in their own distinct section of the Formica family tree, Romiguier says he suspects temporary parasitism was a “preadaption” to slavemaking behavior.
Not everyone is convinced. Christian Rabeling, an evolutionary
biologist at Arizona State University in Tempe, says that although the
study furthers our understanding the evolutionary history of the Formica
genus, the family tree included less than 10% of the 175 known species,
a major limitation. “The picture is just more complex than they outline
in the paper,” he says.
To address that issue, the team redid their analyses with a bigger
data set—one that included more species, but lower-quality genetic data.
Romiguier says their results still held, but further research is needed
to be certain.
Another outstanding question is more basic: How could the ants’ genes
have enabled slavery to evolve? Susanne Foitzik, an evolutionary
biologist at Johannes Gutenberg University Mainz in Germany who was not
involved in the new research, has discovered a few candidate genes in
another slavemaking group of ants—the Myrmicinae subfamily. The genes
that she has found are involved in creating a chemical disguise that
tricks neighboring ants into welcoming slavemakers into their nests.
Romiguier is also on the hunt for similar genetic adaptations that have
helped some ants foray into the ruthless world of slavemaking.
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