Tiny ants enslaved inside acorns across the northeastern United States could be resisting their captors with a covert army of killer nannies.
About the size of newspaper commas, ants in the genus Temnothorax fall prey to a marginally larger ant species that doesn’t do its own housework.
Instead the do-little ants, Protomognathus americanus, raid smaller species’ nests and steal babies in the larval and pupal stages. The youngsters grow up inside the acorn home of the slave-makers’ queen, doing her housework and nursemaiding her young.
Biologists have seen that the species vulnerable to enslavement evolve ways to try to fight off raids. But ways for the kidnapped youngsters to resist captivity haven’t shown up. Theorists have even argued that post-enslavement resistance couldn’t evolve. But observers are giving up on the slaves too fast, says Susanne Foitzik of Ludwig-MaximiliansUniversity in Munich.
Kidnapped workers of two Temnothorax species kill off a good portion of their charges in the nurseries of slave-maker colonies, Foitzik said at the 12th International Behavioral Ecology Congress held August 9 through 15 at CornellUniversity.
Yet, in their birth colonies, Temnothorax ants readily nurture their baby sisters and half-sisters to adulthood.
Killing sprees by slave nannies could be an overlooked form of resistance, Foitzik suggests. The baby-killing offers any kin in nearby colonies some protection from slave-makers, since the kidnapper queen’s offspring make up the raiding parties. Paring back their number cuts back the raiding power. Foitzik proposes that this benefit to kin could drive the evolution of the trait.
“This is evolution to be a bad nanny,” says Peter Nonacs of the University of California, Los Angeles.
He compares the ant dynamics to other resistance puzzles that have intrigued evolutionary biologists. A wide variety of bird species, for example, seem able to evolve the urge to kill the eggs of parasitic cowbirds, but hardly any species kills cowbird hatchlings.
Foitzik began to wonder about baby-killing among ants, she says, because the slave-maker colonies contain surprisingly few workers of their own species. The kidnapper queens do lay plenty of eggs. If tended properly, they grow into workers that don’t do a lot of work, depending on slaves for food even as adults. These slave-makers attack other colonies to refresh the supply of household help.
At a West Virginia study site, slave-maker nests averaged only two worker adults of the queen’s species, and several dozen slaves. “That’s not a raiding party — that’s a raiding duet,” Foitzik says. New York colonies averaged only five slave-maker adult workers. The slave-maker ants have large, fierce jaws and use chemical weaponry during attacks, but these reduced numbers can still make raids iffy.
When Foitzik brought colonies into her lab, the slave-maker queen’s young failed to thrive. She discovered that slave nursemaids care for the eggs and young larvae but turn into horror nannies once slave-maker young reach the pupal stage. “They take pupae and dump them in some corner. Mold grows on them and they die,” Foitzik says. Or the slaves rip apart other pupae and eat the chunks.
Overall, slave nursemaids kill some 80 percent of their captors’ young queens and some 60 percent of the young workers, Foitzik reports. To see if lab life, rather than enslavement, was driving the ants to such extremes, Foitzik also kept in the lab colonies of the slave species that hadn’t been raided. There, more than 90 percent of the young survived to adulthood.