The team，led by Inon Scharf and Susanne Foitzik from the Johannes Gutenberg University of Mainz in Germany，wanted to find out how these socially complex insects reacted to intruders of different species.

　　To do this，they brought ant colonies into their laboratory.In the wild Temnothorax longispinosus inhabits forests across the northeast of the US，where leaves of trees fall annually in autumn.They nest in cavities(洞穴)of rotten leaves or sticks in the leaf litter.

　　“We introduced workers of four different ant species into the ant colonies，”Dr.Scharf explained.The first was an unfamiliar species—one that didn’t share the ant’s habitat.The second was from the same species and the third was a familiar competitor—a related species that did inhabit the same areas.Finally，the ants encountered the most dangerous intruder—the slavemaker ant.

　　Slavemakers invade other ant colonies and steal their brood，often killing the host queen and workers in the process.The enslaved workers emerge from the stolen host brood inside the slavemaker nest.They then carry out routine tasks for the slavemaker—tending the queen，raising the young and hunting for food.

　　When the little Temnothorax longispinosus ants encountered a slavemaker，they attacked it—biting and stinging.When they met less threatening opponents，they simply tried to drag them from the nest.

　　“It is surprising—they’re making a clear distinction，”said Dr.Scharf.“But it makes sense in terms of evolution.They don’t waste their energy responding aggressively to every single intruder.It’s logical to adjust their response to the threat.”

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