"A. texana provides a great subject for studying mutualism because it's isolated from closely related species, and it lives at the northern extreme of the leaf-cutters' range," Solomon said. "We wanted to find out how it could survive the winters here, and given that it can, we wanted to know why it hasn't expanded even farther north."
Leaf-cutters live in large colonies that contain up to 5 million members, and their subterranean nests have been found to extend almost 100 feet underground. Within these warrens, they dig thousands of tunnels that connect various chambers, where football-sized gardens of Attamyces are cultivated.
Over several years, Mueller's team systematically mapped the range of the ants and collected samples of live Attamyces fungus from dozens of nests throughout the range. Tests in the lab revealed that the Attamyces in more northerly ant colonies resists cold better than samples from southern nests, where winters are milder. The studies also found that the ants aid the cold-tolerant fungi by moving them during the coldest months of the Texas winter into deeper garden chambers where conditions are milder.
The researchers were able to show that the fungus's resistance to cold is based on genetic differences; this suggests that it has evolved during the several million years since the ants first arrived from the south with fungus in tow.
In addition to determining how leaf-cutter ants can survive outside the tropics, the study also found that the ants have been prevented from spreading farther north by the physical limitations of their fungal crop. According to Solomon, the finding that a species is limited by its mutualist is of particular interest to biologists.
"The range of the ants is not limited by their own tolerance to the cold but by the tolerance of the
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