Texas leafcutter ants farm crops of fungus that evolved cold tolerance to Texas winters, just as northern farmers cultivate cold weather crops, researchers from The University of Texas at Austin show in a new paper published in the journal PNAS Early Edition.
Though the cold tolerant fungus gives the ants the ability to maintain winter gardens, the fungus is still sensitive enough to cold, it limits the ant's ability to spread farther northward.
"The same is true for human farmers," says Ulrich Mueller, professor of biology. "Some of our crops come originally from the tropics, and humans have had to select them over time to grow in colder climates. But we are still limited by our abilities to select and adapt crops to local conditions."
Mueller and his colleagues found that even within Texas the fungus is more tolerant of cold at its northern edge near Dallas, and less tolerant of cold at its southern edge near Brownsville.
At Fort Belknap, just northwest of Dallas, Mueller says the ants "are just hanging on."
Leafcutter ants are largely tropical, and the Texas leafcutter ant, Atta texana, is one of only three leafcutter ant species found in the United States. The species arrived in the region about 10,000 years ago after the retreat of the glaciers and the end of the last Ice Age.
"The ants may have only been in Fort Belknap for a few hundreds or thousands of years," Mueller says.
The ant's symbiotic relationship with the cold tolerant fungus clearly permits it to survive in the more temperate environments of Texas.
The finding provides a perspective on symbiotic relationships, which are normally thought of as being beneficial to both organisms.
"We normally think that forging a symbiotic relationship enriches lives -- that each organism is helping the other," says Mueller. "But we have found that this can be the opposite. In the tropics, the symbiosis between the leafcutter ants and their fungal crops helped to broaden the ants' ecological niches. In the Texas leafcutter, the symbiotic relationship also constrains them."
Texas is a particularly interesting laboratory for studies of local species adaptations because of its unique ecological conditions. It is the only state in the U.S. where an unusually steep precipitation gradient from east to west crosses a steep temperature gradient from north to south, ranging from temperate to subtropical.
"Texans are uniquely positioned to monitor the effect of environmental change on U.S. biodiversity," says Mueller. "It will be interesting to see what happens with these ants over the next 10 to 20 years with global warming. Will they expand to Oklahoma and across the Mississippi River, or will cold snaps like those we just experienced knock them back?"
Mueller's research was supported by the National Science Foundation, the W.M. Wheeler Endowment and numerous Texas landowners who allowed ant collection on their properties.