February 22, 2011

Texas Leaf-Cutter Ant Has Adapted To Texas Cold

A new study by biologists at Rice University and the University of Texas at Austin finds that the Texas leaf-cutter ant, Atta texana, adapted to the harsh Texas winters through their food.

Although adapting to Texas winters may seem mild to those from northern climates, the ants' ancestors emigrated from the tropics.

A texana, like all leaf-cutter ants, cuts leaves but does not eat them.

"Leaf-cutters can't digest the nutrients of leaves directly, so they use a fungus called Attamyces as a kind of external digestive system," co-author Scott Solomon, a lecturer in ecology and evolutionary biology at Rice, said in a statement.

"It's an example of a relationship that biologists call mutualism. The ants are completely reliant on the fungus, and the fungus -- which only occurs in leaf-cutter colonies -- is likewise reliant on the ants."

The new study found that leaf-cutter colonies in northern Texas and Louisiana, where winter temperatures regularly get below freezing, have found new ways to cope with the cold.

Most leaf-cutter ant species are native to the predictably warm tropics, so the Attamyces fungus has adapted to a narrow range of temperatures. 

Previous studies have found the ants are attentive gardeners and they keep an ever-present watch on their subterranean gardens.  The ants are careful to regulate humidity in the gardens and to weed out anything that threatens the crop.

"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 in a statement. "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. 

The researchers systematically mapped the range of the ants and collected samples of live Atttamyces fungus from dozens of nests throughout the range over several years. 

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 team was able to show that the fungus' resistance to cold is based on genetic differences.

The study also found that the ants have been prevented from spreading farther north by the physical limitations of their fungal crop.

Solomon said the discovery 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 fungus," Solomon said.

"The range of one species is often influenced by the range of another -- particularly harmful ones like predators or competitors -- but it is really rare to find a case where a species' range is completely dependent upon the evolution of a species that is helpful."

The researchers published their findings in the journal PNAS Early Edition


Image Caption: These are workers of the Texas leafcutter ant species, Atta texana. Credit: Alexander Mikheyev


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