Leaf-cutting ants are the farmers of the ant world. But unlike real farmers, the leaf-cutters raise only one crop: a strain of fungus particular to each colony. Now researchers have figured out why: different strains war with each other, forcing the ants to grow just one.
Leaf-cutting ants have been cultivating fungi for millions of years. The benefits are mutual: Ants nourish the fungi with pieces of leaves and defecate on them to provide other nutrients. In turn, the fungi flourish and are carried far and wide by virgin ant queens when they leave to build a new colony. But each colony deals with only one strain of fungus, and scientists didn't understand why or how this was.
Evolutionary biologist Michael Poulsen and fellow researchers at the University of Copenhagen, Denmark, took a crack at the mystery by observing the fungus gardens of 18 ant colonies. When they placed ant feces from one colony onto the fungus of another, the fungus became discolored and grew slower. In addition, when ants from one colony were fed fungi from another, their feces caused a similar reaction on their own fungi. This did not happen when they were fed fungus from their own garden. These findings led the researchers to suspect that compounds specific to the foreign fungus were inhibiting the growth of the in-house fungus.
"This makes it necessary that ants rear clones of the same fungi," says Poulsen, whose team reports its findings today in Science. Otherwise, the competition would stunt fungal growth and reduce the ants' food supply. So in terms of farming, says Poulsen, it's really the fungus that's running the show.
"This is a great advance," says Ulrich Mueller, an evolutionary biologist at the University of Texas at Austin. "We usually think of a mutual relationship as one in which both partners gain and there is no internal conflict. But this paper elegantly shows there can be a relationship even with the conflict."