Farmer ant species may have lost all its males.

Minuscule gardeners that grow fungus for food may be the first ant species that scientists have discovered to have no power of sexual reproduction. Several lines of evidence suggest that the species Mycocepurus smithii consists only of females that produce daughters from unfertilized eggs, says Anna Himler of the University of Texas at Austin.

Even standard ant reproduction has some asexual aspects. Worker ants typically don't reproduce but care for the offspring of their queen. The queen lays both fertilized and unfertilized eggs, the latter turning into males.

Scientists already knew of five unusual ant species that produce females from unfertilized eggs, says Himler. In these species, however, occasional males still appear.

In Himler's studies of M. smithii, she collected more than 100 nests without finding a male. A colleague likewise collected some 200 nests but failed to find any males. Checks of two large collections in museums also turned up no undisputed males.

In Himler's work, queens isolated as virgins laid eggs that developed into healthy daughters. Genetic analysis suggested that these daughters were clones of their mothers.

After 15 months of testing, Himler concluded that switching the fungus that the ants eat doesn't induce sexual reproduction. Neither does treating the ants with antibiotics, in case some infection had rendered them sterile.