Eating seeds shifts ant sex ratios.

Researchers have new details on why ants plant flowers.

Ants play a major role in dispersing seeds of such woodland delights as trilliums, violets, spring beauties, bloodroot, and hepatica. The seeds release a chemical that interests ants, who will lug home even lumps of Teflon coated with the attractant. The ants pull nutritious outer tidbits off the seeds and throw away-that is, plant-the part with the embryo. A nutrient-rich ant garbage pile offers a great place for a plant to start new roots. Seeds collected by ants also escape the sharp teeth of foraging mice.

This ant-plant tale is an old standby in discussions of mutualisms, relationships that confer reproductive advantages on both partners. Nonetheless, the story troubled E. Raymond Heithaus of Kenyon College in Gambier, Ohio. Studies had documented the benefits to plants but not the benefits to ants, he says.

To see whether the alleged mutualism was just a trick on the ants, Heithaus and Manuel A. Morales compared 24 ant colonies satiated with bloodroot seeds to 27 colonies deprived of seeds. In the March Ecology, the researchers report that the seed-fed colonies did not grow unusually big, but they did have 3.5 times as many reproductive females, a clear benefit for producing offspring in later generations.

"I almost wish it had come out the other way," Heithaus says. He's still crusading for a tougher look at alleged mutualisms.