Freeloader. Parasite. Sponge. We all know someone whose special talent is getting something for nothing. But what about plants and animals engaged in cooperative relationships—think of bees pollinating flowers in return for nectar? The extent of mooching in nature remains a subject of debate. Evolutionary biologist Ellen Simms believes organisms keep cheating in check by detecting true cooperators and rewarding them, a mechanism she calls "partner choice."

Simms tested her theory, which has its origins in economics, by studying the relationship between wild legume plants and the rhizobial bacteria species that colonize their roots. The bacteria "fix" nitrogen, taking it from the air and making it accessible to the plant, which needs nitrogen to grow and can't get it any other way. In return, legumes may provide bacteria with sugar, a regular flow of oxygen, and a place to call home: root nodules, which look "like little tumors," says Joel Sachs, a postdoctoral researcher in Simms's lab. Nodules come in a range of sizes—the bigger the nodule, the bigger the benefit to bacteria.

Simms and her team gathered legume roots from test plots at Bodega Bay and isolated the bacteria species living in the roots' nodules. They discovered that bacteria fixing the most nitrogen lived in the largest nodules. "If [bacteria] are good to the plant, [they] get rewards back," explains Sachs. "This may help us understand what sort of traits we should focus on to improve nitrogen fixation in agriculture," adds Simms.

This understanding of how cooperation evolves and persists can be applied across a range of fields. For example, studying partner choice may shed light on the evolution of—and, perhaps, cures for—viruses. "Understanding how cooperators avoid being cheated is similar to understanding how hosts avoid being exploited by a pathogen," explains Simms.

Do slacker bacteria ever escape plants' punishment? "We don't really know," says Simms. "We think there are [successful] cheaters, but they are not as common as we've all thought."