The researchers, from the Universities of Edinburgh and Oxford, explored the impact of cheats in populations of the notorious pathogen, Pseudomonas aeruginosa. These bacteria cooperate to scavenge iron from their environment, but mutant cheats do not contribute their fair share of scavenger molecules and instead simply steal the iron supplies of others.
"Cheats are kept in check by simple frequency dependence. When rare, cheats prosper at the expense of co-operators, but as they become more common, the profitability of their strategy declines. At equilibrium, neither strategy has the upper hand, so the two coexist," says Adin Ross-Gillespie, lead author of the study in The American Naturalist.
The authors say that this pattern arises only under certain conditions. They said, it arose in this case because population productivity was sensitive to the frequency of cheats.
Cultures with few cheats grew rapidly and achieved larger absolute sizes in the time available, providing greater opportunity for cheats to exploit the situation. However, cultures comprising high proportions of cheats grew poorly, they said.
"Too many cheats spoil the broth," Ross-Gillespie added.
The researchers say that studies like this help them to better understand how cooperative traits evolve and persist.