The persistence of cooperation between species, or mutualism, despite clear incentives for members to "cheat," is a classic problem in ecology. Traditional explanations focus on short-term, pairwise relationships between partners, even though many mutualists interact with multiple species throughout a lifetime. Todd Palmer et al. (pp. 17234-17239) monitored annual survival, growth, reproduction, and ant occupancy of 1,750 Acacia drepanolo-bium trees over 8 years, and constructed demographic models that related the trees' lifetime fitness to occupation by four mutualistic species of ants.
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