Both costs and benefits determine the removal of Datura (Solanaceae) seeds by seed-dispersing ants

Mutualisms confer both benefits and costs to participants, but costs have been largely ignored when considering how mutualisms function and evolve. Plants that are dispersed by ants produce seeds with attached nutrient-rich food rewards (elaiosomes). When ants approach a seed, they likely assess both the benefits (elaiosome mass) and costs (mass of the inedible seed) of moving it. We hypothesized that the masses of both the seed and elaiosome would affect diaspore removal rate, and predicted that when given a choice, ants would remove diaspores with higher benefits and diaspores with lower costs more quickly. To test these hypotheses, we manipulated the elaiosomes of Datura wrightii and Datura discolor (Solanaceae) and conducted choice experiments where we presented diaspores with variable benefits and costs to colonies of the seed-dispersing ant Novomessor cockerelli (Formicidae). D. discolor has a larger elaiosome-to-seed ratio since its seeds are half the mass of D. wrightii. Consistent with our hypotheses, ants removed seeds with heavier elaiosomes (larger rewards) and lighter seeds (lower costs) more quickly. Our study provides new evidence for seed dispersal costs by quantifying a cost of seed dispersal to ants and underscores the necessity of measuring both the benefits and costs of mutualistic interactions.