COS 291-4 - Why keep investing in seed dispersal when the costs are high and resources low?: Ant preferences for elaiosome mass in the seeds of Datura wrightii

Abstract: Elaiosomes are nutrient-rich fat bodies attached to seeds that act to entice and reward seed-dispersing ants. The cost of producing an elaiosome can be high, up to 10% of the total energy a plant invests into seed production and dispersal. Previous experiments with Datura wrightii (Solanaceae) have shown that even when nutrients are limited, plants continue to produce large elaiosomes. Why these plants continue to produce large elaiosomes even when resources are limited remains a mystery. One potential explanation is that seed-dispersing ants preferentially choose seeds with larger elaiosomes. In previous experiments, ants have been found to prefer species with larger seeds with larger elaiosomes. However, few studies have measured ant preferences within a species, and none has directly manipulated elaiosome size. I artificially manipulated the elaiosomes on D. wrightii seeds to create three treatments: seeds with no elaiosome, one unmanipulated elaiosome, and one large elaiosome (two elaiosomes combined together). In the field, these seeds were offered, cafeteria-style, one meter outside colonies of the seed-dispersing ant Novomessor cockerelli. The number of seeds remaining of each type was recorded every five minutes for a total of 30 minutes. We hypothesized that elaiosome mass would affect the removal rate of the seeds. We tested the prediction that seeds with twice the natural elaiosome mass would be removed at a higher rate than unmanipulated seeds (with one elaiosome), which in turn would be removed at a higher rate than seeds lacking an elaiosome. Consistent with our prediction, the elaiosome treatment did affect the rate at which seeds were removed by ants. Seeds with large elaiosomes were removed at a significantly higher rate than seeds with an unmanipulated elaiosome, which in turn were removed at a higher rate than seeds with no elaiosome. Rodents are major seed predators for D. wrightii, and the ability to be moved quickly once the seeds drop from the fruit is paramount for their success. In combination, the importance of rapid removal and ant preferences for larger elaiosomes can help explain why, even under resource-poor conditions, plants invest heavily in large elaiosomes.