Importance of elaiosome size to removal of ant-dispersed seeds

Several studies relate removal of diaspores from different ant-dispersed plant species to the size of their elaiosomes. This study is the first one to relate intraspecific variation in elaiosome size to removal of diaspores by ants. This approach circumvents the problem posed by interspecific variation in chemistry and morphology of elaiosomes. We used the system Hepatica nobilis Mill. (Ranunculaceae) and Myrmica ruginodis Ny1. The elaiosome was the attractive part of the diaspore and its attractiveness decreased quickly after release from the fruit. In experimental runs where ants collected diaspores, the elaiosome size of accepted diaspores was larger than of those rejected and the largest diaspores were collected first. Since size of diaspore, elaiosome and achene were correlated, the diaspores that were removed first had both the largest elaiosome and achene. However, our experiments suggested that elaiosome size was more important to removal than achene size or the elaiosome:achene size ratio. If ant dispersal improves plant fitness, elaiosome size and hence diaspore size would be expected to increase over time. However, such directional selection mediated by the ants is probably counterbalanced by the plant. Seed predators and a negative trade-off between number and size of seeds would, among other factors, select for smaller diaspore size, i.e. counteract the effect of the ants' preference for larger elaiosomes.