Questioning the mutual benefits of myrmecochory: a stable isotope-based experimental approach

1. Mutualisms play a key part in ecological systems and drive the evolution of much of the world's biological diversity. Among them, myrmecochory, seed dispersal by ants, is a worldwide mechanism throughout many ecosystems. However, the classic representation of myrmechocory as a mutualism could be put into question if one of the two players did not garner a real advantage. 2. A controlled diet experiment was conducted in which ants were given five diets (supplemented or not with elaiosomes). First, using an ant-seed mutualism system, the aim was to understand if elaiosome consumption could modify reproductive output and sex allocation. Second, nitrogen isotopic values were used as a nutrient tracer in the brood to estimate the consumption and selection of elaiosome in comparison to other diets. 3. A significant difference was found in the production of pupae between some diets, but pupae production was not linked to the elaiosome supplementation. Repartitions between pupa type and the effect of the diets were also not significantly different. Moreover, the nitrogen isotopic values of pupae differed among diets but not pupa types. 4. The mutualistic aspects of myrmechory remain unclear, especially when it comes to estimating the benefits to ants. It is clear that ants assimilate the elaiosome nutrients, but the effect of this assimilation on the reproductive output or sex allocation appears limited. Elaiosomes could provide a nutritional advantage under certain conditions when resources are scarce (qualitative or quantitative), but they are not nutritionally required, and thus represent a food source with no specific advantage to the ant.