The role of symbiotic micro-organisms in the regulation of terrestrial food webs has only recently being addressed. One potential group of such symbionts are systemic and asymptomatic Neotyphodium fungal endophytes that form mutualistic associations with a variety of grasses. The effects of endophytes on higher trophic levels are still little known. In particular, no study has yet examined the effects of endophytes at the higher trophic levels of vertebrate food chains. We examined how feeding on Neotyphodium endophyte infected (E+) and endophyte free (E-) meadow ryegrass (Scherodonus pratensis) affects body mass and population size of sibling voles (Microtus levis), and whether the diet mediates the vulnerability of voles to least weasel (Mustela nivalis nivalis) predation. Because high vole mobility increase risk of encounters with weasels, that are known to be olfactory hunters, we also examined whether endophytes influence vole mobility and whether least weasels are able to distinguish olfactory cues of the voles fed on the two diets.

Results/Conclusions The performance and reproduction of voles did not differ between the two diets suggesting that voles appear relatively insensitive to endophyte consumption in this respect. However, E+ diet reduced vole activity and protected voles from least weasel predation. Even so, the reduced mobility of voles fed on E+ grass was unrelated to risk of predation. Voles exhibit an array of behaviours in their avoidance of predators. Many of these are related to mobility, for example fleeing and freezing. Especially the latter behaviour was frequently observed in encounters between E+ voles and weasels. It is plausible that reduced mobility was related to freezing under predation risk, thus explaining our counter-intuitive results.  Whatever the mechanism is, our study is the first to demonstrate that plant-associated microbial symbionts may affect top-down trophic interactions in vertebrate communities.