Running Head: Parasitism during rodent range expansion Parasites and invasions: changes in gastrointestinal helminth assemblages in invasive and native rodents in Senegal

Understanding why some exotic species become widespread and abundant in their colonized range is a fundamental issue that still needs to be addressed. Among many hypotheses, newly established host populations may benefit from a "parasite release" through impoverishment of their original parasite communities or reduced infection levels. Moreover, the fitness of competing native hosts may be affected by the acquisition of exotic taxa from invaders ("parasite spill-over") and/or by an increased transmission risk of native parasites due to their amplification by invaders ("parasite spill-back"). We focused on gastrointestinal helminth (GIH) communities to determine whether these predictions could explain the ongoing invasion success of the commensal house mouse (Mus musculus domesticus) and black rat (Rattus rattus), as well as the associated drop of native Mastomys species, in Senegal. A decrease of overall prevalence and individual species richness of GIH were observed along the invasion gradients as well as lower specific prevalence/abundance (Aspiculuris tetraptera in M. m. domesticus, Hymenolepis diminuta in R. rattus) on the invasion front. Conversely, we did not find any strong evidence of GIH spill-over or spill-back in invasion fronts, where native and invasive rodents co-occurred. For both invasive species, our results were consistent with the predictions of the parasite release hypothesis. Further experimental research is needed to determine whether and how the loss of GIH and reduced infection levels along invasion routes may result in any advantageous effects on invader fitness and competitive