Spillover of an alien parasite reduces expression of costly behaviour in native host species

We used a natural experiment of populations of native red squirrels (<em>Sciurus vulgaris</em>) that were colonised by the alien grey squirrel (<em>Sciurus carolinensis</em>), comparing repeated measurements of red squirrel parasite infection and personality with those taken in sites where only the native species occurred.<br> Using data from 323 arena tests across three experimental (native species and IAS) and three control sites (only native species), we found negative correlations between native species’ activity and infection with <em>Strongyloides robustus</em> in the sites invaded by the alien species. Activity was also negatively correlated with infection by its native helminth (<em>Trypanoxyuris sciuri</em>) but only when grey squirrels were present, while in the red-only sites there was no relationship of <em>T. sciuri</em> infection with any of the personality traits. Moreover, individuals that acquired <em>S. robustus</em> during the study reduced their activity after infection, while this was not the case for animals that remained uninfected.