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

<p>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&rsquo; 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.</p>