Microbial surveillance versus cytokine responsiveness in native and non-native house sparrows

The success of introduced species often relies on flexible traits, including immune system traits. While theories predict non-natives will have weak defences due to decreased parasite pressure, effective parasite surveillance remains crucial, as infection risk is rarely zero and the evolutionary novelty of infection is elevated in non-native areas. This study examines the relationship between parasite surveillance and cytokine responsiveness in native and non-native house sparrows, hypothesizing that non-natives maintain high pathogen surveillance while avoiding costly inflammation. We made this specific prediction, as this pattern could enable invaders to effectively mitigate pathogen risk in a manner commensurate with the life-history priorities of a colonizing organism (i.e. rapid maturation and high reproductive effort). To test this hypothesis, we measured TLR-2 and TLR-4 expression, markers of pathogen surveillance and cytokine responses (changes in IL-1β and IL-10), regulators of inflammation, to a simulated bacterial infection. In non-native sparrows, we found that as TLR-4 expression increased, IL-1β and IL-10 responses decreased, a relationship not observed in native sparrows. Additionally, higher body condition predicted larger IL-1β and IL-10 responses in all birds. These findings suggest that high TLR-4 surveillance may mitigate strong inflammatory responses in non-native sparrows, with pathological and resource-based costs driving immune variation among and within populations.