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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 measuredTLR-2 andTLR-4 expression, markers of pathogen surveillance and cytokine responses (changes inIL-1βandIL-10), regulators of inflammation, to a simulated bacterial infection. In non-native sparrows, we found that asTLR-4 expression increased,IL-1βandIL-10responses decreased, a relationship not observed in native sparrows. Additionally, higher body condition predicted largerIL-1βandIL-10responses in all birds. These findings suggest that highTLR-4surveillance may mitigate strong inflammatory responses in non-native sparrows, with pathological and resource-based costs driving immune variation among and within populations. 

Phenotypic plasticity is a major mechanism whereby organisms adjust their traits within‐generations to changes in environmental conditions. In the context of range expansions, plasticity is thought to be especially important, as plastic changes in traits can lead to rapid adaptation. One epigenetic process in particular, DNA methylation, enables organisms to adjust gene expression contingent on the environment, which suggests it may play a role in range expansions. At present, we know little about how methylation is regulated in wildlife, especially expression of the enzymes responsible for altering methyl marks on the genome. In this study, we compared expression of three epigenetic regulator genes (DNA methyltransferase 1, DNMT1; DNA methyltransferase 3, DNMT3; and one ten‐eleven translocation methylcytosine dioxygenase, TET2) in three tissues (gut, liver, and spleen) of house sparrowsPasser domesticusfrom nine countries. Some countries are in the native range of the species (Israel, the Netherlands, Norway, Spain, and Vietnam) whereas others are sites the species has colonized in the last 150 years (i.e. Australia, Canada, New Zealand, and Senegal). In this exploratory study, we asked whether non‐native birds and/or birds from sites with comparatively unpredictable climates would express different levels of these genes. We found that all three genes were expressed more in sparrows from the native range and from areas with more stable temperatures. Expression of all three genes was also strongly correlated among‐locations and within‐individuals, but mean expression was quite different among tissues. Many factors (e.g. urbanization of the capture site, sex of the bird) did not significantly affect gene expression, but others surprisingly did (e.g. latitude). Our results suggest that these enzymes could be important in range expansions or geographic distribution generally, but more detailed investigations will be insightful.