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A central assumption in ecological immunology is that immune responses are costly, with costs manifesting directly (e.g., increases in metabolic rate and increased amino acid usage) or as tradeoffs with other life processes (e.g., reduced growth and reproductive success). Across taxa, host longevity, timing of maturity, and reproductive effort affect the organization of immune systems. It is reasonable, therefore, to expect that these and related factors should also affect immune activation costs. Specifically, species that spread their breeding efforts over a long lifetime should experience lower immune costs than those that mature and breed quickly and die comparatively early. Likewise, body mass should affect immune costs, as body size affects the extent to which hosts are exposed to parasites as well as how hosts can combat infections (via its effects on metabolic rates and other factors). Here, we used phylogenetic meta-regression to reveal that, in general, animals incur costs of immune activation, but small species that are relatively long-lived incur the largest costs. These patterns probably arise because of the relative need for defense when infection risk is comparatively high and fitness can only be realized over a comparatively long period. However, given the diversity of species considered here and the overall modest effects of body mass and life history on immune costs, much more research is necessary before generalizations are appropriate.

There are at least two reasons to study traits that mediate successful range expansions. First, dispersers will found new populations and thus impact the distribution and evolution of species. Second, organisms moving into new areas will influence the fate of resident communities, directly competing with or indirectly affecting residents by spreading non-native or spilling-back native parasites. The success of invaders in new areas is likely mediated by a counterbalancing of costly traits. In new areas where threats are comparatively rare, individuals that grow rapidly and breed prolifically should be at an advantage. High investment in defenses should thus be disfavored. In the present study, we compared the energetic, nutritional and collateral damage costs of an inflammatory response among Kenyan house sparrow (Passer domesticus) populations of different ages, asking whether costs were related to traits of individuals from three different capture sites. Kenya is among the world's most recent range expansions for this species, and we recently found that the expression of Toll-like receptors (TLRs), leukocyte receptors that instigate inflammatory responses when bound to microbial elements, was related to the range expansion across the country. Here, we found (contrary to our expectations) that energetic and nutritional costs of inflammation were higher, but damage costs were lower, in range-edge compared to core birds. Moreover, at the individual level, TLR-4 expression was negatively related to commodity costs (energy and a critical amino acid) of inflammation. Our data thus suggest that costs of inflammation, perhaps mediated by TLR expression, might mitigate successful range expansions.

Hosts can manage parasitic infections using an array of tactics, which are likely to vary contingent on coevolutionary history between the host and the parasite. Here we asked whether coping ability of congeners that differ in host-parasite coevolutionary history differed in response to experimental infections with a coccidian parasite. House sparrows (Passer domesticus) and gray-headed sparrows (Passer griseus) are sympatric and ecologically similar, but house sparrows are recent colonizers of Kenya, the site of our comparison, whereas gray-headed sparrows are native. We evaluated three variables as barometers of infection coping ability: vertical flight, pectoral muscle size, and fat score. We also measured routing of a dose of (13)C-labeled leucine, an essential amino acid, among tissues to compare resource allocation strategies in response to infection. We found that burden effects on performance were minimal in both species, but house sparrows maintained considerably higher burdens than gray-headed sparrows regardless of exposure. House sparrows also had more exogeneous leucine tracer in all tissues after 24 h, demonstrating a difference in the way the two species allocate or distribute resources. We argue that house sparrows may be maintaining larger resource reserves to mitigate costs associated with exposure and infection. Additionally, in response to increased parasite exposure, gray-headed sparrows had less leucine tracer in their spleens and more in their gonads, whereas house sparrows did not change allocation, perhaps indicating a trade-off that is not experienced by the introduced species.

Summary Two adaptationist hypotheses have been proposed to explain why stress, particularly elevation of stress hormones (i.e., glucocorticoids), tends to suppress immune functions. One is that immune suppression represents efforts to minimize autoimmune responses to self-antigens released as organisms cope with stressors (i.e., the autoimmune-avoidance hypothesis). The other is that immune suppression occurs to promote a shunting of resources to life processes more conducive to survival of the stressor (i.e., the re-allocation hypothesis). Here in wild-caught house sparrows (Passer domesticus), we tested the second hypothesis, asking whether sustained elevation of baseline glucocorticoids, due to captivity, caused a greater rate of decline in immune functions than flight performance. A greater decline in immune functions than flight performance would support the re-allocation hypothesis. As in previous studies, we found that captivity tended to alter baseline corticosterone, suggesting that house sparrows experience captivity as a stressor. Captivity also affected several constitutive and induced innate immune metrics: bacterial (Escherichia coli) killing activity (BKA) of blood and oxidative burst of leukocytes both changed in a manner consistent with immune disregulation. In contrast, breast muscle size and vertical flight (hovering) duration improved over captivity. Collectively, these changes provide indirect support for the re-allocation hypothesis, although within individuals, changes in immune and physical performance were unrelated.