SUMMARY
Immune responses benefit hosts by clearing pathogens, but they also incur physiological costs and tissue damage. While wild animals differ in how they balance these costs and benefits, the physiological mechanisms underlying such differential investment in immunity remain unknown. Uncovering these mechanisms is crucial to determining how and where selection acts to shape immunological defense. Among free-living song sparrows (Melospiza melodia) in western North America, sickness-induced lethargy and fever are more pronounced in Southern California than in Washington and Alaska. We brought song sparrows from two populations (Southern California and Washington) into captivity to determine whether these differences persist in a common environment and what physiological signals facilitate such differences. As in free-living sparrows, captive California birds exhibited more pronounced fever and lethargy than Washington birds in response to lipopolysaccharide, a non-pathogenic antigen that mimics bacterial infection. After treatment, the two populations showed similar reductions in luteinizing hormone levels, food intake and body mass, although treated birds from California lost more breast muscle tissue than treated birds from Washington. Moreover, California birds displayed higher bioactivity of interleukin-6, a pro-inflammatory cytokine, and marginally higher levels of corticosterone, a steroid hormone involved in stress, metabolism and regulating inflammatory responses. Our results show that immunological differences between these populations cannot be explained by immediate environment alone and may reflect genetic, maternal or early-life effects. Additionally, they suggest that cytokines play a role in shaping immunological variation among wild vertebrates.
FOOTNOTES
-
B9 cells were provided by Narayan C. Rath of the USDA and the University of Arkansas with the permission of Lucien Aarden of Sanquin Research and the University of Amsterdam. Robin Corcoran and Isabelle Bisson provided field assistance. Cassandra Nuñez and Siva Sundaresan helped transport and house birds. Nathan Gregory, Sergio Córdoba-Córdoba, Maria Echevery, Adrian deFroment, Christie Riehl and Ryan Chisholm helped measure anorexia. Thanks to Cassandra Nuñez, Christie Riehl, David Pattemore and two anonymous reviewers for comments on prior versions. This work was funded by the Max Plank Institute for Ornithology. J.S.A. was supported by the NSF Graduate Research Fellowship Program. J.C.W. was supported by NSF grant number IOS-0750540. L.B.M. was supported by NSF IOS-0920475.
-
Supplementary material available online at http://jeb.biologists.org/cgi/content/full/213/23/4099/DC1
- © 2010.