Abstract
Chronic exposure to stressors has been shown to suppress immune function in vertebrates, making them more susceptible to pathogens. It is less clear, however, whether many natural stressors are immunosuppressive. Moreover, whether stressors make disease more likely or more severe in populations is unclear because animals respond to stressors both behaviorally and physiologically. We tested whether chronic exposure to three natural stressors of wood frog tadpoles—high-densities, predator-cues, and low-food conditions—influence their susceptibility to a lethal ranavirus both individually in laboratory experiments, and collectively in outdoor mesocosms. Prior to virus exposure, we observed elevated corticosterone only in low-food treatments, although other treatments altered rates of growth and development as well as tadpole behavior. None of the treatments, however, increased susceptibility to ranavirus as measured by the proportion of tadpoles that became infected or died, or the time to death compared to controls. In fact, mortality in the mesocosms was actually lower in the high-density treatment even though most individuals became infected, largely because of increased rates of metamorphosis. Overall we find no support for the hypothesis that chronic exposure to common, ecologically relevant challenges necessarily elevates corticosterone levels in a population or leads to more severe ranaviral disease or epidemics. Conditions may, however, conspire to make ranavirus infection more common in metamorphosing amphibians.
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Acknowledgments
We thank Robin Warne for providing RIA training and Keri VanCamp and Vassar College for the use of the Collins Field Station. Thanks also to Lynne Beatty, Deanna Russell, and Alex Guitard for their assistance with the mesocosm experiment. This research was funded by National Science Foundation grant IOS-0818212 to EJC and DEB1139199 to JLB and a Sigma Xi Grant in Aid of Research to BCR.
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Reeve, B.C., Crespi, E.J., Whipps, C.M. et al. Natural Stressors and Ranavirus Susceptibility in Larval Wood Frogs (Rana sylvatica). EcoHealth 10, 190–200 (2013). https://doi.org/10.1007/s10393-013-0834-6
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DOI: https://doi.org/10.1007/s10393-013-0834-6