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      Investment in Constitutive Immune Function by North American Elk Experimentally Maintained at Two Different Population Densities

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          Natural selection favors individuals that respond with effective and appropriate immune responses to macro or microparasites. Animals living in populations close to ecological carrying capacity experience increased intraspecific competition, and as a result are often in poor nutritional condition. Nutritional condition, in turn, affects the amount of endogenous resources that are available for investment in immune function. Our objective was to understand the relationship between immune function and density dependence mediated by trade-offs between immune function, nutritional condition, and reproduction. To determine how immune function relates to density-dependent processes, we quantified bacteria killing ability, hemolytic-complement activity, and nutritional condition of North American elk ( Cervus elaphus) from populations maintained at experimentally high- and low-population densities. When compared with elk from the low-density population, those from the high-density population had higher bacteria killing ability and hemolytic-complement activity despite their lower nutritional condition. Similarly, when compared with adults, yearlings had higher bacteria killing ability, higher hemolytic-complement activity, and lower nutritional condition. Pregnancy status and lactational status did not change either measure of constitutive immunity. Density-dependent processes affected both nutritional condition and investment in constitutive immune function. Although the mechanism for how density affects immunity is ambiguous, we hypothesize two possibilities: (i) individuals in higher population densities and in poorer nutritional condition invested more into constitutive immune defenses, or (ii) had higher parasite loads causing higher induced immune responses. Those explanations are not mutually exclusive, and might be synergistic, but overall our results provide stronger support for the hypothesis that animals in poorer nutritional condition invest more in constitutive immune defenses then animals in better nutritional condition. This intriguing hypothesis should be investigated further within the larger framework of the cost and benefit structure of immune responses.

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          Most cited references 31

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          How should pathogen transmission be modelled?

           H. McCallum (2001)
          Host-pathogen models are essential for designing strategies for managing disease threats to humans, wild animals and domestic animals. The behaviour of these models is greatly affected by the way in which transmission between infected and susceptible hosts is modelled. Since host-pathogen models were first developed at the beginning of the 20th century, the 'mass action' assumption has almost always been used for transmission. Recently, however, it has been suggested that mass action has often been modelled wrongly. Alternative models of transmission are beginning to appear, as are empirical tests of transmission dynamics.
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            Population biology of infectious diseases: Part I.

            If the host population is taken to be a dynamic variable (rather than constant, as conventionally assumed), a wider understanding of the population biology of infectious diseases emerges. In this first part of a two-part article, mathematical models are developed, shown to fit data from laboratory experiments, and used to explore the evolutionary relations among transmission parameters. In the second part of the article, to be published in next week's issue, the models are extended to include indirectly transmitted infections, and the general implications for infectious diseases are considered.
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              Ecological immunology: costly parasite defences and trade-offs in evolutionary ecology.

              In the face of continuous threats from parasites, hosts have evolved an elaborate series of preventative and controlling measures - the immune system - in order to reduce the fitness costs of parasitism. However, these measures do have associated costs. Viewing an individual's immune response to parasites as being subject to optimization in the face of other demands offers potential insights into mechanisms of life history trade-offs, sexual selection, parasite-mediated selection and population dynamics. We discuss some recent results that have been obtained by practitioners of this approach in natural and semi-natural populations, and suggest some ways in which this field may progress in the near future.

                Author and article information

                [1 ]Department of Natural Resources and Environmental Sciences, University of Nevada, Reno, Nevada, United States of America
                [2 ]Pacific Northwest Research Station, United States Forest Service, La Grande, Oregon, United States of America
                Sonoma State University, UNITED STATES
                Author notes

                Competing Interests: The authors have declared that no competing interests exist.

                Conceived and designed the experiments: CJD KMS. Performed the experiments: CJD KMS BLD. Analyzed the data: CJD. Wrote the paper: CJD KMS.

                Role: Academic Editor
                PLoS One
                PLoS ONE
                PLoS ONE
                Public Library of Science (San Francisco, CA USA )
                20 May 2015
                : 10
                : 5

                This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication

                Figures: 3, Tables: 0, Pages: 17
                This study was supported by a Hatch grant awarded by Nevada Agriculture Experimental Station and funding from the University of Nevada Reno, and US Forest Service grant awarded to KMS. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Research Article
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