Abstract
Evolutionary ecologists have long been interested by the link between different immune defenses and fitness. Given the importance of a proper immune defense for survival, it is important to understand how its numerous components are affected by environmental heterogeneity. Previous studies targeting this question have rarely considered more than two immune markers. In this study, we measured seven immune markers (response to phytohemagglutinin (PHA), hemolysis capacity, hemagglutination capacity, plasma bactericidal capacity, percentage of lymphocytes, percentage of heterophils, and percentage of eosinophils) in tree swallow (Tachycineta bicolor) nestlings raised in two types of agro-ecosystems of contrasted quality and over 2 years. First, we assessed the effect of environmental heterogeneity (spatial and temporal) on the strength and direction of correlations between immune measures. Second, we investigated the effect of an immune score integrating information from several immune markers on individual performance (including growth, mass at fledging and parasite burden). Both a multivariate and a pair-wise approach showed variation in relationships between immune measures across years and habitats. We also found a weak association between the integrated score of nestling immune function and individual performance, but only under certain environmental conditions. We conclude that the ecological context can strongly affect the interpretation of immune defenses in the wild. Given that spatiotemporal variations are likely to affect individual immune defenses, great caution should be used when generalizing conclusions to other study systems.
Highlights
Wild organisms are continuously exposed to pathogens such as viruses, bacteria, or macroparasites
The aim of this study was to assess the effects of environmental quality and annual variation on the correlations among seven immune markers in tree swallow nestlings raised in contrasted agro-ecosystems of southern Quebec, Canada
We measured the immunity of 210 tree swallow nestlings over the 2 years of the study (Table 1)
Summary
Wild organisms are continuously exposed to pathogens such as viruses, bacteria, or macroparasites. Several studies have shown that environmental conditions can affect an organism’s immune defenses (Neve et al 2007; Arriero 2009). The fact that environmental conditions can interact with an organism’s immune defenses has prompted the emergence of a new field often referred to as ecological immunology. Given that immune defenses are costly to develop, maintain, and use (reviewed in Lochmiller and Deerenberg 2000), individuals are expected to trade-off limited resources between immunity and other costly functions such as growth and reproduction. Maximization of an organism’s immune defenses is limited due to the risk of an over-responsive immune system, which can lead to autoimmune pathology (Graham et al 2005; Sadd and Siva-Jothy 2006). Immune defenses are expected to be under stabilizing selection.
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