Fitness outcomes in relation to individual variation in constitutive innate immune function

Abstract

Although crucial for host survival when facing persistent parasite pressure, costly immune functions will inevitably compete for resources with other energetically expensive traits such as reproduction. Optimizing, but not necessarily maximizing, immune function might therefore provide net benefit to overall host fitness. Evidence for associations between fitness and immune function is relatively rare, limiting our potential to understand ultimate fitness costs of immune investment. Here, we assess how measures of constitutive immune function (haptoglobin, natural antibodies, complement activity) relate to subsequent fitness outcomes (survival, reproductive success, dominance acquisition) in a wild passerine (Malurus coronatus). Surprisingly, survival probability was not positively linearly predicted by any immune index. Instead, both low and high values of complement activity (quadratic effect) were associated with higher survival, suggesting that different immune investment strategies might reflect a dynamic disease environment. Positive linear relationships between immune indices and reproductive success suggest that individual heterogeneity overrides potential resource reallocation trade-offs within individuals. Controlling for body condition (size-adjusted body mass) and chronic stress (heterophil-lymphocyte ratio) did not alter our findings in a sample subset with available data. Overall, our results suggest that constitutive immune components have limited net costs for fitness and that variation in immune maintenance relates to individual differences more closely.

Keywords: constitutive; fitness; induced; maintenance cost; resource reallocation; trade-off.

Figure 1.

The effect of individual variation in immune indices on subsequent fitness-related outcomes in purple-crowned fairy-wrens. Fitted lines show the predicted probability of survival (a–c), successful reproduction of at least one recruited offspring (d–f), and acquisition of a dominant breeding position (g–i), in response to (from top to bottom) haptoglobin (Hp), natural antibodies (NAbs) and complement activity (CA). Ribbons show confidence intervals based upon error of fixed effects only, while all continuous variables are held at median values. Lines are shown for males in May only, as reference categories for ‘sex' (all responses) and ‘time of year' (reproductive success response only). Lines and ribbons are derived from models fitted independently with a single index and maximum sample. Data ticks bordering each plotting space show raw data values at binary 1 and 0 outcomes; y-axes are cropped to improve visualization of the fitted lines. Plots labelled with ‘*' denote significant effects of indices in final models.