Meeting Abstract
The impacts of body mass on the immune system are fairly unknown. Recent research on mammals found that neutrophil concentration scaled hypermetrically with body mass (that is, larger mammals had disproportionately higher concentrations), a surprising result not predicted by any scaling framework for immunity. Although we can predict the form of relationships between leukocyte cell concentrations and body mass in birds based on previous mammalian slopes, fundamental physiological differences (e.g. cell differentiation/storage, anatomical structures to accommodate flight, etc.) and evolutionary histories between taxa might have produced differences in their form. Here, we examined whether existing scaling hypotheses accurately predicted form of relationships of lymphocytes, eosinophil, and heterophils, the avian functional equivalent of neutrophils. We also examined the predictive ability of body mass, life-history variation and phylogenetic relationships for variation in the three cell types (i.e., an omnibus model). An intercept-only model best explained lymphocyte and eosinophil concentrations, supporting hypotheses with a slope of zero. The omnibus model that included body mass and life history variables best-explained variation in heterophils, however. In this model, body mass explained over 30% of the variation in heterophils with life-history traits providing comparatively little explanatory power (~8%). No a priori hypothesis predicted the hypermetric scaling we observed for heterophils (b=0.19 +/- 0.05). These results reveal surprising effects of body size on avian heterophil concentrations, consistent with observations in mammals, and imply that large organisms, generally, might require larger reserve pools of broadly protective immune cells.
