The impacts of body mass on immune cell concentrations in birds

Body mass affects many biological traits, but its impacts on immune
defenses are fairly unknown. Recent research on mammals found that
neutrophil concentrations disproportionately increased (scaled
hypermetrically) with body mass, a result not predicted by any existing
theory. Although the scaling relationship for mammals might predict how
leukocyte concentrations scale with body mass in other vertebrates,
vertebrate classes are distinct in many ways that might affect their
current and historic interactions with parasites and hence the evolution
of their immune systems. Subsequently, here, we asked which existing
scaling hypothesis best-predicts relationships between body mass and
lymphocyte, eosinophil, and heterophil concentrations—the avian functional
equivalent of neutrophils—among >100 species of birds. We then
examined the predictive power of body mass relative to life-history
variation, as an extensive literature indicates that the timing of key
life events has influenced immune system variation among species. Finally,
we ask whether avian scaling patterns differ from the patterns we observed
in mammals. We found that an intercept-only model best-explained
lymphocyte and eosinophil concentrations among birds, indicating that the
concentrations of these cell types were both independent of body mass. For
heterophils, however, body mass explained 31% of the variation in
concentrations among species, much more than life-history variation (4%).
As with mammalian neutrophils, avian heterophils scaled hypermetrically
(b=0.19 +/- 0.05), but more steeply than mammals (~1.5x;
0.11+/- 0.03). As such, we discuss why birds might require more
broadly-protective cells compared to mammals of the same body size.
Overall, body mass appears to have strong influences on the architecture
of immune systems.