Leucocytes are mass-invariant in reptiles unlike in endotherms

How do large and small animals defend against parasite challenges given the consequences of body mass on physiology and disease transmission? Functionally equivalent mammalian and avian granulocytes increased disproportionately with mass (i.e., hypermetrically), such that large organisms have higher concentrations than expected by mass alone. However, how ectothermic leucocyte concentrations scale is understudied. We hypothesized that differences exist in the scaling of leucocyte concentrations between endotherms and ectotherms as these groups have substantial differences in their physiology and immunology. Analysis was conducted using MCMCglmm, phylogenetically informed models to determine which existing scaling hypothesis best predicted relationships between mass and lymphocyte and heterophil concentrations among >120 species of reptiles and compared our results with those from birds and mammals. We also compared body mass and life history as a predictor of variation in immune cell concentrations. Reptilian lymphocyte concentrations scaled close to isometrically (i.e., mass-invariant; phylogenetic MCMCglmm: b(mean, 95% CI) = -0.040, [-0.072, -0.007]), and the slope was indistinguishable from the slope found for mammals and birds. In contrast, reptilian heterophil concentrations scaled isometrically (b =0.006, [-0.028, 0.040]), whereas functionally equivalent mammalian and avian granulocytes scaled hypermetrically. Although reptilian leucocytes were mass-invariant, the slope was greater than predicted if metabolic rate serves as a pace marker for immune defenses, suggesting that large organisms have greater protection than expected by the Rate of Metabolism Hypothesis. Life history was also a poor predictor of variation in reptilian heterophil and lymphocyte concentrations. Our results provide novel insight into large scale patterns of immune defenses of endothermic and ectothermic vertebrates and have direct implications for modeling the evolution of immune defenses and identifying parasite reservoirs.