Body size affects immune cell proportions in birds and non-volant mammals, but not bats
Powered flight has evolved several times in vertebrates and constrains morphology and physiology in ways that likely have shaped how organisms cope with infections. Some of these constraints likely have impacts on aspects of immunology, such that larger fliers might prioritize risk reduction and safety. Addressing how the evolution of flight may have driven relationships between body size and immunity could be particularly informative for understanding the propensity of some taxa to harbor many virulent and sometimes zoonotic pathogens without showing clinical disease. Here, we used a scaling framework to quantify scaling relationships between body mass and the proportions of two types of white blood cells--lymphocytes, and granulocytes (neutr-/heterophils)--across 60 bat species, 414 bird species, and 256 non-volant mammal species. By using phylogenetically-informed statistical models on field-collected data from wild Neotropical bats, data gleaned from other wild bats available in the literature, and data from captive non-volant mammals and birds, we show that lymphocyte and neutrophil proportions do not vary systematically with body mass among bats. In contrast, larger birds and non-volant mammals have disproportionately higher granulocyte proportions than expected for their body size. Future comparative studies of wild bats, birds, and non-volant mammals of similar body mass should aim to further differentiate evolutionary effects and other aspects of life history on immune defense. Summary statementPowered flight might constrain morphology such that certain immunological features are prioritized. We show that bats largely have similar cell proportions across body mass compared to strong allometric scaling relationships in birds and non-flying mammals.