The Evolution of Mammalian Blood Parameters: Patterns and Their Interpretation

It is suggested that small cell size in mammals evolved as a means of facilitating rapid increase in blood hemoglobin through facultative increase in total erythrocyte number, possibly via splenic contraction, without the risk of reaching critical levels of viscosity.

Abstract

Taxonomic and allometric differences in blood parameters were studied in 206 species of mammals. Interspecific tests using the standard cross-species method and recently developed comparative methods that control for phylogenetic effects due to nonindependence of species emphasized the need for correct analytical methods. Contrary to previous studies, this study found that body mass was a signifcant predictor of blood parameters among mammals; body mass was positively and significantly correlated with red-cell volume and cell hemoglobin content and negatively correlated with red-cell count, platelet count, and percentage reticulocytes. Red-cell count and cell size were inversely correlated, such that hemoglobin levels tended to be relatively constant across species. Furthermore, this work suggests that, within species, high hemoglobin levels are maintained through high cell count rather than through large cell volume. The hypothesis is suggested that small cell size in mammals evolved as a means of facilitating rapid increase in blood hemoglobin through facultative increase in total erythrocyte number, possibly via splenic contraction, without the risk of reaching critical levels of viscosity.