Different constraints shaped red blood cell characteristics in vertebrates

The correlation between body mass and haematological parameters has been investigated in mammals many times in the past, and to a much lesser degree in other vertebrates. Given the pivotal role of red blood cells (RBCs) in delivering oxygen to tissues, a correlation between haematological parameters and metabolic rates could also be expected. In the present study, I investigated these correlations using data from 513 vertebrate species. Most parameters showed poor (but significant) correlation with either body mass or metabolic rates. Nonetheless, haematocrit was significantly higher in endotherms compared with ectotherms. One of the most interesting parameters is RBC volume (mean corpuscular volume, MCV), also because this, together with haemoglobin concentration, determines all other haematological parameters. MCV is correlated with metabolic rates (and thus, indirectly, with body mass) and haemoglobin-oxygen affinity. Moreover, in species with nucleated RBCs (i.e. non-mammals), it is strongly correlated with genome size. This study shows how haematological parameters co-evolved with metabolic demands and the constraint posed by a factor extrinsic to the gas exchange system (i.e. genome size).