Constraints on vertebrate athleticism: The temperature dependence of red blood cell volume

Abstract

The ability to perform at high levels of aerobic activity (i.e. athletic ability) increases with temperature among vertebrates. These differences in species' activity levels, from highly active to sedentary, are reflected in their ecology and behavior. Yet, the changes in the cardiovascular system that allow for greater oxygen supply rates at higher temperatures, and thus greater activity levels, remain unclear. Here we show that vertebrates provide more oxygen to tissues at higher temperatures in part by increasing the total volume of red blood cells in the body. Across 60 species of vertebrates (fishes,amphibians, reptiles, birds and mammals), whole-body red blood cell volume increases exponentially with temperature after controlling for effects of body size and taxonomy. These changes are accompanied by increases in relative heart mass, an indicator of athletic ability. The results help explain how temperature-dependent changes in cardiovascular design allow species to overcome the constraints of passive diffusion on oxygen supply.