The Functional Significance of Variation in Hematological Traits that Determine Aerobic Capacity

A principal aim of evolutionary physiology is to understand how physiological variation affects the fitness and distribution of organisms. Physiological traits underlie performance, behavior, and life histories and exhibit variation at every taxonomic level. Studies within and between species can reveal whether variation in a physiological trait is currently, and/or was historically, functionally significant. Intraspecific variation is regarded as the raw material on which selection acts, and studying it in relation to measures of fitness can help identify the targets of selection that underlie life history strategies. Comparing interspecific variation with ecological factors can help clarify whether the physiological variation was either shaped by adaptation in response to different selective pressures or influenced differences in habitat and life histories.Traits related to energy expenditure are believed to play a principal role in shaping the evolution of life histories. Hematocrit, the percent of red blood cells per unit volume of blood is a measure of an individual’s oxygen carrying potential and therefore is pivotal in determining endurance. Given that birds show extensive variation in hematocrit within and between species and that flight imposes high energetic costs, variation in hematocrit in birds is likely to be functionally significant. I used observational, experimental, and comparative approaches to investigate the functional significance of intraspecific and interspecific hematocrit variation in birds. Intraspecific hematocrit variation could influence fitness if it plays a role in regulating eggshell colouration or by limiting physical endurance, thereby shaping reproductive decisions. Experimental manipulation of hematocrit in free-living European starlings provided no evidence that hematocrit variation affects eggshell coloration. However, natural and experimental variation in hematocrit influenced fitness measures, suggesting that hematocrit variation, if heritable, is acted upon by natural selection. Comparative analyses indicated that hematocrit variation across passerines was related to habitat altitude, latitude and migration, suggesting adaptive or exaptive significance. Overall, the results suggest that variation in hematocrit within and between avian species is functionally significant. Such studies can help to understand population dynamics, demography, biodiversity, and responses to climate change.