Interaction of allometry and development in the mouse Mus musculus: heart rate and hematology

The contribution of body mass changes to developmental adjustments in heart rate and hematology has been investigated in the mouse Mus musculus. Both resting heart rate (fH) and hematological variables including erythrocyte concentration, hemoglobin concentration, blood oxygen capacity, hematocrit, mean corpuscular hemoglobin and mean corpuscular volume, changed considerably during the increase in body mass from birth (1g) to adulthood (maximum of 50 g). There were two phases of change, one characteristic of preweaned mice (approximately less than 10 g) and the other of postweaned mice (approximately 10-50 g). In preweaned mice resting fH was about 1/2 of the value predicted on the basis of interspecific allometric data from mammals. fH increased steadily until body mass reached 10 g, then began to decrease with further mass increase at the same rate as predicted from interspecific allometric data. Erythrocyte concentration, hematocrit, hemoglobin concentration and blood oxygen capacity were all significantly lower in preweaned mice compared with postweaned mice. It is suggested that the progressive heart rate increase in very young mice may be to increase cardiac output to compensate for the neonatal anemia. After weaning, hematological variables showed little or no further change with increasing body mass. Collectively, these data indicate that during the early phases of postnatal growth, developmental factors other than body mass have the greatest influence on heart rate and hematology, and allometric data derived from interspecific studies on adults have little predictive value in neonates. After weaning, however, body mass is the major influence on these variables, and allometric data derived from interspecific studies on adults are reasonably accurate as predictors. We conclude that interspecific allometric studies must be properly regarded as the study of adult animals of different body sizes, and that untested assumptions about the applicability of these data to intraspecific studies of immature specimens should be made with extreme caution.