Abstract
While metabolism is a fundamental feature of all organisms, the causes of its scaling with body mass are not yet fully explained. Nevertheless, observations of negative correlations between red blood cell (RBC) size and the rate of metabolism suggest that size variation of these cells responsible for oxygen supply may play a crucial role in determining metabolic rate scaling in vertebrates. Based on a prediction derived from the Cell Metabolism Hypothesis, metabolic rate should increase linearly with body mass in species with RBC size invariance, and slower than linearly when RBC size increases with body mass. We found support for that prediction in five species of eyelid geckos (family Eublepharidae) with different patterns of RBC size variation during ontogenetic growth. During ontogeny, metabolic rate increases nearly linearly with body mass in those species of eyelid geckos where there is no correlation between RBC size and body mass, whereas non-linearity of metabolic rate scaling is evident in those species with ontogenetic increase of RBC size. Our findings provide evidence that ontogenetic variability in RBC size, possibly correlating with sizes of other cell types, could have important physiological consequences and can contribute to qualitatively different shape of the intraspecific relationship between metabolic rate and body mass.
Highlights
IntroductionOur understanding as to the determinants of metabolic rate scaling with body size remains unsatisfactory
Despite decades of research, our understanding as to the determinants of metabolic rate scaling with body size remains unsatisfactory
The negative relationship between genome size and mass-corrected metabolic rate in both passerine birds and mammals [17,18] together with the fact that genome size strongly correlates with red blood cell (RBC) size in both groups [19,20] suggest that just RBC size variation may be crucially connected with metabolic scaling, possibly due to their direct role in oxygen supply
Summary
Our understanding as to the determinants of metabolic rate scaling with body size remains unsatisfactory This has recently become a subject of renewed intense debate among the proponents of several models put forward to explain patterns of metabolic allometry [1,2,3,4,5]. In the latter two experimental studies, red blood cell (RBC) size was taken as a proxy of general cell size of an organism. It is possible that the link between metabolic rate and cell size expected under the Cell Metabolism Hypothesis as formulated by Kozłowski et al [2] holds in vertebrates only for RBCs rather than for organismal cells in general
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