Economies of scaling: More evidence that allometry of metabolism is linked to activity, metabolic rate and habitat

Organismal metabolic rates influence many ecological processes, and the mass-specific metabolic rate of organisms decreases with increasing body mass according to a power law. The exponent in this equation is commonly thought to be the three-quarter-power of body mass, determined by fundamental physical laws that extend across taxa. However, recent work has cast doubt as to the universality of this relationship, the value of 0.75 being an interspecies 'average' of scaling exponents that vary naturally between certain boundaries. There is growing evidence that metabolic scaling varies significantly between even closely related species, and that different values can be associated with lifestyle, activity and metabolic rates. Here we show that the value of the metabolic scaling exponent varies within a group of marine ectotherms, chitons (Mollusca: Polyplacophora: Mopaliidae), and that differences in the scaling relationship may be linked to species-specific adaptations to different but overlapping microhabitats. Oxygen consumption rates of six closely related, co-occurring chiton species from the eastern Pacific (Vancouver Island, British Columbia) were examined under controlled experimental conditions. Results show that the scaling exponent varies between species (between 0.64 and 0.91). Different activity levels, metabolic rates and lifestyle may explain this variation. The interspecific scaling exponent in these data is not significantly different from the archetypal 0.75 value, even though five out of six species-specific values are significantly different from that value. Our data suggest that studies using commonly accepted values such as 0.75 derived from theoretical models to extrapolate metabolic data of species to population or community levels should consider the likely variation in exponents that exists in the real world, or seek to encompass such error in their models. This study, as in numerous previous ones, demonstrates that scaling exponents show large, naturally occurring variation, and provides more evidence against the existence of a universal scaling law. © 2012 Elsevier B.V.

A comparison of Diamond Forrester and coronary calcium scores as gatekeepers for investigations of stable chest pain

To determine if calcium scores (CS) could act as a more effective gatekeeper than Diamond Forrester (DF) in the assessment of patients with suspected coronary artery disease (CAD). A sub-study of the Cardiac CT for the Assessment of Chest Pain and Plaque (CAPP) study, a randomised control trial evaluating the cost-effectiveness of cardiac CT in symptomatic patients with stable chest pain. Stable pain was defined as troponin negative pain without symptoms of unstable angina. 250 patients undergoing cardiac CT had both DF scores and CS calculated, with the accuracy of both evaluated against CT coronary angiogram. Criteria given in UK national guidelines were compared. Of the 250 patients, 4 withdrew. 140 (57 %) patients were male. The mean DF was 47.8 and mean CS 172.5. Of the 144 patients with non-anginal pain 19.4 % had significant disease (>50 % stenosis). In general the DF over estimated the presence of CAD whereas the CS reclassified patients to lower risk groups, with 91 in the high risk DF category compared to 26 in the CS. Both receiver operating curve and McNemar Bowker test analysis suggested the DF was less accurate in the prediction of CAD compared to CS [Formula: see text] Projected downstream investigations were also calculated, with the cost per number of significant stenoses identified cheaper with the CS criteria. Patients with suspected stable CAD are more accurately risk stratified by CS compared to the traditional DF. CS was more successful in the prediction of significant stenosis and appears to be more effective at targeting clinical resources to those patients that are in need of them.