Activity alters how temperature influences intraspecific metabolic scaling: testing the metabolic-level boundaries hypothesis.

A common belief is that the body-mass scaling of metabolic rate is the result of intrinsic (physical) constraints related to body design. However, many recent studies have shown that extrinsic (ecological) factors significantly affect metabolic scaling relationships, both within and among species. One of these factors is ambient temperature (T), but its influence on the intraspecific (ontogenetic) scaling slope (b) of metabolic rate varies widely. I tested whether the metabolic-level boundaries hypothesis (MLBH) can explain this variation, at least in part. The MLBH predicts that b should correlate negatively with T for resting metabolism, but show variable associations (from negative to positive) for metabolism measured during varying levels of locomotor activity. I tested the MLBH by using a literature survey of T effects on b for resting or routine metabolic rates of 179 animal and plant species. As predicted, sedentary species of ectothermic animals and plants exhibiting no locomotor activity mostly show negative associations between b and T, whereas mobile species exhibiting various low levels of uncontrolled, spontaneous, routine locomotor activity during measurements of resting or routine metabolism show varied associations. A similar difference occurs between teleost fish species exhibiting no locomotor activity versus varying levels of routine locomotor activity. These results show that intrinsic and extrinsic factors (e.g., activity and ambient temperature, respectively) can interactively affect metabolic scaling. Metabolic scaling is highly malleable, and not the simple result of universal physical constraints.

A Perspective on Body Size and Abundance Relationships across Ecological Communities.

Recently, several studies have reported relationships between the abundance of organisms in an ecological community and their mean body size (called cross-community scaling relationships: CCSRs) that can be described by simple power functions. A primary focus of these studies has been on the scaling exponent (slope) and whether it approximates -3/4, as predicted by Damuth's rule and the metabolic theory in ecology. However, some CCSR studies have reported scaling exponents significantly different from the theoretical value of -3/4. Why this variation occurs is still largely unknown. The purpose of our commentary is to show the value of examining both the slopes and elevations of CCSRs and how various ecological factors may affect them. As a heuristic exercise, we reanalyzed three published data sets based on phytoplankton, rodent, and macroinvertebrate assemblages that we subdivided according to three distinctly different ecological factors (i.e., climate zone, season, and trophic level). Our analyses reveal significant variation in either or both the CCSR slopes and elevations for marine phytoplankton communities across climate zones, a desert rodent community across seasons, and saltwater lagoon macroinvertebrate communities across trophic levels. We conclude that achieving a comprehensive understanding of abundance-size relationships at the community level will require consideration of both slopes and elevations of these relationships and their possible variation in different ecological contexts.

Activity affects intraspecific body-size scaling of metabolic rate in ectothermic animals.

Metabolic rate is commonly thought to scale with body mass (M) to the 3/4 power. However, the metabolic scaling exponent (b) may vary with activity state, as has been shown chiefly for interspecific relationships. Here I use a meta-analysis of literature data to test whether b changes with activity level within species of ectothermic animals. Data for 19 species show that b is usually higher during active exercise (mean +/- 95% confidence limits = 0.918 +/- 0.038) than during rest (0.768 +/- 0.069). This significant upward shift in b to near 1 is consistent with the metabolic level boundaries hypothesis, which predicts that maximal metabolic rate during exercise should be chiefly influenced by volume-related muscular power production (scaling as M (1)). This dependence of b on activity level does not appear to be a simple temperature effect because body temperature in ectotherms changes very little during exercise.

Pulse oximetry vascular assessment in patients with leg ulcers.

Arterial screening of the lower limbs is recommended before compression therapy is initiated in patients with ulceration or dermatitis. Hand-held Doppler ABPI is the accepted assessment tool but has limitations. Pulse oximeters have potential as alternative screening instruments with some advantages over Doppler. This article reviews the mode of action, application and limitations of pulse oximeters. A pulse oximetry toe/finger arterial screening index, which may be used as an alternative to Doppler ABPI, is described. Case studies are reported in which arterial blood flow which was easily detected by pulse oximetry could not be detected by Doppler.