Sustained metabolic scope.

Abstract

Sustained metabolic rates (SusMR) are timeaveraged metabolic rates that are measured in free-ranging anhuals maintaining constant body mass over periods long enough that metabolism is fueled by food intake rather than by transient depletion of energy reserves. Many authors have suggested that SusMR of various wild animal species are only a few times resting (basal or standard) metabolic rates (RMR). We test this conclusion by analyzing all 37 species (humans, 31 other endothermic vertebrates, and 5 ectothermic vertebrates) for which SusMR and RMR had both been measured. For all species, the ratio of SusMR to RMR, which we term sustained metabolic scope, is less than 7; most values fall between 1.5 and 5. Some of these values, such as those for Tour de France cyclists and breeding birds, are surely close to sustainable metabolic ceilings for the species studied. That is, metabolic rates higher than 7 times RMR apparently cannot be sustained indefinitely. These observations pose several questions: whether the proximate physiological causes of metabolic ceilings reside in the digestive tract's ability to process food or in each tissue's metabolic capacity; whether ceiling values are independent of the mode of energy expenditure; whether ceilings are set by single limiting physiological capacities or by coadjusted clusters of capacities (symmorphosis); what the ultimate evolutionary causes of metabolic ceilings are; and how metabolic ceilings may limit animals' reproductive effort, foraging behavior, and geographic distribution.Most studies of peak metabolic rates have dealt with brief bursts of activity. As is well known, the shorter the burst, the higher the metabolic rate or power output that can be sustained (see Fig. 1 Burst metabolic rate equals maximum power output maintained by a human (e.g., a runner) or animal, as a declining function of the duration that the individual is able to sustain that output. The dashed line is the RMR of the same individual. The present paper argues that power output declines to an asymptotic value at long times-a maximal achievable value of time-averaged SusMR-that is only a few times the RMR. stored energy reserves, which eventually become depleted. Is there a ceiling on the time-averaged metabolic rate that an animal or human can sustain indefinitely, for days or weeks, while remaining in energy balance through food intake? (Naturally, to pursue the example of a human athlete, we are not picturing a runner as jogging uninterruptedly day and night for weeks, but instead as alternately running, resting, eating, and sleeping and thereby achieving some timeaveraged metabolic rate on a maximal training regimen.) In other words, does the curve of Fig. 1 eventually decline to an asymptote that could be termed the sustained metabolic rate (SusMR)?Human experience makes it obvious that such an asymptotic ceiling must exist. For example, the athletes who compete in the annual Tour de France bicycle race are highly motivated to maximize their time-averaged power output. The race ...

Associations between basal metabolic rate and reproductive performance in C57BL/6J mice

“…SusMR is presumed to be limited by an animal's morphology. A big gut, and associated organs (such as the liver) that can process energy faster, makes more energy available to support SusMR but also requires a greater amount of maintenance (BMR) (Drent and Daan, 1980;Hammond and Diamond, 1992;Hammond and Diamond, 1997;Peterson et al, 1990;Weiner, 1989;Weiner, 1992). Therefore, an individual with a higher BMR will have greater capacity for SusMR and, if available energy is unlimited, they may be able to sustain greater reproductive output.…”

Energetic costs of activity by lizards in the field

“…The TRM for an ectothermic animal incorporates the energy expenditure of a resting animal with the daily body temperature pattern found in animals in the field (Anderson & Karasov 1981;Congdon & Tinkle 1982;Peterson, Nagy & Diamond 1990;Benabib & Congdon 1992). Although %AR is a more aggregated index than ECT, and therefore provides less detailed information about the subdivision of an animal's energy budget, it is easier to obtain because it does not require direct measurements of time, distance travelled and/or energy expended in locomotion.…”

Limits to sustained energy intake. XIV. Heritability of reproductive performance in mice

“…adipose tissue) (Drent and Daan, 1980;Peterson et al, 1990;Weiner, 1992). SusEI is an important parameter because it may provide an upper bound that constrains many aspects of animal performance, like reproductive output, migration behaviour and thermoregulatory capabilities (Drent and Daan, 1980;Hammond and Diamond, 1997;Johnson et al, 2001a;Piersma, 2011;Speakman and Król, 2005a).…”