We summarize the recent information on field metabolic rates (FMR) of wild terrestrial vertebrates as determined by the doubly labeled water technique. Allometric (scaling) relationships are calculated for mammals (79 species), reptiles (55 species), and birds (95 species) and for various taxonomic, dietary, and habitat groups within these categories. Exponential equations based on body mass are offered for predicting rates of daily energy expenditure and daily food requirements of free-ranging mammals, reptiles, and birds. Significant scaling differences between various taxa, dietary, and habitat groups (detected by analysis of covariance with P < or = 0.05) include the following: (a) The allometric slope for reptiles (0.889) is greater than that for mammals (0.734), which is greater than that for birds (0.681); (b) the slope for eutherian mammals (0.772) is greater than that for marsupial mammals (0.590); (c) among families of birds, slopes do not differ but elevations (intercepts) do, with passerine and procellariid birds having relatively high FMRs and gallinaceous birds having low FMRs; (d) Scleroglossan lizards have a higher slope (0.949) than do Iguanian lizards (0.793); (e) desert mammals have a higher slope (0.785) than do nondesert mammals; (f) marine birds have relatively high FMRs and desert birds have low FMRs; and (g) carnivorous mammals have a relatively high slope and carnivorous, insectivorous, and nectarivorous birds have relatively higher FMRs than do omnivores and granivores. The difference detected between passerine and nonpasserine birds reported in earlier reviews is not evident in the larger data set analyzed here. When the results are adjusted for phylogenetic effects using independent contrasts analysis, the difference between allometric slopes for marsupials and eutherians is no longer significant and the slope difference between Scleroglossan and Iguanian lizards disappears as well, but other taxonomic differences remain significant. Possible causes of the unexplained variations in FMR that could improve our currently inaccurate FMR prediction capabilities should be evaluated, including many important groups of terrestrial vertebrates that remain under- or unstudied and such factors as reproductive, thermoregulatory, social, and predator-avoidance behavior.

Abstra~t. Field m~tabolic rates (FMRs or HF), all measured using doubly labeled water, o~ 23 species of eut_henan mammals, 13 species of marsupial mammals, and 25 species of birds were sumi?anzed and ana_lyzed allometrically (log 10 -log, 0 regressions). FMR is strongly correlated with body mass m each of these groups. FMR scales differently than does basal or standard met~bol~c rate in eutherians (FMR slope= 0.81) and marsupials (FMR ~lope= 0.58J, but not m birds (FMR slope= 0.64 overall, but 0.75 in passerines and 0.75 mall other birds). Medium-sized (240-550 g) eutherians, marsupials, and birds have similar FMRs, and these are ~ 1 7 times as high as FMRs of like-sized ectothermic vertebrates ~uch as iguanid lizards. Fo~ endothermic vertebrates, the energy cost of surviving in nature Is. enormous compar.ed With that for ectotherms. Within the eutherians, marsupials, or b~rds, FMR sca!es differently for the following subgroups: rodents, passerine birds, her-bivor~us euther~ans, herbivorous marsupials, desert eutherians, desert birds, and seabirds. Equat~ons are give~ for use in predicting daily and annual FMR and food requirement of a species of terrestnal vertebrate, given its body mass.

more than sixfold (ratio of highest over lowest). Some of this variation is associated with affiliations with lower taxonomic levels (Infraclass: eutherian vs metatherian mammals; Family: passerine, procellariform and galliform birds vs other birds), some is associated with habitat (especially desert vs nondesert), and some with differences in basic diet preference and foraging mode and season. The scaling slopes for FMR often differ from BMR slopes for the same Class of animals, and most differ from the theoretical slope of 0.75. Differences among slopes and intercepts that were detected using conventional regression analyses were largely confirmed upon reanalysis using Independent Contrasts Analysis to adjust for phylogenetic biases.

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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 ...