Physiological convergence amongst ant-eating and termite-eating mammals

Ant- and termite-eating are among the few food habits common to monotremes, marsupials, and eutherians. Data are reported on the rate of metabolism and temperature regulation of 14 species of mammals having these food habits, including two monotremes, one marsupial and 11 eutherians. Small mammals with these habits have comparatively high body temperatures and high basal rates of metabolism, but ant- and termite-eaters that weigh more than 1 kg generally have low body temperatures and low basal rates of metabolism. The higher basal rates in small species ensure effective temperature regulation. Low body temperatures in large species principally result from low rates of metabolism. Rates of metabolism are low in these mammals because they use a food that has a limited availability and a low energy density, the density being further decreased in large species by the ingestion of non-nutritive material during feeding. Burrowing habits in some large species also contribute to low rates of metabolism. The combination of body size, food habits, and presence or absence of burrowing behaviour can account for all but about 6% of the range in basal rate in ant- and termite-eaters. Ants and termites, because of their locally clumped distributions, permit a larger mass in terrestrial predators than do other invertebrate prey. The reason why so many "primitive" mammals feed on ants and termites is that, once evolved, mammals with these habits are nearly impossible to displace ecologically, because much of ecological replacement is associated with high rates of reproduction, which are themselves correlated with high rates of metabolism in eutherians. Consequently, the ecological replacement of ant- and termite-eaters is inhibited, because this food habit does not permit high rates of metabolism, except at small masses.     

Complications inherent in scaling the basal rate of metabolism in mammals

The scaling of the basal rate of metabolism in mammals is reexamined. Both the power and level of the scaling function are sensitive to various factors that interact with body mass and rate of metabolism, including the precision of temperature regulation, food habits, and activity level. This sensitivity implies that the rate of metabolism is a highly plastic character in the course of evolution. Consequently, the singular effect of mass on the rate of metabolism is most effectively analyzed in ecologically and physiologically uniform sets of species, rather than in taxonomically defined groups, which often are ecologically and physiologically diverse. Otherwise, all fitted curves for mammals integrate a variety of competing factors, thereby reflecting the species used and denying unique analytic significance to the power in scaling relations. Kleiber's eutherian curve may represent a relatively uniform set of data because all the species included were domesticated and because selection for high rates of production (and high rates of metabolism) occurred in the process of domestication. In the analysis of scaling relationships, the standard error of estimate (Sy.x) is a more valuable measure of the residual variation than is (1.0-r2) because r2 is a non-linear measure of the conformation of data to the relation and because Sy.x, unlike r2, is independent of the units used in the scaling relationship. At present the best estimate indicates that total rate of metabolism scales proportionally to approximately m0.60 at small masses (less than 300 g), as long as small species do not enter torpor, and scales proportionally to approximately m0.75 at large masses (greater than or equal to 300 g). Physiological properties other than metabolism are potentially sensitive to secondary factors, so their scaling functions also would be most clearly defined for physiologically uniform groups of species. This view suggests that insight into the significance of scaling relations can be obtained by examining the residual variation around a scaling function as well as by examining conformation to the function.     

The evolution of energetics in eutherian “insectivorans”: an alternate approach

An analysis of standard energetics in 57 species of “insectivorans”, small eutherians that preferentially feed on soil invertebrates, indicated that a combination of climate, the use of torpor, substrate, food habits, and log₁₀ body mass accounted for 92.5% of the variation in log10 basal rate of metabolism in insectivorans, whereas log₁₀ body mass alone accounted for 76.7% of the variation. With the addition of subfamily affiliation, this analysis accounted for 95.5% of the variation in log₁₀ basal rate, the most distinctive subfamilies being Soricinae and the Talpinae, which have equally high basal rates.Sorex species have basal rates that average 2.5 times those of tropical crocidurines, reflecting an approach bySorex to life in cold climates that does not include the use of torpor, a stratagem widely used by crocidurines in warm-temperate and tropical climates. The absence of torpor inSorex may facilitate a high reproductive rate through a high basal rate of metabolism, a combination that may be incompatible with a small mass, insectivorous food habits, and life in the lowland tropics, but required in cold-temperate environments. Insectivorans other than shrews, moles, and cold-temperature hedgehogs have low basal rates principally in association with tropical distributions and the use of torpor. Basal rate of metabolism in insectivorans also correlated with ordinal, familial, subfamilial, and tribal affiliations. The suggestion that phylogeny is an important determinant of performance characters like rate of metabolism ignores the requirement that performance must be compatible with conditions in the environment and with a species’ other characteristics. The principal reason why performance characters are correlated with taxonomic affiliation is that many clades consist of species that share a common approach to the environment. Thus, clades not only represent evolutionary histories, they also are crude measures of physiological and behavioral performances.     

Standard energetics of leaf-nosed bats (Hipposideridae): its relationship to intermittent- and protracted-foraging tactics in bats and birds

Basal rates of metabolism within the insectivorous genera Hipposideros and Ascelliscus, Old World leaf-nosed bats (Hipposideridae), ranged from 58% to 77% of the mammalian standard. The larger species, Hipposideros diadema and Hipposideros maggietaylori, effectively thermoregulated at ambient temperatures down to 9 degrees C, whereas two smaller species, Hipposideros galeritus and Hipposideros cervinus, occasionally permitted body temperatures to fall below 32 degrees C. The low basal rates of metabolism in hipposiderids correlated with a predatory life-style characterized by intermittent flight from a perch to capture insects, a correlation similar to that found in nonpasserine birds. Intermittent-foraging bats and nonpasserines collectively had basal rates of metabolism that averaged 75% of those that pursue insects during protracted flight. However, no difference in basal rate was found between protracted- and intermittent-foraging passerines, which had basal rates 1.8- and 2.4-times those of protracted-foraging and intermittent-foraging bats and nonpasserines, respectively. Bats, swifts, and caprimulgids that enter torpor have basal rates that are 85% of those of similar species that do not enter torpor. Body mass, order affiliation, foraging mode, and propensity to enter into torpor collectively account for 97% of the variation in basal rate of metabolism in insectivorous bats and birds. Foraging style therefore appears to be a factor contributing to the diversity in endotherm energetics. Minimal thermal conductance in the genus Hipposideros ranged from 75% to 102% of the mammalian standard. Birds have minimal thermal conductances that are 75% of mammals and intermittent foragers have minimal conductances that are 78% of protracted foragers.     

The energetics of New Zealand's ducks

Measurements on rates of metabolism and temperature regulation are presented from nine populations of seven species of ducks resident in New Zealand. An analysis of these data and those from 18 additional species obtained from the literature indicates that basal rate of metabolism in anatids correlates with body mass and restriction to the Australian-New Zealand region: these 'southern' species have basal rates that average 70% of those from the Northern Hemisphere. The low basal rates of southern anatids may reflect reduced pectoral muscle masses in association with the absence of migratory habits and/or life on land masses without eutherian predators. New Zealand flightless teal (Anas aucklandica nesiotis, Anas aucklandica aucklandica) do not have mass-independent basal rates that differ from those found in flighted ducks living in the same region, although flightless teal have lower total basal rates than most ducks as a result of small masses. Minimal thermal conductance in this sample is determined by body mass alone. Regulated body temperature is negatively correlated with body mass.     

Development of thermoregulation and torpor in a marsupial: energetic and evolutionary implications

Altricial mammals and birds become endothermic at about half the size of adults and presumably would benefit energetically from entering torpor at that time. Because little is known about torpor during development in endotherms, we investigated whether after the establishment of endothermic thermoregulation (i.e. the ability to maintain a high body temperature during cold exposure), Sminthopsis macroura, a small (∼25 g) insectivorous marsupial, is capable of entering torpor and whether torpor patterns change with growth. Endothermic thermoregulation was established when the nest young reached a body mass of ∼10 g, and they were capable of entering torpor early during development at ∼10–12 g, lending some support to the view that torpor is a phylogenetically old mammalian trait. Torpor bout length shortened significantly and the minimum metabolic rate during torpor increased as juveniles approached adult size, and consequently total daily energy expenditure increased steeply with age. Relationships between total daily energy expenditure and body mass during development of S. macroura (slope ∼1.3) differed substantially from the relationship between basal metabolism and body mass in adult endotherms (slope ∼0.75) suggesting that the energy expenditure–size relationship during the development differs substantially from that in adults under thermo-neutral conditions. Our study shows that while torpor can substantially reduce energy expenditure during development of endotherms and hence is likely important for survival during energy bottlenecks, it also may enhance somatic growth when food is limited. We therefore hypothesize that torpor during the development in endotherms is far more widespread than is currently appreciated.     

Ecological factors affect the level and scaling of avian BMR

The basal rate of metabolism (BMR) in 533 species of birds, when examined with ANCOVA, principally correlates with body mass, most of the residual variation correlating with food habits, climate, habitat, a volant or flightless condition, use or not of torpor, and a highland or lowland distribution. Avian BMR also correlates with migratory habits, if climate and a montane distribution is excluded from the analysis, and with an occurrence on small islands if a flightless condition and migration are excluded. Residual variation correlates with membership in avian orders and families principally because these groups are behaviorally and ecologically distinctive. However, the distinction between passerines and other birds remains a significant correlate of avian BMR, even after six ecological factors are included, with other birds having BMRs that averaged 74% of the passerine mean. This combination of factors accounts for 97.7% of the variation in avian BMR. Yet, migratory species that belong to Anseriformes, Charadriiformes, Pelecaniformes, and Procellariiformes and breed in temperate or polar environments have mass-independent basal rates equal to those found in passerines. In contrast, penguins belong to an order of polar, aquatic birds that have basal rates lower than passerines because their flightless condition depresses basal rate. Passerines dominate temperate, terrestrial environments and the four orders of aquatic birds dominate temperate and polar aquatic environments because their high BMRs facilitate reproduction and migration. The low BMRs of tropical passerines may reflect a sedentary lifestyle as much as a life in a tropical climate. Birds have BMRs that are 30-40% greater than mammals because of the commitment of birds to an expensive and expansive form of flight.     

Allometries of the durations of torpid and euthermic intervals during mammalian hibernation: A test of the theory of metabolic control of the timing of changes in body temperature

Summary The durations of the intervals of torpor and euthermia during mammalian hibernation were found to be dependent on body mass. These relationships support the concept that the timing of body temperature changes is controlled by some metabolic process. Data were obtained from species spanning nearly three orders of magnitude in size, that were able to hibernate for over six months without food at 5°C. The timing of body temperature changes was determined from the records of copper-constantan thermocouples placed directly underneath each animal. Because all species underwent seasonal changes in their patterns of hibernation, animals were compared in midwinter when the duration of euthermic intervals was short and relatively constant and when the duration of torpid intervals was at its longest. Large hibernators remained euthermic longer than small hibernators (Fig. 2). This was true among and within species. The duration of euthermic intervals increased with mass at the same rate (mass^0.38) that mass-specific rates of euthermic metabolism decrease, suggesting that hibernators remain at high body temperatures until a fixed amount of metabolism has been completed. These data are consistent with the theory that each interval of euthermia is necessary to restore some metabolic imbalance that developed during the previous bout of torpor. In addition, small species remained torpid for longer intervals, than large species (Fig. 3). The absolute differences between different-sized species were large, but, on a proportional basis, they were comparatively slight. Mass-specific rates of metabolism during torpor also appear to be much less dependent on body mass than those during euthermia, but the precision of these metabolic measurements is insufficient for them to provide a conclusive test of the metabolic theory. Finally, small species with high mass-specific rates of euthermic metabolism are under tighter energetic constraints during dormancy than large species. The data presented here show that, in midwinter, small species compensate both by spending less time at high body temperatures following each arousal episode and by arousing less frequently, although the former is far more important energetically than the latter.     

An analysis of the factors that influence the level and scaling of mammalian BMR

The factors influencing the basal rate of metabolism (BMR) in 639 species of mammals include body mass, food habits, climate, habitat, substrate, a restriction to islands or highlands, use of torpor, and type of reproduction. They collectively account for 98.8% of the variation in mammalian BMR, but often interact in complex ways. The factor with the greatest impact on BMR, as always, is body mass (accounting for 96.8% of its variation), the extent of its impact reflecting the 10(6.17)-fold range of mass in measured species. The attempt to derive mathematically the power relationship of BMR in mammals is complicated by the necessity to include all of the factors that influence BMR that are themselves correlated with body mass. BMR also correlates with taxonomic affiliation because many taxa are distinguished by their ecological and behavioral characteristics. Phylogeny, reflecting previous commitments, may influence BMR either through a restriction on the realized range of behaviors or by opening new behavioral and ecological opportunities. A new opportunity resulted from the evolution by eutherians of a type of reproduction that permitted species feeding on high quality resources to have high BMRs. These rates facilitated high rates of gas, nutrient, and waste exchange between a pregnant eutherian and her placental offspring. This pattern led to high rates of reproduction in some eutherians, a response denied all monotremes and marsupials, thereby permitting eutherians to occupy cold-temperate and polar environments and to dominate other mammals in all environments to which ecologically equivalent eutherians had access.     

Food habits and the basal rate of metabolism in birds

Summary The correlation of basal rate of metabolism with various factors is examined in birds. Chief among these is body mass. As in mammals, much of the remaining variation in basal rate among birds is associated with food habits. Birds other than passerines that feed on grass, nectar, flying insects, or vertebrates generally have basal rates that are similar to mammals of the same mass and food habits. In contrast, most invertebrate-eating birds that weigh over 100 g have higher basal rates than equally-sized, invertebrate-eating mammals. The high basal rates of small passerines equal those of small mammals that do not enter torpor and represent the minimal cost of continuous endothermy. Large passerines and small procellariiforms, charadriiforms, and psittaciforms generally have higher basal rates than mammals with the same mass and food habits. The high basal rates of passerines (in combination with altricial habits) may have significance in permitting high post-natal growth rates and the exploitation of seasonally abundant resources. These interrelations may contribute to the predominance of passerines in temperate land environments.     

Food habits, energetics, and the reproduction of marsupials

Basal rate of metabolism in marsupials and in eutherian mammals is principally correlated with body mass, food habits and activity. Feeding on fruit, the leaves of woody plants, or invertebrates is associated with low basal rates, especially at large masses, in both groups of mammals. These foods lead to low basal rates because they are seasonally unavailable, are indigestible, or need to be detoxified. The depression in basal rate associated with frugivory and folivory is increased when coupled with sedentary, arboreal habits in both marsupials and eutherians. In contrast, eutherians that feed on vertebrates or herbs generally have high basal rates, while marsupials that eat these foods do not have high basal rates. These foods permit high basal rates, which are exploited by eutherians because high basal rates in these mammals lead to high rates of reproduction. Marsupials have, at best, a limited correlation of reproduction with rate of metabolism, so that feeding on vertebrates or herbs does not lead to high basal rates in these mammals. This difference between marsupials and eutherians in the coupling of reproduction to energetics has at least two ecological consequences. 1) Marsupials generally do not tolerate cold-temperate environments because they do not accelerate growth and development to complete reproduction within a short spring and summer. 2) Marsupials coexist with ecologically similar eutherians as long as marsupials have food habits that are correlated with low rates of metabolism in eutherians (i.e. they feed on fruit, the leaves of woody plants, or invertebrates), but they tend to be displaced by eutherians when marsupials have food habits that are associated with high rates of metabolism in eutherians (i.e. when they feed on vertebrates and, probably, herbs).     

Daily torpor and hibernation in birds and mammals

Many birds and mammals drastically reduce their energy expenditure during times of cold exposure, food shortage, or drought, by temporarily abandoning euthermia, i.e. the maintenance of high body temperatures. Traditionally, two different types of heterothermy, i.e. hypometabolic states associated with low body temperature (torpor), have been distinguished: daily torpor, which lasts less than 24 h and is accompanied by continued foraging, versus hibernation, with torpor bouts lasting consecutive days to several weeks in animals that usually do not forage but rely on energy stores, either food caches or body energy reserves. This classification of torpor types has been challenged, suggesting that these phenotypes may merely represent extremes in a continuum of traits. Here, we investigate whether variables of torpor in 214 species (43 birds and 171 mammals) form a continuum or a bimodal distribution. We use Gaussian‐mixture cluster analysis as well as phylogenetically informed regressions to quantitatively assess the distinction between hibernation and daily torpor and to evaluate the impact of body mass and geographical distribution of species on torpor traits. Cluster analysis clearly confirmed the classical distinction between daily torpor and hibernation. Overall, heterothermic endotherms tend to be small; hibernators are significantly heavier than daily heterotherms and also are distributed at higher average latitudes (～35°) than daily heterotherms (～25°). Variables of torpor for an average 30 g heterotherm differed significantly between daily heterotherms and hibernators. Average maximum torpor bout duration was >30‐fold longer, and mean torpor bout duration >25‐fold longer in hibernators. Mean minimum body temperature differed by ～13°C, and the mean minimum torpor metabolic rate was ～35% of the basal metabolic rate (BMR) in daily heterotherms but only 6% of BMR in hibernators. Consequently, our analysis strongly supports the view that hibernators and daily heterotherms are functionally distinct groups that probably have been subject to disruptive selection. Arguably, the primary physiological difference between daily torpor and hibernation, which leads to a variety of derived further distinct characteristics, is the temporal control of entry into and arousal from torpor, which is governed by the circadian clock in daily heterotherms, but apparently not in hibernators.     

Radiant heat affects thermoregulation and energy expenditure during rewarming from torpor

The high expenditure of energy required for endogenous rewarming is one of the widely perceived disadvantages of torpor. However, recent evidence demonstrates that passive rewarming either by the increase of ambient temperature or by basking in the sun appears to be common in heterothermic birds and mammals. As it is presently unknown how radiant heat affects energy expenditure during rewarming from torpor and little is known about how it affects normothermic thermoregulation, we quantified the effects of radiant heat on body temperature and metabolic rate of the small (body mass 25 g) marsupial Sminthopsis macroura in the laboratory. Normothermic resting individuals exposed to radiant heat were able to maintain metabolic rates near basal levels (at 0.91 ml O(2) g(-1) h(-1)) and a constant body temperature down to an ambient temperature of 12 degrees C. In contrast, metabolic rates of individuals without access to radiant heat were 4.5-times higher at an ambient temperature of 12 degrees C and body temperature fell with ambient temperature. During radiant heat-assisted passive rewarming from torpor, animals did not employ shivering but appeared to maximise uptake of radiant heat. Their metabolic rate increased only 3.2-times with a 15- degrees C rise of body temperature (Q(10)=2.2), as predicted by Q(10) effects. In contrast, during active rewarming shivering was intensive and metabolic rates showed an 11.6-times increase. Although body temperature showed a similar absolute change between the beginning and the end of the rewarming process, the overall energetic cost during active rewarming was 6.3-times greater than that during passive, radiant heat-assisted rewarming. Our study demonstrates that energetic models assuming active rewarming from torpor at low ambient temperatures can substantially over-estimate energetic costs. The low energy expenditure during passive arousal provides an alternative explanation as to why daily torpor is common in sunny regions and suggests that the prevalence of torpor in low latitudes may have been under-estimated in the past.     

Daily torpor in mice: high foraging costs trigger energy-saving hypothermia

Many animal species employ natural hypothermia in seasonal (hibernation) and daily (torpor) strategies to save energy. Facultative daily torpor is a typical response to fluctuations in food availability, but the relationship between environmental quality, foraging behaviour and torpor responses is poorly understood. We studied body temperature responses of outbred ICR (CD-1) mice exposed to different food reward schedules, simulating variation in habitat quality. Our main comparison was between female mice exposed to low foraging-cost environments and high-cost environments. As controls, we pair-fed a group of inactive animals (no-cost treatment) the same amount of pellets as high-cost animals. Mice faced with high foraging costs were more likely to employ torpor than mice exposed to low foraging costs, or no-cost controls (100% versus 40% and 33% of animals, respectively). While resting-phase temperature showed a non-significant decrease in high-cost animals, torpor was not associated with depressions in active-phase body temperature. These results demonstrate (i) that mice show daily torpor in response to poor foraging conditions; (ii) that torpor incidence is not attributable to food restriction alone; and (iii) that high levels of nocturnal activity do not preclude the use of daily torpor as an energy-saving strategy. The finding that daily torpor is not restricted to conditions of severe starvation puts torpor in mice in a more fundamental ecological context.     

The metabolism of New Guinean pteropodid bats

Rate of metabolism and body temperature were measured in eight species of pteropodid bats that live in Papua New Guinea. These data and those from 13 other species available in the literature are examined to determine the factors with which the energetics of pteropodids are correlated. Lowland populations of species that weigh < 35 g generally have low basal rates and often become torpid, whereas their highland populations had standard or high basal rates and were precise thermoregulators, as were all adult pteropodids that weighed more than 60 g. In large pteropodids belonging to the genera Dobsonia and Pteropus, females are smaller and consequently have lower total basal rates than males. Compared to species found on continents and large islands, species limited in distribution to small islands have lower basal rates, both because of a smaller mass and a reduction of metabolism independent of body mass. These trends are most marked in females that belong to small-island species, adjustments that may facilitate reproduction and survival on small oceanic islands with a limited resource base. Minimal thermal conductances are usually low in species that roost exposed in trees, whereas large species that roost in caves have either standard or high conductances.     

Reduction of metabolism during hibernation and daily torpor in mammals and birds: temperature effect or physiological inhibition?

Summary The present study addresses the controversy of whether the reduction in energy metabolism during torpor in endotherms is strictly a physical effect of temperature (Q₁₀) or whether it involves an additional metabolic inhibition. Basal metabolic rates (BMR; measured as oxygen consumption, ), metabolic rates during torpor, and the corresponding body temperatures (T _b) in 68 mammalian and avian species were assembled from the literature (n=58) or determined in the present study (n=10). The Q₁₀ for change in between normothermia and torpor decreased from a mean of 4.1 to 2.8 with decreasingT _b from 30 to <10°C in hibernators (species that show prolonged torpor). In daily heterotherms (species that show shallow, daily torpor) the Q₁₀ remained at a constant value of 2.2 asT _b decreased. In hibernators with aT _b<10°C, the Q₁₀ was inversely related to body mass. The increase of mass-specific metabolic rate with decreasing body mass, observed during normothermia (BMR), was not observed during torpor in hibernators and the slope relating metabolic rate and mass was almost zero. In daily heterotherms, which had a smaller Q₁₀ than the hibernators, no inverse relationship between the Q₁₀ and body mass was observed, and consequently the metabolic rate during torpor at the sameT _b was greater than that of hibernators. These findings show that the reduction in metabolism during torpor of daily heterotherms and large hibernators can be explained largely by temperature effects, whereas a metabolic inhibition in addition to temperature effects may be used by small hibernators to reduce energy expenditure during torpor.Abbreviation BMR basal metabolic rate     

Energetics of New Zealand's temperate parrots

Abstract

As in most parrots (Order: Psittaciformes) studied, New Zealand species have, independent of body mass, high basal rates of metabolism, low thermal conductances, and precisely regulated body temperatures. An analysis of covariance showed that basal rate in parrots correlates with body mass and thermal climate; temperate species have basal rates that are 21% higher than those of tropical species; and New Zealand's parrot have basal rates 32% higher than expected from body mass. Present information suggests that basal rate in parrots appears to be correlated neither with water availability in the environment nor with food habits. High basal rates in parrots are associated with large pectoral muscle masses. The very small pectoral muscle masses of the kakapo, Strigops habroptilus, predict that this flightless parrot has a low basal rate.     

Cool running: locomotor performance at low body temperature in mammals

Mammalian torpor saves enormous amounts of energy, but a widely assumed cost of torpor is immobility and therefore vulnerability to predators. Contrary to this assumption, some small marsupial mammals in the wild move while torpid at low body temperatures to basking sites, thereby minimizing energy expenditure during arousal. Hence, we quantified how mammalian locomotor performance is affected by body temperature. The three small marsupial species tested, known to use torpor and basking in the wild, could move while torpid at body temperatures as low as 14.8-17.9°C. Speed was a sigmoid function of body temperature, but body temperature effects on running speed were greater than those in an ectothermic lizard used for comparison. We provide the first quantitative data of movement at low body temperature in mammals, which have survival implications for wild heterothermic mammals, as directional movement at low body temperature permits both basking and predator avoidance.     

The comparative energetics of 'caviomorph' rodents.

The energetics of 11 species of New World hystricognath ('caviomorph') rodents are presented and compared with data from the literature on 19 additional species. Log(10) body mass alone accounts for 94% of the variation in the log(10) basal rate of metabolism in caviomorphs. The residual variation in basal rate is correlated with the stratum on which species live: arboreal species have low basal rates; terrestrial and fossorial species have intermediate basal rates; and aquatic species have high basal rates. When stratum is not included in the analysis, folivores, especially those that are arboreal, have lower basal rates than species with other food habits when combined with log(10) body mass. Small island endemics, all of which are folivores, have basal rates that are 61% of continental species. Log(10) basal rate correlates with family affiliation when combined with log(10) mass, but only if no other factor is included. Therefore, caviomorphs with low basal rates are arboreal, folivorous, live on small islands and belong to the Capromyidae, whereas other character combinations are associated with higher basal rates. These observations demonstrate that the basal rates of caviomorphs reflect many factor interactions. No differences in basal rate were found to reflect climate. Log(10) mass, the only factor to correlate with conductance, accounts for 82% of the variation in log(10) minimal thermal conductance. Mean interspecific body temperature was 36.9 degrees C; it was lowest in aquatic and fossorial species.