Mass-specific Metabolism Research Articles

Snail shell shape, force of attachment, and metabolic rate together cope with the intertidal challenge

Inhabitants of rocky intertidal shores, including gastropods, require specific adaptations to cope with numerous challenges that vary across the intertidal levels. We collected Stramonita biserialis snails from upper (wave-protected and intense predation) and lower (wave-exposed and low predation) intertidal sites to compare the following traits: shell skeleton (ventral and abaxial lateral views of shell shape, thickness, and mass), foot size, energy metabolism, and attachment strength to determine whether the trait values of snails from each zone fit the environmental challenges they face. We used a Principal Component Analysis to reduce the dimensionality of the data. Multivariate Analysis of Covariance (MANCOVA) for comparing characteristics between the two intertidal zones, and Partial Least Squares (PLS) analyses for testing the integration of overall snail characteristics. The traits of the snails of the two intertidal sites matched with the adaptations expected to allow them to cope with their contrasting challenges. The snails from the lower intertidal had more streamlined shells (which reduces drag forces) and a larger aperture and foot extension (which increase the strength of their attachment to the substrate) compared to snails from the upper sites. Snails from the lower intertidal also had a high mass-specific metabolism and soft body proportion, indicating that these snails from the wave-exposed sites have an energetically active musculature that matches their strong substrate attachment. The thin shell walls of the snails of the lower intertidal match the relatively low predatory pressure there.

On the natural selection of body mass allometries

I use data based life history models to illustrate natural selection causes for the evolution of inter-specific body mass allometries in birds and mammals. This illustrates i) how the primary selection of resource handling and mass-specific metabolism generates net energy for individuals, ii) how the selected net energy generates a population dynamic feedback selection where intra-specific interactive competition selects body masses that scale in proportion with net energy on the timescale of natural selection, iii) how the feedback selection of body mass buffers ecological variation in survival, iv) how the exponents of body mass allometries are selected from the dominant spatial dimensionality of the foraging ecology, v) how the population density allometry is affected by inter-specific competition, and vi) how primary selected metabolism bends the metabolic allometry and explains a metabolic invariance across major taxa of vertebrates.

Effect of different masses, ages, and coats on the thermoregulation of dogs before and after exercise across different seasons.

Domestic dogs demonstrate a positive correlation between body mass and whole-animal metabolism but a negative relationship between body mass and lifespan in dogs. Additionally, essential physiological mechanisms in domestic dogs, such as the relationships between thermal relations and body size and age, remain poorly understood. In this study, we looked at thermoregulation in dogs of different sizes, ages, coat types, and head morphologies across three different seasons. We used tympanic membrane temperatures (Tear) and infrared thermography to observe temperature regulation in pet dogs before and after a 45-min moderate walking exercise trial. We hypothesized that Tear and heat dissipation is positively correlated with body mass. Using network analysis, we found that body mass was among the most central features for spring and summer trials, but not for the winter trials. Similarly, leg length, snout length, and paw width were the central predictors in two of the three seasons. Mediation analysis demonstrated that nose and snout length act as significant mediators of the effects of body mass on mouth temperatures in the spring. For the summer trials, nose length and paw width significantly mediated the effect of body mass on mouth temperatures. Age, however, does not seem to be a major determinant of thermoregulation in dogs according to best subset models. A cross-seasonal examination of repeated measurements showed that mouth temperature heat dissipation rates decreased with increasing temperature and humidity. Overall, our findings support our hypothesis that Tear and heat dissipation rates are positively correlated with body mass in dogs, thus, negatively correlated with mass-specific metabolism. This finding suggests that small dogs allocate a bigger proportion of their metabolism to "inefficiencies" of heat production to offset greater heat loss.

Genome size influences adaptive plasticity of water loss, but not metabolic rates, in lungless salamanders.

Many expressions of phenotype, such as physiological performance, integrate multiple underlying traits to function. Linking component traits to adaptive physiology thus gives insight into mechanisms of selection acting on performance. Genome size (C-value) is a trait that influences physiology in multiple taxa by exerting a nucleotypic effect, constraining cell size and cellular physiology such that whole-organism mass-specific metabolism is reduced with increasing C-value. We tested for this mechanism of C-value function acting in lungless salamanders, plus an unexplored potential mechanism of C-value effects constraining water transport across the body surface to influence cutaneous water loss rates. We found no evidence for a nucleotypic effect on metabolic rates, but we demonstrate a relationship between C-value and water loss physiology. Under warmer experimental conditions, C-value was inversely correlated with water loss and positively correlated with resistance to water loss, which demonstrated adaptive plasticity at higher temperatures. We hypothesize that this pattern results from differences in cell size constraining diffusion and evaporation of water from the skin under warm conditions when cutaneous perfusion is reduced. Testing this hypothesis may confirm a previously unappreciated adaptive role for C-value variation in this group, and reveals the possibility that genome size influences physiological exchange across transport barriers more broadly.

Untangling life span and body mass discrepancies in canids: phylogenetic comparison of oxidative stress in blood from domestic dogs and wild canids.

Canids are a morphological and physiological diverse group of animals, with the most diversity found within one species, the domestic dog. Underlying observed morphological differences, there must also be differences at other levels of organization that could lead to elucidating aging rates and life span disparities between wild and domestic canids. Furthermore, small-breed dogs live significantly longer lives than large-breed dogs, while having higher mass-specific metabolic rates and faster growth rates. At the cellular level, a clear mechanism underlying whole animal traits has not been fully elucidated, although oxidative stress has been implicated as a potential culprit of the disparate life spans of domestic dogs. We used plasma and red blood cells from known aged domestic dogs and wild canids, and measured several oxidative stress variables: total antioxidant capacity (TAC), lipid damage, and enzymatic activities of catalase, superoxide dismutase, and glutathione peroxidase (GPx). We used phylogenetically informed general linear mixed models and nonphylogenetically corrected linear regression analysis. We found that lipid damage increases with age in domestic dogs, whereas TAC increases with age and TAC and GPx activity increases as a function of age/maximum life span in wild canids, which may partly explain longer potential life spans in wolves. As body mass increases, TAC and GPx activity increase in wild canids, but not domestic dogs, highlighting that artificial selection may have decreased antioxidant capacity in domestic dogs. We found that small-breed dogs have significantly higher circulating lipid damage compared with large-breed dogs, concomitant to their high mass-specific metabolism and higher growth rates, but in opposition to their long life spans.

The Natural Selection of Metabolism Explains Curvature in Fossil Body Mass Evolution

The natural selection of metabolism and mass can explain inter-specific body mass allometries from prokaryotes to mammals (Witting in Theor Popul Biol 117:23–42, https://doi.org/10.1016/j.tpb.2017.08.005, 2017a), with allometric exponents that depend on the selected metabolism and the packing of home ranges in predominately one (1D), two (2D), or three (3D) spatial dimensions. The predicted exponent for total metabolism for a 2D packing of home ranges increases from 3/4 to 7/4 when the fraction of the inter-specific body mass variation that follows from the natural selection of metabolism increases from zero to one. While a 3/4 exponent is commonly observed for inter-specific comparisons in mammals, a 7/4 exponent has so far not been reported. Yet, I detect the full range of exponents for evolution over time in the fossil record. There are no fossil data for allometric correlations between metabolism and mass, but I show that the allometry $$ \left( {\dot{w} \propto w^{{\hat{\dot{w}}}} } \right) $$ for the rate of evolution $$ \left( {\dot{w} = {\text{ d }}w/{\text{ d }}t} \right) $$ in mass (w) in physical time (t) is given by the underlying set of allometries for life history parameters, including mass-specific metabolism. The $$\hat{\dot{w}}$$ exponent describes the curvature of body mass evolution in time, with predicted values including: 3/2 (2D) for within niche evolution in small horses over 54 million years. 5/4 (2D) and 9/8 (3D) for across niche evolution of maximum mass in four mammalian clades. 3/4 (2D) for fast evolution in large horses, and maximum mass in trunked and terrestrial mammals. And 1 for maximum mass across major lifeforms during 3.5 billion years of evolution along a metabolic bound. These results integrate the inter-specific allometries of existing species with a deeper understanding of their natural selection during evolutionary diversification over millions of years.

Cellular metabolism and IL-6 concentrations during stimulated inflammation in primary fibroblasts from small and large dog breeds as they age.

The immune system undergoes marked changes during aging characterized by a state of chronic, low-grade inflammation termed 'inflammaging'. We explore this phenomenon in domestic dogs, which are the most morphologically and physiologically diverse group of mammals, with the widest range in body sizes for a single species. Additionally, smaller dogs tend to live significantly longer than larger dogs across all breeds. Body size is intricately linked to mass-specific metabolism and aging rates, which suggests that dogs are exemplary for studies in inflammaging. Dermal fibroblast cells play an important role in skin inflammation, making them a good model for inflammatory patterns across dog breed, body sizes and ages. Here, we examined aerobic and glycolytic cellular metabolism, and IL-6 concentrations in primary fibroblast cells isolated from small and large dog breeds, that were either recently born puppies or old dogs after death. We found no differences in cellular metabolism when isolated fibroblasts were treated with lipopolysaccharide (LPS) from Escherichia coli to stimulate an inflammatory phenotype. Unlike responses observed in mice and humans, there was a less drastic amplification of IL-6 concentration after LPS treatment in the geriatric population of dogs compared with recently born dogs. In young dogs, we also found evidence that untreated fibroblasts from large breeds had significantly lower IL-6 concentrations than observed for smaller breeds. This implies that the patterns of inflammaging in dogs may be distinct and different from other mammals commonly studied.

Energetic costs of locomotion in bears: is plantigrade locomotion energetically economical?

Ursids are the largest mammals to retain a plantigrade posture. This primitive posture has been proposed to result in reduced locomotor speed and economy relative to digitigrade and unguligrade species, particularly at high speeds. Previous energetics research on polar bears (Ursus maritimus) found locomotor costs were more than double predictions for similarly sized quadrupedal mammals, which could be a result of their plantigrade posture or due to adaptations to their Arctic marine existence. To evaluate whether polar bears are representative of terrestrial ursids or distinctly uneconomical walkers, this study measured the mass-specific metabolism, overall dynamic body acceleration, and gait kinematics of polar bears and grizzly bears (Ursus arctos) trained to rest and walk on a treadmill. At routine walking speeds, we found polar bears and grizzly bears exhibited similar costs of locomotion and gait kinematics, but differing measures of overall dynamic body acceleration. Minimum cost of transport while walking in the two species (2.21 J kg-1m-1) was comparable to predictions for similarly sized quadrupedal mammals, but these costs doubled (4.42 J kg-1m-1) at speeds ≥5.4 km h-1 Similar to humans, another large plantigrade mammal, bears appear to exhibit a greater economy while moving at slow speeds.

Body mass and correlated ecological variables in the North American muskrat lineage: evolutionary rates and the tradeoff of large size and speciation potential

This study tracks evolutionary change in body mass (W) and correlated ecological variables over the 3.75 million year history of the North American muskrat (Ondatra zibethicus). A new model is presented suggesting muskrat body mass has been in equilibrium for most of its history. Four pulses of pronounced size increase are correlated with glacial dynamics and volcanic events. Ranges of evolutionary rates in darwins and a new metric based on percent change in W document episodic size change. Proportional size change is independent of interval length, with a background range attributed to natural selection ≤25–30%. In increasing body mass by a factor of ten to about 1 kg mass-specific metabolism was halved, home range quadrupled, population density decreased fourfold, and average biomass more than doubled. Estimates of species diversity in ancient cotton rats (Sigmodon) and muskrats are calculated from a function derived from the correlation of numbers of North American rodent species and mean W. The phyletic mode of muskrat body size increase is explained as a combination of large body size reducing speciation coupled with an aquatic lifestyle. To the ecological consequences of large size in evolving clades (Cope’s rule) we can now add reduced speciation potential.

The natural selection of metabolism and mass selects allometric transitions from prokaryotes to mammals

The exponents of inter-specific allometries for several life history (metabolism, lifespan, reproductive rate, survival) and ecological (population density, home range) traits may evolve from the spatial dimensionality (d) of the intra-specific interactive competition that selects net assimilated energy into mass, with 1∕4 exponents being the two-dimensional (2D) case of the more general 1∕2d (Witting, 1995). While the exponents for mass-specific metabolism cluster around the predicted -1/4 and -1/6 in terrestrial and pelagic vertebrates, the allometries of mobile organisms are more diverse than the prediction. An exponent around zero has been reported for protists and protozoa (Makarieva et al., 2005, 2008), and the exponent appears to be strongly positive in prokaryotes with a value of about 5/6 (DeLong et al., 2010).I show that the natural selection of metabolism and mass is sufficient to explain exponents for mass-specific metabolism that decline from 5/6 over zero to −1∕6 in 3D, and from 3/4 over zero to −1∕4 in 2D. These results suggest that mass-specific metabolism is selected as the pace of the resource handling that generates net energy for self-replication and the selection of mass, with the decline in the metabolic exponent following from a decline in the importance of mass-specific metabolism for the selection of mass. The body mass variation in prokaryotes is found to be selected from primary variation in mass-specific metabolism, while the variation in multicellular animals is selected from primary variation in the handling and/or densities of the underlying resources, with protists and protozoa being selected as an intermediate lifeform.

Negative relationships between population density and metabolic rates are not general.

Population density has recently been suggested to be an important factor influencing metabolic rates and to represent an important 'third axis' explaining variation beyond that explained by body mass and temperature. In situations where population density influences food consumption, the immediate effect on metabolism acting through specific dynamic action (SDA), and downregulation due to fasting over longer periods, is well understood. However, according to a recent review, previous studies suggest a more general effect of population density per se, even in the absence of such effects. It has been hypothesized that this results from animals performing anticipatory responses (i.e. reduced activity) to expected declines in food availability. Here, we test the generality of this finding by measuring density effects on metabolic rates in 10 clones from two different species of the zooplankton Daphnia (Daphnia pulex Leydig and D.magna Straus). Using fluorescence-based respirometry, we obtain high-precision measures of metabolism. We also identify additional studies on this topic that were not included in the previous review, compare the results and evaluate the potential for measurement bias in all previous studies. We demonstrate significant variation in mass-specific metabolism among clones within both species. However, we find no evidence for a negative relationship between population density and mass-specific metabolism. The previously reported pattern also disappeared when we extended the set of studies analysed. We discuss potential reasons for the discrepancy among studies, including two main sources of potential bias (microbial respiration and declining oxygen consumption due to reduced oxygen availability). Only one of the previous studies gives sufficient information to conclude the absence of such biases, and consistent with our results, no effect of density on metabolism was found. We conclude that population density per se does not have a general effect on mass-specific metabolic rate.

Comparative locomotor costs of domestic dogs reveal energetic economy of wolf-like breeds.

The broad diversity in morphology and geographic distribution of the 35 free-ranging members of the family Canidae is only rivaled by that of the domesticated dog, Canis lupus familiaris. Considered to be among nature's most elite endurance athletes, both domestic and wild canids provide a unique opportunity to examine the variability in mammalian aerobic exercise performance and energy expenditure. To determine the potential effects of domestication and selective breeding on locomotor gait and economy in canids, we measured the kinematics and mass-specific metabolism of three large (>20 kg) dog breed groups (northern breeds, retrievers and hounds) of varying morphological and genomic relatedness to their shared progenitor, the gray wolf. By measuring all individuals moving in preferred steady-state gaits along a level transect and on a treadmill, we found distinct biomechanical, kinematic and energetic patterns for each breed group. While all groups exhibited reduced total cost of transport (COT) at faster speeds, the total COT and net COT during trotting and galloping were significantly lower for northern breed dogs (3.0 and 2.1 J kg-1m-1, respectively) relative to hound (4.2 and 3.4 J kg-1m-1, respectively) and retriever dogs (3.8 and 3.0 J kg-1m-1, respectively) of comparable mass. Similarly, northern breeds expended less energy per stride (3.5 J kg-1stride-1) than hounds or retrievers (5.0 and 4.0 J kg-1stride-1, respectively). These results suggest that, in addition to their close genetic and morphological ties to gray wolves, northern breed dogs have retained highly cursorial kinematic and physiological traits that promote economical movement across the landscape.

Effects of body mass and water temperature on routine metabolism of American paddlefish Polyodon spathula

This study quantified the effects of temperature and fish mass on routine metabolism of the American paddlefish Polyodon spathula. Thermal sensitivity, as measured by Q(10) value, was low in P. spathula. Mean Q(10) was 1·78 while poikilotherms are generally expected to have Q(10) values in the 2·00-2·50 range. Mass-specific metabolism did not decrease with increased fish size to the extent that this phenomenon is observed in teleosts, as evidenced by a mass exponent (β) value of 0·92 for P. spathula compared with 0·79 in a review of teleost species. Other Acipenseriformes have exhibited relatively high β values for mass-specific respiration. Overall P. spathula metabolism appears to be more dependent on body mass and less dependent on temperature than for many other fishes. An equation utilizing temperature and fish mass to estimate gross respiration for P. spathula was derived and this equation was applied to respiratory data from other Acipenseriformes to assess inter-species variation. Polyodon spathula respiration rates across water temperature and fish mass appear most similar to those of Atlantic sturgeon Acipenser naccarii and white sturgeon Acipenser transmontanus.

Intrinsic vs. extrinsic influences on life history expression: metabolism and parentally induced temperature influences on embryo development rate

Intrinsic processes are assumed to underlie life history expression and trade-offs, but extrinsic inputs are theorised to shift trait expression and mask trade-offs within species. Here, we explore application of this theory across species. We do this based on parentally induced embryo temperature as an extrinsic input, and mass-specific embryo metabolism as an intrinsic process, underlying embryonic development rate. We found that embryonic metabolism followed intrinsic allometry rules among 49 songbird species from temperate and tropical sites. Extrinsic inputs via parentally induced temperatures explained the majority of variation in development rates and masked a relationship with metabolism; metabolism explained a minor proportion of the variation in development rates among species, and only after accounting for temperature effects. We discuss evidence that temperature further obscures the expected interspecific trade-off between development rate and offspring quality. These results demonstrate the importance of considering extrinsic inputs to trait expression and trade-offs across species.

Comment on “Energy Uptake and Allocation During Ontogeny”

We demonstrate that the model of energy allocation during ontogeny of Hou et al. (Reports, 31 October 2008, p. 736) fails to account for the observed elevation of metabolic rate in growing organisms compared with similarly sized adults of different species. The basic model assumptions of the three-quarter power scaling for resting metabolism and constancy of the mass-specific maintenance metabolism need to be reassessed.

The Ecology of Body Size and Depth Use by Bloater (Coregonus hoyi Gill) in the Laurentian Great Lakes: Patterns and Hypotheses

Deepwater ciscoes (genus Coregonus, subgenus Leucichthys) radiated into six phenotypes that occupy different depths in the Great Lakes, based upon body size and lipid content. Large, lipid-dense ciscoes occupy greater depths than small, lean ciscoes. This relationship is observed between adults and juveniles of the most prevalent deepwater cisco, the bloater (C. hoyi Gill). Lipid-dense adult bloaters are restricted to the hypolimnion, whereas lean juveniles are found primarily in the epilimnion. This article critically reviews, synthesizes, and provides hypotheses from the literature on the ecology of body size and depth use of bloater. Factors influencing depth use in bloater are categorized by Fry's (1971) environmental factors. The case is made for two parsimonious hypotheses to explain the depth distribution by body size of bloater: (1) the optimal foraging-antipredation (OFA) hypothesis, and (2) the mass-specific metabolism hypothesis. The OFA hypothesis relates abundance of piscivores with abundance and extent of diel vertical migration of bloater. The mass-specific metabolism hypothesis relates body size to density-dependent growth, metabolism, swimming activity and hence depth distribution of bloater. Follow-up hypotheses and predictions are presented and may broaden our understanding of trophic ecology and adaptive radiation of deepwater ciscoes in the Great Lakes.

Energetics and longevity in birds

The links between energy expenditure and ageing are different at different levels of enquiry. When studies have examined the relationships between different species within a given class the association is generally negative--animals with greater metabolism per gram of tissue live shorter lives. Within species, or between classes (e.g. between birds and mammals) the association is the opposite--animals with higher metabolic rates live longer. We have previously shown in mammals that the negative association between lifespan and metabolic rate is in fact an artefact of using resting rather than daily energy expenditure, and of failing to adequately take into account the confounding effects of body size and the lack of phylogenetic independence of species data. When these factors are accounted for, across species of mammals, the ones with higher metabolism also have the largest lifetime expenditures of energy-consistent with the inter-class and intra-specific data. A previous analysis in birds did not yield the same pattern, but this may have been due to a lack of sufficient power in the analysis. Here we present an analysis of a much enlarged data set (>300 species) for metabolic and longevity traits in birds. These data show very similar patterns to those in mammals. Larger individuals have longer lives and lower per-gram resting and daily energy expenditures, hence there is a strong negative relationship between longevity and mass-specific metabolism. This relationship disappears when the confounding effects of body mass and phylogeny are accounted for. Across species of birds, lifetime expenditure of energy per gram of tissue based on both daily and resting energy expenditure is positively related to metabolic intensity, mirroring these statistical relationships in mammals and synergizing with the positive associations of metabolism with lifespan within species and between vertebrate classes.

Kv1.3 gene-targeted deletion alters longevity and reduces adiposity by increasing locomotion and metabolism in melanocortin-4 receptor-null mice

Gene-targeted deletion of the voltage-gated potassium channel, Kv1.3, results in 'super-smeller' mice that have altered firing patterns of mitral cells in the olfactory bulb, modified axonal targeting to glomerular synaptic units, and behaviorally have an increased ability to detect and discriminate odors. Moreover, the Kv1.3-null mice weighed less than their wild-type counterparts, have modified ingestive behaviors, and are resistant to fat deposition following a moderately high-fat dietary regime. In this study, we investigate whether or not gene-targeted deletion of Kv1.3 (Shaker family member) can abrogate weight gain in a genetic model of obesity, the melanocortin-4 receptor-null mouse (MC4R-null). Mice with double gene-targeted deletions of Kv1.3 and MC4R were generated by interbreeding Kv1.3 (Kv)- and MC4R-null mouse lines to homozygosity. Developmental weights, nose to anus length, fat pad weight, fasting serum chemistry, oxygen consumption, carbon dioxide respiration, locomotor activity and caloric intake were monitored in control, Kv-null, MC4R-null and Kv/MC4R-null mice. Physiological and metabolic profiles were acquired at postnatal day 60 (P60) in order to explore changes linked to body weight at the reported onset of obesity in the MC4R-null model. Gene-targeted deletion of Kv1.3 in MC4R-null mice reduces body weight by decreasing fat deposition and subsequent fasting leptin levels, without changing the overall growth, fasting blood glucose or serum insulin. Gene-targeted deletion of Kv1.3 in MC4R-null mice significantly extended lifespan and increased reproductive success. Basal or light-phase mass-specific metabolic rate and locomotor activity were not affected by genetic deletion of Kv1.3 in MC4R-null mice but dark-phase locomotor activity and mass-specific metabolism were significantly increased resulting in increased total energy expenditure. Gene-targeted deletion of Kv1.3 can reduce adiposity and total body weight in a genetic model of obesity by increasing both locomotor activity and mass-specific metabolism.

Ontogeny of predator-sensitive foraging and routine metabolism in larval shorthorn sculpin, Myoxocephalus scorpius

Most animals will reduce foraging activity in the presence of a predatory threat. However, little is known about the onset of this decision-making ability during the early life stages of fishes, and how the trade-off between foraging and predator-avoidance may be affected by changes in metabolic demand during ontogeny. To examine these issues, the foraging behaviour of larval shorthorn sculpin Myoxocephalus scorpius was monitored during visual exposure to a predatory threat (juvenile Atlantic cod, Gadus morhua) throughout development at 3°C (March–April, 2004). Larvae did not respond to predatory exposure during the first week post-hatch, but thereafter showed drastic reductions in foraging activity when exposed to predators. During early development, the mass-specific routine metabolism of shorthorn sculpin larvae displayed a triphasic ontogeny and peaked during metamorphosis. This high mass-specific metabolic demand could make reduced foraging under predation threat very costly during this stage of development. To further investigate this possibility, additional experiments were performed (March–April, 2005) where larvae were reared with visual exposure to predators for 6 h day−1 during the feeding period. At 7-week post-hatch, larvae exposed to predators were smaller (wet mass and SL), showed decreased levels of whole-body lipids and certain fatty acids, and experienced higher rates of mortality as compared to control larvae. In environments where abundant predators cause larval fish to reduce their foraging rate, growth and survival of larvae may be negatively affected.

Mammalian metabolism: insights from arid zone reptiles.

Mammals, being endotherms have very high metabolic rates compared to ectothermic reptiles. Similarly, small mammals have high rates of mass-specific metabolism compared to larger mammals. This review examines the mechanistic basis of why particular mammal species have a specific metabolic rate. Initial studies compared mammals with arid zone reptile species of the same size and Tb. Mammals have larger internal organs, with more mitochondrial membrane surface area than the reptiles. The cells of mammals are leakier to Na+ ions and their mitochondrial membranes are leakier to H+ ions than in reptile cells. These leakier membranes have membrane lipids that are polyunsaturated and less monounsaturated than their less leaky counterparts. Examination of the cellular basis of allometric variation in metabolism in mammals reveals very similar findings with polyunsaturated membranes associated with the high mass-specific metabolic rates of small mammal species and monounsaturated membranes with low rates of metabolism of large mammals. These findings have resulted in the development of the ?membrane pacemaker? theory of metabolism, which proposes that membrane bilayer composition is regulated in animals and that highly polyunsaturated membranes result in enhanced molecular activity of membrane proteins and in turn this results in an elevated metabolic rate of cells, tissues and consequently whole animals. This theory is also supported by the recent examination of the basis of body-size variation in the metabolic rates of birds. The ?membrane pacemaker? theory of metabolism is currently the only explanation of the mechanisms determining the metabolic rate and thus the cost of living of animals. It has implications for the effect of food habits on metabolism and the relationship between metabolism and lifespan.