Seasonal variation in the resting metabolic rate of male wood mice Apodemus sylvaticus from two contrasting habitats 15 km apart

Diurnal and nocturnal resting metabolic rates of winter- and summer-acclimatized adult male wood mice Apodemus sylvaticus from two adjacent populations, 15 km apart, were measured. One population lived in deciduous woodland, and experienced a narrower daily range of temperatures than the second population, which inhabited maritime sand-dunes. Ambient temperature and body mass had significant effects on the resting metabolism of mice, excluding winter-acclimatized sand-dune animals where only temperature explained significant amounts of the observed variation. Only in this latter group could a thermoneutral zone be determined, with a lower critical temperature of ca. 25 °C and resting metabolism of 0.155 W. Nocturnal resting metabolic rates were significantly greater than diurnal levels. Winter acclimatization was associated with reductions in thermal conductance and resting metabolism, thus minimizing energy expenditure at rest. Site differences in thermoregulatory strategies were only found in winter, thermal conductances remained similar but mice from the sand-dunes had significantly lower metabolic rates than those from the woodland. Winter acclimatization in wood mice was influenced by factors in addition to photoperiod. Intra-specific and individual variations in resting metabolism, as shown in this study, potentially have a pronounced effect on the daily energy expenditure of a free-living animal. Accepted: 6 September 1996     

Energetic costs and thermoregulation in northern fur seal (Callorhinus ursinus) pups: the importance of behavioral strategies for thermal balance in furred marine mammals

Behavioral thermoregulation represents an important strategy for reducing energetic costs in thermally challenging environments, particularly among terrestrial vertebrates. Because of the cryptic lifestyle of aquatic species, the energetic benefits of such behaviors in marine endotherms have been much more difficult to demonstrate. In this study, I examined the importance of behavioral thermoregulation in the northern fur seal (Callorhinus ursinus) pup, a small-bodied endotherm that spends prolonged periods at sea. The thermal neutral zones of three weaned male northern fur seal pups (body mass range = 11.8-12.8 kg) were determined by measuring resting metabolic rate using open-flow respirometry at water temperatures ranging from 2.5° to 25.0°C. Metabolic rate averaged 10.03 ± 2.26 mL O?kg?1 min?1 for pups resting within their thermal neutral zone; lower critical temperature was 8.3° ± 2.5°C , approximately 8°C higher than the coldest sea surface temperatures encountered in northern Pacific waters. To determine whether behavioral strategies could mitigate this potential thermal limitation, I measured metabolic rate during grooming activities and the unique jughandling behavior of fur seals. Both sedentary grooming and active grooming resulted in significant increases in metabolic rate relative to rest (P = 0.001), and percent time spent grooming increased significantly at colder water temperatures (P < 0.001). Jughandling metabolic rate (12.71 ± 2.73 mL O?kg?1 min ?1) was significantly greater than resting rates at water temperatures within the thermal neutral zone (P < 0.05) but less than resting metabolism at colder water temperatures. These data indicate that behavioral strategies may help to mitigate thermal challenges faced by northern fur seal pups while resting at sea.     

Counter-Gradient Variation in Respiratory Performance of Coral Reef Fishes at Elevated Temperatures

The response of species to global warming depends on how different populations are affected by increasing temperature throughout the species'' geographic range. Local adaptation to thermal gradients could cause populations in different parts of the range to respond differently. In aquatic systems, keeping pace with increased oxygen demand is the key parameter affecting species'' response to higher temperatures. Therefore, respiratory performance is expected to vary between populations at different latitudes because they experience different thermal environments. We tested for geographical variation in respiratory performance of tropical marine fishes by comparing thermal effects on resting and maximum rates of oxygen uptake for six species of coral reef fish at two locations on the Great Barrier Reef (GBR), Australia. The two locations, Heron Island and Lizard Island, are separated by approximately 1200 km along a latitudinal gradient. We found strong counter-gradient variation in aerobic scope between locations in four species from two families (Pomacentridae and Apogonidae). High-latitude populations (Heron Island, southern GBR) performed significantly better than low-latitude populations (Lizard Island, northern GBR) at temperatures up to 5°C above average summer surface-water temperature. The other two species showed no difference in aerobic scope between locations. Latitudinal variation in aerobic scope was primarily driven by up to 80% higher maximum rates of oxygen uptake in the higher latitude populations. Our findings suggest that compensatory mechanisms in high-latitude populations enhance their performance at extreme temperatures, and consequently, that high-latitude populations of reef fishes will be less impacted by ocean warming than will low-latitude populations.     

Thermal limits in young northern fur seals,Callorhinus ursinus

The thermoregulatory abilities of northern fur seals (Callorhinus ursinus) during their first two years in the frigid waters of the North Pacific Ocean may limit their geographic distribution and alter the costs for exploiting different species of prey. We determined the thermoneutral zone of six young northern fur seals by measuring their metabolism in ambient air and controlled water temperatures (0°C–12°C) from ages 8 to 24 mo. We found that the ambient air temperatures within our study (overall 1.5°C–23.9°C) did not affect resting metabolic rates. Calculated lower critical temperatures in water varied between 3.9°C and 8.0°C, while an upper critical temperature in water was only discernible during a single set of trials. These thermal responses provide insight into the possible physiological constraints on foraging ecology in young northern fur seals, as well as the potential energetic consequences of ocean climate change and altered prey distributions.     

Respiratory Q10 varies between populations of two species of Myrmica ants according to the latitude of their sites

Metabolic respiration by groups of resting Myrmica ruginodis and M. scabrinodis worker ants from five sites representing a range of latitudes, have been compared by measuring rates of CO(2) production-standardised by fat-free weight-at 5 and 25 degrees C. M. ruginodis which lives in cooler habitats than M. scabrinodis consistently produced more CO(2). At 5 degrees C ants of both species from southern latitudes were metabolically more active than those from more northerly latitudes, whereas at 25 degrees C the situation was reversed. Estimates of Q10 were positively correlated with latitude indicating that the respiratory metabolism of northern populations increases relatively more in response to rising temperatures than southern populations. Values of Q10 at different latitudes were the same for both species. The results are discussed in terms of seasonal fluctuations of temperature at different latitudes.     

Determinants of inter-specific variation in basal metabolic rate

Basal metabolic rate (BMR) is the rate of metabolism of a resting, postabsorptive, non-reproductive, adult bird or mammal, measured during the inactive circadian phase at a thermoneutral temperature. BMR is one of the most widely measured physiological traits, and data are available for over 1,200 species. With data available for such a wide range of species, BMR is a benchmark measurement in ecological and evolutionary physiology, and is often used as a reference against which other levels of metabolism are compared. Implicit in such comparisons is the assumption that BMR is invariant for a given species and that it therefore represents a stable point of comparison. However, BMR shows substantial variation between individuals, populations and species. Investigation of the ultimate (evolutionary) explanations for these differences remains an active area of inquiry, and explanation of size-related trends remains a contentious area. Whereas explanations for the scaling of BMR are generally mechanistic and claim ties to the first principles of chemistry and physics, investigations of mass-independent variation typically take an evolutionary perspective and have demonstrated that BMR is ultimately linked with a range of extrinsic variables including diet, habitat temperature, and net primary productivity. Here we review explanations for size-related and mass-independent variation in the BMR of animals, and suggest ways that the various explanations can be evaluated and integrated.     

Longevity and metabolism in Drosophila melanogaster: genetic correlations between life span and age-specific metabolic rate in populations artificially selected for long life

We measured age-specific metabolic rates in 2861 individual Drosophila melanogaster adult males to determine how genetic variation in metabolism is related to life span. Using recombinant inbred (RI) lines derived from populations artificially selected for long life, resting metabolic rates were measured at 5, 16, 29, and 47 days posteclosion, while life spans were measured in the same genotypes in mixed-sex population cages and in single-sex vials. We observed much heritable variation between lines in age-specific metabolic rates, evidence for genotype x age interaction, and moderate to large heritabilities at all ages except the youngest. Four traits exhibit evidence of coordinate genetic control: day 16 and day 29 metabolic rates, life span in population cages, and life span in vials. Quantitative trait loci (QTL) for those traits map to the same locations on three major chromosomes, and additive genetic effects are all positively correlated. In contrast, metabolic rates at the youngest and oldest ages are unrelated to metabolic rates at other ages and to survival. We suggest that artificial selection for long life via delayed reproduction also selects for increased metabolism at intermediate ages. Contrary to predictions of the "rate of living" theory, we find no evidence that metabolic rate varies inversely with survival, at the level of either line means or additive effects of QTL.     

Oxygen consumption and the energetics of island-dwelling Florida cottonmouth snakes

We measured oxygen consumption (Vo(2)) to estimate standard metabolic rates (SMR) in cottonmouth snakes (Agkistrodon piscivorus conanti) from Seahorse Key and the adjacent peninsula of northern Florida. The island population is unusual because adult snakes feed on fish that are dropped by colonial nesting birds, and food resources are temporally limited relative to that of mainland populations. We found no differences in SMR between island and mainland snakes at any of four experimental temperatures (15 degrees -30 degrees C), suggesting that any adjustments to energy limitations involve other aspects of physiology or behavior. As with other viperid species, the SMR of cottonmouths is about one-half of that expected from interspecific allometric regressions previously reported for snakes generally. Allometric mass exponents of SMR averaged 0.76 and were not affected by temperature. We found that Vo(2) increased with temperature (Q(10) = 2.4-2.8) and was elevated 29% during scotophase compared with photophase. Neonates exhibited elevated Vo(2)compared with older juveniles of similar size, apparently due to assimilation of yolk that is present in the neonatal gut. In adult snakes, specific dynamic action (SDA) following feeding resulted in four- to eightfold increases in Vo(2), with magnitude and duration related positively to relative meal size. The total energy devoted to SDA increased with meal size and averaged 32.8%+/-4.4% of total ingested energy. We estimate that a nonreproductive 500-g adult cottonmouth at Seahorse Key uses 3,656 kJ of assimilated energy annually for maintenance and activity, which requires ingestion of approximately 1 kg of fish.     

Colder environments did not select for a faster metabolism during experimental evolution of Drosophila melanogaster

The effect of temperature on the evolution of metabolism has been the subject of debate for a century; however, no consistent patterns have emerged from comparisons of metabolic rate within and among species living at different temperatures. We used experimental evolution to determine how metabolism evolves in populations of Drosophila melanogaster exposed to one of three selective treatments: a constant 16°C, a constant 25°C, or temporal fluctuations between 16 and 25°C. We tested August Krogh's controversial hypothesis that colder environments select for a faster metabolism. Given that colder environments also experience greater seasonality, we also tested the hypothesis that temporal variation in temperature may be the factor that selects for a faster metabolism. We measured the metabolic rate of flies from each selective treatment at 16, 20.5, and 25°C. Although metabolism was faster at higher temperatures, flies from the selective treatments had similar metabolic rates at each measurement temperature. Based on variation among genotypes within populations, heritable variation in metabolism was likely sufficient for adaptation to occur. We conclude that colder or seasonal environments do not necessarily select for a faster metabolism. Rather, other factors besides temperature likely contribute to patterns of metabolic rate over thermal clines in nature.     

Energetics and water flux of the marbled velvet gecko (Oedura marmorata) in tropical and temperate habitats

The gecko Oedura marmorata was studied in two different climatic zones: the arid zone of central Australia and in the wet-dry tropics of northern Australia. Doubly labelled water was used to measure field metabolic rate (FMR) and water flux rates of animals in the field during the temperate seasons of spring, summer and winter, and during the tropical wet and dry seasons. FMRs were highest in the tropical wet season and lowest in the temperate winter. The geckos in central Australia expended less energy than predicted for a similarly sized iguanid lizard, but geckos from the tropics expended about the same amount of energy as predicted for an iguanid. Water flux rates of geckos from the arid zone were extremely low in all seasons compared to other reptiles, and although water flux was higher in tropical geckos, the rates were low compared to other tropical reptiles. The standard metabolic rates (SMRs) of geckos were similar between the two regions and among the seasons. Geckos selected higher body temperatures (T _bs) in a laboratory thermal gradient in the summer (33.5°C) and wet (33.8°C) seasons compared to the winter (31.7°C) and dry (31.4°C) seasons. The mean T _bs selected in the laboratory thermal gradient by geckos from the two regions were not different at a given time of year. The energy expended during each season was partitioned into components of resting metabolism, T _b and activity. Most of the energy expended by geckos from central Australia could be attributed to the effects of temperature on resting lizards in all three seasons, but the energy expended by tropical geckos includes a substantial component due to activity during both seasons. This study revealed variability in patterns of ecological energetics between populations of closely related geckos, differences which cannot be entirely attributed to seasonal or temperature effects. Received: 14 November 1997 / Accepted: 4 May 1998     

Changes in metabolism in response to fasting and food restriction in the Steller sea lion (Eumetopias jubatus)

Many animals lower their resting metabolism (metabolic depression) when fasting or consuming inadequate food. We sought to document this response by subjecting five Steller sea lions to periods of: (1) complete fasting; or (2) restricting them to 50% of their normal herring diet. The sea lions lost an average of 1.5% of their initial body mass per day (2.30 kg/d) during the 9-14-day fast, and their resting metabolic rates decreased 31%, which is typical of a "fasting response". However, metabolic depression did not occur during the 28-day food restriction trials, despite the loss of 0.30% of body mass per day (0.42 kg/d). This difference in response suggests that undernutrition caused by reduced food intake may stimulate a "hunger response", which in turn might lead to increased foraging effort. The progressive changes in metabolism we observed during the fasts were related to, but were not directly caused by, changes in body mass from control levels. Combining these results with data collected from experiments when Steller sea lions were losing mass on low energy squid and pollock diets reveals a strong relationship between relative changes in body mass and relative changes in resting metabolism across experimental conditions. While metabolic depression caused by fasting or consuming large amounts of low energy food reduced the direct costs from resting metabolism, it was insufficient to completely overcome the incurred energy deficit.     

Bioenergetics of reproduction and pup development in a subterranean rodent (Ctenomys talarum)

The objective of this study was to evaluate the maternal costs of reproduction and pup development in the subterranean rodent Ctenomys talarum (Thomas 1898). Statistical differences were detected in whole-animal metabolic rates between nonreproductive and pregnant or lactating females. Whole-animal metabolic rates during pregnancy and lactation were 128% and 151% of the resting metabolic rate (RMR) observed in nonreproductive females. The total additional energy cost of reproduction (above the nonreproductive level) was similar for both the gestation and lactation periods. Mass-specific RMR revealed an upregulation of cell or tissue metabolism during lactation but not during gestation. The mass-specific metabolic rate of pups was 237% of the adults' metabolic rates. No differences were observed in body temperature among nonreproductive, pregnant, or lactating females. No differences were detected in body mass at birth among pups from litters with different numbers of nestlings. Pups increased their body temperature, reaching adult temperature at 30 d of age, when they were near weaning. Milk constituted the exclusive food for pups until they started eating solid food at 10 d old. Suckling time decreased with age of pups, and at the same time, mother chases directed toward their pups increased. These reproductive characteristics may contribute to successful existence in a subterranean habitat.     

Metabolic response to changes in temperature in northern wheatears from an arctic and a temperate populations

Until recently it had been widely accepted that birds are energetically adapted to the latitude they inhabit, having an increased basal metabolic rate (BMR) at higher latitudes. Latterly, this general view has been questioned and the influence of phenotypic flexibility, due to factors such as habitat, life‐history or acclimatization has received increased attention. In particular, focus has been directed towards comparing species from arid and mesic habitats, but less attention has been given to species which breed in cold climates. We chose to study northern wheatears Oenanthe oenanthe from two populations at different latitudes (southern Norway, Iceland), but with similar life‐histories and habitat requirements throughout the year, in a common‐garden experiment. In order to assess true latitudinal trends in metabolic rate, we estimated the nocturnal resting metabolic rate (RMR) of northern wheatears from southern Norway and Iceland at different temperatures from 0° to 30°C. We found that Norwegian birds had overall lower metabolic rates than Icelandic birds, which were also slightly larger. This difference was not observed at 0°C, which might indicate that Icelandic birds might rely on better feather insulation reducing metabolic costs at very low temperatures. At temperatures above 10°C birds of both populations had constant metabolic values, indicating that their thermoneutral range almost completely covered the temperatures experienced during the breeding period. This study shows that the northern wheatear, which is one of only a few insectivorous long‐distance migratory songbirds occurring at such high latitudes, has evolved metabolic adaptations to life at cold temperatures which are endogenously determined.     

Life history traits associated with body size covary along a latitudinal gradient in a generalist grasshopper

Animal body size often varies systematically along latitudinal gradients, where individuals are either larger or smaller with varying season length. This study examines ecotypic responses by the generalist grasshopper Melanoplus femurrubrum (Orthoptera: Acrididae) in body size and covarying, physiologically based life history traits along a latitudinal gradient with respect to seasonality and energetics. The latitudinal compensation hypothesis predicts that smaller body size occurs in colder sites when populations must compensate for time constraints due to short seasons. Shorter season length requires faster developmental and growth rates to complete life cycles in one season. Using a common garden experimental design under laboratory conditions, we examined how grasshopper body size, consumption, developmental time, growth rate and metabolism varied among populations collected along an extended latitudinal gradient. When reared at the same temperature in the lab, individuals from northern populations were smaller, developed more rapidly, and showed higher growth rates, as expected for adaptations to shorter and generally cooler growing seasons. Temperature-dependent, whole organism metabolic rate scaled positively with body size and was lower at northern sites, but mass-specific standard metabolic rate did not differ among sites. Total food consumption varied positively with body size, but northern populations exhibited a higher mass-specific consumption rate. Overall, compensatory life history responses corresponded with key predictions of the latitudinal compensation hypothesis in response to season length.     

Thermoregulation of weaned northern elephant seal (Mirounga angustirostris) pups in air and water

Fasting weaned northern elephant seal pups (Mirounga angustirostris) experience diverse environmental conditions on land and in water on a daily basis. Each environment undoubtedly induces distinct energetic costs that may vary for pups of differing body condition. To determine the energetic costs associated with different environmental conditions and whether costs vary between individuals, body mass, surface area, volume, body composition, resting metabolic rate, and core body temperature were determined for 17 weaned northern elephant seal pups from A?o Nuevo, California. Metabolic rate and body temperature were measured for pups resting in air (20.9 degrees +/-0.8 degrees C), cold water (3.8 degrees+/-0.4 degrees ;C), and warm water (14.5 degrees+/-0.2 degrees C). Resting metabolic rate increased with body mass (range: 62.0-108.0 kg) and was also correlated with lean mass and lipid mass. Metabolic rates ranged from 293.6 to 512.7 mL O(2) min(-1) and were lowest for pups resting in cold water. Thermal conductance, calculated from metabolic rate and core body temperature, ranged from 3.1 to 15.2 W degrees C(-1), with the highest values in air and the lowest values in cold water. Metabolic responses to the three environmental conditions did not differ with individual variation in body condition. For all elephant seal pups, a consequence of high lipid content is that thermoregulatory costs are greatest on land and lowest in cold water, a pattern that contrasts markedly with terrestrial mammals.     

Resting metabolic expenditure as a potential source of variation in growth rates of the sagebrush lizard

Along an elevational gradient on SW Utah, sagebrush lizards (Sceloporus graciosus) exhibit an unexpected pattern of growth. Lizards from a high elevation population grow faster than lizards from two populations at lower elevations despite shorter daily and seasonal activity. Results from a common environment study of growth suggest that the differences in growth are not due to adaptation to local environmental conditions. In this study, I test the hypothesis that higher growth rates in lizards from high elevation may be attributable to reduced resting metabolic expenditure compared to that of lizards from populations at two lower elevations. Resting metabolic rates were measured for individuals from each of the study populations across different times of day and over a broad range of temperatures. Under the same laboratory conditions, field-caught lizards from the high elevation population exhibited lower metabolic rates when compared to lizards from lower elevations. Daily resting metabolic expenditures were calculated using the observed metabolic rates coupled with estimates of daily activity. Daily resting metabolic expenditure was 50% greater for individuals from the two lower elevation populations, which could result in 12.5% more energy that could be potentially allocated to growth for lizards from high elevation. Such energetic savings may be able to explain differences in the patterns of growth observed in nature.     

Multigenerational exposure to increased temperature reduces metabolic rate but increases boldness in Gambusia affinis

Acute exposure to warming temperatures increases minimum energetic requirements in ectotherms. However, over and within multiple generations, increased temperatures may cause plastic and evolved changes that modify the temperature sensitivity of energy demand and alter individual behaviors. Here, we aimed to test whether populations recently exposed to geothermally elevated temperatures express an altered temperature sensitivity of metabolism and behavior. We expected that long‐term exposure to warming would moderate metabolic rate, reducing the temperature sensitivity of metabolism, with concomitant reductions in boldness and activity. We compared the temperature sensitivity of metabolic rate (acclimation at 20 vs. 30°C) and allometric slopes of routine, standard, and maximum metabolic rates, in addition to boldness and activity behaviors, across eight recently divergent populations of a widespread fish species (Gambusia affinis). Our data reveal that warm‐source populations express a reduced temperature sensitivity of metabolism, with relatively high metabolic rates at cool acclimation temperatures and relatively low metabolic rates at warm acclimation temperatures compared to ambient‐source populations. Allometric scaling of metabolism did not differ with thermal history. Across individuals from all populations combined, higher metabolic rates were associated with higher activity rates at 20°C and bolder behavior at 30°C. However, warm‐source populations displayed relatively bolder behavior at both acclimation temperatures compared to ambient‐source populations, despite their relatively low metabolic rates at warm acclimation temperatures. Overall, our data suggest that in response to warming, multigenerational exposure (e.g., plasticity, adaptation) may not result in trait change directed along a simple “pace‐of‐life syndrome” axis, instead causing relative decreases in metabolism and increases in boldness. Ultimately, our data suggest that multigenerational warming may produce a novel combination of physiological and behavioral traits, with consequences for animal performance in a warming world.     

The relationship between female brooding and male nestling provisioning: does climate underlie geographic variation in sex roles?

Comparative studies of populations occupying different environments can provide insights into the ecological conditions affecting differences in parental strategies, including the relative contributions of males and females. Male and female parental strategies reflect the interplay between ecological conditions, the contributions of the social mate, and the needs of offspring. Climate is expected to underlie geographic variation in incubation and brooding behavior, and can thereby affect both the absolute and relative contributions of each sex to other aspects of parental care such as offspring provisioning. However, geographic variation in brooding behavior has received much less attention than variation in incubation attentiveness or provisioning rates. We compared parental behavior during the nestling period in populations of orange‐crowned warblers Oreothlypis celata near the northern (64°N) and southern (33°N) boundaries of the breeding range. In Alaska, we found that males were responsible for the majority of food delivery whereas the sexes contributed equally to provisioning in California. Higher male provisioning in Alaska appeared to facilitate a higher proportion of time females spent brooding the nestlings. Surprisingly, differences in brooding between populations could not be explained by variation in ambient temperature, which was similar between populations during the nestling period. While these results represent a single population contrast, they suggest additional hypotheses for the ecological correlates and evolutionary drivers of geographic variation in brooding behavior, and the factors that shape the contributions of each sex.     

Pollinator Abundance and Pollen Limitation of a Solanaceous Shrub at Premontane and Lower Montane Sites

Populations at geographic range limits may frequently encounter changing biotic interactions as well as abiotic constraints. A conundrum in plant population biology, the frequent apparent overproduction of flowers, is thought to be a response by plant populations to unpredictable pollinator environments, such as those that may be expected at the range limit. We studied pollinator visitation rates and pollen limitation of fruit set in populations of Witheringia solanacea , a widespread bee-pollinated species. We compared two populations in the south and two in the north of Costa Rica, with each region encompassing a lower montane site at the upper elevational limit of the species range with small plant populations, and a premontane site where population densities were high. Populations in the south, where the climate was favorable, had higher pollinator visitation and higher natural fruit set, and showed only minor pollen limitation as compared with northern populations at windy sites near the Continental Divide. Lower montane populations had lower pollinator visitation rates than premontane populations but did not show more pollen limitation of fruit set. In general, pollen limitation was not inversely proportional to pollinator abundance but was higher where the climate is unpredictable.     

Metabolic rates,genetic constraints,and the evolution of endothermy

The metabolic distinction between endotherms and ectotherms is profound. Whereas the ecology of metabolic rates is well studied, how endotherms evolved from their ectothermic ancestors remains unclear. The aerobic capacity model postulates that a genetic constraint between resting and maximal metabolism was essential for the evolution of endothermy. Using the multivariate breeders’ equation, I illustrate how the (i) relative sizes of genetic variances and (ii) relative magnitudes of selection gradients for resting and maximal metabolism affect the genetic correlation needed for endothermy to have evolved via a correlated response to selection. If genetic variances in existing populations are representative of ancestral conditions, then the aerobic capacity model is viable even if the genetic correlation was modest. The analyses reveal how contemporary data on selection and genetic architecture can be used to test hypotheses about the evolution of endothermy, and they show the benefits of explicitly linking physiology and quantitative genetic theory.