Ectothermy

The evidence for ectothermy is also weak, although it has been claimed by some researchers that the absence of nasal passages large enough to house respiratory turbinate bones is the 'Rosetta Stone' that demonstrates dinosaurian ectothermy.

From: Encyclopedia of Geology , 2005

Endotherm☆

Marta K. Labocha , Jack P. Hayes , in Encyclopedia of Ecology (Second Edition), 2019

Energetic Influence of Endotherms on Ecosystems

Endothermy is more energetically costly than ectothermy. Because endotherms use more energy than ectotherms, the same amount of food can maintain a larger population of similar-sized ectotherms than endotherms. Moreover, 90% or more of the energy assimilated by endotherms is converted to heat, so only a small percentage of the food energy drawn from the ecosystem by endotherms is converted to biomass (i.e., to grow tissue or produce offspring). In other words, endotherms have lower production efficiency than ectotherms.

Because of the high energetic cost of endothermy, endothermic carnivores require higher prey densities than ectothermic carnivores. In systems with low primary productivity they will be absent or rare. Even folivorous endotherms may be absent in habitats with extremely low productivity.

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Heterotrophic Energy Flows

Kenneth A. Nagy , in Encyclopedia of Energy, 2004

3.5 Rates of Feeding

Feeding rates that are actually achieved in the field are influenced not only by intrinsic factors, such as endothermy or ectothermy, activity level, reproductive status, and digestive physiology, but also by extrinsic factors, such as season, food availability, competitive and social interactions with other individuals of the same or other species, and reduction of feeding opportunities due to the presence of predators or inclement environmental conditions (e.g., excessive midday heat, darkness, rainstorms). Over a period of several days or weeks, animals are usually able to compensate for short-term difficulties in getting food and are able to obtain approximately enough food to maintain energy balance. Body size is the most important determinant of feeding rate; bigger animals eat more food than do smaller animals. However, this is not a one-to-one relationship. An animal weighing 10 times more than another animal does not eat 10 times more food each day; rather, it eats only approximately 5 to 6 times more, in accordance with the scaling of metabolic rate. When differences in body mass are accounted for, feeding rates still vary by more than 25 times among species during their activity seasons. For example, a representative insectivorous lizard that weighs 100 g consumes about 0.7 g of food (dry matter intake [DMI]) per day, whereas a typical 100-g bird living in a marine habitat consumes about 18 g of dry matter per day ( Table II). Both may be living in the same seashore habitat, eating a similar diet (arthropods), and maintaining the same body temperature during the day, but the lizard's metabolic energy expenditures over a 24-h period are only about 4% those of the bird, so the lizard's food needs are proportionately lower as well. Within various groups of endothermic vertebrates, there is a 230% variation in feeding rates among same-sized animals. Desert-dwelling mammals and birds have relatively low feeding rates, and marine birds have relatively high feeding rates. Desert mammals and birds are known to have lower metabolic rates (basal and field) than do related species living in other habitats, so their food requirements should be correspondingly lower. The high feeding rates of herbivorous mammals, relative to other mammals, most likely result in part from their low MEE (Table I), meaning that relatively more plant food must be eaten to obtain a given daily rate of metabolizable energy intake.

Table II. Feeding Rates of Wild Vertebrates, Summarized in Allometric Equations Derived from Field Measurements of Energy Metabolism, for Various Groups of Mammals, Birds, and Reptiles

Animal group a b Expected feeding rate (g dry matter/day) for a 100-g animal
Eutherian mammals (58 species) 0.299 0.767 10.2
Marsupial mammals (20 species) 0.483 0.666 10.4
Herbivorous mammals (26 species) 0.859 0.628 15.5
Desert mammals (25 species) 0.192 0.806 7.9
Birds (95 species) 0.638 0.685 15.0
Passerine birds (39 species) 0.630 0.683 14.6
Desert birds (15 species) 0.407 0.681 9.4
Marine birds (36 species) 0.880 0.658 18.2
Reptiles (55 species) 0.011 0.920 0.77
Herbivorous reptiles (9 species) 0.033 0.717 0.91
Insectivorous lizards (27 species) 0.011 0.914 0.73

Note. The equations have the following form: y=axb , where y=feeding rate (in grams dry matter consumed per day), x=body mass (in grams), a is the intercept at body mass=1 g, and b is the allometric slope.

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Thermoregulation in Animals: Some Fundamentals of Thermal Biology☆

Udo Gansloßer Gianna Jann , in Encyclopedia of Ecology (Second Edition), 2019

Evolutionary Considerations

This already leads to the question of phylogenetic development of thermoregulation. In order to understand this tradition, it may be helpful to look at some taxa that are somewhere in between ecto- and endothermy. Some insects, for example, large, nocturnal moths (Sphingidae), bees, dragonflies or wasps, are able to regulate thoracic and in some cases also abdominal temperature. However, this endothermy is only achieved when they are active, they perform wing-movements, called shivering, decoupled from flight. Moths at least, due to their hairy scales, have values of thermal conductance similar to birds and mammals, and they can keep large differences between Ta and Tb (some North American moths can fly at a core body temperature of around 30°C at Ta   =   0°C). However, these small animals cannot achieve continuous endothermy similar to same-sized vertebrates if they are not active day and night (which insects are not).

Larger species in the continuum between ecto- and endothermy are found among fish. Bluefin tuna (Thunnus thymnus) of 200–350   kg can uphold temperature differences of up to 20°C. In these fish, contrary to "cold- bodied" species, we find large amounts of red (=   aerobic) skeletal muscles near the body core (along the vertebral column) instead of under the skin. Also a high BMR, and a countercurrent heat-exchanger in the circulatory system are further characteristics of these endothermic fish. Besides red skeletal muscles, endothermic fish also have local heat sources in stomach, gut and liver tissue. Also (again kept up by retia mirabilia   =   countercurrent heat exchangers) in the eyes and the brain of warm-blooded fish such as Mako sharks (Isurus oxyrhynchus) there is a temperature difference to the environment of >   5°C. However, there are no heat generating tissues in the sharks, heads, instead warm blood from abdominal red muscles is transported directly to the eye and brain regions. In some bony fish (e.g., Swordfish, Xiphias gladius) contrary to sharks, eye muscles are working as local heat sources, the whole complex of heater muscles, brain and eyes is thickly isolated in fat, and temperature differences of up to 14°C can be upheld between brain and surrounding water.

There is also evidence for mechanisms of physiological and behavioral temperature control in these fish. Also some python snakes and Leatherback turtles (Dermochelys coriacea) are able to obtain a certain control over their body temperatures.

Thus, the question of when and why endothermy could evolve has to be approached very broadly. The adaptive value of real endothermy and effective thermoregulation could have been to allow a decrease in body size at a constant body temperature. This would not only have allowed an increase in activity, but also an increase in reproduction. However, endothermy also is costly, and thus certain preconditions had to be met before achieving it. On the biochemical level, changes in membrane permeability for ions, are discussed as necessary preconditions for increasing metabolic rates. On the organismic level, it seems plausible at least in the evolution of mammals to assume that the large (up to 250   kg) therosaurus reptiles, the ancestors of mammals, had, due to their large size, achieved a certain degree of thermal independence, and that a whole array of morphological and physiological changes (development of isolating fur, increasing efficiency of ventilation by developing a bony palate and diaphragm, etc.) then allowed the transition from large reptile (with so called inertial homeothermy, which means they simply were too large to lose enough heat for being poikilotherms) to small mammal with an active, regulatory endothermy.

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Physiological Ecology

B.K. McNab , in Encyclopedia of Ecology, 2008

Physiological ecology deals with the adjustments, that is, the adaptations that organisms make to the physical and biological environments in which they live. They include the modification of rates of metabolism; the differential use of ectothermy and endothermy; the ability to balance salt and water budgets in terrestrial and aquatic environments; the adjustment of gas exchange in hypoxic, hyperbaric, and hypobaric environments; and the evolution of photosynthesis relative to water availability and the gas composition of the atmosphere. Geographical distributions may be limited by cold in montane and polar environments, water presence or absence in terrestrial regions, and the abundance of oxygen and carbon dioxide in aquatic and terrestrial environments, as well as the presence and the absence of competitors and selective food types. Many aspects of physiological ecology are associated with the ability to maintain an acceptable internal physiological state, which ultimately must be paid for by the acquisition of resources from the environment and by adequate energy expenditures. If this cannot be accomplished while maintaining a standard physiological state, many species reduce energy expenditure, such as entering torpor, but often with consequences for their life history, as with a reduction in reproductive output.

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Reptiles, Biodiversity of

F. Harvey Pough , in Encyclopedia of Biodiversity (Second Edition), 2013

Glossary

Ancestral

Describes a character or character state of the organism being considered that retains the primitive condition for its evolutionary lineage.

Derived

Describes a character or character state of the organism being considered that has changed from the ancestral condition for its evolutionary lineage.

Ectothermy

Deriving the energy needed to raise body temperature from sources outside the body.

Endothermy

Deriving the energy needed to raise body temperature from within the body – i.e., from metabolic heat production.

Heliothermic

Regulating body temperature primarily by moving between sun and shade.

Operative temperature

A measure of environmental temperature that combines the effects of heat exchange via radiation, convection, with metabolic heat production, and evaporative heat loss.

Paraphyletic

A taxonomic grouping of animals that does not meet the cladistic criterion of including the most recent common ancestor and all its descendants.

Sister group

The evolutionary lineage most closely related to the one being discussed.

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Vertebrates, Overview

Carl Gans , Christopher J. Bell , in Encyclopedia of Biodiversity (Second Edition), 2001

Glossary

Chordate

A member of the group Chordata. The Chordata includes the most recent common ancestor of tunicates and cephalochordates and all of that ancestor's descendants. Tunicates, lancelets, hag-fishes, and vertebrates are all chordates.

Ectoderm

An embryonic tissue that provides the future outside layer of the animal.

Ectothermy

A method of body temperature control in which the animal utilizes external sources for gaining and giving up heat, thus achieving temperature control without affecting metabolic rate.

Endothermy

A method of body temperature control in which the animal modifies its metabolic rate to achieve the desired body temperature.

Neural crest

An embryonic tissue intermediate between neurectoderm and ectoderm, with cells migrating widely to their final destination. This tissue gives rise to anterior skeletal elements, many portions of the future head and pharynx, and all pigment cells. Sometimes also referred to as mesectoderm.

Neurectoderm

An embryonic tissue that gives rise to the central tube of the nervous system.

Notochord

A stiff, flexible, longitudinal rod running along the middorsal portion of the chordate body. It is situated dorsal to the coelom and ventral to the central tube of the nervous system.

Pharynx

The anterior portion of the alimentary canal, characterized by lateral buds that provide skeletal support for the gill region.

Tuberculum interglenoideum

An anterior projection of the first (cervical) vertebra in salamanders. The tuberculum interglenoideum bears articular facets that insert into the foramen magnum of the skull and provide additional articulation points between the skull and the vertebral column.

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Reptiles, Biodiversity of

F. Harvey Pough , in Encyclopedia of Biodiversity, 2001

II.C. Active Reptiles and the Evolution of Endothermy

In an ecological context, the generalization that reptiles are ectotherms with low metabolic rates and low levels of activity applies to extant species, although some modifications of that characterization will be addressed in subsequent sections. In an evolutionary context, however, the generalization is patently false because birds are reptiles and they are endotherms with high metabolic rates. Clearly the reptilian lineage has a capacity for endothermy that is barely expressed in nonavian reptiles. An examination of the evolution of endothermy explains that dichotomy and emphasizes how tightly anatomical and physiological characteristics are linked to thermal ecology.

Ectothermy is the ancestral condition for vertebrates, and the derived condition of whole-body endothermy has evolved at least twice, in mammals and in birds. In a broader view, regional endothermy has evolved independently in sharks, tunas, and billfishes, and whole-body endothermy may have been characteristic of pterosaurs (flying archosaurian reptiles of the Mesozoic) and some lineages of dinosaurs.

Ectotherms and endotherms have very different relationships to their physical environments: Ectotherms rely primarily on behavioral thermoregulation to raise their body temperatures because they have low metabolic rates, and the absence of insulation facilitates uptake of heat from the environment. In contrast, endotherms use internal heat production from high metabolic rates to regulate body temperature, and they require insulation to retain metabolic heat in their bodies.

The evolutionary transition from ectothermy to endothermy is impeded by a Catch-22—adding insulation to an ectotherm impedes its behavioral thermoregulation, but in the absence of insulation any heat produced by increasing its resting metabolic rate is lost to the environment. The solution to this paradox lies in finding a basis for the evolution of insulation or the evolution of an increased metabolic rate that does not depend on the preexisting occurrence of the other character.

Mammals are the sister group of reptiles (including birds) and the common ancestor of mammals and reptiles was ectothermal. Thus, the evolution of endothermy in the mammalian lineage may provide a model for the evolution of endothermy among reptiles. Anatomical changes seen in the fossil record of predatory synapsids, the sister group of mammals, strongly support the hypothesis that the initial step in the evolution of mammalian endothermy was selection for increased locomotor capacity. These changes include the evolution of a cursorial body form, changes in the rib cage that suggest the presence of a diaphragm, and increased surface area in the nasal passages to warm and humidify large volumes of air. Increasing levels of locomotor activity require an increase in metabolic rate, and internal heat production would create a selective value for insulation (see Pough et al., 1999, Chapters 4 and 19, for details and references).

Some features of the fossil record of birds suggest that a similar scenario can be applied to the evolution of avian endothermy, but others appear to contradict that interpretation. Like the predatory synapsids, the small maniraptoran dinosaurs that form the sister group of birds appear to have been fleet-footed predators that pursued their prey. That interpretation suggests that these dinosaurs may have evolved the metabolic capacity for endothermy just as synapsids did, and if the recent report of feathers in fossil dinosaurs is correct, it would support that interpretation. However, two lines of evidence cast doubt on the hypothesis that the dinosaurian precursors of birds had high metabolic rates. Examination of an excellently preserved specimen of the small dinosaur Sinosauropteryx suggests that it had simple septate lungs that were ventilated by a pistonlike movement of the liver like those of living crocodilians. Lungs of this sort would not support high rates of oxygen consumption. That interpretation is supported by CAT scans of the nasal passages of dinosaurs that reveal no trace of modifications of the nasal passages to warm and humidify large volumes of air (see Pough et al., 1999, Chapter 13, for details and references).

This evolutionary perspective emphasizes intricate interconnections among the anatomical and physiological characters of extant reptiles and their ecology and behavior, as well as the evolutionary capacity for breaking those links. An examination of living reptiles reveals additional connections among anatomy, physiology, ecology, and behavior.

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Thermoregulation, Performance, and Energetics

Laurie J. Vitt , Janalee P. Caldwell , in Herpetology (Third Edition), 2009

Synthesis

Water balance, respiration, thermoregulation, and energetics are tightly linked in ectothermic vertebrates. For amphibians, rates of water loss can be extremely high, and most species select microhabitats that minimize water loss. Such microhabitats are usually relatively cool or enclosed. Most amphibians take in large amounts of water and produce dilute urine, although there are some notable exceptions. One consequence of activity at low temperatures and of ectothermy in general is that metabolic rates are low (no metabolic cost of heat production). For many reptiles, activity occurs at high body temperatures, but during periods of inactivity, body temperatures are much lower. Reptiles in general take in much less water than amphibians and are capable of retaining more of what they take in. As a result, they produce relatively concentrated urine, often including uric acid as a concentrated waste product. Like amphibians, metabolic rates of reptiles are low because there is no cost of heat production (with a few exceptions); however, overall, reptilian metabolic rates are higher than those of amphibians. Because nearly all energy acquired is directed into low-cost maintenance, growth, reproduction, and storage, amphibians and reptiles can occur at high densities in environments that limit densities of homeothermic vertebrates that expend much of their ingested energy on heat production. Amphibians and reptiles can also persist through long periods of energy shortages.

Although the interplay between temperature, water economy, and energetics is well documented from a physiological perspective, the correlated evolution of these important physiological traits is only beginning to be appreciated. The evolutionary history of geckos in the genus Coleonyx exemplifies the possibilities an evolutionary approach to the interplay between water economy, temperature, and metabolism can have in understanding physiological processes. The ancestor of Coleonyx in North America appears to have had relatively low body temperature (26°C), high evaporative water-loss rate (2.5 mg/g/hr), and a low standard metabolic rate (0.07 mg/g/hr) and lived in a relatively moist, forested habitat. Two extant species, C. mitratus and C. elegans, retain these characteristics, and they are members of the earliest lineage (Fig. 7.27). During the evolutionary history of Coleonyx, species moved into more arid environments, ultimately into the deserts of North America. Correlated with that shift are increases in body temperatures (above 31.0°C), reductions in evaporative water loss (< 0.1 mg/g/hr), and increases in standard metabolic rate (> 0.15 mg/g/hr). In this example, the set of predictions based on a shift from mesic to xeric habitats holds true, indicating that these are indeed adaptations to life in specific environments. Finally, this example points to the importance of maintaining physiological homeostasis for amphibians and reptiles occupying diverse environments.

Figure 7.27. An hypothesis of physiological–ecological character state evolution in lizards in the genus Coleonyx. Four equally parsimonious hypotheses were found based on physiological data alone, but when coupled with biogeographic data, the other three were rejected. EWLR = evaporative water-loss rate, TP = temperature preference, SMR = standard metabolic rate, H = high, L = low. Solid bars indicate acquisition of a new state, and crosshatched bars indicate independent evolution of a derived state. The genera Eublepharis, Hemitheconyx, and Holodactylus make up the outgroup. Presumably, a shift occurred in the selective regime (SR) from an energy-rich to an energy-poor microhabitat during the evolutionary history of Coleonyx.

 Adapted from Dial and Grismer, 1992.

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WATER BALANCE AND THE PHYSIOLOGY OF THE AMPHIBIAN TO AMNIOTE TRANSITION

Karen L.M. Martin , Kenneth A. Nagy , in Amniote Origins, 1997

CONCLUSION

Gans and Pough (1982, p. 8 ) wrote, "We suggest that the absence of a unifying morphological scheme, rather than being an incidental by-product, is an important aspect of the reptilian grade. The lives of Recent reptiles are shaped by a set of shared characteristics that need not produce obvious structural features." The features they suggest as definitive are physiological features: The amniotic egg, ectothermy, low metabolic rate (compared with endotherms), reliance on anaerobiosis for activity, and behavioral temperature regulation. We propose that a similar case may be made for the traits that unite the "amphibian" grade: Great morphological diversity with no obvious unifying morphological characters, but physiological features that set them apart from the bony fishes and from the reptiles. These features include increased variability or relaxed regulation of body hydration levels, high cutaneous permeability to water, relatively low body temperatures for activity, and extremely low metabolic requirments. Pough (1983) described amphibians as being "well known for their generally quiescent and inconspicuous lifestyles and for their low annual use of energy." Bartholomew (1982, p. 344) noted "present-day members of the class Amphibia are morphologically different from the Paleozoic forms which were transitional between fish and reptiles. Nevertheless, they demonstrate clearly the high level of success with which animals are sometimes able to exploit a harsh and demanding environment, despite physiological adjustments that appear at first glance to be modest and ineffective."

Initially we stated that one of the basic issues in comparing amphibians and reptiles is their water budget, and that this affects the whole animal's physiology in a variety of ways. The synergistic effects of providing a dry outer surface are the possibility of warmer body temperatures, higher metabolism, and increased activity and growth rates. This could lead to increased tolerance of high temperatures but perhaps decreased tolerance of low temperatures.

What are the advantages of remaining an amphibian, and not completing the transition to land? The low energy strategy enables amphibians to survive with a very short growing season (Pough, 1983). The geographic range may be limited only by the duration of development from larva to adult for hibernation or aestivaton (Pinder et al., 1992). Some species of the desert or in the arctic may be inactive as much as 6–10 months of the year, a life-style that does hold a certain appeal. With a very low metabolic rate, amphibians can survive on a limited food source. Adult Rana muscosa can survive an eight month inactive period with only 4–6% body fat (Bradford, 1983). In addition, adults and larvae do not compete for food; therefore, more members of the population may be supported at once in the same habitat. Temperature selection by habitat is reflected in the biogeography of extant amphibians, which extend to higher altitude and latitude than reptiles; amphibians show greater tolerance to colder climates than reptiles (Pinder et al., 1992). Amphibians can be active at much cooler body temperatures than reptiles (Hutchison and Dupre, 1992); for example, there are observations of frogs swimming under ice (as do some turtles) and salamanders walking in snow. Several species of frogs tolerate being frozen during winter (Storey and Storey, 1986). By becoming inactive, amphibians can tolerate wide fluctuations in internal conditions during drought or very cold conditions.

By releasing the constraints of the hydric niche, the earliest amniotes increased body temperature and metabolic rate, but in so doing may have limited themselves to a more narrow thermal niche. In essence, the reptile by comparison has a higher metabolism and a more narrowly regulated, homeostatic internal milieu that enables it to be more active than an amphibian in a much wider range of terrestrial habitats. In this manner, reptiles have obtained greater independence from the hydric environment by means of a more fixed internal milieu than was the case in anamniote tetrapods.

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Thermoregulation in Animals

U. Ganslosser , in Encyclopedia of Ecology, 2008

Adaptation to Cold Seasons

In temperature-conformic ectotherms that are generally unable to increase their temperatures by internal means, torpor or hibernation in winter or estivation in hot, dry summers (the latter being shown, for example, by lungfish or desert snails) is superficially similar to torpor in endotherms; however, external energy sources are needed to end this state. In insects, a special form of arrested development called diapause, triggered by combination of hormonal, photoperiodic, and nutritional factors, is a common strategy. Heterotherm animals are also characterized by changing their body temperatures and basal metabolism according to external conditions. Torpor is characterized by a low body temperature and low BMR. In so far, it is probably a variation of temperature conformity instead of regulation. However, there is an important difference between heterothermy and ectothermy: heterotherms are capable of increasing body temperatures (mostly by increasing BMR) by their own, internal means, whereas ectotherms need some external source of warmth or other energy for this waking up. Thermoregulation is never totally switched off during torpor; instead, the set point for the onset of thermoregulatory activities is only temporarily lowered. Heterothermy has long been regarded as a primitive character of animals not yet ready for real homeothermy, but it is a finely tuned adaptive strategy.

Torpor is further divided by the regularity and seasonality of its occurrence and triggering mechanisms. Long torpor, mostly hibernation, is often extended for several months and is characterized by a lowering of body temperatures under 10   °C, and metabolic rate is about 5% of BMR during active phases. However, even deep hibernating torpor in all species studied so far is interrupted by short periods of activity at normal body temperature, and these intervals are internally triggered.

Large mammals, such as bears, also go into torpor. However, this is only shallow torpor; with a reduction of their body temperatures by about 5   °C, heart rates and metabolic rates are reduced by up to 30%. Nevertheless, hibernating bears can stay in their dens for several months, and their energy needs are covered by burning fat. Some other physiological adaptations, such as recycling urea into essential amino acids, and most probably also calcium storage and recycling, have been developed in these large carnivores as well. Large bears are not the only carnivores capable of larger torpor. Raccoons and raccoon dogs, at least in parts of their range, also enter torpor for several weeks.

Short-term torpor of several days or even daily torpor is much more widespread also among larger mammals – both American and European badgers enter daily or short-term torpors, with body temperatures of about 28   °C. Daily or short-term torpor, in general, reduces body temperatures to c. 10–30   °C; metabolic rates are reduced to values of about 30%. Most mammal species entering daily torpor are small and nocturnal such as small marsupials (dasyurids, petaurids, and didelphids), mouse lemurs, hedgehogs, tenrecs, shrews, or bats. However, in almost all these taxa (except primates), we also find species exhibiting deep torpor with body temperatures around 5   °C and durations of 10 days to several months (marsupials: Cercatetus nanus, a burramyid, reaches values of 2% of its normal BMR for several weeks, European hedgehog: energy of about 4% normal rate, T b c. 5   °C for at least 10 days, bats: Myotis −2 to +5   °C T b, energy about 1% BMR, etc.). Heterothermy among birds is different in several aspects: it mostly occurs during the night, T b is lowered by c. 5   °C, it also occurs in rather large species such as turkey vultures, but, and this is a phenomenon whose adaptive significance is still unclear, energetic demand is mostly higher than BMR. Only few species, such as some members of Colibri, tend to reduce T b to values below 18   °C, some below 10   °C, and only one species of bird, a nightjar from North America, Phalaenoptilus nuttallii, goes into torpor for several days in a row, and also reaches a T b as low as 6   °C. It is not yet totally clear, neither for birds nor for mammals, which physiological mechanisms are responsible for reawakening. One hypothesis assumes a combination of low blood pressure and accumulation of toxic metabolic products in the blood to cleanse the blood from these waste products, another assumes a biological clock (maybe even the circadian one which is also being slowed by lower body temperatures). In any case, the end of a torpor phase is achieved by active warming, the velocity of which mostly depends on body size: small animals of about 10   g body weight can gain almost 1   °C per min, species of about 1   kg only achieve 0.5   °C min−1, and species over 10   kg are real slow wakers, with increases of about 0.1   °C min−1. This is a constraint on the capability for deep torpor in large species.

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