Temperature and animal energetics

1. TEMPERATURE AND ANIMAL
ENERGETICS
Few environmental factors have a larger influence on animal energetics than
temperature.
Those animals whose body temperature fluctuates with that of the
environment experience corresponding temperature-induced changes in
metabolic rate, whereas those that can maintain a constant body temperature
in fluctuating environmental temperatures have to expend metabolic energy
to do so.

2.  Chemical reaction rates, especially those of enzymatic reactions, are highly temperature
dependent.
Therefore, tissue metabolism and, ultimately, the life of an organism depend on maintenance of the
internal environment at temperatures compatible with metabolic reactions facilitated by enzyme.
When we consider the effect of temperature on the rate of a reaction, it is useful to obtain a
temperature quotient by comparing the rate at two different temperatures.
A temperature difference of 10 celsius degrees has become a standard (if arbitrary) span
over which to determine the temperature sensitivity of a biological function. It is calculated by
using the van't hoff equation:
• Temperature Dependence of Metabolic Rate

3. Where k1 and k2 are rates of reaction (rate constants) at temperatures t1, and t2, respectively.
Van't Hoff Equation

4. • The metabolic rates in most animals with variable body temperature increase two- to
threefold for every 10 degree (Celsius) increase in ambient temperature, in accordance with
what would be predicted for the Q10 of enzymes.
• Yet, the metabolic rates of some ectotherms exhibit a remarkable temperature
independence
• For example
some intertidal invertebrates that experience large swings in ambient temperature with the ebb
and flow of the tides have metabolic rates with a Qlo very close to 1.0, so the rate of
metabolism changes very little with temperature changes as large as 20 degrees. These animals
appear to possess enzyme systems with extremely broad temperature optima, which prevents
their inactivation during environmental temperature swings.

5. • Such enzyme systems may be due to a staggering of the temperature optima
of sequential enzymes in a reaction, such that a drop in the rate of one step
in a sequence of reactions "compensates" for an increased rate of other
steps in the sequence.

6. Enzymatic acclimation
• Acclimation occurs in individual tissues as well as in whole animals.
• For instance, at a given experimental temperature, winter acclimated frogs
and summer acclimated frogs have different contractile properties of skeletal
muscles and different heart rates.
• Similarly, nerve conduction persists at low temperatures in cold-acclimated
fishes, but it is blocked at these same temperatures in warm-acclimated ones.
• How can this be explained? It is reasonable to suppose that enzymatic
reactions have been affected

7. • A change in the rate of enzymatically controlled reactions can indicate a
change either in the molecular structure of one or more enzymes or in some
other factor that affects enzyme kinetics.
• In some instances of acclimation, however, thermal compensation appears to
result simply from a change in the quantity of an enzyme rather than its
characteristics.

8. Homeo viscous membrane adaptations
• The cell membrane, which is composed largely of a bilayer of lipids with embedded
proteins, is very sensitive to temperature change.
• Low temperatures can cause the membrane to enter a gel-like phase with very high
membrane lipid viscosity, whereas high temperatures can cause the membrane to become
"hyperfluid" with very little viscosity.
• Either situation can cause increasingly disruptive changes in physical properties as
temperatures move away from optimal values for a particular animal.
• The many functions of the cell membrane, which range from forming a physical barrier to
general solute diffusion to facilitating transmembrane movement of specific solutes, can be
in jeopardy if the membrane lipid viscosity becomes too high or too low.

9. • We can imagine the effect of temperature on lipid viscosity by recalling that room
temperatures lie below the melting point of a cooking grease but above the melting
point of a cooking oil.
• The difference between the oil and the grease lies in the degree of hydrogenation of
the carbon backbone.
• The greater the proportion of unsaturated (i.e., double unhydrogenated) carbon-
carbon bonds of a lipid's fatty acid molecules, the lower its melting point.
• At temperatures above the melting point, the lipid is less viscous, or "oily"; below
the melting point, it is more viscous, or "waxy."

12. acclimatization of ectothermic animals
• Part of the acclimatization of ectothermic animals to cold or hot
environments is that the membrane lipids become more saturated during
acclimatization to warmth and less saturated during acclimatization to cold,
helping to stabilize the form of the lipids and thus the cellular functions that
spring from them.
• This phenomenon is called homeoviscous adaptation, referring to
adaptations at the molecular level through natural selection that help
rninimize temperature-induced differences in viscosity.

13. • Unfortunately, there is no simple measure of membrane fluidity.
Most often used as an index is the steady-state fluorescence
anisotropy (a measure of the lack of symmetry of a molecule or
structure). 1,6-Diphenyl-l,3,5-hexatriene (DPH) is a commonly used
probe of membrane fluidity.

14. • Initially, acute temperature changes are accompanied by changes in
membrane polarization and fluidity.
• However, with time, homeoviscous adaptation of the membrane lipids
results in a lipid polarization and membrane viscosity after acclimation at 5°C
that are similar to those after acclimation at 20°C.

15. Determinants of Body Heat and Temperature
• The temperature of an animal depends on the amount of heat (calories)
contained per unit mass of tissue. Because tissues consist primarily of water,
the heat capacity of tissues between 0°C and 40°C approximates 10 cal .
• It follows that the larger the animal, the greater its body heat content at a
given temperature.

16. The rate of change of body heat depends on
• (1) the rate of heat production through metabolic means.
• (2) the rate of external heat gain.
• (3) the rate of heat loss to the environment.

18. • body heat = heat produced + (heat gained - heat lost)
= heat produced + heat transferred
• Numerous factors affect the rate of body heat production. Behavioral mechanisms such as
simple exercise cause an increase in heat production by elevating metabolism.
• The activation of autonomic mechanisms leading to release of hormones can produce
accelerated metabolism of energy reserves.

19. • The total heat content of an animal is determined by the metabolic
production of heat and the thermal flux between the animal and its
terrestrial surroundings
• The relation between these factors can be represented as in which Htot is the
total heat, Hv is the heat produced metabolically, Hc is the heat lost or gained by
conduction and convection, Hr is the net heat transfer by radiation, He is the heat lost by
evaporation, and Hs is the heat stored in the body.

20. Conduction
• The transfer of heat between objects and substances that are in contact with
each other is conduction.
• It results from the direct transfer of kinetic energy of the motion from
molecule to molecule, with the net flow of energy being from the warmer to
the cooler region.

21. • in which Q is the rate of heat transfer (in joules per centimeter per second)
by conduction.
• k is the thermal conductivity of the conductor; A is the cross-sectional area
(in square centimeters); and 1 is the distance (in centimeters) between points
1 and 2, which are at temperatures t, and t,, respectively.
• Conduction is not limited to heat flow within a given substance; it may also
be between two phases, such as the flow of heat from skin into the air or
water in contact with the body surface.

22. Convection
• The transfer of heat contained in a mass of a gas or liquid by the movement of that
mass is convection.
• Convection may result from an externally imposed flow (e.g., wind) or from the
changes in density of the mass produced by heating or cooling of the gas or fluid.
• Convection can accelerate heat transfer by conduction between a solid and a fluid,
because continuous replacement of the fluid (e.g., air, water, or blood) in contact
with a solid of a different temperature maximizes the temperature difference
between the two phases and thus facilitates the conductive transfer of heat between
the solid and the fluid.

24. Radiation
• The transfer of heat by electromagnetic radiation takes place without direct contact between
objects.
• All physical bodies at a temperature above absolute zero emit electromagnetic radiation in
proportion to the fourth power of the absolute temperature of the surface.
• As an example of how radiation works, the sun's rays may warm a black body to a
temperature well above the temperature of the air surrounding the body. A dark body both
radiates and absorbs more strongly than does a more reflective body having a lower
emissivity.
• For temperature differences between the surfaces of two bodies of about 20 Celsius degrees
or less, the net radiant heat exchange is approximately proportional to the temperature
difference.

26. Evaporation
• Every liquid has its own latent heat of vaporization, which is the amount of
energy required to change it from a liquid to a gas of the same temperature-
that is, to evaporate.
• The energy required to convert 1 g of water into water vapor is relatively
high, approximately 585 cal.
• Many animals dissipate heat by allowing water to be evaporated from body
surfaces.

27. Heat storage
• Heat storage leads to an increase in temperature of the heat-storing mass.
• The larger the mass, or the higher its specific heat, the smaller its rise in
temperature (in "C) for a given quantity of heat (in joules) absorbed.
• Thus, a large animal that has a small surface-to-mass ratio tends to heat up
more slowly in response to an environmental heat load than does a small
animal that has a relatively high surface-to-mass ratio.

28. • The rate of heat transfer (kilocalories per hour) into or out of an animal also
depends on several factors.
• Surface area per gram of tissue decreases with increases in body mass, providing
small animals with a high heat flux per unit of body weight (as already noted).
Animals can sometimes control their apparenc surface area by changing posture
(e.g., by extending limbs or drawing them close to the body).
• Temperature difference between the environment and the animal's body has a large
effect by altering the temperature gradient (i.e., change in temperature per unit
distance) for heat transfer. The closer an animal maintains its temperature to the
ambient temperature, the less heat will flow into or out of its body.

29. • Specific heat conductance of the animal's surface varies with the nature of the body
surface.
• Animals with high heat conductances in surface tissues are typically close to the
temperature of their surroundings, with some exceptions, such as the elevation of
body temperature when an animal basks in sunlight.
• . Animals that actively maintain a constant body temperature (birds, mammals) have
feathers, fur, or blubber that decrease the heat conductance of their body surfaces.
• An important feature of fur and feathers is that they trap and hold air, which has a
very low thermal conductivity and therefore further retards the transfer of heat

30. Mechanisms to regulate the exchange of heat
• Animals use several different mechanisms to regulate the exchange of heat between
themselves and the environment:
• Behavioral control includes moving to a part of the environment where heat
exchange with the environment favors attaining optimal body temperature.
• For instance, a desert ground squirrel retires to its burrow during the midday heat; a
lizard suns itself to gain heat by radiation from its surroundings, raising its body
temperature well above ambient temperature.
• Animals also control the amount of surface area available for heat exchange by
adjusting their postures.

31. • Autonomic control of blood flow to the vertebrate skin affects the
temperature gradient and, hence, the heat flux at the body surface.
• For example, the activation of piloerector muscles increases the extent of
fluffing of pelage and plumage, which increases the effectiveness of
insulation by increasing the amount of trapped, unstirred air).
• Sweating and salivation during panting cause evaporative cooling

33. • Acclimatization includes long-term changes in pelage or subdermal, fatty-
layer insulation, as well as changes in the capacity for autonomic control of
evaporative heat loss through sweating.
• Acclimatization can also include the capacity for metabolic heat generation,
as in finches.

34. Temperature Classifications of Animals
• It should be clear to you now that animals deal with variation in the thermal
characteristics of their environment in a variety of ways.
• The "traditional" scheme used by comparative physiologists to classify
thermoregulatory modes of animals is based on the stability of body temperature.
• When exposed to changing air or water temperatures in the laboratory,
homeotherms (or homoiotherms) maintain body temperatures above ambient
temperatures and regulate their body temperatures within a narrow physiological
range by controlling heat production and heat loss.
• In most mammals, the normal physiological range for core body temperature is
typically from 37°C to 38°C; whereas, in birds, it is closer to 40°C.

35. • Some vertebrates other than birds and mammals and some invertebrates also
can control their body temperatures in this manner, although such control is
often limited to periods of activity or rapid growth in these organisms.

36. Poikilotherms
• Poikilotherms are those animals in which body temperature tends to fluctuate more
or less with the ambient temperature when air or water temperatures are varied
experimentally.
• The colloquial terms "warm blooded" for homeotherms and "cold blooded" for
poikilotherms are unsatisfactory because many poikilotherms can become quite
warm.
• For example a locust sustaining flight in the equatorial sun or a lizard running
across the sand at midday in a hot desert may have blood temperatures exceeding
those of warm blooded mammals.

37. • Early comparative physiologists considered all fishes, amphibians, reptiles,
and invertebrates to be poikilotherms, because all of these animals were
thought to lack the high rates of heat production found in birds and
mammals.
• Moreover, numerous birds and mammals are now known to allow their body
temperatures to vary widely, either regionally in the body or in the whole
body over time.

39. Endotherms
• Endotherms are animals that generate their own body heat through heat production
as a by-product of metabolism, typically elevating their body temperatures
considerably above ambient temperatures.
• Most produce heat metabolically at high rates, and many have relatively low thermal
conductivity because of good insulation (fur, feathers, fat), which enables them to
conserve heat in spite of a high temperature gradient between body and
environment.
• Mammals and birds exemplify animals that regulate their temperatures within
relatively narrow limits and are therefore said to be homeothermic endotherms.

40. • A few large fishes (sharks and larger tuna) and some flying insects are termed
regional heterothermic endotherms because they maintain regions of their
body above ambient temperatures, sometimes for short periods of time
under specific circumstances, as in flying insects.

41. Ectotherms
• Ectotherms produce metabolic heat at comparatively low rates-rates normally too
low to allow for endothermy
• . Often, ectotherms have low rates of metabolic heat production and high thermal
conductances-that is, they are poorly insulated.
• As a result, heat derived from metabolic processes is quickly lost to cooler
surroundings.
• Accordingly, heat exchange with the environment is much more important than
metabolic heat production in determining an ectotherm's body temperature

42. • On the other hand, the high thermal conductance allows ectotherms to
absorb heat readily from their surroundings.
• Behavioral temperature regulation is the principal means by which
ectotherms regulate their body temperatures.

43. Heterotherms
• Heterotherms are those animals capable of varying degrees of endothermic
heat production, but they generally do not regulate body temperature within
a narrow range.
• They may be divided into two groups, regional and temporal heterotherms.
• Temporal heterotherms constitute a broad category of animals whose
temperatures vary widely over time.
• Monotremes (egg-laying mammals) such as the echidna are temporal
heterotherms as are other mammals and birds in torpor and hibernation.

45. • Temporal heterothermy is also shown by many flying insects, pythons, and some
fishes, which can raise the temperature of their bodies (or regions of their bodies, as
in the fishes) well above ambient temperature by virtue of heat generated as a by-
product of intense muscular activity.
• Some insects prepare for flight by exercising their flight muscles for a time to raise
their temperatures before takeoff.
• In hot environments, this flexibility gives certain large animals, such as camels, the
ability to absorb great quantities of heat during the day and to give it off again
during the cooler night.

46. • Regional heterotherms are generally ectotherms that can achieve high core
(i.e., deep-tissue) temperatures through muscular activity, while their
peripheral tissues and extremities approach the ambient temperature.
• As mentioned earlier, examples include mako sharks, tuna, and many flying
insects.
• Elevated temperatures generally allow higher metabolic rates than would be
achieved at ambient environmental temperatures

47. • Fishes that are regional heterotherms depend on counter current heat
exchangers.
• Heat is conserved in the body core by a specialized parallel arrangement of
incoming arteries and outgoing veins in the case of heat exchangers,
facilitates heat transfer between blood vessels and retains heat in the body
core.
• Some large billfin fishes (e.g., marlin) use specialized ocular muscle called
"heater tissue" to elevate brain temperature

50. TEMPERATURE RELATIONS OF
ECTOTHERMS
• Ectotherms occupy a wide variety of environments- both hot and cold.
• A few very specialized environments have highly stable temperatures, varying no
more than a degree or two throughout the year.
• Examples are the shallow marine waters under the Arctic and Antarctic ice, the deep
regions of the seas, the air deep within the interior of many caves, and the
microenvironments within deep groundwater.
• Such variation is maximized in terrestrial temperate environments, some of which
may have daytime summer surface temperatures of nearly 40°C and night-time
winter temperatures of -40°C.

51. Ectotherms in Freezing and Cold
Environments
• Because the body temperature of many ectotherms depends to a considerable extent on the
ambient temperature, freezing is a threat to those species living in environments with
ambient temperatures below freezing.
• The formation of ice crystals within cells is usually lethal because, as the crystals grow in
size, they rupture and destroy the cells.
• No animal is known to survive complete freezing of its tissue water, but some come close
• The freshwater larvae of the midge Chironomus, which survive repeated freezing, yield
some unfrozen liquid at temperatures as low as -32°C.
• If ice crystals form and grow within cells, they damage the tissue by breaking the cells.
• In contrast, ice crystals that form outside the cells do little damage.

52. • Red blood cells, yeast, sperm, and other cell types also can withstand freezing
damage, provided intracellular ion concentrations do not rise above those
levels that cause damage to cell organelles.
• Some animals can undergo supercooling, in which the body fluids can be
cooled to below their freezing temperature, yet remain unfrozen because ice
crystals fail to form.
• Ice crystals will not form if they have no nuclei (mechanical "seeds," so to
speak) to initiate crystal formation.

53. • The body fluids of some cold-climate ectotherms contain antifreeze substances.
• For example, the body fluids of a number of arthropods, including mites and
various insects, contain glycerol, the concentration of which typically increases in
the winter.
• Glycerol, acting as an antifreeze solute, lowers the freezing point to as low as -17°C.
• The tissues of larvae of the parasitic wasp Brachon cephi can withstand even lower
temperatures; they have been supercooled to -47°C without ice crystal formation.

54. Ectotherms in Warm and Hot Environments
• Heat exchange with the environment is closely related to body surface area,
so the temperature of a small ectotherm (which has a relatively large surface
area) rises and falls rapidly as environmental temperatures undergo daily
fluctuations.
• All ectotherms have a critical thermal maximum, a temperature above which
long-term survival is not possible.
• Generally, this is determined by measuring the temperature at which 50%
mortality occurs in a population.

55. • The critical thermal maximum varies enormously, depending on the organism.
Some thermophilic bacteria can thrive at temperatures above 9O°C, although
almost all metazoans have critical thermal maxima below 45°C.
• The physiological causes for a critical thermal maximum are varied.
• Most ectotherms never experience such extreme temperatures. Yet, even in
temperate climates, many experience general environmental temperatures that are
high enough to require active responses to prevent unacceptably elevated body
temperatures.
• Many ectotherms expose their bodies to sun or to shade to absorb more or less
heat, respectively, from the environment.

57. • Certain reptiles, however, additionally use nonbehavioral (i.e., physiological) means
to control the rates at which their bodies heat and cool.
• For example, the diving Galapagos marine iguana Amblyrhynchus can permit its
body temperature to rise at about twice the rate at which it drops by regulating both
heart rate and flow of blood to its surface tissues.
• When the iguana wishes to warm up, it basks in the sun and simultaneously diverts
cooler core blood to the surface.
• The net effect is a large difference between body and environmental temperature.

58. • The increased blood flow increases the skin's heat conductance and speeds
absorption of heat into the animal.
• Increased pumping of blood accelerates the removal of heat from surface
tissues to deeper tissues.
• During the iguana's prolonged feeding dives in the cool ocean, loss of body
heat is slowed by a reduction in blood flow to the surface tissues and by a
general slowing of the circulation.

59. Costs and Benefits of Ectothermy: A
Comparison with Endothermy
• Early comparative physiologists assumed that ectothermy was inferior to endothermy as a
way of life.
• The endothermal vertebrates (primarily birds and mammals) were viewed as more complex
recent evolutionary arrivals than the primarily ectothermal "lower vertebrates" (fishes,
amphibians, and lizards).
• More recently, however, the term "lower vertebrates" has fallen out of favor as we realize
that they are as highly adapted for their way of life as are birds and mammals for theirs.
• Indeed, the endotherms and ectotherms do represent different life-styles, the former
representing a fast, high-energy way of life and the latter representing a slower, low-energy
approach.

60. • Thus, in considering the "costs and benefits" of ectothermy relative to endothermy in
animals, the following generalizations can be made:
• Because their body temperatures are generally closer to ambient temperatures, ectotherms
generally live at a lower metabolic rate.
• As a consequence, ectotherms can "invest" a larger proportion of their "energy budget" in
growth and reproduction.
• Ectotherms require less food, and so they can spend less time foraging and more time
quietly avoiding predators.
• They also need less water, because they lose less by evaporation from their typically cooler
surfaces, and they need not be massive for the purpose of reducing surface-to-volume ratio.

61. • The benefits in item 1 are balanced by certain costs, among which is the
inability of ectotherms to regulate their body temperatures (unless their
environments permit behavioral thermoregulation) .
• For example, a lizard can elevate its temperature by basking only if there is
sufficient solar radiation, which limits the times of day and seasons of the
year when such activity is possible.

62. • Endothermic animals can do certain things on a bigger, faster scale, but they do so
at a price.
• The field metabolic rates (daily costs of survival in their natural habitats) of
endotherms are as much as 17 times as high as the field metabolic rates of
ectotherms.
• Thus, a 300 g rodent needs 17 times as much food per day as does a 300 g lizard
living in the same habitat and having the same diet of insects.
• The high rate of respiratory gas exchange makes endotherms susceptible to
dehydration in hot, dry climates.

63. TEMPERATURE RELATIONS OF
HETEROTHERMS
• Heterotherms are animals intermediate between pure ectotherms and
endotherms. As mentioned earlier in this chapter (in Temperature
classifications of animals), certain insects and fishes are heterotherms. Some
flying insects, in .
• Cluding locusts, beetles, cicadas, and arctic flies, can be considered both
temporal and regional heterotherms because, when preparing to fly, they
raise the core temperatures in their thoracic parts to more or less regulated
levels.

64. • Like endotherms in general, endothermic flying insects face problems of regulating
their body temperature when their environments have large temperature gradients.
• At ambient temperatures approaching O°C, convective heat loss is generally so
rapid that flight temperatures cannot be maintained.
• High ambient temperatures, on the other hand, place the insect in danger of
overheating.
• Thus, at ambient temperatures above 20°C, the hovering sphinx moth Manduca
sexta prevents thoracic overheating by regulating the flow of warm blood to the
abdomen.

65. • Another important factor is that these regional heterotherms swim
continuously, so the red muscle never cools down to the ambient
temperature.
• One of the implications of regional heterothermy is the energy savings in
the cool tissues while the temperatures of only certain tissues, such as the
swimming muscles, are being elevated.

67. TEMPERATURE RELATIONS OF
ENDOTHERMS
• In homeothermic endotherms (most mammals and birds), body
temperature is closely regulated by homeostatic mechanisms that
regulate rates of heat production and heat loss so as to maintain a
relatively constant body temperature independent of environmental
temperatures.
• As mentioned earlier, core temperatures are maintained nearly
constant between 37°C and 38°C in mammals and about 40°C in
birds.

68. Mechanisms for Body Temperature Regulation
• Endotherms use a wide variety of both physiological and behavioral
mechanisms to maintain body temperature within a narrow range.
• Before these mechanisms are considered, however, the concept of thermal
neutral zone must be introduced.

69. Thermal neutral zone
• The degree of thermoregulatory activity that homeotherms require to maintain a
constant core temperature increases with increasing extremes of environmental
temperature.
• At moderate temperatures, the basal rate of heat production balances heat loss to
the environment.
• Within this range of temperatures, termed the thermal neutral zone, an endotherm
does not need to expend energy to maintain its body temperature; it can regulate its
body temperature by adjusting the rate of heat loss through alterations in the
thermal conductance of the body surface.

70. Thermogenesis
• When the ambient temperature drops below the lower critical temperature,
an endothermic animal responds by generating large amounts of additional
heat from energy stores, thereby preventing a decrease in the core
temperature.
• There are two primary means of extra heat production other than exercise:
shivering and nonshivering thermogenesis.
• Both processes convert chemical energy into heat by a normal energy-
converting metabolic mechanism that is adapted to primarily produce heat.

71. • Shivering is a means of using muscle contraction to liberate heat.
• Shivering thermogenesis occurs in some insects as well as in endothermic
vertebrates.
• The nervous system activates groups of antagonistic skeletal muscles so
that there is little net muscle movement other than shivering.
• The activation of muscle causes ATP to be hydrolyzed to provide energy
for contraction.
• Because the muscle contractions are inefficiently timed and mutually
opposed, they produce no useful physical work, but the chemical energy
released during contraction appears as heat.

72. • In nonshivering thermogenesis, enzyme systems for the metabolism of fats
are activated throughout the body, so conventional fats are broken down and
oxidized to produce heat.
• Very little of the energy released is conserved in the form of newly
synthesized ATP.
• A specialization found in a few mammals for fat-fueled thermogenesis is
brown fat, also called brown adipose tissue (BAT)

74. Endothermy in cold environments
• Endotherms adapted to cold environments have necessarily evolved a number of
mechanisms, both temporary and permanent, that help them retain body heat.
• For example, an animal sensing heat loss in a windy place will fluff its fur or
feathers and move to a more sheltered area.
• This reduces convection and the dissipation of body heat by the wind.
• More enduring responses to cold include the thick layers of insulation in many
arctic animals, in the form of subcutaneous fat or a thicker pelage or plumage

76. • The specific conductance's of homeotherms vary over a large range and
decrease with body size.
• Larger animals have lower specific heat conductance owing to their generally
thicker coats of fur or feathers.
• In addition, they face smaller heat loss in cold climates because of their
relatively smaller surface areas.

78. Counter current heat exchange
• Effective locomotion requires that the limbs of endotherms not be
mechanically hindered by a massive layer of insulation.
• The flukes and flippers of cetaceans and seals and the legs of wading birds,
arctic wolves, caribou, and other cold weather homeotherms require blood to
nourish cutaneous tissue and limb muscles used in locomotion.
• The well vascularized limbs are potential major avenues of body heat loss
because they are thin and have large surface areas.

79. • Arterial blood, originating in the animal's core, is warm. Conversely, the
venous blood returning from peripheral tissues may be very cold.
• As blood flows from the core, it enters arteries in the limb that lie next to
veins that carry blood returning from the extremity.

81. DORMANCY SPECIALIZED METABOLIC
STATES
• Dormancy is a general term for reduced body activities, including reduced
metabolic rate.
• It often includes heterothermy. Dormancy can be variously classified
according to its depth (in reference both to ability for arousal and to decrease
in Tb) and its duration, and it includes sleep, torpor, hibernation, winter
sleep, and estivation.

82. Sleep
• Studied intensively in human beings and other mammals, sleep entails extensive
adjustments in brain function.
• In mammals, slow wave sleep is associated with a drop in both hypothalamic
temperature sensitivity and body temperature, as well as changes in respiratory and
cardiovascular reflexes.
• During rapid-eye-movement (REM) sleep, hypothalamic temperature control is
suspended.
• Although there may be a variety of triggers of sleep, in mammals there is evidence
of sleep-inducing substances that build up during wakefulness, accumulating in
extracellular fluids of the central nervous system

83. • Seals resting on pack ice sleep for only a few minutes at a time before
rousing to scan the ice for approaching polar bears.
• Human beings and many other mammals sleep for hours at a time.
• Many of the big carnivores (e.g., lions and tigers) sleep for as long as 20
hours a day, especially after a meal.

84. Torpor
• The lower the Tb, the lower the basal metabolism and the lower the rate of
conversion of energy stores, such as fatty tissues, into body heat.
• Thus, it is generally advantageous to allow body temperature to decrease
during periods of nonfeeding and inactivity.
• Small endotherms, because of their high rates of metabolism, are subject to
starvation during periods of inactivity when they are not feeding.
• During those periods, some animals enter a state of torpor, in which
temperature and metabolic rate subside.

85. • Then, before the animal becomes active, its body temperature rises as a result
of a burst of metabolic activity, especially through shivering or oxidation of
brown-fat stores or both (if a mammal)
• Daily torpor is practiced by many terrestrial birds.
• The hummingbird is a classic example, allowing its body temperature to fall
from a daytime level of about 40°C to a night time level as low as 13°C (in
the rufous hummingbird when a low ambient temperature permits).

87. Hibernation
• A period of deep torpor, or winter dormancy, hibernation lasts for weeks or even
several months in cold climates.
• It is entered into through slow wave sleep and is devoid of rapid-eye-movement
sleep.
• Hibernation is common in mammals of the orders Rodentia, Insectivora, and
Chiroptera, which can store sufficient energy reserves to survive the periods of
nonfeeding.
• Many hibernators arouse periodically (as often as once a week or as infrequently as
every 4-6 weeks) to empty their bladders and defecate.

88. • During hibernation, the hypothalamic thermostat is reset to as low as 20 Celsius
degrees or more below normal.
• At ambient temperatures between 5°C and 1S0C, many hibernators keep their
temperatures as little as 1 degree above ambient temperature.
• If the air temperature falls to dangerously low levels, the animal increases its
metabolic rate to maintain a constant low Tb or becomes aroused.
• Thermoregulatory control is not suspended during torpor and hibernation-it merely
continues with a lowered set point and reduced sensitivity (gain), as in slow wave
sleep.

89. • All true hibernators are midsized mammals weighing at least several hundred
grams and large enough to store sufficient reserves for extended hibernation.
There are no true hibernators among large mammals.
• Bears, which were once thought to hibernate, in fact simply enter a "winter
sleep" in which body temperature drops only a few degrees, and they remain
curled up in a protected microhabitat such as a cave or hollow log.
• With its large body mass and low rate of heat loss, a bear can store sufficient
energy reserves to enter winter sleep without dropping body temperature.

90. Estivation
• The poorly defined term estivation, which has been called "summer sleep," refers to
a dormancy that some species of both vertebrates and invertebrates enter in
response to high ambient temperatures or danger of dehydration or both.
• Land snails such as Helix and Otala become dormant during long periods of low
humidity after sealing the entrance to the shell by secreting a diaphragm-like
operculum that retards loss of water by evaporation.
• Many land crabs similarly spend dry seasons in an inactive state at the bottom of
their burrows.

92. • Well known as estivators are African lungfish, Protopterus.
• These air-breathing fish survive periods of drought in which their ponds dry
up by estivating in the semidry bottom until the next rainy season floods the
area.
• The lungfish seals itself inside a "cocoon," in which a small tube leads from
the fish's mouth to the exterior to allow ventilation of the lungs.
• Interestingly, chemical estivation-inducing factors in the plasma of estivating
lungfish produce a torporlike state when injected into mammals

93. • Some small mammals, such as the Columbian ground squirrel, spend the hot
late summer inactive in their burrows, with their core temperatures
approaching the ambient temperature.
• This state is probably similar physiologically to hibernation, but it differs in
seasonal timing.