Temperature

Temperature is an important environmental factor. It influences the biology of an animal through changes in the rates of biochemical and physiological processes and in the stability of biomolecules. An animal's Tb is the result of thermal balance between the rates of heat gain and loss.

Fourier Equation for Heat Balance

Heat balance of an animal is determined by the net exchange of heat. Heat balance is decided through avenues of heat gain or loss and metabolic heat production. The general equation describing heat balance is

prod i Hcond i Hconv i Hrad i Evap i Hst

Hprod is metabolic heat production, Hcond is heat transfer by conduction through physical contact, Hconv is heat transfer across the fluid boundary layer at the surface of the animal, and Hrad is radiative heat transfer between the animal and its surrounds; all depend on temperature gradients, either between the animal's body or surface temperature and the ambient temperature or the temperature of the surrounding objects. Evap is the heat that is lost during evaporation (or heat gained during condensation) and depends on the difference in vapor pressure between the animal and the surrounding air, assuming the air at the animal's surface is saturated. Hstore is the heat that is gained or lost by a change in Tb, and must equal zero for an organism to be in heat balance and thus maintain a constant Tb.

All avenues of heat transfer depend on the surface area of the animal, being greater the larger the surface area. Therefore, by reducing surface area, thermal conductivity, or the coefficient of radiation, an animal can reduce heat transfer to the environment. For a homeotherm to maintain constant Tb the equilibrium between heat loss and gain or production must be maintained at all ambient temperatures (Figure 2). For an ectotherm at a given Tb, if the properties of heat transfer remain unaltered, there can only be a single ambient temperature at which the rates of heat loss and gain with the environment are in equilibrium. An endotherm, without altering the heat transfer properties, at the same ambient temperature, would have a higher Tb; the exact Tb would be determined when equilibrium is established between the rates of heat production and transfer. Under these conditions, Tb will remain constant over time. At ambient temperatures lower than the point of equilibrium the animal must increase heat gain or production to match heat loss, reduce heat loss or a combination of the two to prevent a decline in Tb. At warmer temperatures, the animal must increase heat loss and reduce heat gain or production to maintain Tb. Many endothermic homeotherms extend the single ambient temperature at which equilibrium between heat production and loss occurs into a zone, the thermoneutral zone, without activating regulatory heat production. This is achieved by altering heat transfer (e.g., conductance through changes in surface area achieved through posture or peripheral adjustments to the circulation) or by utilizing heat from obligatory heat-generating processes (activity, postprandial metabolism).

Sources of Heat

External heat. Generally, metabolic heat production by ectotherms is negligible. If Tb equals ambient temperature (Ta) then the animal must thermoconform. However, Tb can be elevated above ambient temperature through behavioral adjustments (e.g., basking). In addition, Tb can be elevated by increasing radiative heat gain (e.g., black bodies absorb more radiant heat than white) and/or conductive heat gain (e.g., ventral contact with solar-heated substrate) and can be maintained upon removal of the heat

Mammalia Aves

All mammals ^ All birds

Gnathostomota

Sarcopterygii (lobe-finned fish)

Sauropsida

Crocodylia Squamata

■ Sphenodontia Testudines

Crocodylidae Esturine crocodile -.'J Boidae Brooding pythons

Varanid lizards

Some other squamates

Actinopterygii (ray-finned fish)

Neopterygii

Acanthopterygii

Dermochelyidae Leatherback turtle

Amphibia Sphyraenidae Gempylidae - Trichiuridae Scombridae Xiphiidae Istiophoridae

Tunas, butterfly mackerel Swordfish billfish

Marlin, sailfish

-Other Perciform fishes

Chondrichthyes

Elasmobranchii

Hexapoda

Pterygota

Nelumbonaceae (e.g., Lotus)

Araceae

(e.g., Philodendron)

^ Regional heat production General heat production External heat

Diptera (flies)

Coleoptera (beetles) Hymenoptera (bees and ants) Lepidoptera (moths) Odonata (dragonflies) Hemiptera (bugs) Orthoptera (grasshoppers) - Other insects

Thresher sharks

Alopiidae

Odontaspididae

- Megachasmidae

Pseudicarchariidae

Mackerel sharks (e.g., Mako, salmon ^ shark)

Lamnidae

Carcharhiniformes (Ground sharks)

Squaliformes (dogfish) Other sharks

Cetorhinidae Mitukurinidae

Carcharhinidae Blue shark

- Myliobatidae Manta ray Xp

-Other rays and skates

Chordata

Chelonioidae

Vertebrata

Scombroidei

Perciformes

Percoidei

Other fishes

Lamniformes

Insecta

Plants

Figure 1 Cladograms showing proposed phylogenetic relationships among homeothermic organisms. Homeothermy may be achieved in organisms that rely predominantly on external heat obtained from the environment (ectotherms) or in organisms in which the majority of heat is metabolically generated (endotherms). The metabolically generated heat can be either confined to specific areas of the body (e.g., thorax in insects, cranial regions in fish) or more generally distributed and may be purposely generated for thermoregulation (thermogenesis) or as a by-product of other activities.

Heat transfer

'Endotherm'

Heat loss T

'Ectotherm'

Tb = constant Obligatory homeotherm thermogenesis t iM

Heat loss e

Heat gain Heat production (Regulatory thermogenesis)

'Biological relevant' temperatures

Ambient temperature

Figure 2 Body temperature is established as a balance between heat input and heat loss. Heat input occurs through heat transfer or from obligatory or regulatory thermogenesis. Heat transfer, either loss or gain, between the animal and its environment can occur via conduction, convection, radiation, or evaporation/condensation. The rate of heat transfer for each mode is proportional to the surface area and, except for evaporation/conduction, is proportional to the temperature gradient between the animal and the environment. For an 'ectotherm' at an established body temperature (Tb) in equilibrium with a given ambient temperature (Ta) heat gain equals heat loss (intersection with Tb = Ta). At temperatures on either side of this single Ta, for Tb to remain constant changes in heat transfer need to occur; at lower Ta heat loss will increase and heat needs to be added (or heat loss reduced) and for higher Ta heat gain will increase and heat loss needs to increase (and/or heat input reduced). The increase in cellular metabolism associated with leakier membranes in 'endotherms' is coupled with increased heat production (obligatory thermogenesis). The production of heat will result in an increase in Tb to a temperature where heat loss from the increasing Tb-Ta gradient will reestablish equilibrium at the single Ta in which equilibrium was originally established in the 'ectotherm'. To maintain Tb with changing Ta again requires adjustments in heat transfer. In the case of an 'endotherm', a decrease in Ta is initially met over a narrow temperature range through changes in heat transfer to offset the increasing heat loss (known as the 'thermal neutral zone', TNZ), after which further decline in Ta is countered with increased heat production (regulatory thermogenesis).

source by minimizing conductive heat loss (e.g., reduced peripheral circulation, addition of insulative fat layers).

Internal heat. Obvious examples of endotherms are mammals and birds but other examples can be found among the reptiles, fish, insects, and plants. In all these examples sufficient heat can be metabolically produced to elevate tissue temperatures above that of the ambient temperature. In many of these organisms the increase in temperature is reflected as a general increase in overall Tb (mammals, birds, tuna, brooding pythons), whereas in others the temperature increase is regional (lamnid sharks, swordfish, many insects, the flowers of some plants). Heat generated from the activity of exercising aerobic skeletal muscle can be retained; this can be an important source of heat in small endotherms and in some endothermic fish. In tuna and lamnid sharks the red muscle is located in a more medial and anterior body position and the perfusion ofthis muscle enables countercurrent heat entrapment of metabolic heat that accompanies continual locomotion and the maintenance of a stable Tb despite excursions by these fish into cold water (Figure 3 a). In lamnid sharks (e.g., Salmon shark) that inhabit cold waters the red muscle has specialized to function within an elevated temperature range (20-30 °C).

In other cases, the skeletal muscles can be activated for nonlocomotory isometric high-speed contractions, that is, shivering, to generate heat. An example is provided by the sphinx moth that requires a thoracic temperature higher than 35 ° C for flight, achieved by shivering of wing muscles prior to flight. During flight, heat is produced as a by-product of flight metabolism and is independent of ambient temperature across the range of 15-35 °C, yet thoracic temperature is held reasonably constant by adjusting blood flow and hence heat transfer from the pubescent abdomen. At warm temperatures a similar situation occurs in the dragonfly. At cooler Ta the dragonfly and orchid bee, which are both poorly insulated, appear to modulate heat production. In the orchid bee this is achieved by beating their wings at elevated frequencies, reducing flight muscle efficiency, and producing more

Figure 3 Body (red line) and water (blue line) temperatures as a function of time in (a) an endothermic fish, the bigeye tuna and (b) a large ectothermic shark. Being an endotherm the tuna has the capacity to elevate Tb above that of the environment, whereas the ectothermic shark adopts a temperature closer to the average water temperature that it is exposed to while diving through the water column. In the case of the shark regulation is dependent on the diving behavior to contain Tb within limits. In the tuna Tb is largely independent of the temperatures encountered. (a) Redrawn from Holland KN and Sibert JR (1994) Physiological thermoregulation in bigeye tuna, Thunnus obesus. Environmental Biology of Fishes. 40: 319-327. (b) Redrawn from Carey EG and Scharold JV (1990) Movements of blue sharks (Prionace glauca) in depth and course. Marine Biology 106: 329-342.

Figure 3 Body (red line) and water (blue line) temperatures as a function of time in (a) an endothermic fish, the bigeye tuna and (b) a large ectothermic shark. Being an endotherm the tuna has the capacity to elevate Tb above that of the environment, whereas the ectothermic shark adopts a temperature closer to the average water temperature that it is exposed to while diving through the water column. In the case of the shark regulation is dependent on the diving behavior to contain Tb within limits. In the tuna Tb is largely independent of the temperatures encountered. (a) Redrawn from Holland KN and Sibert JR (1994) Physiological thermoregulation in bigeye tuna, Thunnus obesus. Environmental Biology of Fishes. 40: 319-327. (b) Redrawn from Carey EG and Scharold JV (1990) Movements of blue sharks (Prionace glauca) in depth and course. Marine Biology 106: 329-342.

heat. Either strategy achieves the goal of regulating Tb, particularly the thorax, throughout the flight (Figure 4).

Another mechanism for generating heat is nonshivering thermogenesis (NST). Only two animal tissues are specialized for NST: brown adipose tissue in small eutherian mammals and cranial heater tissue in billfishes and the butterfly mackerel. Brown adipose tissue contains uncoupling protein 1 (UCP 1) that permits futile cycling of the mitochondrial electron transport chain to produce heat without ATP synthesis and degradation. Fish cranial heater tissue have lost their myofibrillar contractile apparatus and participate in futile cycling of Ca2+ between the cytoplasm and the sarcoplasmic reticulum which is mediated in the ryanodine receptor by Ca2+-ATPase. In birds it would appear that NST also occurs via Ca2+ release uncoupled from ATP synthesis. However, this occurs in the skeletal muscle and is owing to isoforms of the sarcoplasmic/endoplasmic reticulum Ca2+-ATPase not associated with muscle fiber contraction. Interestingly, UCPs have also been identified in plants. However, in these cases most of the heat is generated via an alternative, cyanide-insensitive oxidase pathway that reduces the electrochemical proton gradient across the mitochondrial membrane. These later cases are examples of regulatory thermogenesis.

A further source of obligatory heat can arise from postprandial metabolism following ingestion of a meal (often referred to as specific dynamic action). The mechanical processing of food contributes little, whereas the high energetic cost associated with the synthesis of macromolecules (e.g., proteins) can be a substantial contributor owing to its high energetic cost. The resulting heat can warm both the viscera and the body core (e.g., albacore tuna).

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