Thermoregulation PDF

Lecture 1 Thermoregulation Prof McKechnie TEMPERATURE: determined by the average kinetic energy of molecules in a system LOOKING AT THE THERMAL LANDSCAPE OF RED LARKS A thermal landscape refers to the spatial and temporal variation of temperatures within a specific...

Lecture 1 Thermoregulation Prof McKechnie TEMPERATURE: determined by the average kinetic energy of molecules in a system LOOKING AT THE THERMAL LANDSCAPE OF RED LARKS A thermal landscape refers to the spatial and temporal variation of temperatures within a specific environment. It encompasses the different temperature zones and how they fluctuate over time. - The thermal image displays the arid environment with sparse vegetation. The highest surface temperature is observed to be 43°C within the sand dunes. - For a small red lark, traversing short distances around the thermal landscape can put it in very different thermal conditions (ranging from mild to very hot). Their ability to cope in these different areas is a very important determinant of their survival and reproduction. AVENUES OF HEAT TRANSFER IN THERMAL LANDSCAPES Involves heat loss and gain Exclusively heat loss Infrared thermal radiation from atmosphere, Evaporation ground, vegetation (trees) and animal, direct, reflected and scattered sunlight, convection by wind OPERATIVE TEMPERATURE (Te): The temperature experienced by an animal integrating all environmental avenues of heat flux The Te of Night Jars was determined using animal models - Image shows 2 night jar models created by 3D printing. The night jar on the left is identical to that on the right, however synthetic feathers were attached to it. Each of the models had a thermocouple attached to the base. The models were positioned in full sun on a red dune to mimic night jar behaviour when laying on their nests - Huge solar heat blow lead to the birds experiencing body temperatures above 50°C - This is an archetype of trade-off → Areas with vegetation tend to have predators residing around. The night jars have evolved this breeding behaviour to sit on their nests in open, sparse sights to evade predation yet they must endure egregious temperatures. AVENUES OF HEAT EXCHANGE BETWEEN ANIMALS AND THEIR ENVIRONMENTS → The difference between incoming heat and outgoing heat. The longer equation (in the slides) just includes metabolic heat production and evaporation. +/- indicative of heat loss to heat gain Thermoregulation Page 1  CONDUCTION: The transfer of heat between two objects in contact with each other Q(cond) = -kA(T₂-T₁)/x When looking at k values → Water and Human tissue have similar values because we contain water in tissues. Still air is the best insulator as it has a very low heat conductivity value HOW ANIMALS USE CONDUCTION IN THERMOREGULATION Example 1: Red Larks on a hot day - At about 9.00 AM, the Red Larks are found to be hiding in shaded areas. They dig through the surface of the soil and press their bellies into the cool layer of sand beneath. They employ discontinuous gaping to prevent water loss when breathing and close their eyes as well. This mechanism is important in conserving water as Red Larks don't drink water Example 2: Emperor Penguins and their chicks - It is important to note that the chicks reside on the parents' feet to minimize conductive heat loss to the ice below their feet Example 3: Tree-hugging Koalas - Australians observed the tree-hugging phenomenon which was then studied. Results found that koalas do this to maximize conductive heat loss to the significantly cooler tree. - NB NB NB Question → Explain how koalas manipulate the different variables in the conduction equation to maximize their heat conductivity via tree-hugging behaviour. You can speak about how their bellies are pressed up against the cool tree. Their bellies have sparse fur, thus less insulation. The sprawled-out posture maximizes contact area with the tree branch. The koalas also target the coolest areas of the tree branches. CONVECTION: The transfer of heat from one place to another by movement of liquid or gas Free convection (driven by heat transfer) or Forced convection (driven by wind) HOW ANIMALS USE CONVECTION IN THERMOREGULATION Example 1: Hotrod Ants and Thermal Respite Behaviour - Ocymyrmex robustior is subjected to intense solar radiation in the very hot layer of air a few centimetres above the ground (boundary layer) - On the surface of the ground, the ants experience a temperatures of 65°C: The ants adapt by sprinting across the sand very quickly with their skinny, long legs (compared to most ants) or by climbing on top of dung (left by herbivores) to evade these extreme surface temperatures. Example 2: Diving Penguins in the depths of the sea - Penguins are exceptionally insulated and rely on the air trapped between their feathers for warmth. As a byproduct of muscle contractions while swimming, the metabolic heat is sufficient to balance the heat lost to the water - Diving in deeper depths however, their plumage compresses and the warm air beneath gets pushed out of the layers resulting in rapid heat loss. Heat loss is also much faster in water than in air. Forced convection when looking at wind (last 2 slides) The study examined the change in metabolic rates of the birds as a result of changing environmental conditions (wind) - Heat loss increased rapidly with an increase in windspeeds Very important to take all of this into account when attempting to determine/predict operative temperatures of animals. Thermoregulation Page 2 Lecture 2 Thermoregulation Prof McKechnie RADIATION: Transfer of heat through electromagnetic radiation All objects with Tsurface > 0°K emit electromagnetic radiation At biological temperatures: long-wave infrared Why is the eye of the bird cool: moisture due to evaporative heat loss. Plumage decreases amount of heat to the environment. SOLAR RADIATION SPECTRUM Emission of sunlight striking the earth Yellow: sun strikes the atmosphere Red: at ground level Substantial UV component: less strikes the earth, thanks to the Ozone layer A lot of the energy is in the infrared wavelengths, not the visible wavelengths. Most animals absorb the infrared component and some wavelengths are blocked by water vapour in the atmosphere, or CO2, or blocked on their way back out to the atmosphere SOLAR RADIATION - Solar constant = 1360 W m-2- Approx. 42 % of solar irradiance occurs in visible wavelengths (400-700 nm) and approx. 49 % occurs as infrared radiation (> 700 nm). - The major IR component is near - IR, consisting of wavelengths up to ~2,500 nm More solar radiation reaches the ground in arid environments Kalahari vs Mozambique coasts: more heats up the surface SOLAR HEAT GAIN IN ANIMALS - Reflection off ground and objects Indirect (vs direct components) - Heat loss on cold nights: sky temperatures are at -40°C: determines the temperatures beneath - Absorptivity in IR wavelengths generally > 90 % - Influenced by: - size, shape & orientation - fractional absorptivity - insulating coat *Behaviour influences solar heat gain absorption↓ RADIATIVE HEAT TRANSFER CAN BE MODIFIED BY BEHAVIOUR - MINIMIZING OR MAXIMIZING HEAT GAIN EXAMPLE 1 - GREATER ROADRUNNER - raised feathers on dorsal side to expose as much surface area to solar radiation to take advantage of solar radiative heat gain EXAMPLE 2 - OSTRICHES - chicks standing in the shade from their mother, animals take advantage of any shady climates EXAMPLE 3 - BOKKE - standing parallel or perpendicular to the sun depending on environmental sunlight EXAMPLE 4 - NAMAQUA CHAMELEON - change colours in a way that can either minimize or maximize heat gain- absorb the same amount of solar radiation, not manipulating near-infrared absorption Thermoregulation Page 3  PELAGE IN BIRDS (EXCLUSIVELY) Dark Plumage: Cool and reduces heat gain because: Visible light absorption → Dark colours absorb most visible wavelengths, converting them to heat. Heat generation depth →This heat is generated deeper within the feathers, further from the skin. High pigment density → Black plumage has a denser concentration of pigment molecules, maximizing light absorption closer to the surface, preventing further penetration. White Plumage: Positions heat source closer to the skin as visible light is reflected, potentially increasing heat gain. Wind Dissipation and Insulation: Both dark and light plumage can benefit from wind dissipation, as air circulation removes heat. Additionally, feathers themselves offer insulation regardless of colour, providing a barrier between the generated heat and the animal's skin. Heat Absorption and Dissipation in Clothing: Dark robes in hot climates like the Middle East may utilize a similar principle. The dark material absorbs heat, but the loose-fitting design allows for air circulation, potentially increasing heat dissipation before it reaches the skin. BIRDS REGULATE BLOOD FLOW TO THE BEAK TO REGULATE HEAT LOSS - Ground hornbills vasoconstrict the vessels in the beak, the surface temperature tracks that of the air temperature - Adjust vasculature, to allow heat loss across the beak EVAPORATION: Loss of heat through transition of water from liquid to gaseous phase Approx. 2.4 kJ lost from animal for each gram of water: very costly way to regulate temperature - Evaporation is the only mechanism that allows an animal to maintain Tb < Te. Figure showing evaporative water loss in a Sand Grouse - At cool temperatures, E.W remains low and constant (25-35°C) - Temperatures increases past body temperatures: bird is actively trying to cool itself through evaporation; evaporation, panting - Massive increase seen in E.W loss, allows the birds to avoid heat stroke to keep temperatures below lethal levels. At 50°C, the operative temperatures in the arid environments: Piloerection The rate at which an animal loses water by evaporation is dependent on the temperature and humidity of surrounding air, i.e., the “steepness” of the [H₂O vapour] gradient Animals in dry environments lose water much more rapidly than those in humid environments - The [H₂O vapour] in air can be expressed in several ways: HUMIDITY - The amount of water vapour air can hold depends on the temperature of the air - 90% relative humidity in DURBAN, as air temperature changes, relative temperature changes with it. As air warm, water vapour stays constant but the AMOUNT that it can hold changes, and temperature changes - If 90% relative humidity at 10°C: The air is holding 90% of water vapour of it's maximum capacity at 10°C Absolute Humidity- vapour density (mg H2O L-1), partial pressure (kPa) Relative Humidity- % of [water vapour] when the air is saturated (holding all the water it possibly can) Saturation deficit = saturation [H₂O vapour] - actual [H₂O vapour] DIFFERENCE BETWEEN THE CAPACITY AND WHAT IT ACTUALLY IS HOLDING Thermoregulation Page 4 Lecture 3 Thermoregulation Prof McKechnie PHYSICAL PERFORMANCE AND THE "THERMAL CLIFF" OF ANIMALS CTmin: This refers to the critical minimum temperature, the point at which an animal can no longer maintain its body temperature and dies. Asymmetrical Relationship: The critical factor for physical performance is the optimal temperature and the range below it. Even small increases in temperature above the optimal range can be dangerous. Lethal Hyperthermia: As body temperature rises above the optimal range, things become fatal Evaporative cooling mechanisms are crucial for preventing animals from falling off this "thermal cliff" into a critical state. EVAPORATIVE COOLING IN HONEYBEES Under hot conditions: - Workers collect water and distribute around the hive in cells containing eggs and larvae - Fanning → accelerates evaporation - Regurgitation → evaporates the tongue HONEYBEES IN THE SONORAN DESERT When temperatures reach around 37°C, the bees will switch from bringing pollen to the hive and bring diluted water/nectar along with pollen. GOBABEB TRAINING AND RESEARCH CENTRE Kuiseb River's Role: Occasional flooding of the Kuiseb River helps prevent sand dunes from encroaching on the research station's location. Enduring Research Station: The research station continues to operate successfully thanks to the Kuiseb River's influence. WAXY CUTICLE: BARRIER TO EVAPORATIVE WATER LOSS IN NAMIB DESERT BEETLES Secrete wax onto the elytra to prevent evaporative heat loss: not losing water to the environment - TRADE-OFF: no choice but to evaporate to prevent lethal hyperthermia - the wax layer melts completely to evaporate water to the environment AVIAN HEAT DISSIPATION PATHWAYS: Southern Pied Babbler (Turdoides bicolor) A) BEAK - Dissipate through the beak via vasodilation B) RETE OPHTHALAMICUM - Reduces temperature reaching the brain C) SKIN, STRATUM CORNEUM LIPID COMPOSITION - Over short time scales: circulatory adjustments. Low temperatures → vasoconstriction and High temperatures → in subcutaneous capillary beds, partly close sphincter to outgoing veins to increase blood flow and pressure into skin for evaporative heat loss - Over longer time scales: adjust lipid composition to make less waterproof, facilitating evaporation water loss in the skin. - Energetically cheap D) GULAR FLUTTER - Rapid fluttering of the gular membrane causes evaporative increase - Energetically cheap → little increase in metabolic rates E) PANTING - Forced ventilation of lungs provides basis for evaporative water loss across the tongue - Energetically costly → energy to contract and relax muscles, which produces heat - As long as the amount of heat being lost is greater than that being generated → useful F) HYPERTHERMIA - Allowing body temperatures to increase within limits OTHER PATHWAYS: secreting dilute nasal fluid, runs down their beaks. Creates a moist surface Thermoregulation Page 5 RELATIVE COSTS OF EVAPORATIVE COOLING → not in the slides Scholander Curves depict the relationship between resting metabolic rate (RMR) and ambient temperature in endothermic animals. - Maintaining Body Temperature: Endotherms increase RMR at low temperatures to maintain constant internal body temperature. - Thermoneutral Zone: Within a specific temperature range, animals don't need to expend extra energy to dissipate heat (EWL) - this is the thermoneutral zone. - Heat Dissipation at Higher Temperatures: As temperatures rise above the thermoneutral zone, animals must dissipate more heat (EWL) to avoid overheating. - Bird Variations: The slope of the EWL increase varies between bird groups. ○ Cactus Wrens (Panting): Show a rapid rise in EWL due to panting, a highly effective cooling method. ○ Doves and Nightjars: Exhibit a more gradual EWL increase, suggesting different heat dissipation mechanisms or body temperature regulation strategies. ○ Evolutionary Trend: This may be linked to evolutionary history, with older bird groups (nightjars) having less efficient cooling compared to newer groups (songbirds). - Consequences: Rapid EWL increases require significant energy expenditure to compensate for the heat produced (metabolic heat production, MHP). This highlights the diverse thermoregulatory strategies employed by different bird groups. - Exceptional Dissipation: The average passerine bird can dissipate an impressive 140% of its MHP, demonstrating their remarkable thermoregulatory capabilities. BIRDS MAMMALS Panting → widespread in avian phylogeny, Panting → absent in rodents, pronounced in dominant in songbirds ungulates, kangaroos, canids, felids Cutaneous evaporation → doves and pigeons, Sweat glands, saliva-spreading → absent in phenotypic flexibility bats, primary avenue in rodents, canids, felids Gular Flutter → widespread in passerines, Cutaneous evaporation → pronounced in bats hyoid apparatus rapid pulsation METABOLIC HEAT PRODUCTION (MHP) (+) Inherent inefficiency of biochemical reactions means that organisms produce considerable amounts of energy - Associated with activity and processes like digestion that contribute to thermoregulation - Many animals can produce heat without activity for thermoregulatory purposes → more important for endothermic animals - Animals take advantage of internal heat mechanisms → large fish keep heat produced by muscles in the core, allowing maintenance of warm temperatures in cold waters ENDOTHERMY - Also get partially endothermic plants: arum lily flowers. Some insects and reptiles (lizards in south America), Pythons too, female python will shiver but it's vastly energetically expensive METABOLIC RATE (MR) = HEAT PRODUCTION Animals are very inefficient systems and do little real work. That is, most of the energy they consume is lost as heat to the environment - Basal Metabolic Rate (BMR) the minimum cost of living in an endotherm. Measured on resting, fasted animals with no thermal stress during their rest phase. - Standard Metabolic Rate (SMR – useful for ectotherms) the same as BMR except that the temperature at which the measurements are made is specified METABOLIC SCOPE: RANGE OF MR AN ANIMAL IS CAPABLE OF Two different in endotherms: max heat output a resting endotherm is capable of - can't produce anymore heat. 60-70% of VO₂ max Thermoregulation Page 6  MEASURING METABOLIC RATE DIRECT INDIRECT BALANCE-SHEET METHOD Measure heat production Measure rate of O₂ Subtract energy content of consumption and/or CO₂ faeces from energy content of Calculate MR from food information/assumptions about the substrate being metabolised THE METABOLIC CHAMBER OF MEASUREMENT Dual Ice Jacket System: This method uses two ice jackets surrounding a chamber. Heat Measurement: The amount of ice melt in the jackets is measured to calculate the heat generated within the chamber. Ensuring Accuracy: The second ice jacket acts as insulation, preventing external heat from influencing the measurement. This ensures the measured heat originates solely from the chamber itself. FLOW-TRHOUGH RESPIROMETRY → Measurements of O₂ consumption and CO₂ production can be converted to metabolic rates in Joules/second or Watts using the RQ value - Make a reliable assumption about the animals metabolism (what they're metabolising) - Animals will make use of three substrates: carbohydrates, lipids, and proteins - Under normal conditions, the animals won't metabolise proteins - The respiratory quotient varies on what is being metabolized: RQ = 1.00 (carbohydrates) vs 0.71 (lipids) - Indirect measurements using gaseous exchange. - Directly in the middle (RQ) if metabolizing both carbs and lipids in equal amounts - Calculating gut passage time: assume what their metabolizing under certain limits METABOLIC MEASUREMENTS IN ENDOTHERMS METABOLIC RATE SIGNIFICANCE MEASUREMENTS CONDITIONS BASAL METABOLIC RATE Minimum normothermic resting Rest phase, thermoneutral, (BMR) metabolic rate. postabsorptive SUMMIT METABOLISM Maximum normothermic resting Rest-phase, cold / Helox (Msum) metabolic rate. MAXIMUM METABOLIC Maximum metabolic rate. Intense exercise. RATE (MMR) BMR: minimum normothermic (can drop below BMR in hypothermic situations) resting metabolic rate. - Want to measure minimum requirements of metabolic rates. Need to measure at rest and the thermoneurtal zone and when nothing has been digested yet Summit: bird has to be resting, illicit with helox gas and at rest or cold temperatures, illicit max heat production at higher temperatures than colder temperatures Maximum: measured during intense exercise (use wind tunnels in birds to simulate mount Everest. Thermoregulation Page 7 Lecture 4 Thermoregulation in Endotherms 1 - Dr Noakes OUTLINE: HOMEOTHOERMY, SCHOLANDER-IRVING MODEL, VARAITION IN BODY TEMPERATURE, THERMOREGULATION IN NATURAL ENVIRONMENTS ENDOTHERMY HOMEOTHERMY Produce own body heat from metabolism Maintain constant body temperature - Under resting conditions, unlike - Pattern of constant body temperatures ectotherms (conformers) ENDOTHERMIC HOMEOTHERMY - Endotherms are thermoregulators whereas Ectotherms are thermoconformers and exhibit poikilothermy BODY TEMPERATURE: BIRDS VS MAMMALS The graph: - Surveyed literature of different groups of mammals. Key takeaway: variation of over 10°C within each group in body temperatures. - Birds, on average have higher body temperatures, thus a high range between passerines vs non-passerines. - Idea: animals do not have approximately similar body temperatures THE SCHOLANDER-IRVING MODEL Depict the relationship between resting metabolic rate (RMR) and ambient temperature in endothermic animals. The main assumption is that animals maintain a constant body temperature. Thermoneutral zone (TZ) → Range of air temperatures do not need to expend large amounts of energy to maintain body temperature; Regulated via thermal conductance: vasomotor responses, postural adjustments, and insulation Most of the measurements came from animals in metabolic chambers, air temperatures identical to operative temperatures See Appendix (1. Scholander-Irving Model) Thermoregulation Page 8 Body temperatures remain constant until the extremes are reached This refers to the maximum rate at which an animal can produce heat through metabolic processes. Beyond this point, the animal's physiological systems become overwhelmed. Consequences of Exceeding MHP: The Scholander-Irving model predicts that when an animal experiences extreme cold and its resting metabolic rate (RMR) reaches its MHP, it can no longer maintain its body temperature solely through heat production. In this situation, the animal's thermoregulatory strategy shifts. Thermoregulatory Shift: Instead of trying to produce more heat, the animal prioritizes conserving existing heat. This might involve behavioural changes like seeking shelter or huddling with others. Additionally, physiological adjustments might occur, such as reducing blood flow to the extremities to minimize heat loss. IN THE THERMONEUTRAL ZONE BELOW THE THERMONEUTRAL ZONE Animal becomes hypothermic at Cold limit temperature Thermoregulation Page 9 Thermal conductance, which is the opposite of what the model emphasizes. While the model looks at heat production to maintain body temperature, thermal conductance is a measure of how easily heat flows out of an object. Another assumption of the model is that conductance is minimized as soon as you are below the thermoneutral zone Steeper Slope, Faster Heat Loss: The passage mentions a "steep slope" in thermal conductance. This indicates that heat escapes the object rapidly. This rapid heat loss can pose a challenge for endothermic animals (animals that maintain constant body temperature) in cold environments, as they need to generate enough heat to counterbalance the loss and maintain normothermia (ideal body temperature range) as predicted by the Scholander-Irving model. Compare arctic fox and a sparser animal in cold temperatures Ideally, the model would perfectly represent the animal's thermoregulatory behaviour across all temperatures. Conductance is minimized all temperatures below the thermoneutral zone: assumption is tested via extrapolation. Intercept is the body temperature When the line is extrapolated and it surpasses the prediction point → meeting the assumption Normothermic Temperature: The model predicts that at an animal's normothermic temperature (the intercept on the x-axis, here 31°C), its metabolic rate for heat production would be zero. This is because, at its ideal body temperature, the animal wouldn't need to generate extra heat. - suggests that simply extrapolating from the model's trend might not always accurately predict the animal's metabolic rate at its ideal body temperature. Thermoregulation Page 10  ABOVE THE THERMONEUTRAL ZONE Thermoneutral Zone The Scholander-Irving model is based on data from the 1950s and while recent studies apply to some animals, it doesn’t apply to all endotherms. Next up, some examples will be presented. EXAMPLES OF ANIMALS THAT DEVIATE FROM THE SCHOLANDER-IRVING MODEL EXAMPLE 1: Bronze mannikins - BMR hasn’t' been recorded there. There is a wider thermoneutral zone, needs to be recorded with broader temperatures to get more information EXAMPLE: Red-headed finches - Lacks Thermoneutral zone EXAMPLE: African green-pigeons - No thermoneutral zone. Does not conform EXAMPLE: Round-tailed ground squirrel - Highly variable compared to other mammals BODY TEMPERATURE VARIATION Circadian cycle in normothermic body temperature → Typically, body °C is lower at rest Amplitude of circadian cycle (RT) scales negatively with body mass (Mb) - RT = 11.066 * Mb-0.37 Crested barbets (Trachyphonus vaillantii) - Grey is night, temperatures decrease by about 4°C at night and rewarm during the day - Starving response: slightly increase the amplitudes decrease body temperatures in response to food limitation INTER- AND INTRA-POPULATION VARIATION White-browed sparrow-weaver (Plocepasser mahali) - A binomial pattern is obtained from night and day temperatures - Circadian cycle gets smaller during the rainy period compared to the drier one - Second site: more arid, the birds have a larger circadian amplitude and maintain slightly higher daytime temperatures, reducing water costs by allowing temperatures to increase to save water with evaporative cooling Thermoregulation Page 11 Lecture 5 Thermoregulation in Endotherms 2 Dr Noakes OUTLINE: HETEROTHERMY (FACULTATIVE HYPOTHERMIA, TORPOR), EVOLUTION OF ENDOTHERMY ENDOTHERMY → Energetically costly - Produced by homeothermy to result in high, normothermic body temperature Some endotherms are HETEROTHERMS - Capable of facultative (reversible) hypothermic responses: reversible, controlled reduction in body temperature - Low metabolic rates → conserve energy CATEGORIES OF HETEROTHERMIC RESPONSES DURATION DEPTH OF HYPOTHERMIA HIBERNATION Reduction in body temperature for Pronounced (up to a drop in 40°C Days/weeks thanks to antifreeze proteins in their blood) DAILY TORPOR Hours/days Pronounced SHALLOW REST-PHASE Hours/single rest phase Shallow (