The Energetic Costs of Meeting Environmental Challenges

Temperature Impact on Metabolic Reactions

• Endotherms

  • Maintain a stable internal body temperature

  • Their metabolism is optimized for that temperature

  • They must spend energy to keep that temperature constant

• Ectotherms

  • Even small temperature changes can drastially affect metablism

Enzymes

All metabolism depends on enzymes, and enzymes are very temperature sensitive

• Acclimatization: short term adjustment to environmental change

  • Changing enzyme activity

  • Producing more or less enzymes

  • Behavioral changes

  • Seasonal changes (winter to summer coat)

• In ectotherms:

  • Metabolism typically increases 2-3x for every 10 degree C increase, known as the Q10 effect

    • Some organisms, like intertidal invertebrates experience huge temperature swings daily, have very stable enzymes, and their metabolsim barely changes across large temperature changes

• Cells involved in circadian rhythms

  • Have enzymes that don’t change with temperature

Homeoviscous Membrane Adaptation

If membranes are too rigid, proteins can’t move - but if they are too fluid, structure breaks down

• How cells adjust membrane fluidity

  • Cold temperatures: more unsaturated fats

  • Warm temperatures: more saturated fats

• Role of cholesterol: acts as a membrane stabilizer

  • Cold temperature: less cholesterol, more flexibility

  • Warm temperature: more cholesterol, more rigidity

Heat Shock Proteins (Molecular Chaperones)

• When temperatures become extreme, proteins can unfold, misfold, or stop working. To prevent this, cells produce heat shock proteins (HSPs)

  • They help proteins fold correctly, stabilize damaged proteins, and prevent proteins from clumping

• Animals that live in extreme temperatures already have higher baseline levels of HSPs so they can quickly respond to temperature stress

The Physics of Body Heat and Temperature

• Body heat = heat produced + heat gained - heat lost

  • Htot = Hv + Hc + Hr + He + Hs

    • Hv = metabolic heat production

    • Hc = conduction/convection heat transfer

    • Hr = radiation

    • He = evaporation

    • Hs = heat stored

• Body temperature changes when heat production and heat transfer are unbalanced

Where Body Heat Comes From

• Factors that increase heat production:

  • Movement (muscle activity): muscles generate heat

  • Hormones (ANS control): speed up metabolism, burn fat

  • Acclimatization: long-term increase in baseline metabolism

How Heat Moves Between Animals and the Environment

• Conduction (direct contact)

  • Ex: reptile warming on a rock

• Convection (movement of fluids/air)

  • Think of a convection oven or air fryer

  • Heat moves through moving air or water

  • Faster than conduction because fluid keeps moving

  • Ex: wind removing heat from skin

• Radiation (infrared heat transfer)

  • Heat transfer via electromagnetic waves - no contact required

  • Ex: basking lizard absorbing sunlight - body radiating heat in cold air

• Evaporation (heat loss via liquid to gas)

  • Requires heat energy

  • Ex: sweating, panting

Heat Storage and Body Size Effects

• Big animals:

  • Low surface area relative to mass

  • Heat changes slowly

• Small animals

  • High surface area relative to mass

  • Loses and gains heat quickly

  • Needs more energy to regulate temperatuer

Factors Affecting Rate of Heat Exchange

• Surface area

  • More surface area: faster heat exchange

  • Animals can change posture to adjust surface exposure

• Temperature gradient

  • Heat always moves from hot to cold

  • Bigger temperature difference: faster heat transfer

• Insulation/Conductance

  • Fur, feathers, and fat reduce heat loss

  • Animals with high insulation maintain stable body temperatures

  • Animals with low insulation track environmental temperatures

How Animals Regulate Heat Exchange

• Behavioral mechanisms: animals change where they are or how they position themselves

  • Moves into sun or shade

  • Change posture to absorb or reduce heat

  • Migration

• Physiological (short-term internal control)

  • Blood flow control:

    • Vasodilation: more heat lost (warm skin)

    • Vasoconstriction: conserve heat

  • Insulation control:

    • Hair or feathers stand up to trap air

  • In mammals: arrector pili muscles (goosebumps)

  • In birds: arrector plumarum

• Long-term acclimatization (structural changes)

  • Seasonal adjustments: winter coat to summer coat, molting in birds

Classifying Animals Based on Temperature Stability

• Homeotherms (stable temperature)

  • Controlling heat production and heat loss

  • Usually stay within a very narrow range

  • Mammals, birds

• Poikilotherms (variable temperature)

  • Body temperature changes with environment

  • Temperature varies depending on ambient conditions

  • Reptiles, amphibians, fish, most invertebrates

  • Some ectotherms in very stable environments can be homeothermic

Classifying Animals Based on Source of Body Heat

• Endotherms (heat from within)

  • Generate heat using metabolism

  • Maintrain body temperature above environment

  • High metabolic rate (≈5x ectotherms)

  • Require a lot of food/energy

  • Often have insulation

  • Mammals, birds

  • Can live in cold or extreme environments

• Ectotherms (heat from environment)

  • Metabolism produces little heat

  • Low energy requirements

  • Behavior controls temperature

  • Reptiles, amphibians, fish, most invertebrates

Real animals fall on a spectrum

• Homeothermic endotherm: human

• Poikilothermic ectotherm: lizard

  • Homeothermic ectotherm: deep sea fish

Heterothermy (The in-between strategy)

Animals that alternate between:

• Endothermy ↔︎ ectothermy

• Homeothermy ↔ poikilothermy

Types of heterothermy

• Temporal heterothermy (changes over time)

  • Body temperature changes over hours, days, or seasons

  • Bats, hummingbirds, camels, hibernating animals

  • Includes:

    • Torpor (short-term)

    • Hibernation (long-term)

      • Body temperature drops near environmental temperature

      • Metabolism drops to ~1% of normal

      • Occasional arousals bring body temperature back up

• Regional heterothermy (different body parts)

  • Done using countercurrent heat exchangers

  • Penguins: warm body, cold feet

  • Arctic birds: prevent freezing on ice

  • Tuna: keep swimming muscles warm

  • Leatherback turtles: retain heat in flippers

  • Bumblebees and honeybees: warm thorax for flight

Ectotherms in Cold and Freezing Environments

• In cold environments, their body temperature drops, which can cause:

  • Slower metabolism

  • Enzyme malfunction

  • Membrane damage

  • Dangerous electrolyte imbalance

  • Ice crystal formation (cell rupture)

• To survive, theyve evolved three main strategies:

  • Freeze avoidance:

    • Burrow below frost line

    • Stay under water beneath ice

    • Supercool body fluids

    • Produce antifreeze molecules

    • Many insects, spiders, fish, salamanders, and snakes

  • Freeze tolerance:

    • Ice-nucleating agents

      • Promote freezing in extracellular fluid first

      • Pull water out of cells

      • Prevent ice crystals inside cells

    • Cyroprotectants (like glycerol)

      • Lower freezing point

      • Protects cells from dehydration damage

    • Antifreeze proteins

      • Prevents ice crystal growth

      • Found in antarctic fish

  • Supercooling:

    • Body fluids cool below freezing but don’t freeze because ice crystals don’t form

    • Terrestrial insects

    • Arctic fish (must avoid contact with ice nuclei)

Ectotherms in Warm/Hot Environments

All ectotherms have a Critical Thermal Maximum (CTmax), or the temperature above which they lose coordination and risk death. Usually

• Around 45 degrees C for most species

• Deep-sea vent species can tolerate much higher

• High temperatures are dangerous because:

  • Enzymes denature

  • Oxygen transport decreases (hemoglobin binds less O2)

  • Membranes become unstable

• Some adaptations:

  • Heat shock proteins (protect enzymes)

  • Behavioral thermoregulation (move to shade)

  • Physiological regulation (blood flow adjustments)

Ectothermy vs Endothermy

• Endotherms (birds, mammals)

  • Advantage:

    • High activity level in cold

    • Stable body temperature

    • Can live in colder environments (occupy more ecological niches)

  • Disadvantage:

    • HIgh energy demand

    • Need constant food supply

• Ectotherms

  • Advantage:

    • Energy efficient

    • More individuals per area (higher carrying capacity)

  • Disadvantage:

    • Limited activity when cold

    • Restricted by environmental temperature

The Thermal Neutral Zone (TNZ)

the TNZ is the temperatuer range where an endotherm can maintain body temperature without increasing metabolic rate

• Inside the TNZ: heat production = heat loss

• The animal uses low-energy mechanims:

  • Postural changes:

    • Curling up vs stretching out

    • Basking or avoiding sun

  • Vasomator Responses

    • Vasodilation: more heat loss

    • Vasoconstriction: conserve heat

  • Insulation adjustments

    • Raising fur or feathers

    • Goosebumps (arrector pili muscles)

    • Traps warm air layer

• BELOW TNZ: Lower Critical Temperature (LCT)

  • Metabolic rate increases

  • Thermogenesis increases

  • Eventually, hypothermia

• ABOVE TNZ: Upper Critical Temperature (UCT)

  • Evaporative cooling increases

    • Eventually, hyperthermia

Endothermy in Cold Enviornments

• Thermogenesis (Heat Production)

  • Exercise thermogenesis

    • heat from muscle activity

  • Non-exercise thermogenesis:

    • Shivering thermogenesis:

      • Rapid muscle contractions

      • ATP → heat + small movement

      • Used to rewarm hibernators

      • Inefficient, but fast

    • Non-shivering thermogenesis

      • Uses fat metabolism

      • White Fat Thermogenesis

        • Fat → fatty acids → burned in tissues

        • Epinephrine stimulates ATP breakdown → heat

      • Brown Fat Thermogenesis

        • Found in hibernators and newborn mammals

        • Lots of mitochondria, rich blood supply, contains Thermogenin (UCP-1)

        • Protons bypass ATP synthase, no ATP made, energy released as heat

        • Super efficient heat production

• Countercurrent Heat Exchange

  • Used to conserve heat in limbs

  • Warm arterial blood → transfers heat to cold venous blood returning from limbs

  • Arteries and veins run side-by-side

    • Blood reaching limbs is cooler → less heat loss

    • Blood returning to core is warmed

  • Reduces energy needed for thermoregulation

• Insulation Adaptations

  • Short term: Fluff fur/feathers

  • Seasonal: winter coats, molting

  • Structural: blubber in marine mammals

• Body Shape Adaptations

  • Bergmann;’s Rule:

    • Larger body size in colder climates

    • Lower surface area to volume → retain heat

  • Allen’s Rule:

    • Smaller extremities in cold climates

    • Reduce heat loss

      • Ex: arctic fox ears (small) vs desert fox ears (large)

Endothermy in Hot Environments

Must dump excess heat while often conserving water

• Limited Heterothermy

  • Ex: Dromedary camel

  • Camels allow body temp to fluctuate

  • Less sweating, conserves water, uses environment for cooling

  • temporal heterothermy

• Heat windows

  • Special body regions that release heat

  • Characteristics:

    • Highly vascularized

    • Thin or no insulation

  • Examples:

    • Rabbit ears

    • Vicuna axilla (armpit region)

• Evaporative Cooling

  • Highly effective but costs water

  • Sweating:

    • Controlling by ANS

    • Evaporation removes heat

    • Problem in deserts: dehydration

  • Panting:

    • Rapid shallow breathing

    • inhale through nose (conserves water)

    • Exhale through mouth (dump heat)

      • Nasal cavity conserves heat/water, oral cavity allows heat loss

• Dehydration Trade-Off

  • If dehydrated:

    • Reduce evaporation

    • But heat loss also decreases

  • Dangerous for small mammals

    • Solutions:

      • Burrow during day

      • Nocturnal activity

      • Concentrated urine and dry feces

Neuronal Thermoregulation

Both endotherms and ectotherms can control temperature using neural pathways

• The body::

  • Detects temperature (thermoreceptors)

  • Processes it in the brain/spinal cord

  • Activates responses to either:

    • Gain heat

    • Conserve water

    • Lose heat

Thermostatic Regulatiom (The “Set Point” System)

• Mammals: The Hypothalamus is the Body’s Thermostat

  • How temperature is sensed:

    • Thermoreceptors in the skin

    • Thermoreceptors in the body core

    • Temperature-sensitive neurons in the brain and spinal cord

• The hypothalamus is ~20x more sensitive to core temperature than skin temperature

• Three neuron groups in the anterior hypothalamus

  • Heat-Loss activating neurons

    • Triggered when body temperature rises

    • Stimulates sweating, panting, and vasodilation

  • Heat-Production Inhibiting Neurons

    • Triggered when body temperature rises

    • Stops shivering, non-shivering thermogenesis

    • Lowers metabolism

    • Suppresses appetite

  • Heat-Production Activating Neurons

    • Triggered when body temperature drops

    • Increases shivering, metabolism

    • Activates heat conservation

      • Stimulates appetite

Thermoregulation in Non-Mammals

• Birds

  • Hypothalamus has little direct control

  • Main control center is the spinal cord

  • Hypothalamus may control behavioral responses

• Fish

  • Hypothalamus temperature rises, metabolism rises

  • Brainstem temperature changes cause behavior:

    • Warm → swim to cooler water (thermophobic)

    • Cool → Seek warmth (thermophilic

• Reptiles

  • Warm hypothalamus → seek shade

  • Cool hypothalamus → bask in the sun

• Amphibians

  • Mix of fish and reptile strategies

• Invertebrates

  • Not well studied

  • Many still show temperature-driven behaviors

Hypoxia and Temperature Control

• Warm air or water holds less oxygen, so hot envionrments can become oxygen poor

  • Hypoxia resets the thermostat

  • Animals move to cooler, oxygen-rich areas

Temperature Control During Activity (Exercise Physiology)

• Heat production during exercise

  • When muscles work, only ~25% energy goes toward movement - the other ~75% is lost as heat

  • Typically raises 2-4 degrees C

    • A moderate increase in temperature is beneficial to improve heat loss (bigger gradient to environment), improves oxygen delivery to tissues, and speeds up metabolic reactions

How Animals Dump Heat During Activity

• Evaporative Cooling

  • Sweating, panting

  • Red kangaroos lick their forearms

  • Blood vessels dilate, saliva evaporates, cools blood

  • Countercurrent Cooling for the Brain

    • Carotid Rete: a heat-exchange network that cools blood before it reaches the brain

      • Warm arterial blood (to the brain)

      • Passes next to cooler venous blood (from the nose)

      • Heat transfers away

      • Brain stays cooler than the rest of the body

Dormancy (Hypometabolism) in Endotherms

• Used to survive seasonal food shortages

• This reduces energy needs and changes thermoregulation by lowering the body temperature set point

• Types of dormancy vary in depth and duration

  • Sleep

  • Torpor

  • Hibernation (winter dormancy)

  • Aestivation (summer dormancy)

Sleep

• A reversible state of reduced activity, awareness, and muscle movement

• Occurs in vertebrates and some invertebrates

• Two main types in birds and mammals:

  • REM sleep: hypothalamus becomes less sensitive to temperature, and body temperature drops slightly

  • Non-REM sleep

Torpor

• A short-term drop in metabolic rate and body temperature

• Helps animals survive daily or short-term food shortages

• Two forms:

  • Daily torpor (hours; seen in hummingbirds and bats)

  • Long torpor bouts (days-weeks, seen in hibernators)

• Torpor is a controlled thermoregulatory process, not a shutdown.

  • Placental mammals (eutherians) use brown fat to reward

  • Marsupials rewarm using different/unknown mechanisms

Hibernation (Winter Dormancy)

• Long-term, seasonal form of torpor

• Characterized by low body temperature, slow heart rate and breathing, and very low metabolism

• purpose: conserve energy when food is unavailable

• Prepare by:

  • Storing fat (large animals)

  • Caching food (small animals)

• Can last days to months

• Some species give birth during hibernation

Aestivation (Summer Dormancy)

• Dormancy during hot, dry conditions

• Occurs in both aquatic and terrestrial animals, but is rare in mammals

• Main goals:

  • Conserve water

  • Reduce energy use

  • Protect cells and proteins from heat damage

• Physiological strategies include

  • Lower metabolism

  • Increased antioxidants and chaperone proteins to stabilize tissues

• Ex: earthworms, frogs, bees, snails, salamanders, tortoises, and crocodiles

Fever and Pyrogens

• Fever (Pyrexia)

  • An increase in body temperature caused by a higher hypothalamic set point

  • Helps slow pathogen growth

• Pyrogens (Fever-causing substances)

  • Endogenous Pyrogens

    • Produced inside the body

    • Are cytokines released by immune cells

    • They act on the hypothalamus to raise the temperature set point

  • Exogenous Pyrogens

    • Come from outside the body, often bacteria

    • Example: Lipopolysaccharide (LPS) from bacterial cell walls

      • LPS binds to LBP protein

      • This complex activates macrophages

      • Macrophages release cytokines, producing a fever response