Endothermy & Thermoregulation

Endothermy & Thermoregulation

• Modes of Increasing Heat Production– below thermoneutrality (thermogenic

processes)

1) Shivering: high-frequency, relatively uncoordinated contraction of skeletal muscles; convert chemical to thermal energy



processes)

2) Nonshivering thermogenesis (NST):

– increase ion pumping by Na+-K+ active transport pump in cell membranes

– frees catabolism to permit oxidation of food reserves with immediate release of heat



processes)

2) Nonshivering thermogenesis (NST):

– best site = brown adipose tissue or brown fat

– brown fat has: large # mitochondria

large # blood vessels

• Modes of Increasing Heat Production– below thermoneutrality (thermogenic processes)

2) Nonshivering thermogenesis (NST): – brown fat = hibernating gland (misnomer)

– brown fat prominent in:• cold-acclimated or winter-acclimated adults, especially small to

medium body size

• hibernators

• neonates


3) Activity – increase heat production in large but not most small

mammals

– shivering (not NST) is inhibited by activity


4) Regional Heterothermy – common to all mammals – Appendages = poorly insulated; used to shunt heat during activity

or prevent heat loss (via countercurrent exchange)

• Modes of Increasing Heat Production

4) Regional Heterothermy

Countercurrent heat exchange: mechanisms allowing blood to flow to coldest part of extremity without loss of heat; related to vaso-dilation/constriction

- close arrangement of arteries & veins



Countercurrent heat exchange:

e.g., human arms, mammal legs, dolphin flippers, rodent tails, lagomorph ears, foot pads of wolves

- vascular arrangement varies in complexity



Countercurrent heat exchange:

rete mirabile (wonderful net): complex network of veins & arteries; increased efficiency in thermoregulation

e.g., arms of sloths; brains of African antelopes

rete mirabile

RegionalHeterothermy

& Performance

Responding to High Heat Loads

1) first defense = behavioral thermoregulation, therefore conserve water

- nocturnal activity

- occupy burrow

- seek shade

- change body posture

Responding to High Heat Loads

2) alter insulation

- see factor affecting insulation

3) cyclic TB

4) hyperthermia: controlled elevation of TB

5) evaporative cooling

- tremendous water loss


Endothermic Strategies for Coping with Temperature Extremes

• Heterothermy: fluctuating TB

= energy conservation strategy

1) Hypothermia: controlled lowering of TB; approach TA

daily torpor: TB lowered for only part of each day; reduces food intake demands, lowers heat loss

e.g., bats & some rodents

daily torporIs this modern or primitive?



1) Hypothermia:

estivation: summer sleep; common in small, desert mammals; conserves energy & water

hibernation: seasonal lowering of TB in relation to cold temperaturs and/or low food availability



1) Hypothermia

*shallow hibernation – periods of sleep with moderate TB reduction (raccoon, skunk, badger, bear)

*deep hibernation – TB drops within 2-3oC of TA; sleep bouts (entry, deep sleep, arousal) (various bats,

ground squirrels, woodchuck/marmot


Thermoregulation in Bats*large body size = homeothermic*small body size = many heterothermic

– Many with circadian activity cycles, lower TB 2-3oC at day

– Daily torpor & hibernation– Relative to low temps & high

energy expended for flight– Patagial membranes

Excretion &Water Balance

Vertebrate kidney = filtration-reabsorption system

- excrete waste as hypertonic urine relative to blood (because of Loop of Henle)

- longer Loop of Henle = more concentrated urine

Passive, Countercurrent Multiplying Model of mammalian kidney

1) Passive refers to diffusion of NaCl out of ascending limb of Loop of Henle (LOH)

2) Countercurrent refers to opposite direction of flow of filtrate in descending & ascending limbs of LOH

3) Multiplier refers to increase [NaCl] in inner medulla of kidney relative to outer medulla

Endocrine Control&

ADH (vasopressin)

antidiuretic hormone (ADH) - produced by hypothalamus & released by posterior pituitary; key hormone regulating kidney function

ADH & Dehydration

• ADH increases permeability of end of distal tubule & collecting duct of LOH

• Increases multiplier effect

• Concentrates urine; much of remaining H2O removed

ADH & Hydration

• ADH production decreased; not released

• Distal tubule & collecting duct permeability lowered

• Multiplier effect decreases

• [urine] decreases; extra H2O leaves body

Rodents – Arid vs. Mesic Habitats• Rodents in arid habitats have larger pituitary stores of ADH

per unit body weight compared to rodents in mesic habitats• In general, water regulation is relatively simple in mammals

from mesic habitats (e.g., high availability of drinking water, wet food, “low” water loss via evaporation)

• Mammals in arid habitats must contend with stresses on their water balance & must maintain efficient water regulation systems


Rodents – Arid vs. Mesic Habitats

General Sources of Water:

- moist foods - metabolic water

- drinking water

General Ways of Losing Water:

- evaporation

- urination

- defecation

- lactation


Strategies for Water Regulation in Arid Habitats:

1) Consume Wet Food• May not be more efficient at water

regulation

• Must consume large quantities of food with high moisture content (e.g., succulent plants, insects…)

• Many must counter toxins and/or salts in food material, e.g., oxalic acids in succulents or salts in halophylic plants



1) Consume Wet Food• Also may exhibit behavioral mechanisms

to reduce water loss, e.g., burrowing and/or foraging at night thereby balancing evaporative water loss : food water gain

• Variable concentration of urine & feces



2) Thermoregulation Mechanisms

• Hyperthermia = reduce evaporation

• Fewer sweat glands; panting rather than sweating

• Reduce respiratory rate



3) Periodic trips to Water Holes/Rivers (if available)

• Mammals not independent of drinking water

• Must obtain water every 1-2+ days (variations on periodicity of water requirements)

• Variable concentration of urine & feces

Excretion &Water BalanceStrategies for Water Regulation in Arid

Habitats:

3) Periodic trips to Water Holes/Rivers (if available)

e.g., camels• Hyperthermia (7o shifts)

• Concentrate urine & feces

• Tolerate extensive water loss over long periods (25% bw)

• Maintain fluid blood

• Exhale cooled & dehydrated air

• Replace lost water quickly; consume large amounts of water when available


Habitats:

4) “Water Independence”

• Many kangaroo rates = excellent examples

• Low availability of drinking water and/or moist foods; therefore do not rely on these sources

• Rely on water formed via cellular respiration (metabolic water)

Glucose + O2 CO2 + ATP + H2O


Habitats:

4) “Water Independence”• Diet mainly seeds = high in

carbohydrates = can extract high concentrations of water via catabolism, e.g., 2 g of food = 1 g of metabolic water

• “super” concentration of urine via extremely long LOH relative to body size & dry feces (water reabsorption in small & large intestines and less water allocated)

• No sweating

• Most water loss via respiration

Strategies to Reduce Water Loss via Respiration: (“Water-Independent” Mammals)

1) Heat exchange systems• Exhale air cooler than TB results in condensation of water

before air leaves nasal passage (regional heterothermy = nasal passages)

2) Forage at night (respiratory water loss lowest)• Increase metabolism in accordance with low night TA thereby

increasing metabolic water production & need to obtain more seeds


Strategies to Reduce Water Loss via Respiration: (“Water-Independent” Mammals)

3) Rest in burrow during day & plug entrance with soil

• Lower TA & higher humidity in burrow relative to above ground, therefore lower respiratory water loss

Excretion &Water BalanceLactation & Water Balance:• Tremendous seasonal loss of water

for females• Must recycle as much water as

possible (behavioral adaptation) and/or drink frequently (maintain den, nest, etc… relatively close to dependable water source, e.g., wolf dens)

• Recycle water via ingestion of urine & feces from young, thus retrieving some of water lost via lactation (common in “water-independent” mammals and those with altricial young

Endothermy & Thermoregulation

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