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Adaptations for a diverse environments and habitats are among

the most conspicuous features of avian evolution.

Avian adaptations to diverse diets and environments

• Why should physiologists be interested in birds?

• Incredible demands of flight– Weight/Aerodynamics– Metabolic

• Ability to disperse long distances allows birds to utilize a wide range of resource types

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Outline

• Origin of avian physiological characteristics• Muscles• Circulation• Metabolism• Temperature regulation• Water economy• Excretory system

Archaeopteryx

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Birds shared common ancestor with theropod dinosaurs

Endothermysuggested to be requirement for flight—but when did it evolve?

How could we infer from fossil record?

RESPIRATORYTURBINATES(widespread in mammals & birds)

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Hillenius & Ruben 2004

Daily net respiratory water loss rates (respiratory water loss - metabolic water production) for a free-living 1-kg reptile and 1-kg mammal and probable net respiratory water loss for a free-living mammal lacking the use of respiratory turbinates (i.e., with mammalian metabolic rates but with reptile-like nasal anatomy and net respiratory water loss rates per cubic centimeter of O2 consumed). Without the water-conserving function of respiratory turbinates, daily water flux rates in mammals would be out of balance by about 30%.

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therapsids

Archaeopteryx

Therapod dinosaur

Ornithurine bird

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Evidence

Therapod clade was likely endothermic,selection pressures for integument

structures that kept in heat

Seebacher 2003

Outline

• Origin of avian physiological characteristics• Muscles• Circulation• Metabolism• Temperature regulation• Water economy• Excretory system

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Muscle Physiology• Varying composition of

fiber types:• Red = aerobic

metabolism: high myoglobin, mitochondria, fat, metabolic enzymes (sustained flight bower)

• White = anaerobic, little myoglobin, fewer mitochondria (rapid burst of energy)

Only red in supracoracoideus

and pectoralis

Only white in supracoracoideus

and pectoralis

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• Flight creates extreme demands for avian circulatory system• Have 4 chambered heart, though to have evolved from 3

chambered heart in theropod relatives—why?• Bird hearts are 50-100% larger than mammal of corresponding

size

Circulation

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• Flight creates extreme demands for avian circulatory system• Have 4 chambered heart, though to have evolved from 3

chambered heart in theropod relatives—why?• Bird hearts are 50-100% larger than mammal of corresponding

size• Heart rates are about half of similar-sized mammals, but each

stroke is more efficient (ventricles fill and empty more completely), due to more muscular ventricles

• Highest recorded BP of any vertebrate (300-400 mm Hg)• High susceptibility to atherosclerosis and hemorrhaging

Circulation

Life on the metabolic edge

• AP speeds increase ~1.8x for every 10°C

• Allows for prolonged exertion (e.g., migration)

BUT…

•20-30 times more energy than similar sized reptile

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• Levels of metabolism:• Basal Metabolic Rate (BMR)- Measured in

resting birds fasting at nonstressfultemperatures.– In songbirds, average BMR is 50-60x greater than

other birds– Linearly related to mass, but not isometrically

Metabolism

Why?

- Surface area to volume ratio, behavioral differences

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• Levels of metabolism:• Basal Metabolic Rate (BMR)- Measured in

resting birds fasting at nonstressfultemperatures.– In songbirds, average BMR is 50-60x greater than

other birds– Linearly related to mass, but not isometrically

• Activity metabolism: In some birds, is 10-25x BMR (vs. 5-6x BMR in small mammals):– Just being awake = 25-80% above BMR!– Flight = 2-25x BMR (but usually very efficient!! <1%

energy required for same sized mammal to run same distance)

Metabolism (varies w/ level of activity)

Gill 1995

• H = (Tb – Taa) / I) / ITemperature regulation

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• Scholander’s model of endothermyTemperature regulation

• Shivering (mostly pectoralis muscle)• Unlike mammals can’t metabolize brown

adipose tissue (BAT)• Boundaries of thermoneutral zone differ with bird

size and climate• Can also behaviorally regulate temperature…

Temperature regulation: Response to cold

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• Shivering (mostly pectoralis muscle)• Unlike mammals can’t metabolize brown

adipose tissue (BAT)• Boundaries of thermoneutral zone differ with bird

size and climate• Can also behaviorally regulate temperature

(huddling, ‘pyramiding’)• Hypothermia: dropping body temp 2-6 deg C at

night• Torpor: profound hypothermia, incapable of

normal activity, unresponsive to some stimuli.

Temperature regulation: Response to cold

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•Some hummingbirds save 27% energy by allowing temp to drop 20-30 deg C below normal

•Can take up to an hour to warm up and become active

• Behavioral adaptations: avoidance, panting, gular fluttering, ruffling feathers

• Counter-current exchange

• Air movement cools birds in flight, where metabolism can raise body temperatures very rapidly

Temperature regulation: Response to heat

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• High activity = rapid water loss• Turbinates help reabsorb water lost in exhalation

Water economy

Excretory system:

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• Nitrogenous waste excreted as uric acid, (white, insoluble, less toxic than ammonia), but energetically more costly

• Birds require 1 mL to excrete 370 mL of nitrogen as uric acid, whereas mammals require 20x to excrete same amount

• Acid concentration in cloaca can accumulate up to 3000x acid level in blood just prior to defecation (remember: Kangaroo rat concentrates urea up to 20-30x blood levels!!)

Excretory system:

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Excretory system: Limited ability to accumulate solutes: Relative short Loops of Henle & only 10-30% nephrons have LOH

http://people.eku.edu/ritchisong/bird_excretion.htm

Reabsorption in lower intestine limits losses associated with comparatively poor ability to conserve water in kidneys

Urine spheres (uric acid)

Reverse peristalsis pulls urine into colon for reabsorption

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`facultative ammonotely‘ in hummingbirds ?• Ammonia ‘cheaper’, but more toxic—usually only

found in organisms that have high water turnover (e.g., aquatic animals)

• Most birds don’t ingest enough water, except nectar feeding species (Anna’s hummingbird):– @ low temps + high water intake = 50% of nitrogen

excreted as ammonia