Download Physiology I

Physiology I
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In addition to feathers, birds have evolved many skeletal and physiological adaptations for
flight
Skeletal adaptations examined in lab
Lecture will focus on physiological adaptations
• Skeletal Adaptations for Flight:
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Lightweight bones, strong and elastic
Internal struts in wing bones
Highly fused skeleton (synsacrum, head, hands)
Powerful and highly modified joints in the forelimbs (carpometacarpus, alula)
Streamlined shape of body
• Skeletal Adaptations for Flight:
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Keeled sternum or breastbone (carina)
Furcula (clavicle or wishbone)
No teeth, so no need for a heavy jaw (bill, gizzard)
Pelvic and pectoral girdles large and strong
Uncinate processes on ribs
• Physiological Adaptations for Flight:
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Efficient metabolism
Efficient respiratory system (air sacs etc...)
Uric acid excretory system
Endothermy, 4-chambered heart, good circulation
• Physiological Adaptations for Flight:
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Powerful flight muscles, the pectoralis (wings down) and supracoracoideus (wings up)
Shell forms late in egg development
Sex organs atrophy outside breeding season
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Combination of feathers, skeletal, and physiological design strategies
Produced a strong, elastic, lightweight airframe
Produced an efficient power plant to run it
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Birds are very active animals, with amazing stamina
Able to fly for hours at a time, for days on end
Their metabolic furnace burns much brighter than our own
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Average body temperature is about 40 oC (104 oF), compared to our 98.6 oF (37 oC)
Birds live precariously close to the thermal abyss
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Heat denatures proteins, three-dimensional shape uncurls into a string of amino acids
Something you’ve seen every time you’ve fried an egg
At ~ 46o C the rate of protein destruction exceeds the rate of protein synthesis
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Nervous and muscular systems of birds are turbocharged due to high body temp
Supply and transport systems (circulatory, respiratory, excretory), have to keep pace
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High-powered metabolism generates a lot of heat
Can kill them on a hot summer’s day
Can keep them alive through a cold winter’s night
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Variety of physical and behavioral responses to heat and cold
Can change the elevation of their feathers to shed or retain heat
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Fluffing the feathers traps a layer of warm air on a cold day
Holding feathers very close, or extending them far out to expose bare skin, helps cool the bird
down
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Keeled sternum spreads out the attachment point of the flight muscles
Muscles have air channels inside which dissipate heat during flight
Keeled sternum keeps muscle channels from collapsing when muscle contracts
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Heat loss is related to body size, the ratio between surface area and volume
Consider the cline in body size of the English Sparrow?
Hot humid climates favor smaller body (Mexican sparrows)
Colder, dryer climates favor larger body (Canadian sparrows)
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Birds react to cold in many ways
Seek shelter in cavities or dense vegetation
Huddle together to keep warm
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Shiver, the same as we do, to generate additional heat
Can sleep on one leg with their heads tucked under their wings to conserve heat (shorebirds,
wading birds...)
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Most birds drop their body temperatures a degree or two at night
Hummingbirds, chickadees and baby swifts are very tiny, severe SA/vol problem
They have to turn the thermostat way down, leaving only the metabolic “pilot light” on
They enter a state of torpor
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Hummingbirds have a very high surface area to volume ratio, and the highest metabolic rate of
any vertebrate
They don’t fly so much as buzz
Beat their wings up to 80 times per second when they hover
Flight muscles one-third their entire weight!
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Hummers radiate an enormous amount of heat
When the weather gets too cold, they drop their body temperatures as much as 20-32oC
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If body temp falls to ~ 20oC below normal, enters a comatose state which uses an absolute
minimum amount of energy
Hovers unresponsive, on the energetic edge of death
Takes about an hour to fully awaken in the morning from deep torpor
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Chickadees also resort to torpor to survive brief bouts of extreme cold
Baby swifts can enter torpor to get through lean periods of little or no insects to eat
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Too hot is as big a problem for birds as too cold
Birds don’t sweat
Water simply evaporates through the skin, none of those messy sweat glands
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Birds seek shade or cool off in the water, just like us, to keep from overheating
They can also lower their feathers to transfer heat directly from the body
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Birds can also pant, holding their mouths open to evaporate body moisture
Many seabirds (ex. pelican) can also vibrate their gular pouch, which increases the evaporation
of water from the throat and mouth
Birds can also shed large amounts of heat through their legs and feet
Can increase the blood flow to their extremities, especially long-legged birds like herons
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Can also reduce blood flow to the legs by constricting the blood vessels
Direct the return flow of venous blood through a special shunt
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Flying turns up the heat even more
Parakeets flying at 35 km/hr (22 mph) increase their body temperatures to 44oC (111oF)!
Would cook themselves in midair, were it not for the cooling effect of the moving air
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Outstretched wings make nice heat exchangers
Air streaming around them plasters their feathers tightly to their bodies, shedding even more
heat
A flying parakeet sheds three times as much heat as a parakeet at rest
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Passerines have very high basal metabolic rates (BMRs), the metabolic level of the bird at rest
Passerine BMRs are 50-60% higher than non-passerine birds
Birds have the highest BMR of any vertebrate
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But birds are seldom at rest
They are usually expending enormous amounts of energy at a rapid rate
A bird in flight can maintain a metabolic rate 10-25 times its BMR for hours at a time
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One consequence of heightened metabolic rate is that birds produce more metabolic water than
other vertebrates
Water is produced when carbohydrates and other organic compounds that contain hydrogen are
oxidized
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Problem of excess metabolic heat is partly countered by the excess production of metabolic
water
Can be used for evaporative cooling (panting etc.)
Supplement dietary water with visits to open water and using metabolic water
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Birds get water from many sources
Much of their water comes from diet - fruits, meat and insects
Most birds eat high energy foods
Very few are leaf-eaters (folivorous)
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Zebra Finch can survive in the desert without drinking
Only dietary source is dry seeds (10% H2O)
Relies on metabolic water for other 90%!
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Sustaining very high metabolic levels puts great demands on the bird’s transport systems
Bringing fuel and oxygen to the tissues, carrying away waste is a big job for birds
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Mammals don’t have an especially efficient respiratory system
When we exhale, there is always a residual pool of stale air left in the lungs
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Birds completely empty their lungs
Don’t just inflate / deflate a gas bag
Have evolved a flow-through ventilation system
Brings oxygenated air into the lungs while stale air is being forced out
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Lungs are dense, compact structures
Fed by a trachea that splits into two primary bronchi
Each primary bronchus splits into 11 secondary bronchi
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Secondary bronchi split into 1,800 tertiary bronchi
Tertiary bronchi in direct contact with the fine capillaries where gases are exchanged
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In geese, cranes, some shorebirds and other birds, the trachea can be unusually long
Acts like a resonating tube to amplify calls and songs
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Human lungs are inflated by a muscular diaphragm
Birds lower their sternum to increase chest capacity, contract it to squeeze air through the lungs
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Lungs work in conjunction with air sacs
Air sacs are little sacks of tissue that run throughout the chest cavity
Air sacs connect with the primary and second bronchi
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Most birds have 9 air sacs
Big range in number of air sacs - 6 house sparrow; 7 loon, turkey; 12 storks, shorebirds
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Birds breathe in through their nares, or nostrils, usually located at the base of the bill
Each respiratory cycle requires two complete breaths to push a given volume of air through the
birds lungs
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First breath, inhaled air passes through the primary bronchi to the posterior air sacs
When the bird exhales, this air is pushed into the lungs for gas exchange
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Second inhalation pushes air in lungs, now high in CO2, into the anterior part of the lungs and
into the anterior air sacs
Second exhalation pushes out all the stale air from the anterior air sacs
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Air sacs do not have a good supply of blood vessels
Don’t play much of a role in gas exchange
Critical, however, for keeping air flowing through the lungs
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Air sacs serve other functions as well
Help cushion internal organs during flight
Evaporative cooling via air sacs removes some excess metabolic heat, help prevent heat stress
Pigeons and budgies shed 13-22% of their excess heat load through their air sacs
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Air sacs also provide water birds with additional buoyancy
Some birds can control the volume of the air sacs for swimming and diving
Analogous to the way bony fishes use their swim bladders
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Cervical air sacs of the male frigatebird function in courtship
Male inflates his bright red air sacs to use in his courtship display
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Male Prairie Chicken uses his air sacs in the same way (film at eleven...)
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Supplying the bird’s demanding metabolism requires a strong circulatory system, to carry
oxygen and fuel and to remove wastes
Birds, like mammals, have a large 4-chambered heart
Bird’s heart is half again to twice as large as heart of mammal of the same relative size
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Bird hemoglobin levels about the same as ours
Avian hemoglobin is better at oxygen transport than our own
Avian blood sugar levels are twice as high, consistent with their higher metabolic rates
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Resting heart rate of most birds ~ one-half that of mammals of the same size
But birds have more powerful heart muscles
Pump about the same volume of blood as a mammalian heart
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Avian heartbeats average about 220 beats per minute
Some hummingbirds have an incredible 1200 beats per minute (20 beats per second!!)
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Avian blood pressure is also higher than most mammals
Range from about 130 to 220 mm Hg in birds (100 in man, 175 horse)
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Avian blood pressures can run well over 300 mm of mercury
Twice that of a human suffering from high blood pressure
Chronic blood pressure of 150 will cause severe organ damage in humans
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Birds frequently stroke out
If you catch a wild bird, especially a small passerine, they sometimes literally die of fright
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Cardinal was once observed dying of a heart attack during territorial battle with another male
Male Prairie Warbler was seen stroking out during courtship displays
Male birds in general have higher blood pressures than females!
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Pattern of avian blood flow similar to ours
Oxygenated blood leaves the lungs and enters the left atrium
Into the left ventricles and out into arteries
Returning venous blood enters the right atrium, passes through the right ventricle, and out to
the lungs
Bird’s muscles must sustain an incredible rate of firing over prolonged periods to sustain
powered flight
Pectoralis muscle (~1/6th bird’s weight), contracts to create the power stroke
Supracoracoideus muscles raise the wing by pulling down the attached tendon in the recovery
stroke
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In most birds, flight muscles are a mixture of red and white muscle fibers
Red muscle fibers rely on aerobic metabolism
Supply power for endurance
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White muscle fibers rely on anaerobic metabolism
Provides short bursts of energy needed for rapid takeoffs and high speed predator avoidance
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Hummingbirds have unusual flight muscles
They use the recovery stroke as an additional power stroke
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Unusually large supracoracoideus, ½ the size of the pectoralis and (by relative weight) 5 times
larger than other birds (11.5% of body mass!)
Both hummer pectoralis and supra-coricoideus muscles are made up of red fibers
Chickens and turkeys have lots of white fibers in their muscles
White meat of their flight muscles lets them take off quickly
But they quickly tire, can only fly for short distances before too much lactic acid accumulates in
the muscles
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Their legs, on the other hand, are mostly dark meat
Dark meat composed of red muscle fibers (the drumstick)
Chickens have strong legs and can run very quickly
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Such powerful metabolic systems create substantial amounts of wastes
Particular problem for an animal that needs to travel light
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Avian excretory system is a marvelous adaptation to shed weight for flight
Many primitive and aquatic organisms concentrate liquid wastes as ammonia
Ammonia requires large amounts of water to dissolve, not usually a big problem in the ocean
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Terrestrial animals evolved excretory systems relying on urea to concentrate liquid wastes
Urea doesn’t require as much water to dissolve
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Birds have moved to an excretory system based on uric acid
Uric acid requires very little water to dissolve
It is also not toxic when concentrated, as is urea
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You’ve all seen bird wastes, probably a lot closer than you’d like!
Darker portion is solid wastes, but light white portion is crystals of liquid waste, equivalent of
our urine
System may have evolved as an adaptation for egg laying in birds and reptiles
Embryos have no way of getting nitrogen wastes out of the shells
Must store those wastes in a non-toxic concentrated form that requires very little dissolved
water
Seabirds, and other birds with high levels of salt in their diet, can also excrete excess salt
through special glands called salt glands
All birds have them, but they are well developed in birds with high salt content in their diet
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These glands are supraorbital (above the eye)
Glands excrete excrete excess salt from prey or from salt water that seabirds must usually drink
Salt concentration can reach 5% or more, exceeding that of sea water (3.5%)
Birds, in addition to their feathers and skeletal adaptations, have many physiological
adaptations for flight