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Matt Johnson Lecture Notes ORNITHOLOGY (Humboldt State Univ. WILDLIFE 365) LECTURE 5 PHYSIOLOGY I. II. Introduction. A central theme in the success of life on Earth is the capacity for living organisms to maintain homeostasis -- that is, to exert work (physiological or behavioral) and to have adapted (via natural selection) to maintain conditions favorable for survival. The ability of birds to maintain physiological homeostasis is particularly evident in the way their behaviors and physiology enable them to maintain favorable O2 and CO2 concentrations in the blood (respiration and circulation), regulate body temperature, and regulate bi-products of metabolism (excretion and electrolyte regulation). Endothermy and Metabolism. A. Birds are endothermic, maintaining a high body temperature around 40 C (higher than most mammals). This high temperature allows high levels of sustained activity, even when ambient temperatures are low; birds live in the earth's hottest deserts to its coldest seas. B. To maintain this high temperature, birds "pay" an energetic cost. This cost is their basal metabolic rate (BMR), which is the body activity rate required to power the body under normal resting conditions. Metabolic rate is elevated for various activities. For example, flying raises metabolic rate to about 10-25 times that of BMR. C. Scholanders' model of endothermy. 1. Scholanders' model of endothermy has become a classic way to study metabolism and temperature regulation. A bird's thermoneutral zone (TNZ) is the temperature range over which it has little trouble maintaining temperature and metabolism. DRAW 2. At temperature below the TNZ, a bird must take measures to elevate metabolic rate and generate more heat (shivering, hunting, extreme fat metabolism). 3. At temperatures above the TNZ, a bird must also take measures to elevate metabolism so as to cool (it takes work to cool) by evaporation (dilate blood vessels, pant, gullar fluttering, etc.) 4. The shapes of these models vary between species in ways you might expect: a. Surface Area to Volume Ratios (SAVOLRATS) are very important in biology. Increasing body size results in decreasing SAVOLRAT b. Small birds have high SAVOLRATS, so they lose and gain heat rapidly from all their skin, so their TNZ is narrower and their BMR higher than larger birds with lower SAVOLRATS. (with all these abbreviations, this is beginning to read like a government document) D. A useful simplified formula for the transfer of temperature from an animal to its environment is this: Flux = T c SA; where flux is the rate of temperature exchange, T is the difference between body and ambient temps, c is the coefficient of conductance (insulation lowers c and slows flow), and SA is surface area over which temp exchange occurs. E. Birds have many morphological, physiological, and behavioral ways to adjust their body temperatures when ambient temps are unfavorable. Usually these methods affect one of the three variables in the flux formula. OVERHEAD Condition Adaptation or Behavior Energetic Outcome High Temperatures In these cases, birds need to LOOSE body heat to environment (increase flux) 1. Adjust feathers (compact them) 1. inhibits feather insulation, which increases “c”, facilitating heat loss 2. Shunt/bypass counter-current insulation 2. maximizes diff. b/w hot arterial blood and air temp (higher T), facilitating heat loss to environment 3. Find some shade (maximizes T) 4. increases SA of important evaporative heat loss areas 5. increases SA for blood temp exchange 3. Micro-habitat selection 4. Panting, gular flattering If temps are so high that they exceed body temp….they need to absorb LESS heat from environment (decrease flux) Low Temperatures In these cases, birds need to SLOW the loss of body heat to environment (decrease flux) 5. Expose areas with high concentrations of near skin blood vessels 6. Controlled hyperthermia 1. Adjust feathers (puff them up) 1. increases insulation, which decreases c, fighting heat loss 2. Counter-current insulation 2. reduces T between warm arterial blood and cold environment, allows heat that is lost to go to returning cooler venous blood; decreases heat lost to environment 3. Find some sun (lowers T) 4. increases SA in small areas which can absorb heat (this increases flux to yield a positive flux of heat into the body) 3. Micro-habitat selection 4. Orient toward sun & expose skin 5. Controlled Hypothermia/Torpor Outside of TMZ 6. elevates body temperature in hormonally controlled manner…reduces T, helps save water that would otherwise be lost trying to cool the body. 6. Shiver 5. decreases difference between ambient and body temperatures (lowers T) – possible only when inactive 6. increases MR and generates heat III. Getting O2 to and CO2 from tissues. A. Birds have higher sustained metabolic rates than mammals. To support this activity, they need to deliver O2 to lots of tissues very efficiently. This is accomplished with sophisticated respiratory and circulation systems. B. Respiratory system 1. Primary function is to exchange CO2 in body for O2 in air. Secondary function is to aid in temperature regulation. 2. Birds lungs are VERY different from other tetrapods' lungs (like ours). Our lungs are blind two-way air passages. When you exhale, you can't get rid of ALL the air in your lungs, about 20% remains, called residual volume. This makes our lungs inherently inefficient. Birds' lungs are better. 3. Bird lungs are made up of many thin walled tubes, called tertiary bronchi, plus a bunch of air sacs (usually 9) in "strategic" places. OVERHEAD a. Most gas exchange occurs in bronchi. b. It takes two "breaths" for a given volume of air to enter and leave a birds body. OVERHEAD i. In the first inhalation, O2-rich air flows all the way into the birds, through the lungs, into abdominal posterior air sacs. A little gas exchange occurs here. ii. In the first exhalation, this air (still rich in O2) flows forward into the lungs, where it is forced into finer and finer branching bronchioles, that eventually intertwine with blood capillaries and this is where the majority of gas exchange occurs. Nearly all the inhaled air in this way exchanges its O2 for CO2. iii. In the second inhalation, this air, which is now rich in CO2, flows into the anterior air sacs, where just a little more gas exchange occurs, but not much else happens (the air is just getting "out of the way", because remember this inhalation also brings in a new volume of air that goes past the lungs and into posterior air sacs). iv. In the second exhalation, the air (now very O2 depleted) is expelled through the trachea. c. Other functions for air sacs: i. Air bags for diving birds. ii. Mating displays (sage grouse, frigate birds). iii. Puts pressure on syrinx, enabling a greater range of notes for singing birds than could be reached otherwise. C. Circulatory system. 1. Those efficient lungs are only as good as the heart is at getting all that O2 rich blood to the tissues that need it. 2. Birds have a 4-chambered heart like mammals, but it is relatively larger (about twice as big) and delivers larger stroke volumes. This leads to extremely high blood pressure; and a frequent cause of death in birds is aorta rupture. When it works though, it is incredible. Birds are like Indy cars. They run their engines close to the edge. Great performance, great risk. IV. 3. 4 chambers, remember, separates the pulmonary circulatory system (that which goes to and from the lungs) from the systemic circulatory system (that which goes to the rest of the body). Excretory System. A. Like the lungs, the avian kidneys are a unique evolutionary departure from both reptilian and mammalian kidneys. B. Birds need to conserve water; they also can't afford to store lots of it to dilute toxic wastes in a heavy bladder. So they excrete uric acid, not urea. 1. Metabolic activity produces nitrogenous wastes that can become toxic if allowed to accumulate in the body. Many terrestrial animals dilute these wastes and store them (as urea) until they can be excreted. 2. Not birds. They excrete wastes as a semisolid solution of uric acid crystals (white). In this way, they require about 20-40 times less water then mammals to excrete a given amount of nitrogen. Even the famed kangaroo rats that excrete a dry-ish paste can't compare to that. 3. However, the cost is paid in an inability to concentrate unwanted electrolytes (e.g., salt) in the blood. Mammals can do it (via loop of Henle) bird's can't do it very well at all. 4. This is a problem especially for seabirds, which are forced to drink seawater to get their water. So these birds, and others, have evolved special nasal salt glands. 5. These salt glands extract salt and other electrolytes from the body and excrete them down the nose; some birds sneeze them out (storm petrels). "Tube-nosed" birds have especially well developed nasal glands. 6. This active extraction of salts from water takes energy….seabirds’ BMR can increase by up to 7% when their salt glands are actively pulling salt out of seawater.