Avian Circulatory System (2023)

BIO 554/754 Ornithology Avian Circulatory System

Birds have very efficient cardiovascular systems that permitthem to meet the metabolic demands of flight (and running, swimming, ordiving). The cardiovascular system not only delivers oxygen to body cells(and removes metabolic wastes) but also plays an important role in maintaininga bird's body temperature.The avian circulatory system consists of a heartplus vessels that transport:

• nutrients
• oxygen and carbon dioxide
• waste products
• hormones
• heat

Birds, like mammals, have a 4-chambered heart (2 atria & 2 ventricles),with complete separation of oxygenated and de-oxygenated blood. The rightventricle pumps blood to the lungs, while the left ventricle pumps bloodto the rest of the body. Because the left ventricle must generate greaterpressure to pump blood throughout the body (in contrast to the right ventriclethat pumps blood to the lungs), the wallsof the left ventricle are much thicker & more muscular.

Source: chickscope.beckman.uiuc.edu/explore/ embryology/day02/comparative.html

Cross-section through the ventricles of a chicken heart (Source: trc.ucdavis.edu/mjguinan/apc100/modules/ Circulatory/heart/chambers1/chambers.html)

Dorsoventral (A) and lateral (B) thoracic radiographsfrom a grey heron, showing the normal avian cardiac silhouettes,
which are located nearly along the longitudinal axisof the body (Machida and Aohagi 2001).

Heart of a Domestic Chicken. RA, right atrium; RV, right ventricle, LA, left atrium; LV, left ventricle; RAVV, right atrioventricular valve; LAVV, left atrioventricular valve;
IVS, interventricular septum; IAS, interatrial septum; SVC, superior vena cava. The left atrioventricular valve of birds has three cusps whereas the right AV valve is a single section of myocardium
(Figure modified from Lu et al. 1993).

Birds tend to have larger hearts than mammals (relative to bodysize and mass). The relatively large hearts of birds may be necessary tomeet the high metabolic demands of flight. Among birds, smaller birds haverelatively larger hearts (again relative to body mass) than larger birds.Hummingbirds have the largest hearts (relative to body mass) of all birds,probably because hovering takes so much energy.

Electrical activity in the avian heart duringa cardiac cycle -- The P wave begins after sinus node depolarization nearthe top of the right
atrium, and the impulse spreads through the atria towardthe apex of the heart going caudally (down) and to the left (this resultsin a
small upright deflection in theelectrocardiogram, thePwave). The impulse then travels through the AV (atrioventricular) transmission
system to the ventricular myocardium (or heart muscle).Initial activation of the endocardium surrounding the apex of the leftventricle
(in a downward direction) causes a small, upright deflection,or R wave, in the ECG). Depolarization of the remainder of the ventricular
wall during this time is not detected in the ECG becauseextensive and complete penetration of the walls by Purkinje fibers resultedin a
single burst of mutually canceling electrical activity.Next, a large RS wave, or simply S wave, results from rapid, depolarizationof the
ventricle. Surface electrode placement = RA indicatesright axilla, or right wing; LA, left axilla, or left wing; & LL, leftleg (From: Oglesbee
et al. 2001).

Avian hearts also tend to pump more blood per unit time thanmammalian hearts. In other words, cardiac output (amount of blood pumpedper minute) for birds is typically greater than that for mammals of thesame body mass. Cardiac output is influenced by both heart rate (beatsper minute) and stroke volume (blood pumped with each beat). 'Active' birdsincrease cardiac output primarily by increasing heart rate. Ina pigeon, for example (Butler et al. 1977):

	Rest	Active	Increase
Heart rate	115 beats/min	670 beats/min	5.8x
Stroke volume	1.7 ml	1.59 ml	0.9x
Cardiac output	195.5 ml/min	1065 ml/min	5.4x
Oxygen consumed	20.3 ml/min	200 ml/min	10x

In general, bird hearts 'beat' at somewhat lower rates than mammalsof the same size but pump more blood per 'beat.' Among birds, heart ratevaries with size:

Species	Resting heart rate	'Active' heart rate
Turkey	93	-
Herring Gull	130	625
American Robin	570	-
Blue-throated Hummingbird	-	1260

Source: Welty & Baptista. 1988. The Life of Birds.Saunders College Publishing, New York.

Relationship between heart weight and heart rate at restgiven on a
bilogarithmic scale. The mean value for a given speciesis plotted
in this chart (Machida and Aohagi 2001).

Blood pumped by the avian heart enters the blood vessels. Themain types are:

• arteries- carry blood away from the heart & toward the body cells
• arterioles - 'distribute' blood (that is, direct blood where neededwith more going to active tissues & organs & less to less activetissues & organs) by vasodilating & vasoconstricting
• capillaries - exchange of nutrients, gases, & waste productsbetween the blood & the body cells
• venules (small veins) & veins-conduct blood back to the heart

Some of the major arteries in the avian circulatory system:

Carotids deliver blood to the head (& brain). Brachials take blood to the wings. Pectorals deliver blood to the flight muscles (pectoralis). The systemic arch is also called the aorta & delivers blood to all areas of the body except the lungs. The pulmonary arteries deliver blood to the lungs. The celiac (or coeliac) is the first major branchof the descending aorta & delivers blood to organs & tissues inthe upper abdominal area. Renal arteries deliver blood to the kidneys. Femorals deliver blood to the legs & the caudalartery takes blood to the tail. The posterior mesenteric delivers blood to manyorgans & tissues in the lower abdominal area.

Source: http://numbat.murdoch.edu.au/Anatomy/avian/avian3.html

Mitochondria are redistributed towards the cell membrane in the muscle fibers of Bar-headed Geese. (a) The proportion of mitochondria that were subsarcolemmal was higher in Bar-headed Geese than in low altitude species. Grey bar, Bar-headed Goose; unfilled bar, Barnacle Goose; black bar, Pink-footed Foose. (b,c) Representative transmission electron micrographs of muscle fibers from (b) Bar-headed Geese and (c) Barnacle Geese. Scale bar, 2 µm. Arrow, subsarcolemmal mitochondria; arrowhead, intermyofibrillar mitochondrion.

Adaptation for high-altitude flight -- Bar-headed Geese (Anser indicus) migrate over the Himalayas at up to 9000 m elevation, but it is unclear how they sustain the high metabolic rates needed for flight in the severe hypoxia at these altitudes. To better understand the basis for this physiological feat, Scott et al. (2009) compared the flight muscle of Bar-headed Geese to that of low altitude birds (Barnacle Geese, Pink-footed Geese, Greylag Geese, and Mallard ducks). Bar-headed Geese had more capillaries per muscle fiber than expected, and higher capillary densities and more homogeneous capillary spacing. Their mitochondria were also redistributed towards the sarcolemma (cell membrane) and adjacent to capillaries. These alterations should improve O₂ diffusion capacity from the blood and reduce intracellular O₂ diffusion distances, respectively. Bar-headed Geese have, therefore, evolved for exercise in hypoxia by enhancing the O₂ supply to flight muscle.

Bar-headed Geese

Some major veins in the avian circulatory system:

Source: http://numbat.murdoch.edu.au/Anatomy/avian/avian3.html

The jugular anastomosis allows blood to flow fromright to left side when the birds head is turned & one of the jugularsconstricted. The jugular veins drain the head and neck. The brachial veins drain the wings. The pectoral veins drain the pectoral muscles andanterior thorax. The superior vena cavae (or precavae) drain theanterior regions of the body. The inferior vena cava (or postcava) drains theposterior portion of the body. The hepatic vein drains the liver. The hepatic portal vein drains the digestive system. The coccygeomesenteric vein drains the posteriordigestive system & empties in the hepatic portal vein. The femoral veins drain the legs. The sciatic veins drain the hip or thigh regions. The renal & renal portal veins drain the kidneys.

The heart pumps & the vessels carry, of course, blood. Avianblood:

• consists of plasma + formed elements
• plasma is largely water (~85%) plus lots of protein (~9-11%); other constituentsof blood include glucose (blood glucose levels in birds are greater thanin mammals; about 200-400 mg/dl), amino acids, waste products, hormones,antibodies, & electrolytes.
• the formed elements include red blood cells (or erythrocytes), white bloodcells (or leucocytes), and thrombocytes
• bird red blood cells (shown to the right), unlike those of mammals, areelliptical in shape and nucleated. In most species, red blood cells areabout 6 x 12 micronsin size (mammalian RBC's are typically 5.5 - 7.5 microns in diameter).Typical concentrations are 2.5 to 4 million/cubic mm. Avian red blood cellshave a lifespan of 28-45 days (shorter than mammals, e.g., about 120 daysin humans). Red blood cells contain hemoglobin, the molecule responsiblefor transporting oxygen throughout the body, and are produced in the bonemarrow. However, many bird bones are pneumatic (penetrated by air sacs) and do not contain marrow. Hemopoietic bone marrow (red-blood-cell-producing marrow) is located in the radius, ulna, femur, tibiotarsus, scapula, furcula (clavicles), pubis,and caudal vertebrae.

Skeleton of a Rock Pigeon (Columba livia) showing the bones (shaded) that contain red-blood-cell-producing marrow, including the radius and ulna of the wing, femur and tibiotarsus of the leg, furcula and scapula of the pectoral girdle, pubis of the pelvic girdle, and caudal vertebrae. Most other bones (except for very small ones) are pneumatized (Schepelmann 1990).

Examination and discussion of the cells in an avian blood smear, plus examination of a section of the cloacal bursa and discussion of its function.

Differences in the red blood cells of birds and mammals -- Mammals, which had developed an aerobic metabolism, emerged in the Triassic, when the oxygen content in the atmosphere was by approximately 50% lower than current levels and even lower than in the Jurassic period (when birds evolved). Under these conditions, natural selection favored the loss of nuclei in the red blood cells of mammals (making the cells smaller and allowing capilaries to become even smaller in diameter) and change to a biconcave shape (increasing the amount of surface area and enhancing diffusion into and out of the red blood cells). Birds, with their efficient respiratory system, evolved during the Jurassic when the oxygen content in the Earth atmosphere approached the present level, so there was no selective pressure to eliminate nuclei from their red blood cells or change in shape (Gavrilov 2013).

The degree of oxygen saturation of hemoglobin (% of moleculesbinding with oxygen) depends on the partial pressure of oxygen (shown herefor various organisms in oxygen-hemoglobin dissociation curves). The P50is the partial pressure at which 50% saturation occurs; high-affinity hemoglobinhas a low P50 and a curve shiftedto the left, whereas a low-affinity hemoglobin has a high P50and a curve shifted to the right.
(Source: http://www.sfu.ca/biology/courses/bisc445/lectures/respiration_2_circulation.html)

• bird thrombocytes (shown above with two red blood cells), also nucleated,are comparable to the non-nucleated platelets of mammalian blood. Thrombocytesare important in hemostasis (blood clotting).
• White blood cells play an important role in protecting birds from infectiousagents such as viruses and bacteria. Birds have several types of whiteblood cells:

Avian White Blood Cells

The lymphocyte is the most numerous white bloodcell. Lymphocytes are either T-lymphocytes (formed in the thymus)or B-lymphocytes (formed in the bursa of Fabricius). B-lymphocytes produceantibodies; T-lymphocytes attack infected or abnormal cells.

The heterophil is the second most numerousWBC in most birds. Heterophils are phagocytic and use their enzyme-containinggranules to lyse ingested materials. Heterophils are motile and can leaveblood vessels to engulf foreign materials.

Monocytes are motile cells that canmigrate using ameboid movements. Monocytes are also phagocytic.

Eosinophils make up about 2 to 3 %of the WBC population of healthy birds. The function is these cells isunclear.

Scanning electron microscope view of bird thrombocytes adhering to a collagen-lined plate (exposure to collagen causes bird thrombocytes, and mammalian platelets, to release chemicals that make them 'sticky'; the chemicals released by mammalian platelets are different from those released by bird thrombocytes and make platelets 'stickier' than thrombocytes). Avian thrombocytes are larger than mammalian platelets, have a nucleus, and, unlike mammalian platelets, do not form 3-dimensional aggregates. (Credit: Penn Medicine)

Can birds have heart attacks and strokes? -- Mammalian platelets are small, anuclear circulating cells that form tightly adherent (i.e., 'sticky') thrombi (clots or 'plugs') to prevent blood loss after vessel injury. Platelet thrombi that form in the coronary and carotid arteries of humans can also cause common vascular diseases such as myocardial infarction ('heart attacks') and stroke and are the target of drugs used to treat these diseases. Birds have high-pressure cardiovascular systems like mammals, but have nucleated thrombocytes in their blood rather than platelets. Schmaier et al. (2011) found that avian thrombocytes respond to many of the same activating stimuli as mammalian platelets (and so help stop blood loss from damaged vessels) but, unlike mammalian platelets, cannot form tighly adherent thrombi in arteries. Avian thrombocytes are larger than mammalian platelets and are less 'sticky' (because they release different chemicals) than mammalian platelets when exposed to collagen (connective tissue to which thrombocytes and platelets are exposed when there's a break in a blood vessel). When carotid arteries of mice are damaged, platelets form thrombi that can block blood flow (check this video showing the response of human platelets when exposed to a plate covered with collagen); similar damage to the carotid arteries of Budgerigars (similar in size and speed and pressure of blood flow to the carotid arteries of mice) did not cause the formation of thrombi (check this video showing the response of chicken thrombocytes when exposed to a plate covered with collagen). These results indicate that mammalian platelets, in contrast to avian thrombocytes, will form thrombi even in arteries where blood flow is rapid and under high pressure, an essential element in human cardiovascular diseases.

Link:

Heart disease linked to evolutionary changes that may have protected early mammals from trauma

Brain size & immunity -- Parasitism can negativelyaffect learning and cognition. Greater susceptibility to parasitism bymales may impair cognitive ability, and greater male investment in immunitycould compensate for greater susceptibility, in particular when males havea relatively large brain. Why might males be more susceptible to parasites? Because of intense competition for access to mates. Immunosuppression due to the negative effects of androgenson immune function. Møller et al. (2004) analyzed covariationbetween relative size of immune defense organs and brain in birds. Therelative size of the bursa of Fabricius and the spleen in adults covariedpositively with relative brain size across species, and the relationshipbetween immune defense and brain size was stronger for males. Thus, speciesin which males have relatively large brains also have relatively largeimmune defense organs. These findings support the hypothesisthat sex differences in brain function have evolved as a consequence ofdifferences in susceptibility to parasitism. Different components of theimmune system (bursa and spleen) may have evolved to mitigate the negativeimpact of parasites on brain function.

Covariation between relative brain mass of juvenile birdsand relative mass of bursa of Fabricius in different bird species. Relativemass was calculated as residuals from a phylogenetically corrected regressionof log10-transformed organ mass on log10-transformed body mass. The linesare the linear regression lines for males and females, respectively (From:Møller et al. 2004).

B-lymphocytes, the cells that produce antibodies, are initially producedin the embryonic liver, yolk sac and bone marrow, then move through theblood to the bursa of Fabricius (BF).

Birds have a bursa of Fabricius (BF), which is an outpocketingof cloaca. Within the BF, B-lymphocytes mature then migrate to otherbody tissues. The bursa is a blind sac that extends from the dorsal sideof the cloaca, the common portal of the reproductive, urinary, and digestivesystems. Within the bursas of young birds are extensive leaf-like foldscomposed of simple, columnar epidermis and a loose connective tissue withlots of blood vessels and lymph nodules. Atrophy of the BF typically occursaround the time of sexual maturation. (Source: http://www.upei.ca/~morph/webct/Modules/Lymphoid/bursa.html)

In the BF, the B-cells mature and become functional and then move tothe blood, spleen, cecal tonsils, bone marrow, Harderian gland (Hg in diagrambelow), and thymus.

In birds, most of the Ig diversification occurs by gene conversion in the bursa of Fabricius.
However, further Ig diversification is achieved by somatic hypermutation in secondary lymphoid organs (From: Kohonen et al. 2007).

B-lymphocytes produce three classes of antibodies after exposure toa disease organism: IgM, IgY (equivalent to mammalian IgG), and IgA. IgM appears after 4-5 days following exposure to a disease organism and thendisappears by 10-12 days. IgY is detected after 5 days following exposure,peaks at 3 to 3 1/2 weeks, and then slowly decreases. Ig A appears after5 days following exposure. This antibody is found primarily in the mucussecretions of the eyes, gut, and respiratory tract and provides "local"protection to these tissues.

From: Szabo et al. 1998.

(Video) the avian heart

Antibodies do not have the capability to kill viruses or bacteria directly.Antibodies (especially IgY) perform their function by attaching to diseaseorganisms (like bacteria) and blocking their receptors. The disease organismsare then prevented from attaching to their target cells. The attached antibodiescan also facilitate the destruction of pathogens by phagocytes.

IgY Ab = IgY antibody
Source: http://www.genwaybio.com/technology.htm

T-lymphocytes begin as the same stem cells as the B-cells, but are programmedin the thymus rather than the BF. The T-lymphocytes include a more heterogeneouspopulation than the B-cells. Some T-cells act by producing lymphokines(over 90 different ones have been identified); others directly destroydisease organisms. Some T-cells act to enhance the response of B-cells,macrophages, or other T-cells (helpers); others inhibit the activity ofthese cells (suppressors).

This video describes B cell development in the chicken. B cells produce antibodies that bind to infectious organisms (viruses, bacteria, and parasites) and play a vital role for the immune system to protect chickens, as well as us, from infectious disease.

How Did the Peacock Get His Tail? -- It's a questionthat has puzzled zoologists for more than a century. Charles Darwin firstnoted that the choosy peahen plays a crucial role in the evolution of thisextravagant sexual display. "We may conclude that those males which arebest able by their various charms to please or excite the female, are underordinary circumstances accepted. If this be admitted, there is not muchdifficulty in understanding how male birds have gradually acquired theirornamental characters," Darwin wrote. Moller and Petrie (2002) now suggestthat plumage may specifically convey the strength of a male's immunesystem and his desirability as a mate. Hamilton and Zuk (1982) firstsuggested that 'showy' males were signaling to females that they were,if not parasite-free, then parasite "lite." But, there has been littleevidence to support this hypothesis. Moller believes it is because peoplehave been looking at the wrong parasites. "If you look at our own species,we are attacked by hundreds of different species of parasites," said Moller."So if you wanted to study our parasite burden, you'd have to identifyall the parasites, from tapeworms to head lice, see how abundant they areand how they affect us. It would be practically impossible, so we decidedto focus on the immune system." Moller and Petrie took blood samples frommale Blue Peafowl (Pavo cristatus) and recorded the numbers of B-and T-cells, and also measured the peacocks' tails and counted the numberof eye spots. They discovered that the condition and length of the peacock'stail was related to the production of B-cells, and the size of the eyespots to T-cell production. "Our main finding is that females are lookingat different aspects of a male's immune competence," said Moller. Males,in effect, are walking billboards advertising their health and status.And these things matter. Previous research has shown that in chickens andquail, at least, the immune system is under genetic control so offspringwill inherit their parents' ability to fight parasites. Thus, it pays forfemales to be choosy because their chicks, in turn, will survive betterand mate with other, equally picky females. -- SanjidaO'Connell, The Independent (London), September 9, 2002

Tree Swallow

Immunosenescence in some immune components of free-living Tree Swallows -- A wide diversity of free-living organisms show increases in mortality rates and/or decreases in reproductive success with advancing age. However, the physiological mechanisms underlying these demographic patterns of senescence are poorly understood. Immunosenescence, the age-related deterioration of immune function, is well documented in humans and in laboratory models, and often leads to increased morbidity and mortality due to disease. However, little is known about immunosenescence in free-living organisms. Palacios et al. (2007) studied immunosenescence in a free-living population of Tree Swallows (Tachycineta bicolor), assessing three components of the immune system and using both in vivo and in vitro immunological tests. Immune function in female Tree Swallows showed a complex pattern with age; acquired T-cell mediated immunity declined with age, but neither acquired nor innate humoral immunity did. In vitro lymphocyte proliferation stimulated by T-cell mitogens decreased with age, suggesting that reduced T-cell function might be one mechanism underlying the immunosenescence pattern of in vivo cell-mediated response recently described for this same population. These results provide the most thorough description of immunosenescence patterns and mechanisms in a free-living vertebrate population to date. Future research should focus on the ecological implications of immunosenescence and the potential causes of variation in patterns among species.

The avian cardiovascular system is able to quickly respond tochanges in levels of activity (e.g., resting vs. flying) via changes inheart rate, cardiac output, & blood flow (by vasocontriction and vasodilationof vessels).

Measurements of resting heart rate were obtained onlyafter each bird had ceased activity in the dark cage and remained quiet.
The heart rate in an excited state (during excitement)was measured when the animal became maximally excited because its
movement in the cage was restrained manually (Machidaand Aohagi 2001).

Heart rate, diving depth and body angle of a female Common Eider (Somateria mollissima) during dives and when flying. (a, b) heart rates of 250 and 300 beats per minute, (c) heart rate ascending and descending slopes equal to or above 10 beats per minute per second (absolute values), (d) standard deviation of diving depth up to 0.1 m, and (e) change in body angle. The upward and downward pointing arrows indicate the point of take-off and landing, respectively (From: Pelletier et al. 2007).

Common Eider (male)

Many birds forage by diving underwater. So, what happens when a birddoesn't have access to air (oxygen)? The quintessential avian diver isthe EmperorPenguin, which can attain depths greater than500 meters while staying submerged for about 12 minutes. In shallower dives,an Emperor Penguin may stay submerged even longer, over 20 minutes. Divingbirds, including Emperor Penguins, must 'solve' several problems:

• a bird obviously can't breathe when under water, so oxygen levels in thebody begin to decline &, therefore,
• oxygen must be distributed to where it's most needed
• heat conservation (this is a potential problem for deep-diving birds andbirds diving in cold water)

One 'solution' to the oxygen problem is that birds, like penguins, thatspend lots of time underwater store lots of oxygen in their muscles. Themuscles of some diving birds contain lots of myoglobin. Myoglobin bindsoxygen just like hemoglobin but actually has a higher affinity for oxygenthan does hemoglobin. This means that when blood passes through muscle,lots of oxygen is transferred from the blood to the muscle. The chart belowillustrates this. In Emperor Penguins, nearly half (47%) of all oxygenin the body is in the muscles.

Source: http://eee.uci.edu/courses/bio112/diving.htm

Emperor Penguins

Diving beyond the limits (Butler 2001) -- GentooPenguins around South Georgia feed on krill at depths of 80–90 m. KingPenguins feed on myctophid fish at depths of 100–250 m. A large proportionof the dives of both species are longer than their calculated aerobic divelimits. Part of this discrepancy is probably due to inaccuracies in determiningrates of oxygen consumption during diving. That this may well be the caseis suggested by the fact that the temperature in the abdominal cavity ofboth Gentoo and King penguins drops during dives and returns to normalwhen diving behavior ceases. For Gentoo Penguins, abdominal temperatureduring dives was, on average, 2.4°C lower than when the birds werenot diving. The lowest abdominal temperature recorded for each penguinwas, on average, 33.6°C for the Gentoos and 29.7°C for the Kings.So if the birds allow temperature to fall in some parts of the body (i.e.,they do not increase metabolic rate in an attempt to keep abdominal temperaturenormal), there will be a saving in energy both in terms of thermoregulatorycosts and in terms of reduced metabolic rates in those tissues (i.e, aQ10 effect, with Q10 being the factorial change inthe rate of a chemical reaction associated with a 10°C change in temperature).

GentooPenguins Traces of dive depth (top) and temperature in the abdominalcavity (bottom) during a foraging trip of a Gentoo Penguin (A; mass = 6.6 kg) during 1day at sea and a King Penguin (B; mass = 11.7 kg) during 5 days at sea. Note changein scale of the depth traces (Butler 2001).

Gentoo Penguins

Additional physiological responses allow diving birds to make the bestuse of available oxygen & minimize heat loss in cold water:

• peripheral vasoconstriction (less blood & heat go to the surface ofthe body which reduces heat loss)
• vasoconstriction of blood vessels supplying the digestive system (whichmeans less blood is delivered, but when diving the digestive system cantemporarily 'shut down' to conserve energy)
• vasodilation of blood vessels supplying the central nervous system &heart (which means more blood is delivered)

What causes these things to occur? When a bird dives, special receptorsin the body detect an increase in levels of carbon dioxide (because thebird has stopped breathing). These receptors then stimulate the brain which,in turn, send nervous impulses that reduce heart rate & cause bloodvessels in different parts of the body to either vasoconstrict or vasodilate.

Source: http://www.zoo.utoronto.ca/stephenson/Research/Diving.htm

(Video) Bird Circulatory System

Guillemots

How Penguins Avoid the Bends -- Penguins are Olympic-classdivers. After a deep breath, they can plunge hundreds of meters for manyminutes, bob up briefly, and dive again. This ought to cause the bends,or decompression sickness, but penguins seem immune. Now researchers havediscovered a diving habit that may help explain why: On their way up fromthe deep, Adélie and king penguins slow down and surface at an obliqueangle--in effect mimicking the careful decompression of human divers. Marineanimals have a variety of strategies topreventthe bends. In human divers, increased underwater pressure forces nitrogenin the air within body cavities to pass into the blood. If divers surfacebefore the nitrogen is cleared, they can suffer contorted joints, difficultbreathing, and even paralysis. Many whales and seals have blood and musclesadapted to conserve oxygen; they can also collapse their lungs before divingto squeeze out air. Penguins don't have it that easy: Their lungs don'tcollapse, and the buoyant divers need a good dose of oxygen to swim hard.To find out how penguins move about at depth, Katsufumi Sato and colleagues(Satoet al. 2002) attached data loggers to Adélie and king penguinsoff the shores of Antarctica and Crozet Island, about 1000 kilometers away.The instruments measured the depths, speed, and acceleration and decelerationeffects from wing strokes for more than 650 dives. From these data Sato'steam estimated air volumes in penguin lungs during their descents and ascents.The dive profiles revealed that the penguins flapped their flippers continuouslyon the way down. On return trips, after swimming halfway up, they stoppedand let their natural buoyancy give them a free ascent. But surprisingly,instead of shooting straight up the penguins veered at an oblique angle,thus significantly slowing their ascent, the team reports in the May issueof the Journal of Experimental Biology. This increases the amount of timethe penguins spend in shallow water with little prey, but it could providetime for nitrogen, under lower pressure, to return to the air inside bodycavities. Those findings intrigue marine biologist Dan Costa of the Universityof California, Santa Cruz: They've made careful and insightful measurementsof the fine-scale diving behavior of two penguins, supported with verysophisticated models of lung volume, and they may be correct. However,he cautions, there are alternate explanations for why penguins would slowtheir ascents, such as looking out for predators. -- NoreenParks, Academic Press Daily InScight

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Palacios, M. G., J. E. Cunnick, D. W. Winkler, and C. M. Vleck. 2007. Immunosenescence in some but not all immune components in a free-living vertebrate, the Tree Swallow. Proc. Royal Academy of London B, online early.

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Schmaier, A. A., T. J. Stalker, J. L. Runge, D. Lee, C. Nagaswami, P. Mericko, M. Chen, S. Cliche, C. Gariepy, L. F. Brass, D. A. Hammer, J. W. Weisel, K. Rosenthal, and M. L. Kahn. 2011. Occlusive thrombi arise in mammals but not birds in response to arterial injury: evolutionary insight into human cardiovascular disease. Blood 118: 3661-3669.

Scott, G. R. 2011. Elevated performance: the unique physiology of birds that fly at high altitudes. Journal of Experimental Biology 214: 2455-2462.

Scott, G. R., S. Egginton, J. G. Richards, and W. K. Milsom. 2009. Evolution of muscle phenotype for extreme high altitude flight in the Bar-headed Goose. Proceedings of the Royal Society B: online early.

Szabo, Cs., L. Bardos, S. Losonczy, and K. Karchesz. 1998.Isolation of Antibodies from Chicken and Quail Eggs. Presented at INABIS'98 - 5th Internet World Congress on Biomedical Sciences at McMaster University,Canada, Dec 7-16th. Available at URL http://www.mcmaster.ca/inabis98/immunology/szabo0509/two.html

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