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A Review of Flight, Fuel and Blood Chemistry in Racing Pigeons
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A Review of Flight, Fuel and Blood Chemistry in Racing Pigeons
Luckily for us as fanciers, over these many years, pigeons have been the subject of a great deal of investigation, especially in the areas of muscle and the study of energy for flight. As a matter of fact, the very foundations of all modern knowledge of the physiology (refers to the normal chemistry and function) of muscle in many species, including birds, animals and of course, humans, were established by early work conducted by a researcher named Szent-Gyorgyi who used the breast muscles of pigeons in his biochemical experiments. These early investigations by Szent-Gyorgyi ultimately led to the important discovery of the key energy cycle, known as the Krebs Citric Acid Cycle, named for Hans Krebs who also used the breast muscles of pigeons in the universally acclaimed work that led to this major discovery.
In summary, because the biochemical steps in the whole process are extremely complex, the work done by Krebs showed that, within cells of the body, in the presence of oxygen, the major fuels, glucose and fat, are metabolised (roughly means "broken down" or "utilised") in the tissues in a stepwise fashion, to the final products, carbon dioxide and water. Through these complex chemical processes to reach the final products of carbon dioxide and water, important chemical by-products are produced along the way as spin-offs in the whole process. One of these very important compounds represents the key to all work, and is called adenosine triphoshate, ATP for short.
Thus, the energy for work of any kind -- and in particular the energy for flight, which interests us the most -- is developed in muscle fibres through the production of this very important high-energy compound known as ATP. It is a potent compound that in every-day terms is comparable to the steam that once provided the energy to drive the old railroad locomotives. Regardless of the fuel being used -- fat or glucose -- the main purpose is to produce ATP to supply the energy for the different phases of flight, including the launch, rapid sustained cruising flight, dodging bursts of speed at any time, braking to land, etc..
ATP derives its power from the three -- hence, tri phosphate -- important high- energy phosphate bonds that make up this compound. These three high-energy phosphate bonds provide the energy, delivered through the massive breast muscles, that allows the wings of our birds to beat on an average of about 9.5 times per second during the explosive launch phase of flight, and on an average of about 5.5 times per second at cruising speed for the duration of the training toss or race, however long it might be. (Indeed, the two great breast muscles of the pigeon are so powerful that one of them alone is capable of developing a force about ten times the weight of the bird -- definitely, tremendous power.) Through all of this it is significant to realize that it was the pioneering work of Szent-Gyorgyi and Krebs on the breast muscles of pigeons that established the dynamic biochemical foundations of energy production in various species of animals right through to the level of humans.
As noted previously, the major fuels used to provide the energy for short or prolonged flight are glucose and fat, and particularly the latter when prolonged flight is required. In the explosive launch phase of flight -- and indeed whenever explosive or dodging bursts of speed are needed during the course of flight -- glycogen, the storage form of glucose in liver and muscle, is metabolised rapidly to glucose in the white muscle of the breast muscles. In turn, the glucose obtained from the breakdown of glycogen is metabolised in the white muscle to produce the ATP needed to provide energy for this rapid action.
Note that for the launch phase of flight, glucose from the breakdown of glycogen is the chief fuel needed. So powerful and energy-demanding is the launch phase that within 10 minutes or so, all of the glycogen in the white muscle of the breast is completely utilised, and to all intents and purposes, the white muscles cease to function because of the depletion of their main source of energy. Importantly, during the course of the flight, the white muscles are recharged with glycogen to provide the energy for sudden dodging and other explosive bursts of energy, which would be needed if there were attacks by falcons or other raptors, for example, or for any situation requiring a sudden, dramatic increase in speed. The process of re- fueling the white muscle with glycogen originates in the liver which has abundant supplies of glycogen. In the liver, glycogen is broken down to glucose for transport through the blood stream to the white muscles in the breast. In the white muscles, glucose is once more built up into abundant supplies of glycogen that are ready for the kinds of emergencies described earlier.
If the white muscles are once again depleted of glycogen by any of these emergencies, they are subsequently re-fueled, so that even at the prolonged flight time of 18 hours, the white muscles seem to contain useful amounts of this fuel. It is important to be aware as well that some of the glucose liberated to the blood stream from the liver is critically needed as a source of fuel for the brain which can use only glucose as a source of energy. Thus, it is critical that there be a steady flow of glucose from the liver, and is one of the major reasons that birds normally have a high level of glucose circulating in the blood stream.
By contrast, fat in the red muscle of the breast is metabolised to produce the energy from ATP needed for the prolonged effort of flights that range from regular training tosses to short, middle, long distance and endurance performances. For sustained flight, within a short time, there is a shift from the initial use of white muscle which utilises glucose for the launch phase, to the use of red muscle which utilises chiefly fat for the duration of the flight. Supplies of fat are present in the red muscle and as they are depleted during flight, they are replenished from depots in the body cavity. The flow of fat to the red muscle occurs from the fat depots where the fat is converted to fatty acids for transport through the blood stream into the red muscles. Although the white muscles seem to be quite well supplied with glycogen even at 18 hours of flight, by contrast at this same time, the red muscles, which are the real workhorses of the system, are found to be severely depleted of fuel, so that the onset of fatigue becomes imminent and even critical.
As noted in an earlier article, in migratory birds, migratory fat is distinguished from winter fat. Migratory fat is built up rapidly in the body cavity of these birds in preparation for prolonged flight, and is also completely utilised at the end of migration. Winter fat, located under the skin, is used primarily as insulation against the cold. This fat, combined with a certain amount of shivering to create heat helps to keep the bird warm in a cold environment. The use of fat as insulation is a much more efficient method of retaining heat than the continued wasteful use of fuel for shivering to generate heat.
In looking at racing pigeons, it seems likely to me that the fat that is built up rapidly each week in the body cavity among the intestines, etc., is probably of the "migratory" type because it is well utilised and may be almost completely depleted, depending on the severity of the race, by the end of the race. From a biological point of view then, pigeon races each week may well represent a repeated migratory phase of their lives.
As also noted in an earlier article, the breast muscles of the pigeon contain two types of muscle - red and white - with red muscle by far the most abundant. Why does red muscle appear red or red-brown, and white muscle white or at least pale? The difference lies in the fact that red muscle contains myoglobin, a complex, pigmented chemical structure that is similar to haemoglobin, the compound that imparts the red colour to blood. Both haemoglobin and myoglobin contain iron, which gives them their red colour, and both have a great capacity to hold oxygen for transfer to tissues. Red muscle fibres contain abundant myoglobin which holds and then transfers plentiful supplies of oxygen to the muscle structures known as mitochondria -- the powerhouses of all cells, where ATP is generated -- that utilise the fat in the production of energy for flight.
White muscle fibres contain little or no myoglobin, and as a result they have developed the capacity to utilise glycogen in the absence of oxygen (called anaerobic metabolism: "an" = "without"; "aerobic" = "oxygen") in the production of energy for their explosive role in flight. Also, because they don't contain the pigmented compound myoglobin, they are very pale, and are described as white. Thus, the great breast muscles of the domestic chicken or turkey are comprised mainly of white fibres, and appear pale for that reason. By comparison, the massive breast muscles of the pigeon and any cut of beef for that matter, are good examples of muscles that contain mainly red fibres.
To digress momentarily, cells in various organs of the body are of different types, shapes and functions. For example, cells of the liver look like sugar cubes or dice; some brain cells resemble a spider or an octopus; skin cells, especially those near the surface, are flat like pancakes, and so on. When muscle cells are separated out from the main breast muscles by delicate microtechniques, they are seen to be elongated or thread-like, and in fact, resemble fine threads or fibres. Hence, by convention, muscles cells are usually described as fibres.
If a lengthwise section of muscle is cut out of the breast of a pigeon, fixed in formalin to stop all chemical processes, processed in a series of chemicals such as alcohols and xylene, embedded in wax, cut very thinly, placed on a glass slide, and then stained with certain dyes, it can be examined by use of a microscope. A lengthwise section of stained muscle has the appearance of a number of cigar- shaped fibres lying side by side and end to end. Closer inspection reveals that there are fibres of two different diameters, one broad and the other narrow. The broad fibres have a relatively smooth looking interior, whereas the narrow fibres have a rough interior because of the amount of "machinery" and fuel they contain.
To obtain a different look at the microscopic appearance of the muscle, we can also cut our original piece of muscle in cross section and stain it to get an end-on view of these fibres. Here, we see that the fibres are arranged in bundles that are separated from one another by connective tissue. When we look at the muscle in cross section, we see even more clearly that there are fibres of two different diameters, one a broad diameter and the other, a narrow diameter fibre, and further that, for the most part, the broad diameter fibres are located on the outside edge of each bundle, whereas, the smaller diameter fibres are located more deeply within a bundle. (For photos of the microscopic appearance of these fibres, see issues of the ARPJ a couple of years ago.)
The broad diameter fibres are known as white fibres, and the narrow diameter fibres are the red fibres. In diameter, the white fibres are over twice as broad as the red fibres. In cross section, all of the fibres regardless of their diameter, whether they are red or white, are round or oval. Fibers may extend the entire length of a muscle, or may form long strands by connecting to other fibers. If we were to count several series of 100 fibres, we would see that the red fibres vastly outnumber the white fibres; for every 100 fibres, approximately 85-95 are red fibres and only 5-15 are white fibres. Note the point: the great breast muscles of the pigeon contain both red and white fibres, but the muscle appears red to the eye because of the predominance of the red fibres. Both types of fibres are present prior to hatching, a finding that indicates that the differentiation between the two is genetically determined, and not the result of specialised activity after hatching. Incidentally, as an athletic individual becomes conditioned for racing, the number of fibres doesn't increase, but rather the existing fibres simply enlarge to handle the increased work load.
Red fibres are abundantly supplied with microscopic thread-like blood vessels called capillaries that criss cross and interconnect over the surface of each of these fibres. This extensive meshwork of blood vessels is necessary because the red fibres utilise fat in a system that requires oxygen (aerobic metabolism), and a steady flow of oxygen-rich blood to these fibres ensures that, along with good supplies of fuel that is also delivered in these blood vessels, they are able to function rapidly and efficiently for many hours at a stretch. On the other hand, the white fibres are very poorly supplied with blood vessels, since they don't require oxygen to function.
Now, if we cut another thin cross section of muscle, place it on a glass slide, and apply a stain that detects only glycogen, we see that there is very intense staining of the white fibres, which indicates clearly, an abundance of glycogen in these fibres - about 10%. However, there is also some less intense staining of the red fibres, an indication that they contain glycogen also, but much less - about 2.5%.
If we cut another cross section of the breast muscle and this time apply a stain that detects only fat, we see that there is very intense staining of the small red fibres, but no staining whatsoever in the broad white fibres. It becomes immediately clear that the chief fuel for the broad white fibres is glycogen, and for the more abundant narrow red fibres, the chief fuel is fat, although as noted, lesser amounts of glycogen are also present in these red fibres. Overall, analysis of a section of the breast muscles containing a mixture of both red and white fibres, indicates a glycogen content of 3.5% and a fat content that ranges between 10-15%.
As well as their differences in fuel supplies, it is known that the two fibre types have differing speeds at which they operate - called "twitch" speed. Although both twitch very rapidly indeed, white fibres twitch much more rapidly than red fibres. Because white fibres twitch so rapidly, they also utilise their glycogen fuel supplies very rapidly, and so, tire out very rapidly. For this reason, it becomes obvious that white fibres cannot be used for prolonged, hour upon hour flight. So what is their main function? Because they twitch very rapidly and tire out very rapidly as well, white fibres are used for explosive actions, such as the launch phase of flight, or for dodging bursts of speed to escape aerial predators, or in the braking process for landing. So rapidly are the glycogen reserves utilised at launch for example, that it is known that this complete depletion of glycogen in the white fibres occurs within about 10 minutes after the launch. To indicate how rapidly the white fibres can twitch, note that birds shivering from cold, or those whose trembling wing tips indicate birds in top condition, are using their white fibres for these actions.
By contrast, although they truly twitch very rapidly, the red fibres nevertheless twitch much less rapidly than the white fibres, and as a result, they tire out much more slowly. It becomes very obvious then that the red fibres, utilising their abundant stores of fat, and calling on further fat supplies from the body cavity when they are needed, are the real work horses of the breast muscles, and can handle the challenge of rapid, sustained flight over the many miles of the race course for many hours at a stretch.
There are those who believe that glycogen is the fuel needed for training tosses and short races, and that possibly fat is needed only for more distance events. Some even completely discount fat as a fuel, believing (incorrectly) as they do that glycogen is the key fuel for any race of any distance. The well-established scientifically proved facts are, however, that glycogen is the chief fuel for the launch and certain other explosive phases of flight as noted previously, and that fat has been clearly established as the key fuel of the red fibres for any flight from a short training toss to the longest endurance event.
The views of some commercial interests and others who propose "carbohydrate loading" in racing pigeons seem to be at direct variance with the known and well established facts about the fuels needed by pigeons for racing performance. Having said that, I do believe however, based on facts from scientific literature, that the timely use of sugars such as glucose and fructose (in addition to the use of high energy grains) is highly valuable in the preparation of birds for racing, simply because both sugars are known to be converted readily to fatty acids in the liver -- see earlier articles for details. From the liver these fatty acids are exported in the blood stream to the red muscle fibres where they are the major fuel for the sustained efforts of flights ranging from short to long distance.
Now, some other interesting findings related to flight in pigeons. In Canada, experiments on other important aspects of flight were conducted by Dr George and his colleagues, with two flocks of racing pigeons. One flock of birds had been in regular training and were considered to be fit birds. A second flock of birds had not been in training for three months and were very likely in off condition. Both groups were transported to a release point located about 48 km (approx 30 miles) from home. A third flock of birds, known as controls (because they were rested and not exercised), was simply transported to the release point and returned to the loft without being flown. This was done to determine whether there was any effect on birds simply from the trip to the release point alone. The Autumn day chosen for this study was clear, calm and sunny with a temperature of 6oC (about 43oF). Birds in the first two flocks were released in groups of 2-3. Birds in the trained group homed in 60-80 minutes, whereas the group that hadn't been in training for three months homed in 90-160 minutes. (After reading these results, one of my main questions was: why did trained pigeons that should have been in good physical condition take so long to arrive home from such a short distance? In my experience, on a calm day, birds should fly a mile in 1 to 1 1/2 minutes, so from 30 miles I estimated that they should have homed in about 45 minutes maximum. These results are certainly puzzling but that is the way they were reported. I can only conclude that there were restraining effects from factors we don't know about.)
As the birds arrived home, blood samples were collected in separate tubes from a wing vein of each flown bird and from each bird that was just transported and not flown. These blood samples were allowed to clot and the resulting serum (serum is the pale yellow fluid that separates from the clot) was collected and then analysed for a number of constituents, and the results for each of the three groups were compared.
Compared with the rested birds, in those birds in the flock that had been in regular training before this toss, there were significant increases in blood levels of glucose, lactate (lactic acid), fatty acids, uric acid, and in the following hormones: growth hormone, adrenalin, thyroid hormone, AVT (arginine vasotocin), and melatonin. In the flock that had not been in training for three months, there were significant increases in triglycerides (fat), free fatty acids, and the hormone glucagon, but no significant increase in blood levels of corticosterone. Now before anyone begins to panic and stumble over some of these big words, I will try to explain as well as I can, what these results mean! Read on.
To supply energy, fuel is needed for many tissues, among which the brain and breast muscles are highly important. To liberate glucose and fatty acids, the release into the blood stream of two hormones, namely growth hormone and glucagon, is important, so their levels in the blood rise during flight. In turn, these hormones target the liver and the fat depots where they stimulate chemical reactions that liberate glucose and fatty acids into the blood stream for use as fuel by brain and muscle. Blood levels of adrenalin from the adrenal glands located just ahead of the kidneys, also rise and aid in the release of glucose into the blood.
Levels of lactate (lactic acid) in the blood rise as well during the launch phase of flight when the white fibres are working at their maximum, and also if dodging bursts of speed are needed on the way home, or if the birds have to work hard against a head wind, and during the braking phase of landing. The reason is that the white fibres that are likely used to a great extent during these intense muscular efforts, use glucose derived from their stores of glycogen in a chemical reaction that doesn't require oxygen (thus, it is an anaerobic process). In such an anaerobic state, one of the by products of the reaction is lactic acid - hence its elevation in the blood stream.
As discussed in an earlier article, blood levels of uric acid and the hormones AVT and melatonin also rise during flight, and are believed to be useful in preventing the birds from overheating. Recall from an earlier article that the amount of heat produced during flight in the pigeon has been estimated to increase about six times compared with amount of heat produced by a resting bird. The most important site for the loss of most of the heat from the body seems to be the legs - witness the dangling legs of some birds while they exercise around the loft on a hot day. Melatonin seems to exert its emperature-reducing effects by causing blood vessels near the surface of the body to dilate (expand in diameter) in order to bring more heat from the warmer core of the body to the surface. The hormone AVT also seems to be useful in mobilising fat from storage areas for use by the flight muscles, and by playing a role in decreasing the loss of water from the body, along with its role in the regulation of body temperature. Blood levels of uric acid also rise during flight in pigeons and may well be another component of the complex chemical interactions that try to ensure heat stability and water conservation during flight. It was mentioned in an earlier article that uric acid levels are known to increase in wild birds during migration, and in human athletes exercised to exhaustion.
Although blood levels of corticosteroids released from the adrenal glands -- which are indications of a response to stress -- did not rise significantly during the test flights in Canada, another unrelated study in Germany found that there were large increases in the level of this hormone in pigeons flown greater distances than those flown in Canada. In the German study, trained pigeons were flown from distances ranging through 115, 225, 314 and 557 km (about 70, 135, 188 and 330 miles, respectively). Flight times ranged from 315 to 561 minutes (about 5 to 9« hours). Blood samples collected at the end of these flights had a high level of the hormone corticosterone, likely a direct response of the birds to the stress of flying these greater distances. As well, this hormone which is released from the adrenal glands, combined with others mentioned earlier, may help to mobilise glucose from the liver and some say, fat from body fat depots for use during these prolonged flights. The likely reason that blood levels of corticosterone didn't rise in the birds studied in Canada, is that the distance and the time to home were not sufficiently stressful to induce the release of this hormone. By contrast, the time and distances flown by the birds in Germany called for much more strenuous effort, and therefore were sufficiently stressful to cause the release of this hormone.
Sounds complex, doesn't it? Well, it is, and the foregoing explanations merely scratch the surface of all of the events that occur in any living creature, human or otherwise, during work, and indeed during life in general. Like any other living creature, the racing pigeon is a complex animal endowed with beautifully intricate and co-ordinated biochemical systems that convert food into the energy of life. I confess that I know and understand very little about all of the complexity of these marvelously well meshed, intricate systems, but I am fascinated that they work so astonishingly well and efficiently, not just in pigeons but in all creatures, humans included.
In closing, I should say that my lack of solid knowledge on the whole subject reminds me of a perhaps pertinent quotation from the ancient philospher Socrates. It seems that Socrates had an argument or disagreement with a friend or colleague, and after they parted company, Socrates later wrote: "And so I left him, and as I went away, I said to myself, 'Well, although I do not suppose that either of us knows anything, I am better off than he is. For he knows nothing, and thinks that he knows. I neither know nor think that I know. In this latter particular then, I seem slightly to have the advantage of him.'" Like old Socrates, I neither know nor think that I know, but I am willing to expose and share my ignorance with you! Good racing!
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