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Habitat and Environment of the Common Raven
The Common Raven (Corvus corax) lives in a wide variety of geographic regions around the word, but is mostly found in the Northern Hemisphere. Individuals of this species are evenly distributed along coastlines and prefer to build their nest sites along sea cliffs. Common ravens are often located in coastal regions because these areas provide easy access to water and a variety of food sources. Also, coastal regions have stable weather patterns without extreme cold or hot temperatures. The Corvus corax species consumes an omnivorous diet and is adapted to scavenging. Common Ravens are able to adapt to changing seasons and different food demands. Moreover, this species’ ability to adapt is related to its intelligence. In general, Common Ravens live in a wide array of environments but prefer heavily contoured landscapes. When the environment changes in vast degrees, these birds will respond with a stress response. The hormone known as corticosterone is activated by the hypothalamo-pituitary-adrenal axis. Corticosterone is activated when the bird is exposed to stress, such as migrating great distances.

Furthermore, there has been research suggesting that the Common Raven is involved in seed dispersal. In the wild, the Common Raven chooses the best habitat and disperses seeds in locations best suited for its survival.

Habitat Variation and Physiological Regulation
Maintaining homeostasis through internal regulatory mechanisms is directly affected by habitat variation. The Common Raven is considered to be a homeotherm, an endotherm, and a regulator, so it is required to adjust its internal physiological state in response to environmental changes. Food habitats influence the metabolic rate of the Common Raven. Since Common Ravens are omnivores, metabolic rates must fluctuate according to the type of food consumed. According to one study, species that consume only fruit possess lower metabolisms compared to species that ate both fruit and insect material. The high metabolic rate of the Common Raven is partially due to the diversity of its diet.

Altitude is another factor that requires the Common Raven to regulate. Organisms existing at elevations below 1,100 m (3,500 ft) feet have lower metabolisms than organisms living at higher altitudes. Generally, warmer temperatures are associated with lower altitudes, so less energy is required to maintain a constant internal temperature. Bergmann’s rule can also be applied to the Common Raven. Individuals inhabiting higher altitudes and exposed to colder temperatures are usually larger than ravens living at lower latitudes or in warmer temperatures. Also, higher altitudes are associated with lower oxygen partial pressure, so Ravens living at high elevations are confronted with reduced oxygen availability. To compensate for less ambient oxygen, Common Ravens undergo increased respiratory rates, enhanced oxygen loading of hemoglobin at the respiratory surface, and improved oxygen affinity of hemoglobin.

Common Ravens occupy a widespread geographical range and are found in many different habitats, including tundra, seacoasts, cliffs, mountainous forests, plains, deserts, and woodlands. Due to such a diverse habitat, this species is exposed to various temperatures and amounts of precipitation. Individuals that exist in warmer, drier environments have lower basal metabolic rates than organisms inhabiting non-arid areas. Physiologically, a reduced metabolic rate decreases endogenous heat production to prevent evaporative water loss, or more simply evaporation, and conserve energy in an environment with limited resources. A reduction of total evaporative water loss consists of decreases of both respiratory and cutaneous evaporation. In contrast, Common Ravens living at higher latitudes in temperate regions experience high basal metabolic rates. A higher metabolism is related to increased thermogenesis and cold tolerance.

In relation to temperature and precipitation, Common Ravens are exposed to changing seasons with climate extremes. Within the Common Raven species, the degree of climatic seasonality is related to the magnitude of fluctuations in basal metabolic rate and total evaporative water loss. For instance, populations living in Alberta are subjected to both extremely cold temperatures in the winter and very hot and dry weather during the summer months. Furthermore, the Common Raven is not known to migrate long distances to avoid the winter season, so it is required to regulate and cope with the environmental conditions.

Habitat variation often leads to changes in activity levels. Ravens engaged in flight are considered metabolically active. During periods of flight, the cells require more oxygen, and the heat generated must be dissipated to avoid hyperthermia. In response, the Common Raven experiences an increased heart rate and cardiac output. Another method used by many species of birds to regulate thermal conductance is by internally adjusting blood flow through shunt vessels. More specifically, arterial and venous blood vessels are organized to bypass the countercurrent heat exchange occurring in the upper portion of a bird’s legs. Countercurrent heat exchange involves arrangements of blood vessels that allow heat to transfer from warm arterial blood to cooler venous blood travelling to the body’s core. Through this mechanism, arterial blood remains warm before reaching the body’s periphery.

Respiration
Ravens have a high metabolic rate that drives flight. Air flow is directed through the lungs via air sacs. The sacs are used to create a continuous unidirectional flow of fresh air over the respiratory surface. Most birds have 9 air sacs, but the Common Raven as a member of the Passeriformes group only has 7 air sacs (missing 2 cervical air sacs). The sacs are grouped into anterior and posterior sacs.

Respiratory Challenges
The common raven can be found in all parts of the globe. At higher altitudes and in warmer climates the oxygen concentration in the air is lower compared to low altitude or colder climate. This is directly related to Henry's Law. Also, flight is a much more metabolically demanding movement then walking or running, and therefore we see a proportionally larger respiratory system in aves then we do in mammals.

Ventilation


Expansion of posterior air sacs pulls air in through the nares or mouth into the primary bronchus past the mesobronchus toward the posterior air sacs were a proportion of air enters the posterior air sacs while the rest is directed into the posterior secondary bronchi. The expansion of the anterior air sacs pulls the air from the posterior secondary bronchi across the papabronchi (the respiratory surface) into the anterior seconday bronchi and into the anterior air sacs. Exhalation occurs when both posterior and anterior air sacs contract and push air along the same pathway except exiting air leaves via the primary bronchus. The respiratory physiology of the Raven allows for fresh air to be pushed or pulled over the respiratory surface during both inhalation and exhalation.

The main adaptive advantage of avian respiratory systems is the unidirectional flow of air through the respiratory surface. The unidirectional flow of air ensures that the airways always receive air with the same concentration of oxygen as the outside air. There is no mixing of fresh air and stale air in the lungs like there is with mammalian ventilation. Also the tidal ventilation in birds is adaptive because it helps minimize water loss.

Gas exchange
The respiratory surface is made of parabronchi, thin hollow tubes of high surface area, that span the length between the dorsobronchus from the posterior secondary bronchi and the ventrobronchus of the anterior secondary bronchi. Perpendicular to the parabronchi are blood vessels that bring blood of low oxygen concentration and high carbon-dioxide concentration. Gas exchange occurs at each intersection between the parabronchi and blood vessels. This type of perpendicular arrangement of airflow and blood flow is known as cross-current flow. Cross-current flow is adaptive because the blood vessels cross the respiratory surface multiple times allowing for multiple point of gas exchange along each parabronchi. The final oxygen partial pressure of the blood after gas exchange is higher then that of the ambient air.

Physical and Chemical Properties of Pumping Air
The respiratory tract of birds possesses unique air movement properties. Air moves in a unidirectional flow and blood travels in a concurrent direction to air flow. An advantage of this type of system is it minimizes dead space and enables the bird to maintain a highly oxidative, active output. The respiratory system of the Common Raven is no different.

Flight is a unique feat among birds and provides them with many advantages in terms of food, predation, and movement. It is suggested that cardiovascular variables play a large part in avian flight and were naturally selected over time. .Specifically, the avian heart evolved to pump more blood throughout a bird’s body while it is engaged in flight. During rigorous activity, especially when flying, the demand for oxygen is high.

Birds proceed through the four steps of the oxygen cascade: 1. Convection of oxygen to lungs via ventilation 2. The diffusion of oxygen from the lungs into the blood stream 3. Oxygen-rich blood is transported to the peripheral tissues by convection 4. Oxygen diffuses into the mitochondria.

Fick's laws of diffusion of diffusion can be applied to oxygen cascade events in avian species. There is a proportional relationship between the tissue sheet and the surface area. Finally, in the avian respiratory system, the partial pressure of oxygen between the gas, lung, and the vascular capillaries depends upon the ventilation rate and air that is already inhaled.

Physical and Chemical Properties of Pumping Blood
Like all avian species, the blood of the Common Raven transports nutrients, oxygen, carbon dioxide, metabolic waste products, hormones, and heat. Avian blood possesses a more alkaline pH ranging from 7.5 to 7.6, and blood bicarbonate values are between 16 and 32 mmol/L. In addition, pumping blood has a carbon dioxide partial pressure of about 28 mmHg, which is lower than that of placental mammals. Therefore, bird species, including the Common Raven, seem to be in an acute state of respiratory alkalosis relative to mammals. It is also important to note that alterations of respiratory patterns in response to changing oxygen needs do not severely affect the pH of arterial blood.

Glucose, calcium and proteins are other components of avian blood’s chemical properties. Blood glucose levels range from 200 to 400 mg/dL and can increase with stress. Calcium levels are approximately 8 to 12 mg/dL, and total protein, which consists of albumin and globulins, is between 3 and 5.5 mg/dl. Since the Common Raven flies at high altitudes, efficient gas exchange between the respiratory and circulatory systems permits this species to tolerate hypoxia. Due to the unidirectional flow of air and the high oxygen affinity of avian hemoglobin, blood leaving the parabronchi has almost equivalent oxygen partial pressure as inhaled air. Avian hearts pump more blood per unit time than mammalian hearts. Cardiac output (mL/minute) can be calculated by multiplying heart rate (beats/minute) by stroke volume (mL/beat).

Like other vertebrates with closed circulatory systems, pumping blood of the Common Raven can be described by several physiological principles. These principles and laws include diffusion, blood viscosity, osmotic pressure, LaPlace's Law (Young-Laplace Equation), Poiseuille's Law (Hagen-Poiseuille equation), and the Frank-Starling law of the heart. Furthermore, it is important to note that the osmotic pressure of the Common Raven is low compared to mammalian species. Reduced osmotic pressure is due to a lower concentration of plasma albumin protein.

Heart
The heart of the Corvus corax is a four chambered system. This allows for a complete separation of the oxygenated blood from the deoxygenated blood in the heart. This is much like the heart of mammals, but there are key differences in the heart structure to allow better oxygen uptake and dispersal throughout the circulatory system. One key difference in the Corvus corax heart structure is the rightward orientation of the aortic arch. A rightward orientation allows it to function as efficiently as the mammalian aortic arch, but with less actual mass. Another feature of the heart is the branching of two brachiocephalic vessels off the aorta at the base of the ascending aortic arch; both brachiocephalic vessels are quite large and are the same diameter as the aortic arch. Deoxygenated blood is brought into the heart from two precava veins that meet with the sinus venous. The posterior blood is brought into the postcava before entering the right ventricle. Note that the Common Raven’s heart, like most avian organisms, lacks a superior vena cava. On the exterior of the heart, the key structures are the left and right atrium, the left and right ventricles, and the apex of the heart, which is quite muscular on the side of the left ventricle. The final arteries to note are the coronary arteries, which actually deliver oxygen and nutrients to the heart itself in order to maintain homeostasis. Furthermore, the Corvus corax is homoeothermic, so its heart must efficiently pump enough oxygenated blood to the cells to sustain its high metabolism and body temperature.

As you move into the interior section of the heart, there are a series of different valves that help to control the directional movement of blood. These include the pulmonary, aortic, mitral, and tricuspid valves. The valves close when the heart contracts, preventing back flow of blood. As the heart relaxes, the valves expand, allowing blood to flow back into the chambers.

The movement of blood begins as it travels through the tricuspid valve into the right ventricle. In addition, there is a small amount of muscle surrounding the right ventricle. This lack of muscle is required so the blood can pass over the lungs through pulmonary arteries without causing lung damage from high blood pressure. One major adaptation the Corvus corax has is the design of the cross-current flow system. A cross-current flow system occurs where several capillary beds pass over the lungs, allowing more oxygen to diffuse across the membrane and into the bloodstream. Next, the blood returns to the left atrium through the pulmonary veins, which are reddish in coloration compared to the dark blue or purple color of the rest of the venous system. The blood is pumped into the left atrium through the mitral valve. The left atrium, nicknamed the ‘workhorse,’ is the most muscular section of the heart and is responsible for maintaining blood pressure and pumping the oxygenated blood to the rest of the body. The blood finally leaves the heart muscle from the aortic valve and moves through the arteries to its destination.

Arteries
The major arteries of the Corvus corax begin with one major artery that quickly starts to branch off into smaller and smaller arteries. Branching finally ends at the capillary beds, which slows the movement of blood due to a large surface area. Ultimately, a decrease of blood pressure occurs at the capillary beds. This permits the most efficient diffusion possible for the organism. The arteries themselves are composed of many different layers of membrane to allow the blood to pass through at high pressure without rupturing the artery itself. Both arteries and veins have these membranes, but the arteries' muscular membrane portion, known Tunica media, is much larger than the tunica media of the veins. The branching of the arteries begins when the brachiocephalic artery branched off of the ascending aorta. Next, the common carotid arteries branch from the brachiocephalic artery and to the subclavian arteries located under the clavicular region of the body. In the Corvus corax, there is a key difference in the pectoral arteries. Unlike most organisms in which the pectoral arteries branch off quite quickly, the Corvus corax’s pectoral artery remains large in size. This accommodates the enormous supply of oxygenated blood to the pectoral muscles when in flight. As the blood hits the capillaries the venous system begins its work to move the blood through the body back to the heart to be re-oxygenated.

Veins
Like other bird species, the venous system of the Corvus corax is composed of the hepatic portal and renal portal systems. One advantageous adaptation they have developed is the ability to bypass the renal system completely when performing strenuous physical activity. Bypassing occurs by pushing blood through the large venous valves located at the cross between the renal veins and iliac veins. The hepatic portal system uses valves to push blood from the capillaries in the intestines, through the renal vein, and to the capillary bed located in the liver. At the liver, excess fats and carbohydrates are stored in reserve to nourish the rest of the body. As well, the capillary bed helps to deliver nutrients to the liver before moving the deoxygenated blood into another capillary bed. At the second capillary bed, the deoxygenated blood is collected and sent through the renal vein to the inferior vena cava where it begins the cycle again. Note that only veins have valves to stop the back flow of blood and to allow it to move up the body back to the heart.

Blood composition
The blood composition of the Common Raven is similar to that of most avian species. In general, the blood is composed of plasma and cells. Plasma contains approximately 85% water and 9-11% protein. The remaining components include glucose, amino acids, hormones, electrolytes, antibodies and waste. Erythrocytes, or red blood cells, of the Common Raven are elliptical with a centrally located oval nucleus. Concentrations in avian species are between 2.5 and 4 million red blood cells per cubic millimetre. The red blood cells of birds are larger than those of mammals and have a short life span of 28 to 45 days. Common Ravens’ erythrocytes contain two components of hemoglobin. Hemoglobin A accounts for 60% to 90% of the total, and the remainder is comprised of hemoglobin D. Like avian erythrocytes, thrombocytes also contain a nucleus, but instead are involved in hemostasis. Also, the white blood cells include lymphocytes, heterophiles, monocytes, and eosinophils.

Thermoregulation
The main response to body temperature falling below the lower critical temperature is to employ shivering thermogenesis. The large flight and leg muscles undergo rapid cyclical contractions. Muscle contractions require the burning of ATP through inefficient metabolic reactions that produce heat and in turn elevate body temperature. During cold months birds including the common raven will store an increased amount of glycogen in the muscles to be used as fuel for shivering. The catabolism of food inside the raven is also not efficient and releases energy as heat. Also, the increase in fat acts as an insultor. So although generally food is scarcer in the winter, ravens will continue to strive for a large glycogen reserve because of its compounding benefits. The draw back of an increased metabolic rate is the increase in respiration rate due to the metabolic demand for oxygen. An increase in the respiration rate leads to cooling of the bird through evaporative cooling in the lungs. To counter this effect birds can employ a counter current system of blood flow in the nasal passages that prevents inspired air from warming up to much and expired air to remain cool. This is adaptive because birds show an increase in oxygen levels without an increase in their rate of respiration when breathing cold air. The maintenance of cold air during ventilation, minimizes water loss since cold air holds less water, and increases oxygen absorption efficiency, because the bird lungs were shown to absorb more oxygen from cold air then from warm air. Thus the increased demand for oxygen is balanced with little evaporative cooling from water loss. Birds have the ability to raise the feathers on their skin, this ptilomotor response is commonly known as “goose bumps”. Pushing out the feathers creates an air gap for warm air to accumulate rather than have heat lost through conduction of more conductive material like the feathers of the bird. This reduces heat exchange since air is a good insulator. Regulated hypothermia is another adaptation that passerines like the corvus corax process. Regulated hypothermia is initiated in times of low metabolic demand like sleeping. We know that since heat transfer is dependent on the extent of the temperature gradient between the ambient temperature and body temperature, and so a lower body temperature is maintained in order to conserve energy and reduce heat loss over night.

Ravens also employ a host of adaptations for when their body temperature exceeds the upper critical temperature. Panting, an increased breathing rate, increases evaporative cooling and cools the raven. Some birds who have a highly vascular bill use this feature to lose heat. Since the bill has low metabolic needs blood sent to the bill may be used for cooling the blood before it returns to the brain and body.

Bergmann’s rule states that species that inhabit a wide range of climates tend to be larger in cold climates then in warm climates. A larger body size, and there for a larger volume to surface area is adaptive in cold temperatures because there is less surface for which heat exchange can occur. And vice versa for cold climates.