The common raven (Corvus corax), also known as the northern raven, is a large, all-black passerine bird. Found across the Northern Hemisphere, it is the most widely distributed of all corvids. Their Northern range encompasses Arctic and temperate regions of Eurasia and North America, and they reach as far South as Northern Africa and Central America. The common raven is an incredibly versatile passerine to account for this distribution, and their physiology varies with this versatility. This article discusses its physiology, including its homeostasis, respiration, circulatory system, and osmoregulation.
Physiology
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.Berg, R (1999). Corvus corax: Common Raven. Animal Diversity Web. Retrieved 20 October 2013. 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.Eduardo, J., Bicudo, P. W., Buttemer, W. A., Chappell, M. A., Pearson, J. T., Bech, C. (2010). Ecological and Environmental Physiology of Birds. New York: Oxford University Press. pp. 134-186. . 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 nine air sacs, grouped into anterior and posterior sacs, but the common raven as a member of the Passeriformes group only has seven air sacs (missing two cervical air sacs).Christal, Pollock "Passerine Anatomy" 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. Also, flight is a much more metabolically demanding movement then walking or running, and therefore we see a proportionally larger respiratory system in birds than in mammals.
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 can be applied to oxygen cascade events in avian species. There is a proportional relationship between the volume of the pulmonary capillaries and respiratory surface area. Avian respiration follows a system of countercurrent flow in which inspiratory and expiratory processes are dependent on an efficient rate of diffusion to oxygenate blood through the air capillaries. The optimization of the volume of tissue within the capillaries in conjunction with the large respiratory surface area allow for an effective diffusion rate. 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.
Through the respiratory pathway of evaporative water loss, the common raven is able to effectively maintain body temperature within their variable range. The mechanism by which Passeriformes such as the common raven regulate evaporative water loss comes in the form of respiration through panting. Unlike mammalian endotherms, birds do not have sweat glands and instead must rely on respiratory and cutaneous gas exchange to regulate body water. Passeriformees like Corvus corax are also reliant on panting, as they depend much less on supplementary processes such as cutaneous evaporative heat exchange and gular fluttering. This involves a gradual increase in the exchange of gases and breathing rate. This increased breathing rate is induced by an increased tidal volume along with a greater ventilation of the mucosal surfaces. As well, a higher breathing rate leads to an increase in the resting metabolic rate of the individual.
A unique feature of avian respiration involves the usage of turbinates within the nasal cavity during routine breathing. The nasal cavities in avian taxa are the first organ to moderate the inhalation of air humidity during periods of rest. The epithelial-lined turbinates within these cavities act as countercurrent heat exchangers. During this exchange, air inhaled becomes saturated as it brought further down into the respiratory tract. The air being exhaled stays saturated, allowing for a recovery of heat and moisture.
Circulation
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. 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,Hill, R. W., Wyse, G. A., Anderson, M. (2012). Animal Physiology (3 ed.). Massachusetts: Sinauer Associates, Inc. Sunderland, Massachusetts. pp. 617-646. blood viscosity,Sturkie, P. D., Griminger, P. (1976). Avian Physiology. Berlin: Springer. pp. 53-75. osmotic pressure, LaPlace's Law (Young-Laplace Equation), Poiseuille's Law (Hagen-Poiseuille equation), and the Frank-Starling law of the heart.Hill, R. W., Wyse, G. A., Anderson, M. (2012). Animal Physiology (3 ed.). Massachusetts: Sinauer Associates, Inc. Sunderland, Massachusetts. pp. 647-678. 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.
Blood composition