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Physiological challenges placed on organism
Temperature ranges are vital for birds’ survival, and bird migration patterns are the first signal that there is a change in the environment. As food becomes scarce, birds have the option of staying and foraging for food or leaving and looking for better food elsewhere. The common raven lives in a wide variety of environments in which it searches for food and shelter. The common raven must be able to handle varying degrees of seasonal changes and extreme temperatures. In North America, the Common Raven flies at high altitudes where air temperature can reach +45-50 degrees celsius. They also inhabit areas where the temperature drops below freezing, and they can withstand extreme temperatures of – 50 degrees celsius. Common ravens must regulate their body to sustain an optimum temperature while it copes with fluctuating ambient temperatures. Most birds, including the common raven, are small and have low weights. Such low masses involve low thermal inertia and narrow zones of thermal neutrality.

Thermoregulatory laws that govern heat exchange
The common raven is a homeothermic endotherm. Fourier’s law of heat conduction can be applied when discussing avian muscles and fat. Specifically, the raven is allowed to maintain its body temperature regardless of the external temperature. When the common raven is faced with extremely cold temperatures, its body temperature remains high during cold stress. Vascular function of the skin is a small component of the overall insulation. Also, the feathers play a role in mediating insulation and preventing heat loss. The common raven cools its peripheral areas by vasoconstriction. It accomplishes this by increasing the tissue insulation.

We can derermine heat loss through convection using the formula Convection=hc*(Ts-Ta). This formula emphissies the importance of a heat gradient in heat loss through convection. Namely a greater termperature gradient will reasult in greater heat lost or gained through convention. Heat loss is often used as index of insulation. Additionally, convection can be applied to avian species where hc is the convection coefficient.

Studies have shown that the common raven possesses excellent cold tolerance due to its ability to maintian its body tempratrue. This adaptation acts to sustian a consistant basal metabolic rate Not only can the common raven tolerate very cold temperatures, it can cope with extremely hot temperatures, as well. Various species of ravens that thrive in warmer climates do so because of their small body size. One way that ravens can effectively function in hot environments is by maintaining body temperatures that are lower than the ambient temperature.

Behavioral and physiological adaptations for thermoregulation
The Corvus corax uses many different adaptations in order to regulate its body temperature in order to keep it within a desired range for survival in different climates. Since the common raven does not migrate when the climate becomes colder it must use keep itself warm through the harsh winter, it does this with the use of plumage and body size to surface area ratio. Since most of the Corvus corax’s body is covered in feathers its main adaptation is that the Corvus corax uses is its plumage as buffer between the bird and its environment; in colder temperatures such as that of the harsh northern winters the Corvus corax will puff up its feathers in order to trap air between its feathers and skin. Since air has a lower conductance of heat, once the air trapped in the plumage heats up it acts like a coat or bubble of warm air that helps to lower the amount of energy the Corvus corax must use to generate enough heat to maintain a certain body temperature. On the other side of the spectrum, the common raven will plead and prune its feathers in times of increased environmental temperatures in order to release any trapped air that may increase its body temperature. Another behavioral adaptation the Corvus corax uses is bathing which not only cleans its feathers of parasites but also cools its body temperature by the blood in its feet cooling from the water it stands in while bathing. The Corvus corax also will use a technique known as gular fluttering which is the relaxation and contraction of its throat muscles which promotes increased rate of evaporative cooling. This need to keep metabolic rate low is key to survival in the Corvus corax because with a smaller body size to surface area ratio there is greater heat loss to the environment and a higher metabolic rate compared to larger organisms.

Another major adaptation the Corvus corax uses is the implementation of a countercurrent flow system of blood through its body; especially in its legs as well as decreasing the surface area of the leg so less heat is lost to the external environment. Since the legs are designed in a way that there is such little surface area where heat radiation can occur, there is very little heat loss through the legs to the surrounding environment. Using the countercurrent system within such a small area such as the legs of the Corvus corax allows for heat from the blood in the arteries travelling down the leg to be reabsorbed by the blood in the veins travelling from the foot up back to the heart of the organism. This causes the foot of the organism to remain quite cold but still keep the core body temperature of the Corvus corax to remain in a stable degree. This reuptake of heat energy allows the common raven to decrease its metabolism/energy expenditure. Another physiological adaptation the Corvus corax uses is it will induce torpor (will actually decrease its metabolic rate) daily in times of harsh weather, it can do this because over the years the Corvus corax has evolved to have a greater range of its thermoneutral zone. Being able to lower its body temperature closer to that of its surroundings allows the common raven to conserve more energy needed to survive. The final big adaptation the Corvus corax uses is shivering thermogenesis which the rapid random contractions of muscles in order to use up adenosine triphosphate also known as ATP and in turn generate heat.

Specialized organs, anatomy and metabolic regulation
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 insulator. 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.

Special adaptations
Common Ravens inhabit areas affected by seasonal temperature changes. Those living in northern regions are exposed to extremely cold temperatures during the winter months and large fluctuations in ambient temperature throughout the rest of the year. One way the Common Raven’s body copes with the frigid temperatures is by having a lower critical temperature of 0°C.

Researchers investigating Common Ravens inhabiting the interior of Alaska measured ravens’ metabolic and thermal responses to air temperatures between +35°C and -80°C. After analyzing the data, four conclusions were proposed about thermoregulatory and metabolic adaptations in Common Ravens. First, metabolic rates of ravens acclimatized to either summer or winter remained relatively constant at all of the experimental temperatures. For ravens acclimatized to summer climates, the resting metabolic rate during the day was 8.4 kcal/hour. Next, skin and cloacal temperatures demonstrated a very small increase in thermal capacity for subjects used to winter temperatures. The final and most important finding of the study indicated that northern Common Ravens’ cold tolerance is due to continuous high heat production. Cold tolerance was not attributed to a reduction of heat loss through insulative adaptations.

In another study, the maximum ability of passerine and nonpasserine species to dissipate excess heat produced from an increased metabolic rate resulting from exercise and heat stress was measured. Common Ravens, which are a passerine species, had a basal metabolic rate of 6 W during the summer. Also, the ravens were able to increase the quantity of dissipated, nonevaporative heat loss without increasing evaporative heat loss. Through this physiological mechanism, Common Ravens can conserve water and efficiently transform metabolic energy into mechanical work during the summer when temperatures are high. The researcher also suggested that reduced evaporative heat loss is related to the efficient organization of the circulatory and respiratory systems.