Question

Large land mammals of the Arctic endure very low ambient temperatures and stand on frozen ground for long time periods. This can result in large differences in temperature across the body. The tissue temperature in the extremities (feet and lower legs) can be up to 25oC cooler than in the thorax.

(4pts) Draw the Hb-O2 equilibrium curve and indicate on the graph what the usual effect of increases ­and decreases in temperature on Hb-O2 binding. Make sure to label your axes and curves on the graph.

(4pts) What impact would temperature-dependent shifts have on O2 loading in the lungs and O2 delivery to tissues in the extremities?

(8pts) Based on what you know about respiratory and cardiovascular physiology, describe in what ways (how) the anatomy and physiology of these artic mammals might be different from mammals from warmer climates to compensate for this?

(4 pts) Briefly describe why what you propose in (c) is adaptive for these large arctic mammals.

Answer 1

Maintaining core temperature in the cold

The ambient air temperature in the polar regions may persist at −40°C for extended periods, regularly reaching −50°C and occasionally even −60°C, whereas in the oceans, the temperatures are close to freezing. For a homeothermic animal exposed to these conditions, sometimes with a thermal gradient between the body core and the environment of almost 100°C, the challenge is first and foremost to balance heat loss against a minimal rate of metabolic heat production. In the polar regions, where, at least on land, energy is often in short supply, this is preferentially achieved by control of heat loss to the environment. There are three main avenues for heat loss – conduction/convection, radiation and evaporation – each of which must be controlled if the animal is to maintain a stable core temperature.

For example a higher temperature promotes hemoglobin and oxygen to dissociate faster, whereas a lower temperature inhibits dissociation (see Figure 2, middle). However, the human body tightly regulates temperature, so this factor may not affect gas exchange throughout the body. The exception to this is in highly active tissues, which may release a larger amount of energy than is given off as heat. As a result, oxygen readily dissociates from hemoglobin, which is a mechanism that helps to provide active tissues with more oxygen.

Arctic mammals are difference from mammals:

Explanation-

The insulation provided by the fur of different animals varies significantly, and Scholander et al. have shown that the level of insulation increases with the thickness of the fur, which may be further enhanced by piloerection (‘fluffing’) of fur and plumage. The thickness and insulation value of fur and plumage of most polar animals change throughout the year. In muskoxen, all the under-wool, or qiviut, is shed in large sheets every spring and replenished in the autumn, whereas reindeer show similar, but less dramatic, changes, and the insulation by the plumage of ravens does not change at all. The seasonal variations in fur are less notable in small mammals than in larger ones. The fur of ermine, for instance, which has great value to man, has little value as insulation to the animal. Its thermal conductance is higher than that of other mammals of similar size, and although its colour changes seasonally, its thermal conductance does not.

Physiological defences:

Heat loss by conduction may also be reduced by cooling the peripheral tissues while the core temperature is maintained. This is achieved by reducing peripheral and, in particular, cutaneous circulation, and can be visualized by means of infrared photography

Restricting evaporative heat and water loss in the cold and selective brain cooling when warm, by nasal heat exchange.

Most polar animals prepare themselves for periods of reduced food availability by deposition of body fat during times of plenty in summer and early autumn

Adaptation of truly Arctic and Antarctic mammals and birds to the challenges of polar life. The polar environment may be characterized by grisly cold, scarcity of food and darkness in winter, and lush conditions and continuous light in summer. Resident animals cope with these changes by behavioural, physical and physiological means. These include responses aimed at reducing exposure, such as 'balling up', huddling and shelter building; seasonal changes in insulation by fur, plumage and blubber; and circulatory adjustments aimed at preservation of core temperature, to which end the periphery and extremities are cooled to increase insulation. Newborn altricial animals have profound tolerance to hypothermia, but depend on parental care for warmth, whereas precocial mammals are well insulated and respond to cold with non-shivering thermogenesis in brown adipose tissue, and precocial birds shiver to produce heat. Most polar animals prepare themselves for shortness of food during winter by the deposition of large amounts of fat in times of plenty during autumn. These deposits are governed by a sliding set-point for body fatness throughout winter so that they last until the sun reappears in spring. Polar animals are, like most others, primarily active during the light part of the day, but when the sun never sets in summer and darkness prevails during winter, high-latitude animals become intermittently active around the clock, allowing opportunistic feeding at all times. The importance of understanding the needs of the individuals of a species to understand the responses of populations in times of climate change is emphasized.