Thermoregulatory Mechanism in Cold Stress
It is widely accepted that the animals inhabiting higher altitudes or cold environments obey Bergmann and Allen’s rule of adaptation. Bergmann’s rule states that the body surface area of an animal determines its heat dissipation capacity, the volume determines its ability to generate heat and the surface area to volume ratio fixes its thermoregulation capacity.
In simple terms, it can be understood that endothermic animals will have lower surface area to volume ratio in high altitude regions. Allen’s rule proclaims that among the endotherms, length of extremities reduces as the altitude increases as a means of thermoregulatory adaptation mechanism. It is well established that both the rules hold well in mammals and birds. According to both of these thermoregulation hypothesis, larger body size along with smaller extremities helps the animals to conserve more heat in cold environmental conditions and in contrary, smaller body size with larger extremities helps in regulation of heat dissipation in warmer environments.Morphological traits such as plumage are playing role in cold adaptation of birds, however, unfeathered portions of leg and feet are critical during cold exposure. Plumage of birds acts as a critical barrier between the bird’s body surface and the external environment and helps in meeting out 90% of the animal’s requirement of insulation. Feathers are vital importance for birds’ insulation to support thermoregulation of birds. Heat transfer through plumage will occur by conduction and convection via air, conduction through solid portions of feathers and through radiation. The thermoregulatory capacity of feathers is associated with various traits like plumage morphology, density and depth. During cold stress, birds significantly reduce the amount of heat dissipation from legs and feet, which can be clearly explained by the vascular counter current mechanism of heat transfer and conservation.
Bill size also has the potential to influence the heat conservation, in the context of employing thermoregulatory behaviour of back rest to keep birds warmer under cold conditions. As per Allen’s rule of adaptation, higher latitudes with lower temperature tend to bear smaller bills than the birds from lower latitudes. The uninsulated and vascularised bills of birds can serve as thermal window for heat exchange.It is well accepted fact that the thermal needs of birds in response to cold stress are met out largely by shivering thermogenesis. Under immediate exposure to cold environment, sudden increase in metabolic rate associated rigorous activity of muscles leads to shivering thermogenesis and is very much essential for thermoregulation for cold adaptation. But in progress, chronic cold exposure gradually replaces shivering thermogenesis with non-shivering thermogenesis and in fact improves the cold adaptation capacity. Non-shivering thermogenesis generates heat by processes involving chemical energy without muscular contraction. Under very severe cold environments, both shivering and non-shivering thermogenesis mechanisms help the bird to cope with heat generation.
A foremost physiological adaptation to cold stress in birds is accumulation of body fat. This fat accumulation acts as an extra fuel for thermoregulation of birds under cold stress, serves as a reservoir to tackle long winter nights and most importantly helps in to act as a response against sudden drop in temperature. Above all, accumulation of extra layers of fat acts as an added insulation against cold. Another important cold adaptation mechanism observed in birds is development of winter plumage with increase in quantity of feathers for better insulation and was covered briefly in preceding sections. As an extreme response to cold stress, bird chooses migration as a method of adaptation to avoid deleterious effects of winter and for better survival. Migratory behaviour in birds is exhibited as three ways of shifting the positions, such as latitudinal shift, altitudinal shift and habitat shift and a bird chooses the combinations of any of these shifts as an adaptation mechanism in order to provide themselves with maximum chance of survival.
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