Mechanism of thermo-genesis in birds and mammals

Heat Production in Birds and Mammals:

In endotherms heat generation can warm the body as it dissipates throughout tissues and organs. Birds and Mammals can generate heat (thermogenesis) by muscle contraction. ATPase pump enzymes, oxidation of fatty acids in brown fat, and other metabolic processes.

Shivering thermogenesis:

Every time a muscle cell contracts and the hydrolysis of ATP molecules generate heat. Both voluntary muscular work (e.g. running, flying, jumping) and involuntary muscular work like shivering generate heat. Heat generation by shivering is called shivering thermogenesis.

Non Shivering thermogenesis:

Birds and Mammals have unique capacity to generate heat by using specific enzyme the ATPase pump enzymes in the plasma membrane, of most cells. When the body cools, the thyroid gland releases the hormone thyroxine. Thyroxine increases the permeability of many cells to sodium (Na⁺) ions, which leak into the cells. ATPase pump quickly and pump these ions out. In the process ATP is hydrolyzed, releasing heat energy. Hormonal triggering of heat production is called non shivering thermogenesis.

Brown fat:

Brown fat is a specialized type of fat found in newborn mammals; in mammals that live in cold climates and in mammals that hibernate. Brown colour of this fat comes from large number of mitochondria with their iron-containing cytochromes. Deposits of brown fat are beneath the ribs and in the shoulders. Large amount of heat is produced when brown fat cells oxidize fatty acids because little ATP is made. Blood flowing past brown fat is heated and contributed to warming the body. The basal metabolic rate of birds and mammals is high and also produces heat as an inadvertent but useful by product.

Hypothalamic control by thermogenesis:

In amphibians, reptiles, birds and mammals specialized cells in the hypothalamus control thermoregulation. It has heating centre and cooling centre. Heating centre controls vasoconstriction of superficial blood vessels, erection of hair and fur and shivering or non shivering thermogenesis.

The cooling centre controls vasodilation of blood vessels, sweating and panting over all feedback mechanism (with hypothalamus acting as thermostat) trigger either the heating or cooling of body and thereby control body temperature. Specialized neuronal receptors in the skin and other parts of the body sense temperature changes. Warm neuronal receptors excite the cooling centre and inhibit the heating centre. Cold neuronal receptors have opposite effects.

Hibernation:

During winter various endotherms (e.g. bats, wood chucks, chipmunks and ground squirrels) go into hibernation. During hibernation metabolic rate slows, as do the heart and breathing rates. Mammals prepare for hibernation by building up fat reserves and growing long winter pelts. All hibernating animals have brown fat. Decreasing day length stimulates both increased fat deposition and fur growth.

Aestivation:

This state is characterized by slow metabolism and inactivity which allows certain mammals to survive long period of elevated temperature and diminished water supplies.

False hibernation:

Some animals such as badgers, bears, opossums, raccoons and skunks enter a state of prolonged sleep in winter since their body temperature remains near normal; this is not true hibernation.