Birds and mammals have homeothermic endotherms; they can maintain body temperature between 35 and 42°C with metabolic heat and can live in all habitats.
Birds: They have no sweat glands; birds pant to lose heat through evaporative cooling. Some species have highly vacularized pouch (gular pouch) in their throat that they can flutter (process called gular flutter) to increase evaporation from the respiratory system. Feathers especially downy type feathers trap a layer of air next to the body to reduce heat loss from the skin aquatic species who lose heat from their legs and feet, have a rate mirabile in their legs to reduce heat loss.
Mammals: In cold regions the arctic fox and barren ground caribou, also have retemirabile in their extremities (e.g. legs, tails, ears, and nose). Animals in hot climates such as jackrabbits have mechanisms (e.g. large ears) to rid the body of excess heat. Thick pelts and a thick layer of insulating fat called blubber just under the skin help marine animals such as seal and whales, to maintain body temperature of around 36 to 38°C. In the tail and flippers, which have no bludder a counter current system of arteries and veins helps to minimize heat loss.
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.
Fission and Budding:
Fission: Protists and some multi-cellular animals (cnidarians, annelids) may reproduce by fission. Fission is the division of one cell, body or body part into two. In this process, the cell pinches in two by an inward furrowing of the plasma membrane. Binary fission occurs when the division is equal; each offspring contains approximately equal amount of protoplasm and associated structures. Binary fission is common in protozoa. In fission the plane of division may be asymmetrical, transverse or longitudinal, depending on the species e.g. Planarian reproduces by longitudinal fission. Some flatworms and annelids reproduce by constrictions in the middle of the body. This is called multiple fission.
Budding: In cnidarian Hydra and many species of sponges, certain cells divide rapidly and develop on the body surface to form an external bud. The bud cells proliferate to form a cylindrical structure which develops into new animals, usually breaking away from the parent. If the buds remain attached to the parent, they form a colony. Internal budding (freshwater sponges) produces gemmules which are collections of many cells surrounded by body wall. When the body of parent dies and degenerates each gammule gives rise to new individual.
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