Amphibians and Reptiles have air as surrounding medium and face daily and seasonal temperature changes. Mostly they are ectotherms. Most amphibians produce little heat metabolically and rapidly lose it. Behavioural adaptations keep their body temperature within homeostatic range most of the time. Their moist skin can act as a natural evaporative cooling system i.e. heat loss. It limits the habitats and activities of amphibians to warm, moist areas. Some amphibians such as bull frogs can vary the amount of mucus they secrete from their body surface to regulate evaporative cooling.
Reptiles have dry skin which reduces the heat loss through evaporative cooling of the skin. They also have an expandable rib cage which allows for more powerful and efficient ventilation. Reptiles are endothermic. They have low metabolic rate and warm themselves by behavioural adaptations. In addition some of the regulatory mechanisms found in mammals are first found in reptiles. For example diving reptiles (e.g. sea turtles, sea snakes) conserve body heat by routing blood through circulatory shunts into the centre of the body.
These animals can also increase heat production in response to the hormones thyroxime and epinephrine. In addition tortoises and land turtles can cool themselves through salivating and frothing at the mouth, urinating on the back legs, moistening the eyes and painting.
Birds and mammals are 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.
Birds and mammals sun themselves or seek shake as the temperature fluctuates. Many animals hubble to keep warm, others share burrows for protection from temperature extremes. Migration to warm climates and hibernation enable many different birds and mammals to survive the harsh winter months. The desert camel has a multitude of evolutionary adaptations for surviving in some of the hottest and driest climates on earth.
In endotherms, heat generation can warm the body at its dissipates throughout tissues and organs. Birds and mammals can generate heat by muscle contraction. ATPase pumps enzymes, oxidation of fatty acids in brown fat and other metabolic processes.
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