Why mammalian and aves are warm blooded but amphibian and reptiles are cold blooded?

Answer 1

Evolution did not require amphibians nor reptiles to be warm blooded as they began in the ocean/waters making them "ectothermic" they only existed in waters that was best suited for their body temperature. you would never see an amphibian in the poles nor in extreme cold climates. they tend to exist most anywhere near the equator and the tropics of Cancer and Capricorn, areas that are highly tropical to sub tropical regions. those in sub tropical regions and on the boundaries of the Cancer and Capricorn tend to hibernate underground where it is warmer and come out in spring to enjoy the warmth of the sun, to feed and breed. Since the "Cambrian explosion" and they (reptiles and amphibians) moved onto land still remaining cold blooded they do feel the difference in body temperature due to the climatic changes they encounter ie: on warm days you will see them sunning themselves to get warmth from the sun and hibernating underground where it warmer during the winter season. this lead to the evolution of mammals and aves(which are technically Reptiles) which required them to be warm blooded in order to survive the elements.

Mammals and Aves evolved to be warm blood (endothermic) to be better able to battle the climatic elements and also give them a wider range of habitation eg: Polar Bears have warm blood, thick layers of fat and very think waterproof fur to keep them warm and exist in the northern hemisphere in the Arctic Circle where as The Emperor Penguin(an Ave) too is warm blooded, layers of fat covered with very thick downy feathers with a thick layer of outer long waterproof feathers to enable them to endure the unforgiving elements of the Antarctic Circle. Woolly Mammoths during their existence also had evolved to live in unforgiving territories in the Northern Hemisphere eg: Siberia/Alaska with huge layers of fat and very thick woolly fur to keep them warm. this is true for all Mammals and Aves (since late 2011 now officially known only as birds not Aves and listed as Reptiles by Taxonomists and recategorised in the Phylogenic tree. Up until 2011 Birds/Aves lived in their own "CLASS". However due to their prehistoric relations and the discovery of a preserved Ave/Bird in a recent excavation that revealed a Dinosaur with feathers intact they have now been reclassified into the CLASS: Reptilia even though they are warm blooded! their common traits with true Reptiles far outweighed those with mammals and hence were moved officially to the Reptilia CLASS leaving many major UNIVERSITY text books to be updated). where other mammals are concerned in that they belong in the Phylogenic tree under Kingdom: Animalia Phylum: Chordata Class: Mammalia, and in your question "WHY THEY ARE WARM BLOODED" is quite easy, there are many aspects to having warm blood than just location and climate. It comes down to reproduction and birth of new generations. As Mammals give "Vaginal" birth to their young, warm blood is necessary to keep the young warm in the womb and then also is required in the production of breast/mammary gland heat to produce milk to nourish their young. In the case of Marsupial Mammals, they also have a pouch to nurse their young to keep them warm where the teats can be found giving the young a perfect place to nourish and grow safely tucked away and warm till they are ready to leave their mother. Your question is a great question but one that is not simple to answer in just a few lines as it is a very COMPLEX topic and very broad in explanation. i have endeavoured to answer it as broadly as possible. One Mammal i have left out in this answer that does rate a mention of WHY? is Dolphins and Whales, especially Humpback Whales that travel from the Antarctic north to Hawaii every year along the eastern seaboard of Australia to calve in the warm waters of Hawaii, mate, feed and feed enough to reboost their body fat for their trip back with their calf and pregnant once again to the Antarctic where they will remain feeding and nurturing their young calf till it is time for the next season to head north once more for calving. a young Humpback will remain with its mother for approx 2 years before it is big enough and strong enough to do the journey on their own and begin their own generation of Whales. Being warm blooded for them is highly important as they require it for many reasons not just the extremes of temperature that they encounter but for very need of their survival as a species!