Why has natural selection favored the evolution of double circulation in birds and mammals?

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Birds and mammals are endothermic or warm-blooded animals, and they use more energy than reptiles and amphibians. A double circulatory system helps to maintain a higher metabolic rate and meet the oxygen demand required by their active lifestyle. The separation of the systemic and pulmonary circulation in a double circulatory system ensures that oxygenated blood is supplied efficiently to the body tissues. This allows birds and mammals to have a higher aerobic capacity, which results in better endurance and makes them better suited for a variety of ecological niches.

Why Has Natural Selection Favored the Evolution of Double Circulation