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

• A. It allows for direct absorption of oxygen through the skin, supplementing lung function.
• B. It enables the mixing of oxygenated and deoxygenated blood, which helps regulate body temperature.
• C. It reduces the overall energy expenditure required for blood circulation by using a single pump.
• D. It allows for the complete separation of oxygenated and deoxygenated blood, increasing the efficiency of oxygen delivery to tissues.

Answer: (D)

Verified step by step guidance

1. Understand the concept of double circulation: Double circulation refers to the separation of blood flow into two distinct circuits – the pulmonary circuit (blood flow between the heart and lungs) and the systemic circuit (blood flow between the heart and the rest of the body). This system is found in birds and mammals.
2. Recognize the importance of separating oxygenated and deoxygenated blood: In double circulation, oxygenated blood from the lungs is kept separate from deoxygenated blood returning from the body. This ensures that tissues receive blood with the highest possible oxygen content, which is critical for high metabolic demands.
3. Relate this to the metabolic needs of birds and mammals: Birds and mammals are endothermic (warm-blooded) organisms, meaning they maintain a constant body temperature. This requires a high rate of cellular respiration, which depends on efficient oxygen delivery to tissues.
4. Eliminate incorrect options: The first option about direct absorption of oxygen through the skin is incorrect because birds and mammals rely on lungs for gas exchange, not skin. The second option about mixing oxygenated and deoxygenated blood is incorrect because mixing would reduce oxygen delivery efficiency. The third option about reducing energy expenditure is incorrect because double circulation involves two pumps (right and left sides of the heart), not a single pump.
5. Conclude with the correct explanation: Double circulation allows for the complete separation of oxygenated and deoxygenated blood, which maximizes the efficiency of oxygen delivery to tissues. This is essential for meeting the high energy demands of birds and mammals.